Why are gas boilers (for one pipe steam) either on at maximum output or off w nothing in between ?

SlowYourRoll

Jamie Hall said:

Let's consider your two pictures for a moment, here, @SlowYourRoll . There is an unspoken assumption involved in them, which you have to take into account: the assumption is that no more steam is available at the inlet to the radiator. However, this is not the case, so long as the boiler is making steam at one end of the pipes and all the valves are open. If steam is available, it will continue to enter the radiator and condense, thus creating just low enough pressure to encourage more steam to come in. And so on.

Thanks Jamie! So is there an error Dan's book then? I'm specifically looking at about pages 174-179 of The Lost Art Revisited...

Jamie Hall

I believe that I interpret what happens in the system a little differently from @DanHolohan . Perhaps a different way of explaining it; perhaps a slightly different viewpoint. Don't know. Take your pick.

ethicalpaul

I think we are talking about the time just after firing ends

So we have:
- a little air in the radiator
- mostly steam in the radiator (it was constantly condensing and getting replaced with new steam from the supply...now it's about to collapse)
- the vent is closed

The steam starts to collapse. This happens quickly. When it collapses, the remaining air expands to fill the radiator (and some of the supply line too), just as Dan says.

But Dan seems to say that air is under pressure and is holding the vent closed? I don't agree with that. The vent is still hot and is being held closed by the same mechanism that initially closed it. It holds AGAINST the pressure of outside air trying to get in, and when the vent cools, it will start to allow atmospheric air into the radiator. This is how air can get back into the system. But this may not happen during cycling because the vent hasn't cooled yet.

When the vent opens, it's not going to release that "expanded air". "Expanded air" is air that is less dense than room air, ie partial vacuum as I said above. So when the vent opens, the reduced pressure inside the radiator will pull room air into the radiator until the pressure is equal.

In a boiler pressure cycling situation (at least at my house), any vacuum will not have time to equalize before the boiler starts up again and starts supplying steam to the pipes. At that point, the "expanded air" (partial vacuum) in the system will be allowed to go back to its original volume as the steam production raises the pressure of the system back up again. This is where I see a difference between my thinking and Dan's in the page above.

I have several very low pressure gauges installed at different points of my system and I sit next to a radiator where I can hear the vent letting air out and in during heat cycles.

Jamie Hall

Perhaps the distinction is this: so long as the boiler is running the pressure stays up (not high -- a few ounces, perhaps). When the boiler stops, that pressure will fall -- and may, if the boiler stops long enough, drop into a vacuum. When that happens the air will, of course, expand. But as soon as the boiler fires up again, it will compress right back to where it was as the steam pressure comes back up.

Most steam vents will not hold a vacuum, so if the radiator does drop into a vacuum, some air will be drawn in. However, the thermostatic element is still active, so when the pressure comes back up the vent should close again.

Now a question arises in my mind. If this is indeed happening, is there more air in the radiator after such a breath (if you like) than before? I think not -- but I'd have to observe such a situation rather carefully to be positive. My first reaction would be that a certain volume of air is trapped when the vent first closes. If it opens on a vacuum, some air may be drawn in -- but so long as there is still a vacuum, the vent -- if it opens -- will stay open and won't close again until the pressure inside rises above atmospheric. But that is the same pressure it was at when the vent closed in the first place, so the mass of air (and hence the volume (should be close to the same.

Hmm... something to think about.

SlowYourRoll

Jamie Hall said:

Hmm... something to think about.

it's quite the thought experiment. i'll have to think on it myself and reread that section of the book. ultimately that part of the book is explaining the need for a cut-in pressure for the p-trol (in addition to the cut-out pressure), and so one of the things i'm wondering is if your way (for an appropriately sized boiler) would need that cut-in pressure, and if so, why would it need to be so low compared to the cut-out pressure. anyway i'll think on it, and maybe by tomorrow some others might chime in.

kenlmad

That referenced page from the Lost Art is from the section that describes why the vent's specification for "Drop-Away" pressure is important. The particular scenario in the book is 100% accurate in describing what happens when you have a boiler pressurized higher than the radiator vent's "Drop-Away" rating. If the vent stays locked shut due to pressure, you may have ineffective heating in that radiator.

That section is not describing normal operation of a properly designed system, in my opinion.

The section goes on to conclude that the Pressuretrol properly set to 0.5 psig cut-in will allow the radiator vent to reopen, i.e the vent's float will "Drop-Away", and allow air to escape and permit steam to enter the radiator more effectively again, even though the Boiler is likely oversized to create this pressure problem in the first place.

Jamie Hall

Keep in mind that the drop away pressure applies only if the thermal element is not activated. If it is, the vent should stay shut, although many do open under a negative pressure.

jhewings

Hi. In my one-pipe system, I often hear the radiator vents close again (so they must have opened) mid cycle with no pressuretrol cutout. Is this because the steam has condensed and the pressure in the radiator is now less than the operating pressure of the vent, causing the vent to open? Then more steam enters the radiator and causes the vent to close again? Prior to the vent closing the second time mid-cycle I hear a lot of air moving through the vent. Btw I am intending to vent my risers probably today with Gorton "D"s.

ethicalpaul

If yours are the "clink" ones with the liquid/gas alcohol stuff in them, then they are probably cooling during the cycle and then they open and steam reaches them again heating them up and closing them.

Mine are MoM (like Gorton) and are quiet open and close

jhewings

Ok thanks ethicalpaul. I suppose the vent is still hot but not steam hot so it opens mid-cycle after some condensation and then closes again when the steam comes back.

KMSNYC

I have some of those noisy vents - I like the reassuring bright, crisp "clink" sound. It gives one a very definite feeling that things are working! I'm very suspicious of the silence from the others. I haven't tried the smoke test with the silent ones (make some smoke nearby and see which way it blows); maybe that would allay my concerns that they are really doing their job and not silently plotting against me or goofing off. Perhaps in the future when steam comes back into style again (it's bound to, given the enthusiasm shown on this site) the vents will be provided with chips that talk the way smoke and monoxide detectors now do ("steam's on it's way - letting air out now!" or "It's hot and I'm closing" or "steam gone - letting air back in" etc. ) Joking. I have various brands, Maid of the Mist, Gorton, Hoffman and at least one other. I realize that is a violation of the one brand recommendation but I'll correct that (if I can ever can decide) once I have the mains and risers venting properly , which is the present priority and also, I hope, I'll be able to turn the boiler's pressure down a bit. It would be nice to be able to join the < 1 pound club. It's at 2 psi cutout now but cycles too often or more often than I think it should. There is a gyp board ceiling in my basement over the rafters and I'm hesitant to cut it open to provide extra vertical space for these very large vents like Gorton #75 and Hoffman #2. I may put one on the 1st floor almost at floor level, between a radiator and the riser, which is close enough to the main to vent it (I think). I'll also put large vents at the tops of the risers. In the past I replaced an elbow with a tee at that point and used the upward leg of the tee to mount the vents but I used radiator vents that I now think are too small to vent the riser. Interestingly there is a section in "Greening of Steam" pages 128 and following, that point out (one expert opinion) that there really is no difference between "mains" (horizontal with some pitch) and "risers" (vertical); they all need to be vented to get the steam to the radiators. I am kind of using that idea, although the only difference I can see in theory, is that in the horizontal portions the steam and condensate ( in this case) mostly run in the same direction but in the risers they oppose. There's a lot of discussion about that. I'm venting both; finding appropriate and practical places to vent is the key. In the past I have tapped 1/8" npt holes in steam pipe and I think I would go as big as1/4" npt but I'm hesitant to tap in anything larger even though we are talking about very low pressure as piping goes. I'm afraid there will be too few threads in the not so thick metal pipes with these larger sizes. Normally one would plumb in the appropriate fittings but I am not going to take apart this piping system to do that. Interestingly the tables in the back of "Greening Steam" page 131 show that a 1/8" nom pipe can handle the CFM of a Gorton #2, which has a 1/2" male npt fitting on it ....which surprised me. Which means (I think) that I could actually hang a Gorton #2 (a monster) off a 1/8" pipe and not restrict the venting. I probably would not do that, I would go for a 1/4" pipe, substantially stronger, less clog-prone, but it does mean (I think) that I could tap a 1/4" npt into the main or riser for a Gorton #2. Also looking into if they make some kind of compression (non threaded) fitting for steel pipe (like 1.5" or 2" nom). that way I could just cut a section out and put in a new compression piece without involving threading large pipe, removing pipe back to the last threaded connection, or union etc. I'm not set up for those kinds of operations.

SlowYourRoll

kenlmad said:

That referenced page from the Lost Art is from the section that describes why the vent's specification for "Drop-Away" pressure is important. The particular scenario in the book is 100% accurate in describing what happens when you have a boiler pressurized higher than the radiator vent's "Drop-Away" rating. If the vent stays locked shut due to pressure, you may have ineffective heating in that radiator.

That section is not describing normal operation of a properly designed system, in my opinion.

The section goes on to conclude that the Pressuretrol properly set to 0.5 psig cut-in will allow the radiator vent to reopen, i.e the vent's float will "Drop-Away", and allow air to escape and permit steam to enter the radiator more effectively again, even though the Boiler is likely oversized to create this pressure problem in the first place.

Actually I believe the section is describing the operation of all one-pipe steam systems connected to a pressuretrol. Dan even says on page 175 that: "[drop-away pressure] is the pressure at which the float inside the vent will "Drop-Away" after the first heating cycle. Drop-Away pressure is crucial because if the float doesn't "Drop-Away" from the vent hole, the vent won't vent the air that's left in the radiator after that first cycle. And if the air is trapped, the radiator won't heat beyond the first couple of sections. Drop-Away pressure is what makes one-pipe steam work."

Earlier Jamie suggested a way in which one pipe steam is supposed to work. His description ended with "Eventually steam will hit the vent and the vent will close. At that point, the steam supply to the radiator is limited to the rate at which steam can condense in the radiator -- and will continue at that rate so long as the boiler keeps supplying steam." So (as I understand it), in Jamie's ideal (right-sized boiler) system, first off it seems like Jamie is stating that there's little or no air trapped after the air vents close, and furthermore that a properly sized boiler can just keep supplying steam. In other words, it sounds to me like Jamie's system wouldn't need a pressuretrol with a low cut-in point, because there's no need in his system to periodically reopen the air vent. It could still use something to cap the max pressure, but it might as well have the cut-out point be 1.5psig and the cut-in point be 1.25psig, cause there's no need to back the pressure off mid-cycle.

I guess another way of figuring this out would be to ask the question "why does a pressuretrol need a cut-in pressure to be so low?" Dan's explanation gives a reason for that, namely that one-pipe steam is meant to cycle in order to work more trapped air out of the radiator. But if you had an appropriately sized boiler that worked the way Jamie suggested, why would you ever need the pressure to drop 1psig mid-cycle? Wouldn't it be best to get that pressure to the high setpoint and leave it there until the thermostat hits its setpoint?

KMSNYC

I am confused about the question of cycling. Certainly in the old days with coal fired boilers there was not really rapid cycling; the boiler just kept making steam all day. My sense is that tended to "even things out" and some of the issues we now have with modern boilers were not a problem. Good, clean coal. In the case of my gas boiler, which may be oversized, it builds up steam very quickly even when starting from near cold or cool water. I suspect that any modern boiler would as their water content is relatively low. It's like putting a small pot with very little water on the highest burner on the stove. When starting on subsequent cycles when the water is hot to begin with it builds up steam (and pressure) even more rapidly ; hits the cut-out quickly, drops down pressure to cut in quickly, and starts over again. Occasionally these short cycles are also interrupted by the boiler getting to low-water shutoff, momentarily (the water generally comes back presumably from the returns after a short period and almost never trips the timer circuit for adding more water). Depending on the temperature in the house (and the thermostat's setting) it takes a number (3, 4, 5) of these very rapid cycles to get to the thermostat's set point. By rapid I mean mean 5 minutes or less per cycle. Then if it's a colder day it rests for 8, 10 minutes until the thermostat again calls for heat and starts cycling again. I wish it would simply stay on at 1 lb pressure or less, somewhere between cut-in and cut-out and hang in there until the thermostat is happy, with only one cycle instead of many. It may be my own prejudice but I hate seeing equipment cycle on and off like that so rapidly. The part I dislike the most is the little door in the chimney that opens and shuts with every cycle - supposed to improve efficiency but I wonder. It is kind of a cheapo-looking gadget and one day, I feel it will get stuck in the closed position and I won't have heat, when I'm away for the weekend of course. (these kinds of devices always know when you are away. It's the 5th Law of Thermodynamics, aka "The perverseness of inanimate objects". The bread always lands with the butter side down, etc.) I haven't had a chance to investigate it yet. Wish I could increase its timing because it's stupid to have it close just as the pressure is going down to cut-in; then it closes with a scrunch sound. 10 seconds later it opens again with another scrunching sound when the boiler wants to start up. Efficient??? I'm hoping some of this cycling will be remedied by improving the venting on the main and risers. Maybe then the pressure in the system wont rise so rapidly. We'll soon see.

KMSNYC

Another question I have (maybe it belongs elsewhere) is the definition of "clean water" where boilers are concerned. I understand there not being oil in the water and some other things, but I have never seen boiler water that is not brown-rusty in color at least after it runs for a little while. Well, the first time the boiler was brand new, and we had flushed the entire system , it was clear for a little while. But not for long. I cant imagine how one could possibly keep boiler water rust free when running it through steel pipes, the newest of which might be as new as 35 years old but most probably more like 80 or 100. Plus the cast iron radiators?

Jamie Hall

As Paul Harvey used to say, "and now for the rest of the story".

Several items in your post here, @KMSNYC

First. You mention that your boiler runs quickly up to pressure and then cycles 5 or 6 times before the thermostat switches off. This tells me, almost certainly, two things: first, your main venting is inadequate -- possibly woefully inadequate. The pressure should not start to rise beyond 4 ounces to 6 ounces psi (that's ounces, not pounds!) until all the radiators are essentially as hot as they are ever gong to get. What, if any, main venting do you have on this system?

Second, in the same regard, your boiler may be significantly oversized. A properly sized boiler will not run up to a reasonable pressure cutout in any reasonable length of time; indeed, if it is really accurately sized, it may never get to the pressure cutout at all, but just quietly (or not so quietly!) sit there, humming along, until the thermostat is satisfied. An oversized boiler, unhappily, is not at all uncommon.

Third, you mention that the whole thing will start over again in 8 to 10 minutes when the thermostat calls again. That tells me that your thermostat is not programmed for steam heat. Most thermostats have a setting in the setup for "cycles per hour" and often another setting for "steam". Look through the thermostat manual and then set the thermostat for 1 cycle per hour. It's probably set for 6, unless someone did that, as the manufacturers and big box type stores assume everyone has forced air, and that is the setting for forced air.

Fourth, you mention a "little door on the chimney". Is this a motorized damper, or a barometric damper? If it's the motorized variety, they do increase efficiency -- very slightly -- but in my opinion they are an invention of the devil, and very unreliable, and better locked open and left that way. If it's a barometric damper and makes any sort of noise at all, it's not installed correctly and needs to be fixed. It should swing perfectly freely, quite literally when you blow on it.

Fifth, in the next post you mention "clean water". It is perfectly normal for steam boiler water to be the colour of weak tea -- or sometimes not so weak tea. So long as it is free of solids and oils, it's fine.

ethicalpaul

Honestly I got slammed by the wall of text a ways up there and I'm lost in the details and no longer know what we're talking about but as Jamie said there were some issues with some of the ideas up there.

I'll comment on one of them that caught my eye:

Wouldn't it be best to get that pressure to the high setpoint and leave it there until the thermostat hits its setpoint?

This question shows a possible misunderstanding about how the boiler works with the pressuretrol. Most boilers are oversized. That means that, if left unchecked, they will just keep building pressure because the radiators can't condense the steam fast enough to keep up with the boiler's production.

So in those cases, there is no way to "leave it there until the thermostat hits its setpoint"...the pressuretrol doesn't "leave it there". It kills the boiler when it hits a certain pressure (hopefully 1.5psi). I'll say it again to be real clear: The pressuretrol doesn't set the pressure the boiler runs at. It just kills the boiler and then lets the boiler start up again.

Then it has to turn back on at some point, right? Well, over the decades people have decided that .5 is a pretty reasonable time to do it. By the time the boiler starts back up the pressure is likely to be about 0, but it usually won't have dropped into vacuum.

If you had it come back on at say 1psi, then it would only be off for like 15 seconds. People already freak out from "short cycling" phobia with it cutting in at .5--it would be way worse with a higher cut-in.

SlowYourRoll

ethicalpaul said:

Honestly I got slammed by the wall of text a ways up there and I'm lost in the details and no longer know what we're talking about but as Jamie said there were some issues with some of the ideas up there.

I'll comment on one of them that caught my eye:

Wouldn't it be best to get that pressure to the high setpoint and leave it there until the thermostat hits its setpoint?

This question shows a possible misunderstanding about how the boiler works with the pressuretrol.

That question was posed for Jamie's suggested system, not for boiler systems as Dan described. For boiler systems as described in that section of The Lost Art... the answer to that question is "no, you can't do that because one-pipe steam needs the boiler to cycle to operate most efficiently." But the way Jamie described an optimal system, there's no need to cycle the boiler.

Sorry, that wouldn't be clear if you were just skimming my post. The answer is supposed to be "no" based on Dan's description of how it works, but (as far as I can tell) with Jamie's description there essentially isn't a need for a low cut-in pressure. The way Jamie describes it, the steam pushes air out of the radiator, the air vent closes, and then the steam just keeps flowing to the radiator until the thermostat is satisfied.

SlowYourRoll

ethicalpaul said:

Then it has to turn back on at some point, right? Well, over the decades people have decided that .5 is a pretty reasonable time to do it. By the time the boiler starts back up the pressure is likely to be about 0, but it usually won't have dropped into vacuum.

If you had it come back on at say 1psi, then it would only be off for like 15 seconds. People already freak out from "short cycling" phobia with it cutting in at .5--it would be way worse with a higher cut-in.

Actually, that section of The Lost Art describes how and why people settled on 0.5, and it wasn't by just trial and error over the course of decades. It was to specifically address the phenomena mentioned in that section, where trapped air expands once the air vent closes, the expanded air prevents whole sections of the boiler from receiving steam, and the only way to get that trapped air out is to cycle it by a substantial enough amount.

So I'm basically pointing out different ways of confirming that that phenomena occurs. Once such way is to point out that the whole purpose of the pressuretrol and cycling between 1.5 and 0.5psi is to give the system a chance to drive more trapped air out on each subsequent cycle. So if this phenomena didn't exist, there wouldn't be a need to cycle at all. My point is that the way Jamie describes it, there is no need for cycling.

Jamie Hall

@DanHolohan ! Where are you?!

PMJ

No air can ever leave a radiator that is not above atmospheric pressure. Any "expanded" air that filled a greater volume of the radiator with a closed vent that was vacated by collapsing steam must return to its original volume before it can leave. Advancing steam on the next burn accomplishes this volume change quite easily with the vent still closed and with less effort than would be required to push air at atmospheric pressure.

Every radiator in my 2 pipe system is connected to a dry return. All of the dry return and most of the radiators are filled with "expanded" air on the off cycle. This air quickly returns to its original volume and offers no resistance to the flow of new steam prior to opening a vent anywhere.

Jamie Hall

That's what I've been trying to say -- only I talk too much.

KMSNYC

Hi, thanks for all the responses.
First, I am pretty certain that my heating system's vents are not adequate -as I mentioned in my last message, I'm working on it "as we speak" - waiting for some parts. I'm hoping with the mains and risers vented properly, the boiler may not build up as much pressure as it now does. I suspect that what is happening is the air is not getting vented from the system fast enough; the steam is trying to compress the air so pressure is building up in the system. I'm trying to force steam into the system when the air isn't coming out quickly enough. The main trick right now is to find the best places to put those vents as the basement ceiling is low. I just barely fit a Hoffman #75 in one of the lower places and I have 2 Gorton #2s and another Hoffman #75 to go. I'm not going to do any major pipe work now in the middle of heating season (nothing like breaking a pipe or fitting when you need heat!) but I can stick at least one main vent on the 1st floor at floor level just above the basement instead of at the ceiling level in the basement and it should be just as good - and some at the tops of the risers at the top level radiators, which is easy as I already have Tees there. Not sure which one is going where yet. I did all the calculations of riser and main volume (page 130 Greening Steam) so I have at least some idea but there is going to be some trial and error involved. One surprise that I mentioned in my last message was the volume of air (CFM) that could be moved through surprisingly small pipes. (page 131 in Greening Steam). You take a look at a Hoffman #75 with its 3/4" NPT connection or the higher capacity (twice that of the Hoffman 75) Gorton #2 with it's 1/2" NPT connection and you think (or I thought, to be more precise! ) that you'll need a big pipe to connect it to the main without impeding air flow. But even a 1/8" pipe will handle more air (over a short distance, anyway) than even the Gorton #2 can handle -according to the table on pg. 131. I'll use a larger pipe where I can find one or already have a fitting but if I need to tap a hole into the steel main or riser the largest I'm comfortable with is 1/4". Just might not be able to get enough threads into the pipe wall with anything larger than that. Should have some strength even though low pressure.
Anyway once I have good venting in the mains and risers I will see what effect this has on the boiler behavior. I hope but won't know until I try that the boiler will build up pressure much more slowly because air is venting faster. Ideally the steam will get to the top of each riser rapidly and at about the same time although I am less concerned about that initially as long as it's reasonably close - I can always adjust that later by adding /subtracting vents. From what I understand of the systems (and my understanding is definitely not complete!) what I would ideally like to see the boiler do is: (1) thermostat calls for heat , boiler goes on; (2) boiler builds pressure up slowly and then reaches a "plateau" of relatively low pressure and stays there - whether this is 1 lb. or 8 oz. matters less right now than that it stays at some point and doesn't keep getting higher(3) boiler stays at that point producing steam, it does not ever reach the pressuretrol upper cut-out because the amount of steam being produced matches the air going out and also the system's condensing of that steam (system gets rid of heat through the radiators as fast as the boiler is making it; things are balanced); (4) boiler stops because thermostat set point has been reached and for no other reason. One boiler cycle to reach thermostat set point in other words. It may not be possible with the present set up but fewer on-off cycles would be nice and having it keep the pressure most lower for longer cycles. There are a lot of variables. In any case at least on a colder day, the temp in the room where the thermostat is located - which is sometimes but not always the coldest room in the building - soon drops again and the thermostat again calls for heat and the process begins anew. But the rate at which that happens has more to do with factors external to the existing system or to my ability to control it at the moment: number and size of radiators, whether my windows leak, how windy it is outside etc. This is an old house not a perfect modern envelope with triple glazed windows and double walls thickly insulated with minimal thermal bridging where one can do very precise calculations. This is a house with zero insulation and leaky-ish windows where on a calm sunny winter day the upper floor might be the warmest but on a windy grey day it is the coldest. Another ideal: the boiler does not cut off, or rarely cuts off, in the middle of a cycle because of low water. That would mean things are in very nice balance; the amount of condensation returning to the boiler matches the rate at which the boiler is sending steam out so it never runs low. The way things are now the the boiler does tend to cut out for low water not frequently but maybe more often than I would like. That has to do, I think in that it is capable of producing steam very quickly but doesn't hold a lot of water and the system it feeds is fairly big. I realize I could calculate all of that. I am not going to change the boiler or anything and possibly it is oversized. Maybe if the same boiler were added to so that it had the same burner capacity but a > water capacity it would be better suited. Or the other way around, smaller burner, same amount of water. In any event I am stuck with certain parts of the system the way they are. Once (I hope) the venting problem of the mains and risers is addressed the plan is to next attack each radiator and "tune it" by different vent sizing. The goal is once I know each radiator will have all the steam it needs from the mains and risers, I can adjust it however I like (or how the person occupying the particular room likes) - more or less. For example I have one very small room with a small radiator that is probably appropriately sized, or anyone who saw it would probably guess "that's big enough for this room" - but the room has one very large window and the proportion of window to room volume is high. Therefore the room gets relatively cold. So I'll try putting a "faster" vent there. If I increased the size of the radiator the person sleeping in that room would have to rest their feet right on the radiator.

Maybe I am too worried about the system cycling as much as it does but I would like to see it do it less.

KMSNYC

Regarding other questions Jamie asked: the thermostat I have as far as I know is not special for steam and I don't believe it has a special steam setting. it is a normal household electronic thermostat that is programmed to respond to the temperature in the room. You want it to be 70 degrees at 4:00 pm, at 4:00 pm (or maybe 3:50 I cant recall how "smart" it is) if the temp is 68 it tells the boiler to come on. You can have 4 settings per day, etc. Now you post reminded me of something I had long forgotten. A long time ago (late 1970s early 80s) I managed 8 old buildings each had steam heat. There was no thermostat as such that responded to temperature (I think). We had a electro-mechanical device on each boiler that if my memory serves me well was called a "Heat Timer". It was in a metal box about 6" x 10" with a lockable door. It rotated like a clock and had little pegs in it so you could tell the boiler when to come on and when to switch off. It worked on time, not temperature and I think you could control the length of each "on" or "off" period like a normal mechanical timer. There may have been more to it than that, I no longer remember, except figuring out the correct settings for 8 buildings with between 6 and 8 apartments per building was a challenge. In any case all you could do was make the boiler stay on longer or shorter duration or more often. No direct control over temperature- It could in theory reach 100 degrees in one area of the building (it never did - these were old buildings!) and if it did people would open windows, nobody banged on the pipes yelling for us to "send up more steam" - they used the telephone. We did attempt to balance things out through sizing of air vents. But the boiler would, during its "on" periods cut off and on repeatedly for pressure. I think we ran them at much higher pressure like even 10 lbs. (maybe it was 5) but I don't really remember. The Heat Timers were very popular and all the buildings had them.

Yes, the gizmo on the chimney/exhaust is a motorized damper. I think it is there because it allows the boiler manufacturer to claim higher efficiency as building heat doesn't run up the chimney when the boiler is not operating. If the boiler ran for 10 minutes straight then rested for a half hour with the damper closed I could see it. But the thing opening and shutting constantly when the boiler cycles is silly. If it ever gets stuck closed then the boiler won't run.

Thank you for illuminating he concept of "dirty water". The water in the boiler is like strong tea or coffee with a nice iron content or if you prefer like the Mississippi R after a storm or flood. I suspect there is by now a lot of good clean solid ooze and muck sitting in the bottom of the return lines but clearing that will be a non-heating season project. The more fluid parts of the water seem through the gauge glass to be more or less a nice clean dark tea or terra cotta color.

Jamie Hall

I you can give me the make and model of the thermostat I can probably find the manual and tell you how to set it for steam. Somewhere in its menus... buried... they love to do that.

ethicalpaul

Once such way is to point out that the whole purpose of the pressuretrol and cycling between 1.5 and 0.5psi is to give the system a chance to drive more trapped air out on each subsequent cycle. So if this phenomena didn't exist, there wouldn't be a need to cycle at all. My point is that the way Jamie describes it, there is no need for cycling.

There is no need for cycling.

And it wasn’t your text I couldn’t read, @SlowYourRoll 😉

SlowYourRoll

PMJ said:

No air can ever leave a radiator that is not above atmospheric pressure. Any "expanded" air that filled a greater volume of the radiator with a closed vent that was vacated by collapsing steam must return to its original volume before it can leave. Advancing steam on the next burn accomplishes this volume change quite easily with the vent still closed and with less effort than would be required to push air at atmospheric pressure.

Every radiator in my 2 pipe system is connected to a dry return. All of the dry return and most of the radiators are filled with "expanded" air on the off cycle. This air quickly returns to its original volume and offers no resistance to the flow of new steam prior to opening a vent anywhere.

Yes, that is correct. As Dan describes it, the expanded air doesn't leave until the next cycle starts. The problem with the expanded air is that it fills up sections of the radiator prior to cycling. I guess I should've been more clear and say that the cycling allows the previously-expanded air to exit the radiator.

PMJ

SlowYourRoll said:

PMJ said:

No air can ever leave a radiator that is not above atmospheric pressure. Any "expanded" air that filled a greater volume of the radiator with a closed vent that was vacated by collapsing steam must return to its original volume before it can leave. Advancing steam on the next burn accomplishes this volume change quite easily with the vent still closed and with less effort than would be required to push air at atmospheric pressure.

Every radiator in my 2 pipe system is connected to a dry return. All of the dry return and most of the radiators are filled with "expanded" air on the off cycle. This air quickly returns to its original volume and offers no resistance to the flow of new steam prior to opening a vent anywhere.

Yes, that is correct. As Dan describes it, the expanded air doesn't leave until the next cycle starts. The problem with the expanded air is that it fills up sections of the radiator prior to cycling. I guess I should've been more clear and say that the cycling allows the previously-expanded air to exit the radiator.

Once you fill a radiator the first time enough to close the vent how do you ever fill it fuller than that? Any remaining air at that point which expands in vacuum and then contracts again to its original volume when new steam arrives on the next burn will be when the radiator is once again full to the same amount enough to close the vent again. It would seem that this relatively small volume of trapped air can never be removed and is not a problem of any kind.

SlowYourRoll

Thanks @ethicalpaul and @Jamie Hall and everyone else for taking the time to explain. At the very least I now see what the two viewpoints are (I think). Dan's book (as I understand it) is saying that cycling (from about 1.5 to 0.5) is necessary. @ethicalpaul and @Jamie Hall are saying that cycling is not necessary.

To me, the "cycling is not necessary" position raises some questions... Why do you need a low cut-in pressure of 0.5 if cycling is not necessary? Why not set the cut-in pressure to 1.25, since "cycling is not necesary"? Why is short-cycling an issue if "cycling is not necessary"? If you think of cycling as the length of "off" time in between "on" times, isn't the "cycling is not necessary" stance the highest possible form of short-cycling? Isn't "no cycling at all" the same as saying "the off time of each cycle equals zero seconds"? And if cycling is not necessary at all, wouldn't short-cycling be preferable to long-cycling? (for all these questions i'm assuming the boiler is appropriately sized, not oversized)

So, to me, Dan's viewpoint makes the most sense, but I'm kind of at the limit of what I can contribute to the discussion. All I can do is just parrot what he's said basically.

And the only way I can think of testing out the competing theories would be to set the cut-in pressure to just below the cut-out pressure for the "cycling is not necessary" view (which would essentially create a scenario where the boiler isn't cycling), and compare that to a 1.5/0.5 setting as described by Dan. And I can't do that myself because my boiler is most likely oversized.

So I guess that's the question it comes down to for me...if cycling isn't necessary, why not set the cut-out pressure to 1.5 and the cut-in pressure to slightly under 1.5. that's as close to "cycling is not necessary" as you can get.

SlowYourRoll

PS equating "cycling is not necessary" to a p-trol cut-in pressure slightly below the cut-out pressure is my way of grasping the "cycling is not necessary" viewpoint. that's not in Dan's book. for the sake of a clear argument i wanted to clarify where the ideas come from. i think most everything else i've said comes from Dan's book (or at least my understanding of Dan's book)

PMJ

To me cycling is necessary only for comfort. Running a max speed only boiler(even one that is not "oversized") flat out on every call for heat until the thermostat is satisfied on an average day produces a temperature roller coaster I find unpleasant and totally unnecessary.

ethicalpaul

To me, the "cycling is not necessary" position raises some questions... Why do you need a low cut-in pressure of 0.5 if cycling is not necessary? Why not set the cut-in pressure to 1.25, since "cycling is not necesary"? Why is short-cycling an issue if "cycling is not necessary"?

One of us is confused

FIRST, cycling is not necessary to the correct operation of a steam heating system. The boiler will make steam, push it through the pipes to the radiators, and the radiators will fill and...radiate just fine without any cycling. OK?

BUT if a heating system's pressure starts to rise, then it must cut-out, otherwise it will blow the pressure relief. OK?

NOW since almost all boilers are oversized (and even properly sized boilers can be coaxed to higher pressures with for example an extreme recovery from a setback), then we can say that even though cycling isn't NECESSARY, it is quite COMMON.

It's hard for me to tell, but the things you are writing tell me that you don't really have a good understanding of the role of the pressuretrol. It is a safety device that stops the system pressure from going above a desirable/safe value.

Setting the pressuretrol does not set the pressure of the system. My pressuretrol is at 1.5 and my pressure no longer gets near there now that I have a correctly-sized boiler. If I set my pressuretrol to 7psi, nothing is going to change in my system.

And the only way I can think of testing out the competing theories would be to set the cut-in pressure to just below the cut-out pressure for the "cycling is not necessary" view (which would essentially create a scenario where the boiler isn't cycling),

The boiler in this scenario would still cycle. The pressure would get to the cut-out. The pressuretrol would shut down the boiler. The pressure would drop. The pressretrol would allow the boiler to start up again. The pressure would rise. This is called "cycling".

Jamie Hall

In some ways there really are two different problems -- and objective -- which get mashed together here. One is controlling -- modulating, if you like -- the power of the boiler to the demand of the system. The other is making sure that the system can accept and use the steam.

From the standpoint of modulating the boiler, there would be much to be said for keeping the off part of the cycle as short as possible. On the other hand, there are also considerations of the capability of the machinery to accept a short cycle, and the possible effect of short cycles on the longevity of the machinery. Thus, for example, most modern oil burners simply can't have an off cycle less than 30 seconds plus a bit long: the burner is commanded off, there is a 15 second post purge, then a very brief pause, and a 15 second pre-purge built into the control, and for very good reasons. In some systems this is long enough to drop the pressure to atmospheric (or, with sticky vents, into a slight vacuum). In others, the boiler never quite stops due to residual heat in the boiler. It would be nice to have a shorter off cycle and maintain steam production-- but the machinery simply can't do it. Then there is the matter of longevity: starting is harder on the machinery than steady running, which argues for having the whole cycle much longer -- both longer on phases and longer off phases. The same problem exists for gas power burners.

This is not, by any means, an insurmountable problem. Consider, if you will, a gas turbine. Clearly it modulates its power output according to demand, and it does it by modulating the fuel flow and, in tandem, the air flow. In principle this could also be done with oil burners -- and, in fact, it is done with power boilers. The problem there is in that innocent phrase "in tandem, the air flow". There is perfectly good equipment to sense the air/fuel ratio in real time, and control the air flow (it actually uses a predictive algorithm -- this thing is throttling up, increase the air) with a feedback system (oops, a little too much air, dial it back a little). Problem: it ain't cheap, and it requires maintenance -- which ain't cheap, and the cost doesn't scale. What is reasonable for a 10 megawatt power boiler in terms of expense is not even imaginable for a 100 KW residential boiler.

So, to have the modulation we need to match power output to power demand, we stick with an on/off modulation.

I honestly don't know why steam boilers with modulating burners aren't available for gas firing, controlled by boiler pressure (rather than temperature, as hot water mod/cons are) except that I suspect a combination of a lack of demand and a lack of really reliable, inexpensive pressure transducers (thermistors are a dime a dozen -- literally. Pressure transducers aren't).

The other problem -- making sure the system can accept and use the steam properly -- is a little more subtle. In two pipe steam systems, and especially in closely pressure controlled vapour systems, the output of each individual radiator can be adjusted -- either one time or to fit the space -- by adjusting the input control valve (within the limits of the attached radiator, of course). Piece of cake. This is not true in one pipe steam systems. There, as we know, the output of the radiator has to be controlled by limiting the venting rate. Now if the cycle on time is short enough there will be some radiators which are still filling with steam when the burner is turned off. If the cycle is longer, all the radiators will have filled with steam and will be heating at the capacity of the radiator itself. This is a problem: we have lost control of the heat output of the radiator. If we desire to maintain control of the heat output over a longer cycle, then, we have to restore the ability of the vent to control steam input to the radiator -- which means we have to have the off time long enough to drop to atmospheric, reopen the vents, and allow air back in. Then, when the boiler fires back up again, the vents will control the heat output.

Note that the problem is even more severe with thermostatically regulated vents -- there, if the vent is trying to turn the radiator effectively off, it must be allowed to allow air in -- but not out -- to keep the radiator turned off.

I think that it is this requirement to which @DanHolohan is referring when he is talking about a one pipe steam system "breathing"; it is to be able to allow the vents, not the EDR of the radiator, to control the heat output.

You guys make me think too hard. Go do something else for a while...

SlowYourRoll

ethicalpaul said:

It's hard for me to tell, but the things you are writing tell me that you don't really have a good understanding of the role of the pressuretrol. It is a safety device that stops the system pressure from going above a desirable/safe value.

Paul, this is a direct quote from Dan on this board (https://forum.heatinghelp.com/discussion/76149/pressuretrol-dont-quite-understand):

"For the air vents to reopen after the initial firing cycle, the system pressure has to drop below a certain point. That's the pressuretrol's main job - to make the air vents work."

believe me, I'm more than comfortable conceding that I might not understand something (I've only been learning about steam heat for a few months). but what you're saying disagrees with what Dan is saying, and I'm having a harder time believing you over him.

SlowYourRoll

Jamie Hall said:

You guys make me think too hard. Go do something else for a while...

This one sure has my mental hamster-wheel spinning the last couple of days! I'm sort of at the limit of what I can contribute to this anyway, since I'm basically just relaying what I read from one single source. thanks @Jamie Hall and @ethicalpaul for taking the time with this, all the same.

ethicalpaul

SlowYourRoll said:

Paul, this is a direct quote from Dan on this board (https://forum.heatinghelp.com/discussion/76149/pressuretrol-dont-quite-understand):

"For the air vents to reopen after the initial firing cycle, the system pressure has to drop below a certain point. That's the pressuretrol's main job - to make the air vents work."

believe me, I'm more than comfortable conceding that I might not understand something (I've only been learning about steam heat for a few months). but what you're saying disagrees with what Dan is saying, and I'm having a harder time believing you over him.

(shrug emoji) In a properly-sized system the pressure drops after the thermostat stops calling for heat and shuts down the boiler, with the pressuretrol not activating for days, weeks, months or possibly ever. I'm sure Dan wouldn't disagree with that. I think you're putting a little too much weight on a single comment from 17 years ago in a possibly-somewhat-related discussion.

SlowYourRoll

one last post before i go. another one from Dan titled "Why steam boilers short-cycle."

in the article he states that:

"It was only after years of reading technical books and old heating magazines that I learned that the pressuretrol bobs up and down like that because that’s what allows the air vents to reopen once they close on steam temperature. You see the steam pressure inside the pipes and radiators blows closed the little floats inside the air vents, and unless that pressure drops, the vents won’t open. And if no venting takes place after the first cycle, most of the air won’t leave the system. And where there is air, steam will not go. When that happens, the burner short-cycles."

anyway, that's all i got. maybe you're right. maybe Dan is right. i was hoping to get a more definite answer one way or the other, because I believe this is somewhat central to understanding steam heat, since it is one of only two control loops in the heating cycle (the other being the thermostat), but alas...

PMJ

One more thing from someone who runs in vacuum all the time. When the system is sub-atmospheric air is always leaking in somewhere. These are not perfectly sealed systems. This air must be removed. I do it at the tail end of every burn. I open the one solenoid vent at the moment of the burn when the system pressure just goes positive and push out the little bit of air that leaked in during the last off cycle. If I didn't do this I would gradually lose ground on fill level, the percentage of air in the whole system would steadily increase and eventually I would be running into pressure during the burns. Maybe I'll disconnect the solenoid valve after the first burn of a multi burn call and watch the chart. It would be cycling with no venting at all. Cold snap coming up with many really long calls. I'll consider it.

I will also point out that simply spacing all the burns out in a few more pieces spaced more evenly eliminates measurable pressure altogether and thus would end any need for this conversation about pressure dropping enough to let vents open. It is an amazingly hard sell here. You couldn't pay me to run any other way.

ethicalpaul

We can both be right. He is speaking about vents that don't open unless the steam pressure holding them closed goes away. I have no argument with that. Of course there are multiple ways for that pressure to go away. One is due to the pressuretrol cutting out, and one is due to the end of the call for heat from the thermostat. He even says that at the start of this article.

And I am saying "cycling is not required" because in a correctly-sized system there is a call for heat, followed by heating, followed by an end to the call for heat from the thermostat.

Jamie Hall

Folks, I realise my comment up above there was long winded, but on further thinking I do believe that it has merit.

Bottom lines:

A single pipe steam system must cycle the pressure down to maintain control of the heating of individual radiators by the vents.

A two pipe system with the radiators controlled by the inlet valves does not need to cycle to maintain control of the radiators, but may need to cycle if the power output of the boiler exceeds the power demand of the system.

SlowYourRoll

ethicalpaul said:

We can both be right. He is speaking about vents that don't open unless the steam pressure holding them closed goes away. I have no argument with that. Of course there are multiple ways for that pressure to go away. One is due to the pressuretrol cutting out, and one is due to the end of the call for heat from the thermostat. He even says that at the start of this article.

Okay, but what he's saying is that until the pressure goes away to let more air out, that air has expanded to fill most of the radiator sections, and so the steam is only heating a small portion of the radiator directly. So he's saying that if you wait for the end of the call for heat from the thermostat, you spent that entire time directly heating only a couple sections of the radiator. he's saying that if you don't use the pressuretrol to cycle, you are forcing your system to heat the radiators like this


now the radiator will eventually heat because cast iron is a conductor and the heated sections of the radiator will heat the unheated sections, but he's saying that's a bad way to do it.