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

KMSNYC

There have been a few posts about this issue (I think; I'm new here) and some discussion in Dan's book Greening Steam, but: In many happy hours spent watching my 3 year old Burnham gas boiler (5 story row house) cycle off and on, on and off very rapidly getting up to pressure (approx. 2 psi) very quickly, then shutting off, then losing pressure rapidly and cycling back on (0.5) - made me wonder why can't they make a boiler that adjusts it's flame level so that with maximum flame, it reaches pressure: then instead of shutting off and tirelessly repeating the cycle, why cant the flame then be cut back to maintain operating pressure but no more? The analogy is imperfect but who would want a cooking stove with only one heat level? You turn the stove burner on hot to get the water to a boil, then put in the pasta, then turn it back so it doesn't boil over etc. I hate watching a piece of equipment cycle constantly like that - can't help wondering if it increases wear and tear. I realize there are many reasons for rapid cycling; in this case I don't think the boiler is oversized. Does anyone make a boiler that adjusts or can it be adapted to do that ?

Fred

It sounds like your boiler may be over sized and produces more steam than your radiators can condense, hence the short cycling. If properly sized, the boiler will usually only cycle on when there is a call for heat, never reach cut-out pressure during a heat cycle and only shut off when the thermostat is satisfied. Why do yoiu think it is not over sized? Have you calculated the radiator EDR and compared that number to the EDR rating on the boiler plate? To your other question, there are 2 stage gas valves that step the flame up or down based on system pressure. Not all boilers can be adapted to those valves and those that can should be done by a professional to ensure safe combustion.

KMSNYC

Thank you for your reply. Well I did not size the boiler myself but it was sized by a very well known heating company who did look at all the radiators and pipes in the building before sizing it; they have a lot of experience with this type and size of building which are ubiquitous in Brooklyn. Of course that does not prove that it is correctly sized . I could do the calculations myself I suppose. This boiler reaches cut out pressure in a minute or two (I will do some timing and record it here ) maybe slightly longer when starting from cold. Then when it cuts off the pressure drops very rapidly. The total number of such cycles it takes to reach the thermostat's set point depends of course on how cold and windy it is outside but it is always more than one and usually many more. I am wondering if there are other significant factors.

Jamie Hall

Your boiler is wildly oversized i it's doing that... but, to your question:

It's possible, and modulating boilers for hot water heating do just exactly that. For that matter, so do large industrial and power boilers. I expect it is a matter of demand.

In order to keep combustion at an optimum air/fuel ratio, simply turning down the gas isn't enough. Either the burner has to be designed so that on lower gas pressure the airflow is reduced inherently, or there has to be another mechanism to reduce the air supply at the same time -- and by the correct amount. Can it be done? Certainly. Is there a big enough market to justify the research and testing? Probably not.

Second, a properly sized steam boiler -- which is a matter of the initial selection and installation -- won't do what yours is doing anyway. It will, by design, produce exactly (or very close to it) enough steam to match the radiation, and will only cycle on pressure very rarely, if at all. The fact that most steam boilers seem to be more or less wildly oversized is a problem of the initial selection and installation being improper -- and there's nothing the maker can do about it.

Third, it would add complexity to the controls. It is perfectly possible to obtain a variable pressure transducer in the desired range and couple it to the gas/air controls from item 1 up there -- but again, is there a market sufficient to justify the expense?

As a matter of historical interest, a great many coal fired boilers in higher end installations did do exactly what you are suggesting: there was a very simple mechanical arrangement which sensed steam pressure and adjusted the air dampers on the boiler to keep it where it was supposed to be. The efficiency at part fire wasn't all that great, and the emissions were doubtful -- but a hundred years ago that wasn't seen as that much of a problem.

KMSNYC

Thank you for your reply. Well I am probably not going to replace a relatively new boiler at this time but I will do the calculations (out of curiosity) to see if they also indicate an oversized boiler. Or perhaps I should hire a pro to do the evaluation. It does take a while to heat the building on a cold day like today even with this boiler. The building is a masonry row house with exposure on the 2 narrow sides (front and back) and is not insulated much (circa 1875 with a middling 1980s renovation) ) and the windows are not great - double pane wood sash but not integrated frames.

jumper

Back in the seventies some multi-unit buildings with a superintendent onsite replaced old big boiler with multiple smaller boilers. Manually isolated offline boilers. Worked very well for steam heated buildings. Not so well for HHW. Perhaps because recommended method was full flow?

I also noticed that during day the domestic hot water heaters tended to cycle on off often.

JUGHNE

In addition to possible oversized boiler your system may lack adequate steam main line air vents (in basement).
In the days of coal, as Jamie mentioned above, the fire was pretty constantly on. Dampered up and down as needed. Fast main air vents were not needed because with always there being some steam pressure the air did not re-enter the sytem needing to be expelled on every cycle as today with on-off burners.

As modern burners became common, the thought about air removal was overlooked....hey.. it worked before with coal so what is there to change?
Only maybe in the last 50 years or less has it been brought to attention that a major amount of main line venting is needed.

Your pressure could be building rapidly because it is working on pushing air out thru the rad small vents and possibly very undersized main vents.

You should check what you have for main air vents in the basement, show us some pictures of them and boiler piping.

Fred

How many radiators do you have shut off? With each one that you shut off, the boiler becomes incrementally more over-sized.

EBEBRATT-Ed

@KMSNYC

All the above is good advise. Check you boiler capacity versus the radiation installed and make sure it is vented properly. Tha'ts all you can do for now

HVACNUT

Cut in is 1.5 psi with a .5 psi diff?
A cast iron wet base oil boiler with a conversion burner could have gotten you lo,hi,lo operation with A and B pressuretrols.

KMSNYC

Pressuretrol is subtractive in this case. At its present setting cuts out at about 2.2 psi (boiler shuts off); cut-in 0.5 psi (boiler back on). So differential is about 1.7 . I played with cutoff setting trying to turn it down to the ideal 1.5 or less psi or so but that caused way too much cycling at least for now. The fundamental problem (if viewed by modern comfort standards - our cave-dwelling Neanderthal ancestors would have been delighted with this system, although some may have complained about draftiness) with the entire setup is that it is a 5 story row house with a one zone heating system. Top 2 floors are individual apartments. There is no "coldest room" consistently. The thermostat is located on floor 3, as good as anywhere. When the weather is middling like 40-50F, and the day is windy the lower floors stay warm but the top floor has more wind exposure and always get cold so I turn up the thermostat when my top floor tenant inevitably calls. Sometimes I beat him to it. Top floor happy, middle floor too hot. Better windows would of course help when I get $10-20K or so. On cold winter days, unless very windy the top floor tends to be the warmest, even hot. I've thought about thermostatic radiator valves to try to even these things out. Also thought about a thermostat w a remote WIFI sensor to be placed on the top floor that would supposedly average things out between the two. If I were constructing this type of place today I would not choose a one zone system. When I first purchased this place 35 years ago system was totally out of balance. Front radiators barely got warm while rear was too hot. I knew even less about steam systems then than I do today but with a little bit of reading and more guessing I discovered that air valves were the culprit. I adjusted the sizes on radiators and tapped into a riser to add air vents and at the top from radiator added 4 large air vents on top of the riser where the top radiator valve is. I got good exercise running up and down from the basement to floor 5 and back and forth between the front and back ends doing trial-and-error with this. I surmised that these mattered less when the boiler was coal fired as coal could not possibly go off and on as rapidly the way modern boilers did. I was very pleased at the time to subsequently discover an article from HPD that confirmed that thesis. I now have good air valves on the basement mains; I had originally put a bunch of 1/8" radiator valves teed together but now have very large 3/4 Hoffman valve meant for this purpose. I'm in the process of doing the same at the top of the risers. Getting back to the boiler....

KMSNYC

Thank all of you for your comments and help. I did some observation on my boiler with a stop watch which I will spare you for now (I may publish this data later) but it definitely cycles back and forth between pressure-cut out and cut in quite a few times (4 or 5) before it reaches the thermostat set point, depending of course. Then it stays off for 8-10 minutes (today is a cold day!) before repeating the cycle. A few minutes max to get from 0 psi or 0.5 psi to 2.2 psi where it cuts off; a minute or so to drop back to 0.5 psi; then repeats the cycle. Cycle goes a little faster after the first few times presumably because water is very hot; when it starts again after it's 8 minute or so "rest" it takes a little longer but not a lot. It definitely does not hang nicely in the groove between cut in and cut out; it is either on and pressure rising (rising in general; it can stabilize for very short periods during this time) or off and falling, or "resting" when thermostat is not calling for heat. Someone mentioned radiators being valved off and that effectively would reduce the size of the system requiring steam and hence, make the boiler relatively "larger". Indeed I do have one large radiator shut off, causes room to be way too hot. I could instead provide it with a very small air valve but that would be almost the same as shutting it off. I suppose if the boiler size calc was done including this unit, which it was, it would have resulted in an oversized boiler but wouldn't the same be true in reverse if I increased the air vent size on any radiator in order to make it hotter? I guess the calc is really an approximation far better than not doing one but not perfect. I did insulate the basement pipes recently with 1" fg but this cycling was happening before that although now it gets quite cool down there and I may remove a few pieces - I use it as a shop. The last thing in my present minor ":renovation" of the system (after 35 years) and it now being a few years after the new boiler was installed, that could affect this is to replace the smaller air vents I placed at the top of the front riser with a nice big fat 3/4" Hoffman. Maybe the old ones aren't working anymore and they are 1/8" even though there are 4. I did not know about the larger ones at the time. One other note is, when I decided it was time to mess around with the system again, I decided it needed a gauge that would actually do something as opposed to the standard 30 psi gauge evidently put there for purely decorative reasons. I left it there, I understand it's a requirement or something and hung a new 0-3 psi gauge on a stainless siphon (or syphon, or pigtail tube) on the same 3/4 pipe as the boiler controls. Oh I also stuck a needle valve before the gauge to help its longevity and t keep it from having accidents. It doesn't need to show the pressure when there is nobody looking at it. Ignorance was not bliss but now I have this gauge to tell me my problems in better detail. The point of this: in opening some of the piping that also feeds the controls, I discovered they were full - I mean full -of thick, impacted boiler crud. I cleaned them out, amazed that the pressure controls could feel anything through this mass of (I assume) rust from corroded late 19th or early 20th century iron pipe. I cleaned this antique substance out. Is there a market for it? So it did occur to me that my wet return lines could also be filled with this stuff. When the old boiler was removed I think at least a half hearted attempt was made to clean them out but there is a lot of piping there. So that is on my fun to do list for the non-heating months. I don't know if this would affect the situation. Water did eventually seem to find its way back to the boiler. It does occasionally cut out for a few minutes due to low water but it completes most cycles without this happening and almost always the return water returns before the auto feeder feeds water. It's possible the boiler is just too large. It's clear from the above posts that there really aren't normal boilers in this size category readily available that adjust the flame to pressure like I wish mine did. The one thing I like unqualifiedly about this gas fired steam system is that should we lose electric power in very cold weather (it has actually happened 2x in the last 3 winters here, in my NYC neighborhood both times for > a day, once an exploded underground transformer the second corrosion on a power main, both due to wet, salt etc. winter conditions) I could easily meet the very small electric needs of this unit with a small generator or inverter/12 v battery combo.

Jamie Hall

If the pressure is rising steadily between starting up and cutting out you simply don't have anything like enough main venting. But yes, the boiler is just too large. A lot of them are.

nicholas bonham-carter

From your description of the symptoms of some areas too hot, etc, I suspect that you need more main venting. The Hoffman 75 is very good, but rather under sized, so I would replace them, or add several Gorton #2’’s. A low pressure gauge, (0-3 psi) will show you when the air venting takes place at less than 2 ounces of pressure.
Your boiler may have an LWCO which cuts off the boiler every 10 minutes or so to verify the water level, which is not helping.—NBC

KMSNYC

This is air valve on the front main near the lowest point where it joins the riser and the wet return. Furthest from the boiler. I put this large Hoffman on today. Prior to this (for the past 30x years) I had four 1/8" large hole radiator air vents here on tee fittings. Prior to that when I bought the house it was capped and there were no vents on the main but the fitting was there, so who knows. Where the new big air vent is might be a little too close to the end (according to the book) but that's where the fitting is - I also (30x yrs ago) tapped into the main about 3' back from this point uphill towards the boiler and put 2 radiator air vents which are still there (next pic). I have not checked those yet to see if they still work. Putting these in 30x yrs ago plus a main riser vent I also installed then, sat the top of the riser solved at the time the problem of getting almost no heat in the front of the building and too much in back. Front radiators and riser would barely start to get warm before thermostat shut off. Rear ones (almost directly over the boiler) were plenty hot. I checked the pitch of the front main today with a laser level and it looks good (to me) From the boiler a short distance first up and having a wet return attached at the lowest point then up a ways then downwards away from the boiler almost all its length then a straight down wet return on the end. Actually a bit more complicated but looks ok to my amateur eyes compared with what I've looked at tin the book. House was built circa 1875. Original heat I'm sure was coal fired stoves 2 per floor. When central steam heating was installed I don't know. There was an old oil tank when I got the place which I had removed and the owner I bought the place from from had a gas steam system installed a mid 1980s.
The Hoffman vent looks cool, and impressive - like something from a steam locomotive. I hope it also does the job!

KMSNYC

30x yrs ago - tapped into the front main 1/8" npt a few feet back from its end away from the boiler and put on 2 1/8" large hole radiator air vents. Didn't know about vents designed for steam mains at the time.

KMSNYC

Air vent at bottom of rear main in boiler room. There was a fitting there when I got the house 35 yrs ago but no vents. I put one on then and a few days ago swapped it out for these two 1/8" radiator vents. Could not put larger one in now as fittings are totally corroded and would not budge, I'm not going to break something during heating season, too stressful. Pipe insulation not 100% done yet

KMSNYC

new gauge setup. The steel pipe on which the gauge and controls are mounted was totally filled with boiler crud. I extended one section to mount the gauge. I guess plugs on ends were designed for clean out and they served the purpose.

Bushing 3/4 to 1/4" into which 304 ss pigtail is screwed is also stainless, for a few bucks more I wanted to avoid it getting corroded into place.

nicholas bonham-carter

That gauge will be very useful in showing you how easily the air escapes when the steam begins to rise. The desired result is that steam will fill all the supply pipes in the basement, before starting to rise up to the upper floor radiators simultaneously.
If the top floor lags in steam arrival, then the riser tops can be vented. The steam can only travel as fast as the air retreats.—NBC

KC_Jones

The low pressure gauge should stay effectively at zero until the mains are completely full, if not you need more main venting.  This is the advantage the low pressure gauge gives you.  I run a 0-15 ounce gauge and under normal operation it basically reads zero all the time.

KMSNYC

Yes - and thank you. When 35 years ago I got the house, there were no vents on the mains and none on the risers. The back radiators, right over the boilers all got hot esp when I replaced them and adjusted sizing on their air vents but the boiler would have cycled off by the time the front ones (much further from the boiler horizontally) even started to get warm, and increasing the size of the air vents on the front radiators didn't help much. So then I installed two radiator air vents on top of the front riser, and also tapped into the front riser near the top and put two more radiator vents there. I didn't know at the time about the larger vents designed for mains and risers. Doing that helped a lot but it still needed something more so I put more radiator vents in the main in the basement. I don't recall how I figured that out, I think it was total trial and error and by feeling the pipes with my hands. But I eventually managed to get a fairly balanced system where the front radiators heated at about the same rate as those in back and the top floor and lower floors also got reasonably warm together during each cycle. Now I'm revisiting all of that, replacing some old air vents and using proper vents in the mains designed for mains. A few years ago I replaced the boiler and right now it appears what I got was too large (it seems to cycle too much) but I want to get everything else working correctly before I make that conclusion (nothing to do about it anyway, other than selling excess steam to my neighbors!) . But it might be if the air is not exiting the system fast enough, as you say, then pressure will build up more than it should and the gauge will show me that. Tomorrow I will install a large air vent on top of the front main to replace the multiple smaller ones I put in 30x years ago (which may not be working anymore) and we'll see.....

KC_Jones

What size and how long are the mains?  With that information we can recommend and appropriate amount of main venting.

KMSNYC

What is the largest capacity air vent they make for (domestic) steam mains ? Are there other performance characteristics in these that matter ?

KC_Jones

The largest capacity would be the Barnes and Jones big mouth.  After that would be a Gorton #2 vent.

If your mains are long enough, it may require multiple of the above vents, that’s why I asked about pipe size and length.

KMSNYC

Hi , thank you ! I'll measure them today and post them. I installed a Hoffman 75 and have a Gorton #2 ready to go which I intended to put at the top of the front riser. Haven't heard about the "big mouth". A general question I have is "mains vs risers". Clearly the mains being more or less horizontal (horizontal pitched) behave differently in some respects than the vertical risers and in my case (and generally from what I've seen) are larger diameter than the risers. Where air venting is concerned the emphasis seems to be more on mains - although I do have a question about that. Most of the book examples / diagrams I've seen show a building (House) that is more horizontal than vertical. Th mains are longer than the risers; the house is usually shown as 2 stories, the mains stretched out in the basement with relatively short risers going to 1st and 2nd floors. . My building is more vertical than horizontal; it is 5 stories + basement but the envelope is approx. 40' deep x 20' wide (OD).

Jamie Hall

You bring up an interesting point -- and a quite valid one. There really isn't that much difference -- from the standpoint of the steam -- whether it is going along in a "main" or a "riser". For that exact reason, we sometimes recommend -- particularly when the risers are tall -- that they be vented near or at their ends as well. This can be a little difficult to achieve -- as usually they just terminate at a radiator, with no simple way to add a vent. Sometimes, if they are exposed, they can be tapped near the ceiling of the floor below and vented there, but there are other ways.

KMSNYC

Thanks. The top of the front riser had an elbow on it with a nipple and gate valve to the radiator. I swapped out the elbow and replaced it with a tee then put two radiator style large hole air vents on the upwards leg of the tee to vent the riser. I figured that in my setup, with the risers actually longer than the horizontal mains, and probably containing a greater or equal volume of air/steam than the mains it was critical to vent the risers which should actually also vent the mains. At the time I did it (30+) years ago I didn't realize that there were even larger air vents meant specifically for mains. Now I have one so I'll install it there. I also (30 yrs ago) tapped into the riser near the top (one floor below the final radiator) and screwed in a 1/8" nipple with 2 air valves there also. My objective at the time was more to balance the system which was getting no steam to the front riser for lack of proper venting...today I understand the role of venting better than I did then so I'm going to increase the amount of venting. Is there such a thing as too much venting ?

Jamie Hall

Too much venting? Not on mains -- or risers. Although there definitely is a point at which more venting doesn't do anything. But on radiators, yes.

Steamhead

@KMSNYC , I'd put the Gorton #2 on the end of the long steam main and the Hoffman #75 on the top of the riser. The reason is, steam and condensate in the main flow in the same direction (parallel flow) but in the riser they flow in opposite directions (counter-flow) so if the steam moves too fast in the riser it may bang.

KMSNYC

Pipe Dimensions: (mains measured from steam header)
Front main: 2" nom pipe: 45' plus about 6-7' of wet return pipe that is above the boiler waterline
back main: 2" nom pipe: 15' plus about 4-5' of wet return pipe that is above the boiler waterline

Risers (front & back): 1.5" nom pipe 45' each (total 90'). There are also some short legs of anywhere from 6"-3' each that attach radiators to the risers. I didn't count these but they may add another 10 ' to the total

SlowYourRoll

going back to your original point, there's a bit in "The Lost Art" about how radiators need to "breathe"...as in they exhale air at the start of a cycle, and then mid-cycle they need to inhale. i'm at work right now so i can't look up the passage for you. (i recall it having to do with the steam having massive contraction in volume as it condenses, creating a vacuum preventing all the initial air from leaving the radiator, and the only way to completely fill the radiator was to have a pause built in by the p-trol.)

i'm sure one of the experts could explain it better, but the upshot is that there'd need to be changes elsewhere in the system if you were trying to make a continuously-fired boiler.

SlowYourRoll

if you have The Lost Art of Steam Heating Revisited, the relevant section is Air Vents and Drop-Away Pressure on p. 174, but the real "aha!" part comes on page 177 (there might be a similar section in the book you have, i dunno). he gives the full picture in clear detail so i won't even bother paraphrasing, but i will get to the upshot: if there were no cycling, only the first few radiator sections would ever get warm, no matter how long you fired the boiler.

KMSNYC

Thank you for pointing those pages out. I have the revised edition and re-read that section. I am beginning to understand (I hope) what is going on with this system. My first priority is to increase the venting of the mains and risers. I'll then see what effect that has on boiler cycling and whether I can reduce the cut-out to more like 1.5 psi or less. The addition of air vents to the risers and mains I did years ago helped balance the system back to front (or really, to address a very bad imbalance) but is clearly not enough to make the entire system work properly. I am beginning to appreciate the importance of not only steam getting in (and air getting out) but condensate getting out and back and air getting back in. I do have at least 3 brands of radiator air vents at the moment but I'll assume that is less of an immediate problem (to be addressed later) than the correct venting of the mains and risers. After re-reading that section I realized that I may be using "fast" radiator vents improperly in some cases, that is, to try to remedy a problem that is really because the mains and risers are not vented enough. In other words, get the mains and risers full of steam and working correctly, then work on each radiator individually. The latter which may depend on many factors having to do with the radiator, the room it's in, how cold or warm the room tends to get independent of heating, etc., probably a matter of trial and error. At least that is my present thinking.

SlowYourRoll

that's my understanding, get the steam to the radiators as fast as possible to minimize condensation in the pipes. then, once the steam and air are together in each radiator, it's more of a balancing act involving the size of the radiators, the rooms they're in, the speed of the vents, the drop-away pressure of the boiler.

ethicalpaul

I am beginning to appreciate the importance of not only steam getting in (and air getting out) but condensate getting out and back and air getting back in.

I'm confused by this. Personally, I'd rather not let any air back in, but it's not easy to prevent in a 1-pipe system (although I will hold a vacuum for a few minutes after firing). Why do you want to get air back in?

How is getting condensate out related to this? Out of where?

Jamie Hall

There does seem to be some confusion here, @ethicalpaul . The way a one pipe steam system is supposed to work, in the best of all possible worlds, is that the boiler fires up, the steam progresses through the mains (helped by the main vents), the main vents close, the steam progresses through the risers, pushing the air out of the open vents in the radiators. Steam enters the radiators, and -- if the run is long enough -- fills most of the radiator, condensing rapidly as it does so. Condensate formed as a result goes back out the inlet pipe. Air is still being vented from the radiator, so long as the steam supply is faster than the condensation. 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.

Now however... let's suppose that the boiler is oversize, and the pressure starts to rise in the system (all the vents are closed). The boiler reaches cutoff. Now what happens? Well... that depends. If the boiler pressuretrol is arranged so that the cutoff time is relatively short, the pressure drops and the boiler cuts back in. The vents have stayed closed, because they've never actually cooled off enough to open (varies with the vent, but most of them seem to be down around 140 F or so before they open), and because a vacuum has never actually been reached. The boiler fires for a while... cuts off... cuts back in... rinse and repeat until the thermostat is happy or I'm tired.

Now what happens when the thermostat says quit -- or there is a time delay long enough to allow a vacuum to form or the vent temperatures to drop enough to open. Now the vents will all open again, and air will be brought back into the system.

When the thermostat calls again (or the time delay times out), go back to the top and repeat.

Note that during a thermostat call -- provided the off time for a cycling boiler is short enough -- the vents never reopen once closed, and you don't get air back in the system. Meanwhile, the condensate has, hopefully, migrated back out of the radiators and down the various pipes and gone back to the boiler.

SlowYourRoll

Jamie Hall said:

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.

Thanks for the detailed post Jamie. I'm having trouble figuring out how this matches what Dan has in The Lost Art...

The first figure from the book shows the state of things right before steam hits the air vent and shuts it.


The book goes on to explain how the condensation of steam creates a vacuum because of the contraction of the steam, and this contraction causes the trapped air to expand to fill up much of the radiator, at which point the steam is barely giving additional heat to the radiator.


Maybe I'm reading your post wrong, but seems to me like you're implying that the trapped air will stay at the same volume in the radiator, and the steam would keep heating up most of the radiator. Am I missing something? Thanks

ethicalpaul

When the boiler stops, the steam starts collapsing from condensing very quickly. The state of your second picture above I would call "vacuum" where my +- gauge will read negative because the pressure in the system is less than the atmospheric pressure in my house.

But none of this really has anything to do with condensation returning. The condensation will return in positive pressure (during steam production) or negative pressure (during steam collapse) scenarios

During the vacuum phase there may be some small amount of steam drawn into the radiators (because of the larger volume of collapsing steam causing a greater vacuum there), but I can't imagine it's very much heat energy.

PS really nice summary @Jamie Hall

Jamie Hall

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.

KMSNYC

Hi. Someone mentioned there is an article here on how to calculate the correct size of air vents for the mains and/or risers. I haven't been able to locate it. If anyone knows off hand where to find it please let me know

SlowYourRoll

Thanks Paul! I wasn't really talking about condensation returning though. From Jamie's description of how the system is supposed to work, it seemed from the way he described it that any air still trapped in the radiator when the air vent closes would stay at the same (small volume), and so steam could continue delivering heat to the radiator. That was my take on his statement that "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."

But in Dan's book he describes the trapped air expanding to fill a large portion of the radiator, and with the air vent closed that expanded air has nowhere else to go, and on top of that it's applying pressure to the inside of the air vent, which holds the air vent closed.

So here's the page from the book I'm wondering about (@DanHolohan i'll gladly take down the image if it's a copyright issue or something). I just don't see how Paul's description and Dan's description line up. And that reduction in volume is huge (the difference between a gallon and a half a teaspoon, roughly), so Dan's explanation seemed correct to me, since it accounts for that tremendous collapsing of volume.