Steam heating control thoughts

Jamie Hall

Some time ago I threatened to write a little note on some observations and thoughts on the characteristics and control of steam heating systems. I don't write as well as Dan -- sorry, but here they are... I hope you all will enjoy!

EBEBRATT-Ed

A modulating gas burner that could operate like the gas burner in a Mod Con but used in a CI steam boiler (in the residential operating range) would be nice. Not sure if one is made yet or not. Oil, don't think we will ever see it.

Nice job @Jamie Hall very intresting

SeymourCates

Good morning Mr. Hall,

A very well written analysis. However, the conclusions regarding pressure can be a bit misleading depending on the system.

To wit:

For any particular steam system, the pressure during that almost constant pressure interval will be different. It is dependent, more than any other factor, on the flow resistance in the piping. For any particular steam system, however, that pressure will always be the same, or very nearly so. This suggests very strongly that we can use the pressure at that plateau as a very useful indicator of when our boiler is producing steam at the same rate as our radiation is condensing it. That pressure is easily measured; the question becomes what can be done with it.
In

For installations that utilize a boiler that is poorly matched to the radiation, the above is entirely correct. However, if the boiler is properly matched to the radiation (with no more than a 20% pickup factor), it is effectively impossible to utilize pressure as a control parameter. Under 90% of operating conditions, the pressure in the system never rises to a level where control is possible. The radiation is condensing steam at a rate that matches the boiler until the radiators are completely filled...........deep into a cycle. At that point, and only at that point, will be system pressure begin to climb slightly. IF the radiation is somewhat larger than the heatloss, that point may never occur as the boiler will shutdown on the 'stat prior to that point.

The system you describe in your analysis can only operate utilizing pressure under the following scenarios:

1) The boiler is considerably oversized as compared to the radiation.

2) The capability of the radiation close in size to the heatloss


Now, if a situation presents itself where BOTH the boiler and the radiation are significantly oversized, one may still avoid building pressure as the 'stat again opens prior to complete fill of the radiation with the possible exception of near the design day. It is not productive nor desirable to design a control system for a small fraction of the duty cycle.

In conclusion, one needs to be cognizant of the boiler size, radiation size, and building heatloss (relative to radiation size) to determine whether pressure can effectively be utilized as a control parameter. Many times it cannot.

Jamie Hall

I quite agree. In some ways. If the boiler and radiation are perfectly matched the pressure will never rise; control is not needed -- though there is no harm to having a control device present. If the boiler is undersize in comparison to the radiation, the same situation arises. If the boiler is slightly larger than the radiation, at some point in some runs -- and the situation may never occur, I'll grant, in real life -- the pressure will start to rise again. It is at that point that I suggest that it is prudent to shut down the boiler with a control device.

I presume, however, that you are stating that pressure cannot or should not be utilized as a control parameter. May I ask you, then, what parameter you would use for a realistic scenario?

The specific scenario which I will postulate for your consideration is this: the boiler is 10% oversize for the radiation. For reasons which are not relevant to the consideration, a boiler run of approximately 4 hours is required to reach the desired space temperature. It is known that approximately 45 minutes after the start of the boiler that all the radiation will be filled to capacity. Now. If pressure is not to be used, what parameter should be used to modulate the boiler output to match the radiation for the remaining 3 hours and 15 minutes?

Note: timers are excluded, as the times cited in the example are relevant only to the particular instance, and are widely variable. Manual intervention is also excluded.

Note also: the chosen control parameter must be easily and reliably measured with readily available commercial equipment suitable for use in normal heating system environments and power supplies.

SeymourCates

Good evening Mr. Hall,

An interesting scenario that you present.

With your given parameters and without the capability of timers or manual intervention, I must defer to your choice in control strategy as you have effectively placed the observer in a proverbial box (of which I am sure was carefully planned).

I do note for other forum observers that your chosen scenario is within a gnat's knacker of the "perfect" storm.

1) Presuming that your scenario does not involve recovery (as you strictly pronounce against for good reason), the building is operating right at design.

2) The boiler's output is nearly perfectly matched to the radiation.

3) The output of the radiation is nearly perfectly matched to the heatloss.


In all of my years installing and observing steam equipment, I have never observed anything close to that scenario.

And, only on the design day would such a scenario be realistic. On every other day, you face the typical cycling of all steam boilers where the heatloss is a fraction of the output from the radiation. Under these conditions, controlling the boiler via pressure is the wrong choice. The preferred method is to control the system via heatloss. The boiler is operated with outdoor reset on a schedule chosen by the technician with intermittent operation to offer up the precise amount of energy that matches the heatloss. Companies such as Tekmar and HeatTimer produce these products and they are extremely effective (which might explain their relatively high cost). They can take a grossly oversized boiler and cycle it in 10 minute periods, if necessary, to prevent any climb in pressure. We successfully utilize Tekmar's product with indoor feedback with very good results.


You might be aware of @PMJ who has written extensively on this forum with his self-designed system using a PLC that effectively does the exact same thing. I observe extreme reluctance to understand and welcome this technology despite the clear evidence of success. I do grant that the cost is prohibitive for most residential dwellings unless constructed by the technician (ala PMJ).

I will also add one caveat. @PMJ must have an excess of radiation (at least 25% at design) to be successful in cycling an oversized boiler using the electronic controls. Clearly in your scenario, that excess does not exist.

Jamie Hall

The scenario was, I assure you, quite real, as was all the equipment involved. I am pleased to say that the vapourstat performed exactly as was intended.

I also assure you that I understand the technology involved very well. Were it not for the simplicity and reliability of the systems which I use in a number of applications, I might even welcome it. Unfortunately, I am not in a position to regularly oversee the various systems I care for; nor do I believe that most homeowners or small business owners have the time or ability -- or desire to do so.

I might also point out that if the technician is not available to reset the timing for intermittent operation to match the current situation heat loss (which, even with outdoor reset, may be remarkably far from "design" conditions) there is a problem. The technician cannot be assumed to be available in a practical scenario.

The fundamental point -- on which I am sure we agree -- is that unless one happens to be faced with a "perfect storm" situation, where everything matches up, there is a necessity to modulate -- by some means -- boiler output (which necessity, incidentally, is not unique to steam -- hot water has the same problem, though more options). My point -- and where we differ -- is that I am interested in control strategies which accurately measure the controlled parameter and do so with a minimum of equipment, and for which the equipment has maximum reliability at reasonable cost.

I applaud the efforts of experimenters -- such as @PMJ and others -- to find better ways to do things; provided their efforts on their systems are accurately and completely documented so that successive owners and operators can manage the system, and provided that a source for the custom parts continues to exist, I have no doubt that they are a very good thing.

And I admit that I am possibly biased by having had to reverse engineer some exceedingly ingenious systems which ran excellently well when under the care of their inventors. The effort usually resulted in a great appreciation for their ingenuity -- and considerable expense for the successor owners to scrap the whole thing and put in something which worked for the average Joe.

Oh -- and one other thing, as Colombo used to say -- the scenario portrayed involved Cedric.

Jamie Hall

Actually, come to think of it, there is one aspect of the timer and outdoor air temperature sensor control system which I honestly don't understand, and perhaps someone can clarify it for me. How, exactly, does it allow for variations in heat loss due to varying wind loads and solar loads? In one of the places I care for, which has excellent records going back several decades, the variation due to wind load on heat loss has been up to half again the still air heat loss. Makes a difference... (I might add that the dead men designed that system with reference to the wind load/design day losses, bless them).

PMJ

Gentlemen, my goodness. So much that borders on general agreement in one place! And I thought this was shaping up to be quite a scrum.

@SeymourCates , you would be quite correct. I have much excess radiation and excess boiler. But then I think the original system was designed that way. The ultra low pressure controls they employed would have been no use without the radiation and the boiler modulating somewhere in the middle of their capacity. In the case of the coal boiler probably near the lower end on an average day. Though I never operated one it would seem that one could build a smaller fire in milder weather with the damper modulating that up and down. In colder weather increase the size of the fire. But with a boiler on the small side and forced to maintain a fire in it near maximum capacity would seem problematic. I believe that a lot of the oversize conditions we see so much of like mine (the boiler I operate replaced the coal fired one) is simply that contractors tended to replace what was there. But the new gas ones ran on high and full capacity all the time whereas the coal ones were never intended to get close to capacity.

@Jamie Hall , your concern about documentation for my system - if I ever leave I will simply return the system to what I moved into which would take me about an hour. However, I am quite convinced that the steam heat in my house will not survive another transfer at all. They are being ripped out all around me. The heating experts in our government have killed residential steam with their financial incentives. There are so many systems like mine still out there it will take a while, but without any young folks learning how to really operate them( and the bad name they have out there because they are not operated correctly) it is just a matter of time. I, however, plan to enjoy this fine technology as long as I can - along with my slate roof and copper gutters which are also going by the wayside.

You also asked about adjustment to timer systems as conditions change. The basic starting place system needs no adjustment. You start with a burn to satisfied preheat sensor its location of your choice. So every burn has that though few preheats are needed. Then the simplest timer approach just needs to provide a burn/wait percentage that you know from your system will heat on design day wind or whatever. This simply spreads the design day hourly burn rate out evenly rather than running it in straight burns until stat is satisfied. So in my case I estimated 50% worst case burn or 30 minutes per hour worst case with my big boiler ( a coal one of the same size modulating right in the middle of its range). So when I started I simply made the burns 10min and the waits 10 min 3 times an hour. There was no pressure at all then and the heat was much more even. This is the starting place. It is still an hourly burn rate to cover everything. The conventional system does 100% flat out burn until the stat is satisfied. The 10/10 spreads it out a lot and those who have tried it really like the results. But again, nothing to adjust - you are still firing at an hourly rate that will cover anything.

This post is too long now but I will end by saying that I have been experimenting and trying to find something that will adjust itself using the preheat sensor switch and a pressure switch to know when vacuum is gone and doesn't need a PLC. The time from when the burner fires to when these things occur while cycling does change when the weather changes. I have never done anything with an outside temperature sensor. Don't know why that hasn't interested me. Just stuck on stupid maybe.

Anyway, I'm just having fun. I sit in the boiler room with my laptop a lot. My wife is convinced I have lost my mind. She isn't complaining about the heat though!

Eastman

@Jamie Hall

The control systems you're asking about would probably collect data from one or more indoor temperature probes to continuously recalibrate the expected heat loss. Tekmar calls it indoor feedback.

KC_Jones

It's funny I read all these posts and think, wow glad I have the ecoSteam system in my basement.

There seemed to be a lot of interest in it and even now there is, but when people had to spend money or actually install it the crickets showed up.

The ecoSteam works off building heatloss and boiler capacity, but it has so much more.

End of main sensor to track preheat as well as know when to start the timing. It has low and high temperature boost and subtraction. When @ChrisJ and I were playing with it we found that at lower temperatures it was lagging a bit, we felt it was due to infiltration. So at lower temperatures the infiltration had a greater effect and at higher temps a lesser effect. With this control it allowed one to change the curve at the bottom and top.

I would like to report that it works quite well. When we had the really cold temperatures I kept my thermostat at 72 with an actual desired set point (programmed into the ecoSteam) of 70. My thermostat sat at 70 all day and never budged any higher. I had a continuous call for heat, the ecoSteam was set at 3 CPH and cycled the boiler accordingly. This is the highest comfort level I have ever had in the house and it was single digits outside.

On my system it uses the actual heat loss between 25°-50° . Below 25 it brings in some boost to a set max that coincides with the design temperature . Above 50 it's starts subtracting up to WWSD based on a max amount. All parameters, temp levels, boost amount are user defined. I am still tweaking a little to see how much savings and comfort I can wring out of it.

I am sure I missed some points, but those are the big ones for me.

For me it's an awesome system, easy to set up and compared to other systems on the market it was dirt cheap.

Oh one final point for @Jamie Hall the manual and instructions Mark wrote for this are amazing and easy to follow. Of course if I ever sell the house the next owner shouldn't have to mess with a thing.

Eastman

I can't remember anymore: does Mark's EcoSteam system still require a thermostat to initiate calls for heat?

PMJ

@KC_Jones ,

I would be quite confident that the Ecosteam is a very good unit.

I believe it didn't sell as he hoped - I think not due at all to the quality of the product but more that the residential market won't spend anything additional on steam. The market probably won't pay contractors for the extra installation required either even though not that much.

I wanted total freedom to experiment with my own algorithms. I use the same PLC that is in the Ecosteam. Reading the manual I believe it is a very fine unit and a well thought out solution.

And there is something very good about 3CPH. I just always wind up there.

mikeg2015

Controlling a boiler on pressure makes as much sense as controlling an ac condenser by cycling it on pressure.

Even on a vacuum system you still use room thermostats. However it that case the system is self modulating. You start at a deep vacuum and radiator output it’s the lowest. Then once they fill with steam pressure rises and output increases as temp increases. When it shuts down, it goes into vacuum and keeps boiling. If you add a buffer tank you could keep boiling for some time and the system modulates down as the temp drops and natural vacuum increases.

Jamie Hall

Hmm... well, you haven't addressed the problem of matching the burner output to the system demand, but that's OK. I suppose... Parenthetically, keep in mind that the boiling after burner shutdown to which you refer is using heat which was provided by the burner while it was operating. There is no free lunch.

I confess to being delighted with the various responses to my thinking (although a little saddened by @PMJ 's quite correct expectations for steam in general). Shows mostly very good thinking. And it is an interesting problem, with lots of room for creative solutions!

PMJ

@Jamie Hall ,

I'm not agreeing with the post about vacuum with this comment but to me the significance of vacuum and boiling after the burner shuts off has mostly to do with steam continuing to move toward and into the radiators during the off cycle, and much shorter time to new steam on the next burn, rather than anything about a claim of "free" steam. Steam flow is greatly assisted by natural vacuum but it is resisted by air in vented systems. Since vacuum is present some 85% of the total time I am running and is deepest always in the rads, it adds up to a good sized total difference. And I think natural vacuum is actually free.

Eastman

Why have a thermostat cycle at 1cph when the system needs to cycle on pressure at a different frequency? Harmonizing the operating requirements of the system together with the occupants leads to the best outcome.

MarkS

Eastman said:

I can't remember anymore: does Mark's EcoSteam system still require a thermostat to initiate calls for heat?

The commercial units did. The R&D system in my house (which incidentally also has the Midco modulating burner on it) can use the thermostat or an indoor temperature sensor. I generally get better results with the sensor, maintaining temperature within 1/2 degree. On thermostat control the temperature wanders a bit.

mikeg2015

Eastman said:

Why have a thermostat cycle at 1cph when the system needs to cycle on pressure at a different frequency? Harmonizing the operating requirements of the system together with the occupants leads to the best outcome.

Because if sized correctly it never builds pressure. 1 CPH is because of all the stored heat in the mass of the radiators, piping, and depending on location , the boiler too.

Plus frequent thermal cycles is hard on the boiler.

Due to how steam heats a radiator there isn’t much need to modulate a properly sized boiler. If you drop firing rate before radiators are full you will heat unevenly unless you have a vacuum system and can drop vacuum to match firing rate to drop radiator EDR.

I see what your sawing. You want variable capacity system. Could sort of do this with TRVs on every radiator then control burner to maintain a pressure set point. I think there’s an efficiency issue with reducing burner output even with the required Induced draft.

MarkS

Jamie Hall said:

Actually, come to think of it, there is one aspect of the timer and outdoor air temperature sensor control system which I honestly don't understand, and perhaps someone can clarify it for me. How, exactly, does it allow for variations in heat loss due to varying wind loads and solar loads?

So far as I am aware, none of the commercial outdoor reset systems take into account any external factor other than temperature. My R&D system fetches the "feels-like" temperature from a local internet weather station, which helps compensate for wind loads. Imperfect to be sure, as there are many other variables not accounted for, but it does help.

Jamie Hall

MarkS said:

Jamie Hall said:

Actually, come to think of it, there is one aspect of the timer and outdoor air temperature sensor control system which I honestly don't understand, and perhaps someone can clarify it for me. How, exactly, does it allow for variations in heat loss due to varying wind loads and solar loads?

So far as I am aware, none of the commercial outdoor reset systems take into account any external factor other than temperature. My R&D system fetches the "feels-like" temperature from a local internet weather station, which helps compensate for wind loads. Imperfect to be sure, as there are many other variables not accounted for, but it does help.

Thank you, @MarkS . I kind of thought that... your solution to fetch the "feels like" temperature is ingenious. I wonder how difficult it would be to do that locally? Probably not that bad. The only problem I see with your solution -- at least in the areas where I work -- is that the internet around here is erratic at best, and usually fails completely several times a day -- and completely drops out if there is a power failure, which is common enough. In more "civilized" areas I can see that that would work pretty well.

Sort of off topic, but on vacuum -- an experiment which I used to do when I taught thermodynamics was to freeze water with a vacuum pump. Never failed to entertain the troops! The principle, of course, is that when you pull a high enough vacuum, the water still wants to "boil" -- but the only place the required heat can come from is from the heat of fusion of the water, rather than temperature change -- so some of the water "boils" and some of it freezes... and you have a beaker of water happily bubbling away and making "steam" -- while also having ice crystals forming.

BobC

On my area it's the northeast wind that sends chills down your spine. When i was in my apartment 40+ years ago I had to relocate my thermostat because it was located on a living room wall the landlords steam pipe ran up - made for WIDE temperature swings. It didn't help that the house was built in the early 20's and was not insulated.

Right around the corner was a narrow bedroom wall between the door for the LR and a closet that was over the cellar's outside door. I put thermostat on that wall because that wall could feel the NE wind. That became my feels like sensor and the temperature swings got a lot smaller.

Bob

ChrisJ

mikeg2015 said:

Controlling a boiler on pressure makes as much sense as controlling an ac condenser by cycling it on pressure.

Even on a vacuum system you still use room thermostats. However it that case the system is self modulating. You start at a deep vacuum and radiator output it’s the lowest. Then once they fill with steam pressure rises and output increases as temp increases. When it shuts down, it goes into vacuum and keeps boiling. If you add a buffer tank you could keep boiling for some time and the system modulates down as the temp drops and natural vacuum increases.

Some low ambient setups cycle the condenser fan on pressure. My own controls the fan speed by condenser temperature, which is also related to pressure.

Kind of a bad example in my opinion as many things in refrigeration are controlled by pressure.

However, my personal opinion is a steam system's pressure should be limited by the radiation, not a pressuretrol. A properly sized boiler mated to a properly operating system should limit it's own pressure to something very reasonable regardless of the run time.

PMJ

> @ChrisJ said:

> However, my personal opinion is a steam system's pressure should be limited by the radiation, not a pressuretrol. A properly sized boiler mated to a properly operating system should limit it's own pressure to something very reasonable regardless of the run time.

I tend to agree with you here with this addition.

I think we can all agree that insufficient installed radiation would be an insurmountable problem. Fortunately that is rarely the case as continuously firing coal systems relied on getting the job done with the radiation never being full so they installed extra.

When it comes to boiler size it is fine to install one that is too small to actually pressurize an amount of radiation that never needs to be full anyway per the above. I think that is what you are saying. However, the extra boiler issue if it is installed is easily managed to run without pressure simply by limiting its run time. Bottom line this is what your Ecosteam does. In my view there are several benefits to having extra boiler and no negatives.

MarkS

PMJ said:

When it comes to boiler size it is fine to install one that is too small to actually pressurize an amount of radiation that never needs to be full anyway per the above. I think that is what you are saying. However, the extra boiler issue if it is installed is easily managed to run without pressure simply by limiting its run time. Bottom line this is what your Ecosteam does.

True, but a bit misleading. EcoSteam calculates the btu's required to maintain temperature, expressed as a run time based on boiler input. Pressure limiting comes as a consequence, it's not a controlling factor.

PMJ

> @MarkS said:
> When it comes to boiler size it is fine to install one that is too small to actually pressurize an amount of radiation that never needs to be full anyway per the above. I think that is what you are saying. However, the extra boiler issue if it is installed is easily managed to run without pressure simply by limiting its run time. Bottom line this is what your Ecosteam does.
>
> True, but a bit misleading. EcoSteam calculates the btu's required to maintain temperature, expressed as a run time based on boiler input. Pressure limiting comes as a consequence, it's not a controlling factor.

Yes Mark but at the end of the day you stop the boiler from running straight at one time as long as it would with a conventional control. Please don't mistake that as an attempt to minimize the sophistication with which you calculate that required run time. It isn't.

Having no pressure is just a consequence of what I do also for the same reason. I am merely trying to get people to see that a fundamental issue here is that a boiler big enough to get steam in a reasonable time from a cold start to rads at all is too big to be left to run on high after the mains are warm - the only speed it has. The original system with the same size boiler never had it run on high - ever. Downsizing the boiler so it can run on high more reasonably is what most appear to be advocating. I prefer our approach of limiting run time to match what is needed. You do it in a far more sophisticated way than I.

Eastman

What would you guys recommend to an acquaintance that is unable to build there own system steam control system?

PMJ

> @Eastman said:
> What would you guys recommend to an acquaintance that is unable to build there own system steam control system?

Specifically what don't you like about what you have?

SeymourCates

@Jamie Hall

Good afternoon Mr. Hall

Sorry for the delay in my response.

The programmed control systems to which I am referring are not cobbled up arrangements that must be dismantled when the occupant departs the location. They are commercially available products with full manufacturer support. They are as reliable (or more reliable) as your Vaportstat and offer significant efficiency benefits by eliminating the short cycling common with all Vaporstats. Such a solution to minimize pressure in the system is not a good one and reflects 1950's technology in the year 2018.

With regard to the methods to which these controls adapt to wind and sun loading, each has its own preferred solution.

The Tekmar 279 has the option of indoor feedback by several sensors if desired. The control shifts the curve higher, if necessary, if it finds the average indoor temperature has fallen below the target temperature. The exact algorithm utilized is, unfortunately, proprietary to Tekmar.

The Heat-timer MPC Platinum, a very costly control and way out of the price range for residential, utilizes indoor feedback to delay the start of the next cycle if the room temperature matches the target temperature. One would set a slightly higher than anticipated heatloss curve and allow the control to prevent a restart for the next cycle if there is no wind present.

Regarding your concern about your inability to frequent the location to reprogram the unit as required, the Heat-timer offers the option of being fully connected to the Internet allowing the technician to reprogram it from anywhere,

However, if properly setup with indoor feedback, there is very little to manipulate on a weekly basis once the initial setup parameters are established (which admittedly requires some time and effort).

Jamie Hall

As a direct answer to @Eastman -- the answer really depends on how much oversize the existing boiler is relative to the radiation. At least in my opinion. If it is close -- say within 10% -- I still advocate (and will continue to!) the simplest possible system, as outlined in the .pdf in my original post. Properly adjusted vapourstat and a good thermostat in a representative location.

However, if the boiler is really oversized, the problem with my solution is that the run phase of the modulation starts to get too short. At that point, one of the timer delay solutions or timer controlled with outdoor reset may be better for larger systems, while shifting to more cycles per hour on the thermostat may work better for smaller ones.

So I really don't think one size fits all...

Eastman

@PMJ
I have nothing. But there are some common corrective action themes expressed on this website that usually include: main venting, lower operating pressure, etc. Should this include increasing the CPH? Are the systems mentioned by SeymourCates worth the cost? Do they accomplish what is desired?

SeymourCates

@Eastman

The referenced systems are not worth the cost on residential. All efforts on residential should be directed to downfiring the equipment to the greatest degree possible and further allow slightly higher pressures (up to 2 psi) to avoid cycling on pressure for as much of the heating season as possible.

Although I have never attempted it, the use of an inexpensive timer to limit the boiler to a specific on time followed by a specific off time can offer great benefits that avoid the use of a Vaporstat Presuming one does not want to attempt the use of a PLC, this solution is elegantly simple. The timer would be in series with the thermostat, of course. Mr. Hall noted that the boiler must be significantly larger than the radiation to take advantage of such an approach,. In reality, the boiler and the radiation can be close to the same size (his system) but the heatloss must be 20-30% less than the boiler/radiation (to enable cycling with the timer on the design day).

Of course, the above will now enter the realm of "cobbled" for which only the installer can service properly.

PMJ

@Eastman

I meant to ask what do you not like specifically about the performance of your system with an assumed standard tstat/vaporstat or pressuretrol control?

Eastman

@PMJ
I don't have a steam system.

PMJ

> @Eastman said:
> @PMJ
> I don't have a steam system.

I see. Then assuming the hypothetical system that needs a better control already exists it really depends on what the issue is. There are so many different situations as so many modifications have been made to these now old systems.

But I agree with @SeymourCates that the simple delay timer helps greatly with the most common problem of oversize everything. A number of readers here have employed those with good success. They are really not that complicated.

Jamie Hall

And I at least sort of agree -- for systems which are significantly oversized either in radiation or boiler or both. If not... as we all know by now, I'll stick with my old reliable vapourstat, for the boiler on the rare occasions when a run is long enough to fill all the radiators and then some, and a good thermostat.

I might add one note to the various comments about letting the pressure run up, since it is probably obvious to the commenters -- but not necessarily to other interested readers: don't do it on vapour systems. Use something -- doesn't really matter what -- to ensure that the pressure differential between the steam mains and the dry returns stays in the "vapour" range, which is to say between about 3 ounces and 6 ounces. Note that if the system does not go into vacuum on the dry returns that corresponds to a psig reading. But it's differential you are interested in, not absolute pressure.

Why? Because that's where these things are designed to run. A Hoffman Equipped system, and a number of other designs, will stop heating entirely if the differential is greater than that, as they will let boiler pressure into the dry returns. That's the way they are supposed to work. Remember Dan's comment -- you must have a pressure differential to get flow. Others, which depend on a water seal arrangement instead of a trap, will also let boiler pressure into the dry returns, but in an uncontrolled and haphazard way, with somewhat unpredictable -- but poor -- results on the heating. And yet others, which depend on orifices for control (or adjustable valves) will heat unevenly.

gfrbrookline

I agree with Jamie that there is no one size fits all with one pipe steam. I have a burnham V905 heating a my 10K sq. ft. corner condo building with 3 large units sandwich next to another building. My boiler is over sized by about 20%. I have my mains maxed out with 3 big mouths each and have replaced all of my rad vents with hoffman 40's. The building is balanced on normal days within 2 degrees with the vision pro tstat set at 2 cph. The only time we go out of wack is when we have large temperature swings that make the boiler run off the vaprorstat. Since the hoffman 40's open and close throughout the cycling I don't think I am reducing my efficiency running on the vaporstat since I am not making steam that the radiators can't use. I think the key is to have rad vents that can open and close on pressure not temp and have the vaporstat set to work within the operating pressures of the vents. This lets the system breath.

gfrbrookline

Two other observation.

When the boiler was installed with a pressuretrol set at 1.5 cut out and 16oz. subtractive cut in I had water hammers all over the place.

I replaced my original vaporstat with an old school mercury unit and found it is is far more accurate. At 12oz my system is silent and the rads heat all the way through top to bottom.

Jamie Hall

gfrbrookline said:

Two other observation.

When the boiler was installed with a pressuretrol set at 1.5 cut out and 16oz. subtractive cut in I had water hammers all over the place.

I replaced my original vaporstat with an old school mercury unit and found it is is far more accurate. At 12oz my system is silent and the rads heat all the way through top to bottom.

Not surprised -- and I'm delighted your system works so well! It's nice when they do that -- simple solutions, reliable gear...

Those old mercury vapourstats simply can't be beat.

gfrbrookline

I have to give to give a huge shout out to Newenglandsteamworks for most of my fix, Ryan corrected my main vents. Once they were fixed installing the hoffman 40's and lowering the pressure was a no brainer. I never trusted the new style vaporstat so glad I went with my gut.

Eastman

gfrbrookline said:

When the boiler was installed with a pressuretrol set at 1.5 cut out and 16oz. subtractive cut in I had water hammers all over the place.

What is a "subtractive cut in"? The system would drop into vacuum?

Fred

Eastman said:

gfrbrookline said:

When the boiler was installed with a pressuretrol set at 1.5 cut out and 16oz. subtractive cut in I had water hammers all over the place.

What is a "subtractive cut in"? The system would drop into vacuum?

Some Pressuretrols are additive (Cut-in + Differential = Cut-out) others are Subtractive (Main - Differential = Cut-in). Has nothing to do with with vacuum.