How long to expect pressure to hold in one pipe steam system?

Dan_NJ

My question here is how long should one expect pressure to hold after the boiler cycles off? I am getting up to about 2.5 psi and it drops over about 2 minutes to 0 psi after cycle ends. Does this sound right? 9 rads of various sizes and the boiler is 182k btu installed about 14 months ago to replace a Crown that developed a hole. Since then I've dealt with a few deficiencies in the install, mainly failure to adjust pressuretrol (cut in was set to 9 psi). There was surging and some water hammer in the main, and main vent spitting until I got the pressure under control. Replaced main vent and rad vents, and out of 9 rads I'm down to one gurgling rad inlet valve yet to be repacked and one radiator with a hole in it that appears to have been drilled into it at some point - to be patched. Best guess is the original vent was toast and this was someone's fix for it.

KC_Jones

1. Is there a reason you want to run the boiler that high?
2. Why do you want it to hold pressure?
3. That time sounds about right, without the burner things will drop quickly.
4. Was the boiler sized properly for the attached radiation? I only ask because running those numbers indicates you either have ridiculously big rads or the boiler was over sized. Which could be why you are building so much pressure.

Dan_NJ

Thanks, some comments ...
1. It's that high only because I haven't really dialed it in yet (down as low as I can get it). This install has been a learning experience and I hope to have it buttoned up this weekend.
2. I don't know what is normal, just want to know how it "should" be.
3. n/a
4. The boiler is same as the last one, and I don't know the specs of all predecessors, but it was coal fired back in the day, then oil, then (and now) gas. There are few large rads in the system but I expect to get the pressure down quite a bit in the end.

Jamie Hall

If anything your time is longer than usual. The system in the main place I care for drops from 7 ounces (cutout) to 0 in about 30 seconds.

KC_Jones

If the boiler is over sized, which given your answers I bet it is, you will only do so well on pressure.

The pressurtrol is a safety device and if the boiler is sized correctly, really should never come into play.

The true operating pressure is dictated by boiler size and venting.

BTW boilers should never be sized from the previous boiler. You may end up with the same size after the correct calculations, but one should never assume the old one was correct.

You may want to calculate the EDR of your system and compare that to the boiler sizing to see how good or bad you are.

Dan_NJ

Thanks for this, I am partly done with EDR calculation, I will revisit that and complete it. The first Gas fired unit was installed by someone who I am pretty sure knew what they were doing, but I'll go back and see what I can find and do my own calcs too.

jumper

Jamie Hall said:

If anything your time is longer than usual. The system in the main place I care for drops from 7 ounces (cutout) to 0 in about 30 seconds.

Doesn't it depend on at what temperature vents open?

Jamie Hall

jumper said:

Jamie Hall said:

If anything your time is longer than usual. The system in the main place I care for drops from 7 ounces (cutout) to 0 in about 30 seconds.

Doesn't it depend on at what temperature vents open?

Not really -- the pressure drop (at least in my experience) is due almost entirely to condensing and collapsing steam. Of course that may be biased, since most of my experience is in vapour systems and particularly Hoffman, where -- unless you get over exuberant with the pressure in the first place -- the main vents never do close. But the crossover traps do, of course, and some of the radiator traps (I haven't been as fanatical about setting the radiator valves as I might be... )(the radiators where the valves are set properly don't have to -- the trap is a backup. The ones which aren't... well... there are other things which need doing first around here!)

EBEBRATT-Ed

What @KC_Jones said is exactly correct.

A properly sized and vented boiler shouldn't build pressure. All the pressure control adjusting and vapor stats will not fix an oversized boiler. Boiler output should match EDR +pick-up factor as closely as possible

R Dougan

Doesn't this also depend on temperature differential between the space and the thermostat? Under most cases the difference would be between .5 and 1.5 degrees as set on the thermostat but in the case of an overnight setback of 2-4 degrees the burner may have to fire longer for the radiators to meet the heat demand, could it not? So instead of modulating via the thermostat, the pressurtrol would take over until the more extreme differential is closed? Would this scenario still indicate an oversized boiler or just an unusual case of a secondary control taking over?

PMJ

Pressure drop is dramatic and fast. At 1000+ to one volume change steam to water this should be no surprise. Occurs fastest from the rads and in those the fastest in the coldest area ones. You can use this to great advantage when you don't let air fill the space created. When you keep the air out the space created is filled by more steam from the mains. The coldest area rads actually "steal" a higher percentage of the total steam this way on every burner off part of every cycle.

Conversely, when you let air back in the coldest area rads stop heating the fastest! Think of this difference occurring every cycle - adds up to a lot of free balancing lost. It adds up to a large volume of steam that could have gone to your colder areas instead of warmer ones but didn't. And when you increase the number of cycles you actually increase this difference.

Dan_NJ

Going back over the numbers here, my approximate calculations say EDR is in a range of perhaps 450-500 so the boiler at 182k btu seems about right. I'll nail it down when I get to the few remaining radiators to verify.

Jamie Hall

And let the games begin... what @EBEBRATT-Ed said is correct. but @R Dougan ' comment is, sort of, too. And it is one of the reasons for not using excessive overnight setbacks (there are others). The ideal boiler will be sized so that it exactly matches the condensing capability of the radiation, at the coldest space temperatures expected. Right... Even more ideal would be a burner control system which would modulate the firing rate to maintain just the correct pressure for the system, since that varies with space temperature... right...

Most of us are not so blessed, and so the idea is to let the boiler fire at whatever it's happy at until either the thermostat is satisfied or all the radiation is full of steam, and then cut it off until the radiation catches up with the boiler, and then fire the boiler again. Poor man's pulse modulation, if you will.

There are many many threads and even some rather heated debates on the subject!

Curiously, in my opinion at least, it isn't that hard to determine the correct cutout pressure -- although for many systems controlling at that pressure may be (assuming the venting is adequate here!). The procedure I recommend involves a low pressure gauge, preferably on the header but it can be on the boiler. Fire up from a cold start and watch the gauge. It will rise to a low, but remarkably steady pressure -- which should be no more than a few ounces, although very big systems it may be more. And it will stay there for quite some time. At some point, though, the pressure will start to rise again -- and that is when you want the boiler to cut out. Note the pressure just before it starts to rise, give yourself a 50 to 100% margin, and set your cutout at that pressure.

EBEBRATT-Ed

Yeah but don't forget @Jamie Hall & @R Dougan that on a night setback the air in the house will be cooler. Look at how radiator ratings are determined. I think it's 1 psi of steam and at a certain room temperature entering the radiator (I forgot what that temp is) but anyway if the air is cooler on a night set back the radiator will condense more steam when coming out of night setback than it will at normal room temperature. So with a larger load on the boiler the boiler is less likely to build steam pressure, at leats until you approach room temperature.I still say with the average home, and small commercial job unless there is something unusual you should be able to heat the building without the pressure control interfering ...it's a safety control, not an operating control ...unless you add a second control.

Best way is the right size boiler. 2d way is downfire an oversized boiler if combustion tests etc confirm you can do this. To me vapor stats and low pressure gages are not needed. If the 30psi gage needle leaves the pin your building pressure and shouldn't be. Shouldn't be trying to control the pressure...shouldn't have any pressure to control

R Dougan

@EBEBRATT-Ed ...Point well taken. A question though, ...sizing aside, ...say your stuck with your somewhat oversized boiler. Do you think it's hard on the system to cycle on pressure say three or four times during a heat call or are systems designed for this over their lifetime. Just curious what your thoughts on this are. Would it lead to premature failure in one of the other subsystems like the damper control or the automatic gas shutoff valve? Those are probably designed for hundreds of thousands of cycles, but over time even a small increase in cycle percentage could lead to premature failure, ....yes,...no, maybe?

EBEBRATT-Ed

IMHO short cycling kills equipment. Ware and tare on the burner and it's components, ignitors, motors etc.

Boilers and furnaces rapid expansion and contraction by too many firing cycled will fatigue any metal over time. Keeping things at a more constant temperature and slower temperature changes will promote a longer life.

But I am sure others will differ from my opinion.

Oil can usually always be downfired to match the load even if burner components have to be changed...nozzle...blast tube...end cone etc as long as the stack temp isn't dropped to low to condense you would probably gain efficiency.

Gas cannot always be downfired

PMJ

I don't think I have ever seen here a definition of what a short cycle is in minutes on and off.

IMHO the most uneven temps in the structure and of all of the heating components comes from long burns and long offs.

I run 3 cycles per hour and vary the burn/wait percentages in each 20 minute cycle according to the conditions. I try to make it so it takes as many of these cycles as possible to satisfy a call for heat because that will be the most even heat(and btw the most even temperature of all the components). I don't think the total cycles of this operation gets within 1/2 of the rated cycle life of gas valves or anything else in 50 years if I cycled this way continuously all 24 hours - which I don't.

So great is the fear of cycles that so many are willing to run pressure instead. I gave that up long ago for the sake of more even heat. I would gladly pay more for the eveness. The best part is that it turned out I pay even less.

Jamie Hall

It shouldn't surprise you, @EBEBRATT-Ed -- I agree with you. The less cycling one can do on one's machinery, the better. For what it's worth, in my own situation @Charlie from wmass and I have managed to get Cedric dialed in sufficiently closely that the cycling -- except if we have a power failure and it gets cold -- rarely happens, even recovering from a 3 degree setback. And it is also set so that the boiler doesn't cool appreciably when it does.

I still think the best option would be some rather sophisticated control system which would do the same thing the old coal burners did -- but with better efficiency (coal fires under reduced draught are pretty horrible...). That is adjust the firing rate more or less continuously to match the target system pressure. Then if you wanted to add a layer of complexity, run the system under a vacuum, and adjust the target vacuum with outdoor reset...

Someday maybe.

mikeg2015

Modulating an atmospheric boiler doesn’t really work because you can’t control draft very well. Best option is multiple boilers or maybe a single boiler with multiple fire boxes and flue dampers.

A 50/50 split would give you pickup capacity but put you close to heat loss at design.

Jamie Hall

mikeg2015 said:

Modulating an atmospheric boiler doesn’t really work because you can’t control draft very well. Best option is multiple boilers or maybe a single boiler with multiple fire boxes and flue dampers.



A 50/50 split would give you pickup capacity but put you close to heat loss at design.

You have to control both air flow and firing rate, which is what makes it a trifle complex. Not that that can't be done: consider that an aircraft gas turbine producing 70,000 pounds of thrust at full song can dial down to less than a 1,000 at idle. It's just a good deal more complex than simply adjusting the pressure at the nozzle, or something obvious of that sort. The old pressure controlled coal burners controlled only the draught, there being no easy way to quickly control the amount of fuel. Their efficiency at full power wasn't really that bad (for the day...). Their efficiency at low fire was truly horrible.

I have to admit that I can envision the control loops, and could source a good deal of the necessary sensors and computers and drivers for such a system off the shelf (by far the hardest part is to develop a nozzle system which will produce a good pattern over a wide range of flows and pressures). That is not the problem. The problem -- as it is so often -- is that the resulting system is not simple, and making it fail operational (which is required) is even less so -- and the resulting cost does not result in a positive financial benefit.

PMJ

Coal fires ran continuously and modulated because there was no choice. I don't think low fire continuously with all that draft is anywhere near as efficient as shorter evenly spaced full fire cycles with vacuum between them. Not close.

EBEBRATT-Ed

@PMJ ,

well I guess I gotta pick a number so I am going to say anything 5cycles/hour and over is excessive in my opinion. That's once every 12 min which I think is too much.

Obviously the # of cycles depends on a lot of factors. I wouldn't be crazy about 4 times/hr. but not so bad

I think anything 3 and under is fine.

That being said, I am talking about normal cycling to maintain a set temperature. A couple of extra cycles occasionally like when coming out of night setback etc I think s fine

mikeg2015

It goes beyond draft control. Lower air velocity impacts how efficiently the air releases the heat and radiation. I think it’s related to thermal momentum and the fact that air is an insulator.

Let me ask this...

What if I sized a steam boiler only to building heat loss? But stayed generous on pickup? So inmy case, I need 105,000 so with 20% pickup and 83% efficiency it comes to 150,000. Why can’t I just use really slow vents and have a boiler with nice long cycles, running almost continuously in cold weather? No vaporstat ever needed. Nice and slow.

Downside is initial warmup could be 30 minutes before radiators just start to put out heat.

Jamie Hall

PMJ said:

Coal fires ran continuously and modulated because there was no choice. I don't think low fire continuously with all that draft is anywhere near as efficient as shorter evenly spaced full fire cycles with vacuum between them. Not close.

Oh no it's not! Those old coal burners ranged from pretty bad to horrible, depending on firing rate!

PMJ

Jamie Hall said:

PMJ said:

Coal fires ran continuously and modulated because there was no choice. I don't think low fire continuously with all that draft is anywhere near as efficient as shorter evenly spaced full fire cycles with vacuum between them. Not close.

Oh no it's not! Those old coal burners ranged from pretty bad to horrible, depending on firing rate!

Not following you here Jamie. I wasn't saying coal was efficient.

What is your "it" in "Oh no it's not"?

SeymourCates

@EBEBRATT-Ed

You are entirely correct regarding short cycling killing the equipment. But, more importantly, short-cycling kills the efficiency. In the hot water world, we make significant effort to get the burners to fire for a minimum of 10 minutes (and preferably longer). There are solid reasons for this including the gross inefficiency on startup of natural draft appliances and the need to bring the boiler back up to SWT. The suggestion to operate a steam boiler with two minute cycles on pressure is inane when one realizes that the steam collapses inside the mains (and rads) and all that energy is lost to the mains upon shutdown. One could estimate that 50% of the energy is lost in such a scenario. For all the folks that insist on running a pressure of 1 lb. under all conditions with a large boiler (relative to the EDR), they have effectively committed themselves to a miserable short-cycling situation at the end of the cycle. Far more efficient to accept 2 psi and allow the boiler to continue to run until the end of the call. Since the pressuretrol is a limiting device, the setting of 2 psi will only occur when the boiler comes out of setback or is in a recovery for other reasons (presuming, of course, that the boiler is not grossly oversized).

Jamie Hall

@SeymourCates ; I have addressed your errors regarding the operation of steam boilers -- at least on oil -- elsewhere. Suffice it to say that:

1. the stop/start to maintain a correct pressure for most oil burners is not sufficiently long to either drop steam pressure to zero nor to allow the boiler to come completely off the boil. The efficiency loss is minimal.
2. The additional fuel to reach 2 psi is considerably more than necessary to compensate for the loss in in the post purge/pre purge cycle.
3. At least for a vapour steam system, the additional pressure will cause the system to either not work at all (for a Hoffman Equipped system and most orifice systems) or to work poorly.

As @EBEBRATT-Ed has said, and I concur completely, the correct solution to the problem is to attempt to size the boiler and burner to the system load. One can, with a good technician, get remarkably close. The result of doing this will be minimal pressure cycling, and that only when operating near or at design load or coming out of a setback.

SeymourCates

@Jamie Hall

1. The stop/start cycle does not "maintain" anything. Sadly, it allows the boiler to come all the way off any pressure and get just about to zero, with all steam condensing in the mains, before it restarts. At restart, it must reestablish draft and get to a clean combustion. This takes a bit of time on oil. The latest developments in oil are a prepurge and a post purge cycle to assist with this efficiency. If your equipment has these cycles, the delay is much greater than the point where the pressuretrol cuts in and the collapse of all steam has definitely occurred.

2. The additional fuel to reach 2 psi is negligible when compared to the miserable efficiency of the short cycle starting and stopping. If your statement were true, we would not have any care in the world about the short cycling of HW equipment when it is firmly established that such cycling is absolutely terrible for the equipment and for efficiency. We could simply buy the largest boiler available (3X heatloss) and call it a day. But, this approach is not advocated by anyone (AFAIK). You really can't make the argument that such cycling is fine on a steam boiler but totally unacceptable on a HW boiler.

3. The discussion is not related to vapor systems which have their own set of issues. You're apparently stuck with the short cycling inefficiency unless the boiler can be downfired (on oil) or operated in two stage (on gas).

Jamie Hall

" If your equipment has these cycles, the delay is much greater than the point where the pressuretrol cuts in and the collapse of all steam has definitely occurred."

Sorry. Not true. Steam is still being generated, although at a much lower rate.

Please note. I do not advocate cycling on pressure on any system, nor on temperature on hot water systems. If one is so fortunate as to have a system which does not do so, under any conditions, that is to be preferred. Your comment on just going to three times what is needed and calling it a day is, of course, absurd -- but is also deeply insulting to all of us who try to size and maintain systems correctly.

Your comment "the miserable efficiency of start/stop cycling" is, of course, also absurd. I am more than ready to grant that there is a loss of efficiency if a system is stopped and restarted -- at any interval of time, whether 30 seconds for a post/pre or 20 minutes (although the former is far less stressful on the boiler and piping than the latter). To say that it is miserable, however, is unfortunate.

I am much more interested in systems which are reasonably efficient, and which are simple, practical, and above all reliable.

Since you, however, seem to be deeply committed to your position, may I make a suggestion? Actually measure the timings and losses which are involved on a variety of systems -- vapor, two pipe steam, one pipe steam, non-modulating hot water, and modulating hot water. Do this under a variety of conditions -- full design load (design day or deep recovery, for instance) and various part load conditions. Do it also for a variety of stop/start conditions.

I, for one, will be very interested in your results.

PMJ

Having spent many years now adding cycles on purpose while steadily dropping my gas bill I will argue that starting/stopping losses if there are any are negligible on a residential boiler with automatic damper. I will further argue that shorter cycles with natural vacuum between them is more efficient than longer straight runs to any pressure and the system vented to the atmosphere.

Anyone who has actually stood next to a residential boiler that has been off only a few minutes knows a full boil is achieved again very quickly on re-fire so I agree with Jamie on this. In vacuum, boiling is practically instantaneous on refire due to the lower boiling point. Vacuum or not, in just a few minutes the core temperature of a flue damped insulated boiler has hardly changed.

Having deliberately put thousands of "extra" cycles on my equipment over many years now with only positive results in all aspects I believe there is way too much worry about it.

As far as I am concerned the thing that should be thought about by anyone with a system that is already at some pressure and cycling at all to hold it there is that you are maintaining a supply of steam out there well past what is required for the conditions for no good reason. This is what my system was doing when I moved in 25 years ago. Running that way cost me considerably more and was much less comfortable.