How long should it take to build to full pressure of 2# psig?

mickeylewis123

How long should it take to build to full pressure of 2# psig? In a home 1-pipe steam system of average size? And once the system reaches pressure, and the boiler turns off, how long should it normally take for the pressure to drop back to 0.5# psig when the boiler should turn back on?

KC_Jones

If things are correct you never hit pressure. You don't want pressure, the pressuretrol is a safety limit not an operating control. If you are never getting there that is a good thing.

Now if you are getting to that pressure quickly and the boiler is cycling a lot, that isn't the best.

What is the situation you are having that has driven you to ask this question.

Fred

If you're lucky and the boiler is properly sized and you have good main venting your pressure will stay down around just a few ounces. That's what you want!

Jamie Hall

As the guys have said, it should take forever. First place, 2 psi is too high. Set the pressuretrol back down so it cuts off at around 1.5 psi. Second place, if your pressure is rising to the cutout point in less than say 30 to 40 minutes, talk to your boiler technician to see if the firing rate can be reduced some. On the other hand, if you are reaching cutout pressure within say 10 minutes or so from a cold start, that tells me that you are woefully short of main venting.

AnthraciteEnergetics

Once all pipes and rads are hot the boiler overcomes the ability to condense steam (asuuming it is sized accordingly), then pressure builds. Pressure building before the entire system is fully heated indicates problems (venting, etc mentioned).

My system has 98k BTU of boiler (Peerless 63-03) and around 82k of rads. I only build pressure on recovery from temp setbacks. I recently added a 2 stage gas valve to switch to low fire (60k) when the system is hot - now I'm seeing 1/4 psi (no higher) on recovery from setback.

EBEBRATT-Ed

As others have mentioned your boiler should never build pressure if it is sized correctly and you have the proper venting.

That being said, if the boiler runs forever and the house is slow to heat the you could have other problems.

mickeylewis123

Interesting. The reason I asked is that it takes about an hour and fifteen minutes for all 16 of my radiators to heat all the way across, and the last few don't heat all the way across until i hit the limit of the Pressuretrol (1.8#) several times. How long should it take for all the radiators to heat all the way across?

Jamie Hall

mickeylewis123 said:

Interesting. The reason I asked is that it takes about an hour and fifteen minutes for all 16 of my radiators to heat all the way across, and the last few don't heat all the way across until i hit the limit of the Pressuretrol (1.8#) several times. How long should it take for all the radiators to heat all the way across?

Main venting. Lots more main venting. That's your problem... it shouldn't take that long for all the radiators to heat across, provided that the boiler is even remotely closely sized to the installed radiation (not the heat loss of the building!) and the venting is correct. Once all the radiators are hot across, then, and only then, should the pressure begin to rise.

Fred

Are you standing be the boiler the entire time it's running? You do realize if the thermostat is satisfied, in the room where it is located, the thermostat will shut the boiler down before the other radiators heat all the way across. If that's the case, then balancing the radiator venting is the issue. At least slowing the venting on the radiator where the Thermostat is located, for starters.

daveamir

What is bad if u set the pressure to high, will you waste more gas?

Fred

While it seems counter intuitive, steam moves slower the higher the pressure. Yes it will waste fuel. If high enough, it will also ruin both radiator and main vents.

Jamie Hall

daveamir said:

What is bad if u set the pressure to high, will you waste more gas?

Yes. Once the pressure starts to build -- even beyond a few ounces to half a pound -- the additional fuel is just compressing the steam that's already in there, and adds nothing to the space heating. Zero, zip, nada. Every minute of burner time after the pressure starts to build is pure wasted fuel.

And that's not all... excess pressure can damage the vents in the system and the traps. Those don't come cheap, either.

It can also back water into pipes which aren't supposed to have water in them, causing water hammer.

daveamir

Thanks for explaing that, it makes perfect sense now, so what's the best pressure setting?

Fred

As low as you can get it and still reach the furthest radiator. Most of us run in the ounces, certainly 1 pound or less but 1.5 PSI is considered an acceptable max

daveamir

Thanks I appreciate it

mickeylewis123

So now, i'm embarrassed to say that i have several major issues:

1. Two of my mains coming off the header come back on dry returns but I just checked the "A" dimension and it is only 19" from the water line to the lowest point of these returns!!!!
2. And the main vents are at the very end of these dry returns, not set back 15" - although I'm sure this doesn't make a difference until the "A" dimension is fixed, right?
3, My 3rd main has the main vents at the very end of the run, before it turns 90 degrees and drops into a wet return. I've read that the main vents need to be set back 15" from the end.

mickeylewis123

4. Oh, and my boiler is oversized for the EDR of the radiators. EDR of radiators is 413. EDR of the boiler is 746. Not much i can do about this now, is there?

Neild5

The 15 inch set back is to prevent the steam from pushing water into the vent and clogging it.

New England SteamWorks

Never. If properly sized and vented.

Jamie Hall

mickeylewis123 said:

So now, i'm embarrassed to say that i have several major issues:

1. Two of my mains coming off the header come back on dry returns but I just checked the "A" dimension and it is only 19" from the water line to the lowest point of these returns!!!!
2. And the main vents are at the very end of these dry returns, not set back 15" - although I'm sure this doesn't make a difference until the "A" dimension is fixed, right?
3, My 3rd main has the main vents at the very end of the run, before it turns 90 degrees and drops into a wet return. I've read that the main vents need to be set back 15" from the end.

mickeylewis123 said:

4. Oh, and my boiler is oversized for the EDR of the radiators. EDR of radiators is 413. EDR of the boiler is 746. Not much i can do about this now, is there?

1. -- sigh. Well, keep the pressure down as low as you can. You might consider using a vapourstat instead of -- or, better, in addition to -- the pressuretrol, and setting it to cut out at around 11 ounces per square inch. Differential could be 6 ounces per square inch.
2. Sigh again. What you can do, if you have the headroom, is raise the vents above the return or the main on nipples, as high as you can.
4. Ah. Well, no, not much you can do about that. Not that unusual, either, unfortunately (bigger is always better, right?). The poor thing will cycle on pressure, though -- can't help it.

EBEBRATT-Ed

When you get some cold wether shut the boiler down for a couple of hours. After that crank the thermostat up and let her rip. Start timing with your watch.

How long until you can't hold your hand on the steam pipe leaving the boiler?

How long to get steam to the end of each main

How long until all radiators are 3/4 hot

Did the burner run the complete cycle or was it stopped by the pressure control or low water cutoff?

After you do thids we will better know how to help.

also, a few pictures of the boiler and the piping around the boiler

mickeylewis123

Thanks Jamie.

mickeylewis123

Hi Ed, I've done extensive testing and documenting how long it takes from a cold start to when each component heats up, the pressure, and the gas usage. It takes 1 hour and 15 minutes for all 16 radiators to heat up all the way across. 1/3 heat up in 20 minutes, 1/3 heat up in 40 minutes, and a couple don't heat up all the way across until they are hissing and spitting at 1 hr & 15 minutes. I have 3 mains, and there is at least one radiator on each main that doesn't heat up until the very end.

I've also documented the boiler and piping and other than a minor concern about some copper piping being used, the near boiler piping is all to manufacturers spec - 24" from water line to header, riser quantity, riser size, and header size all proper, riser & header piping proper.

BobC

How long (and what diameter) is each of your steam mains?

How many radiators come off each main and are the radiators that are slow to heat near the end of their mains?

What kind of vents do you have on the steam mains?

What kind of vents do you have on each radiator and what is the EDR of each radiator?

If you answer all of the above we can suggest actions to improve things, until then it's all just hot air.

Bob

PMJ

mickeylewis123 said:

4. Oh, and my boiler is oversized for the EDR of the radiators. EDR of radiators is 413. EDR of the boiler is 746. Not much i can do about this now, is there?

Actually, just for the record there is quite a lot you can do about this - but you need a different control which won't be off the shelf. So if you are not comfortable going there then yes you are stuck as others have said.

The trouble with pressure based controls for residential heat is two fold. First, the maximum or cutout pressure on a conventional control fills your system always with a maximum amount of steam even when the conditions don't require nearly that much. And secondly when the burner goes off the pressure drops very quickly because condensing steam takes the volume of 1/1000th in the water form and you find yourself back at the cut in pressure again in short order. This is the short cycling problem people refer to. What is the answer - never go to a pressure stop - ever.

To get out of this problem you need to cycle the boiler on an even timed schedule instead of based on pressure. By controlling the length of each burn and the wait periods in between firings you can run with basically no pressure or any peak amount you wish no matter how much oversized the boiler is. In fact, I find a big boiler actually quite desirable because it recovers a cold delivery system much faster than a "perfectly" sized one. And because I easily manage its size with controlled cycles once the system is warm there is no negative to the big size. My heat is dramatically more even running this way than it was previously running on a vaporstat.

Once again, this approach is for the unusually interested owner who is comfortable making his own control with pretty simple components. If that is not you then stick with the conventional stuff and do the best you can. The folks on this site can help you a lot with all that.

Jamie Hall

@PMJ -- can you quantify, for the record, how much efficiency gain you have with have with your approach vs. the more common -- and unless I am much mistaken -- approach of accomplishing exactly the same thing using a vapourstat and a thetmostat?

AnthraciteEnergetics

Once all pipes and rads are hot the boiler overcomes the ability to condense steam (asuuming it is sized accordingly), then pressure builds. Pressure building before the entire system is fully heated indicates problems (venting, etc mentioned).

My system has 98k BTU of boiler (Peerless 63-03) and around 82k of rads. I only build pressure on recovery from temp setbacks. I recently added a 2 stage gas valve to switch to low fire (60k) when the system is hot - now I'm seeing 1/4 psi (no higher) on recovery from setback.

PMJ

Jamie Hall said:

@PMJ -- can you quantify, for the record, how much efficiency gain you have with have with your approach vs. the more common -- and unless I am much mistaken -- approach of accomplishing exactly the same thing using a vapourstat and a thetmostat?

Jamie,

I didn't see any discussion about efficiency in this thread. I have always maintained that my primary purpose was even heat and operation at no pressure(really the same thing) which I think everyone seems to agree is the most efficient. My point in posting on this thread was only to put out there that no pressure operation with oversize boilers is indeed quite possible.

However, per your request I posted this data previously on another thread comparing where I started vs where I am now. The only large change beyond method of operation then and now was the addition of a vent damper which was big. Here is the link. I think the data was on the first page.

https://forum.heatinghelp.com/discussion/161308/simple-2-pipe-vacuum-setup#latest

Again, I know that you and many others run quite satisfactorily and quite sufficiently efficiently with conventional controls. At the same time I see lots of discussion here about "too big boilers" and "too much pressure". The change in my house moving to this method of operation and then of course on to natural vacuum produced an improvement in how even the heat was that I can only describe as dramatic. I am absolutely convinced it is at the same time more efficient. But I would gladly pay more for the improved comfort had it been the other way. Fortunately it was clearly a win-win.

AnthraciteEnergetics

@PMJ I did it similarly but with 2-stage gas valve and I have a "snap" temp switch at the end of the last (to heat) radiator right above the vent. It opens at 130*F, kicking the valve back to low fire. Once the system is hot the low fire setting makes steam as fast as it can be condensed, nearly eliminating pressure build (< 1/4 psi). A bit more complex as it needs the gas valve change and t-stat wire back to the last rad but it's been working well.

PMJ

AnthraciteEnergetics said:

@PMJ I did it similarly but with 2-stage gas valve and I have a "snap" temp switch at the end of the last (to heat) radiator right above the vent. It opens at 130*F, kicking the valve back to low fire. Once the system is hot the low fire setting makes steam as fast as it can be condensed, nearly eliminating pressure build (< 1/4 psi). A bit more complex as it needs the gas valve change and t-stat wire back to the last rad but it's been working well.

I agree completely with your analysis. 2 stage or modulation will do much the same thing I agree. That plus a closely matched boiler as you have will be very good.

There are a great many installations out there where the boiler is much bigger than needed. I am just pointing out a way to run those too at pressures too low for actual control. Also that big boilers get you to your "once the system is hot" place faster.

Is your system one or two pipe? The real icing on the cake for me performance wise was the natural vacuum. In two pipe that is quite easy to do.

Jamie Hall

@PMJ , @AnthraciteEnergetics and others -- I think we are all going after the same problem in different ways! One is tempted to say "whatever works..."! The fundamental problem -- which we are all addressing -- is trying to match the average boiler firing rate to the condensing rate in the system and, of course, the problem there is that we have to use some surrogate measurement for the condensing rate, since there is no simple way to get at that.

It is worth going way back in time and take a look at some of the controls which were used on coal fired boilers to shed a little more light on what we are doing -- and, not incidentally, to see how it is that we got to where we are usually using pressure (hopefully very low pressure) as the surrogate. One can't just turn the flame off in a coal fired boiler (well, one can, if one is keen on carrying hot coals out of the boiler room in a wheelbarrow...), but what one can do is reduce the draught (or increase it, if you need more oomph) -- and back in the day, several companies had astonishingly ingenious and sensitive controls which related the steam pressure -- in the ounce range -- to draught. They had, of necessity, a lot of damping in the loop as raw steam pressure is jittery (still a problem, if you want to run in the 4 to 6 ounce range), but they could control the draught to control the steam production rate in a very narrow band. Truly a set it and forget it system.

Now the efficiency of those setups was horrendous -- but, honestly, I'm not sure it was that much worse than a coal fired boiler would have been anyway.

I would love to see a system which was able to control the firing rate of a gas or oil burner over a range of at least 10 to 1. Two stage gas is a step towards that, but only a baby step. Part of the problem is keeping the air/fuel ratio correct over that range with equipment which requires little or no maintenance, in boilers which are designed for a very wide range of firing rates (in itself no mean trick). I myself would be inclined to use a highly damped pressure signal as the control for a steam system. I would also use natural vacuum, as @PMJ mentions, in a two pipe system; that wouldn't be difficult at all since one can still buy perfectly good vacuum vents. I haven't really studied it, but I expect one could use delta T as a surrogate for hot water systems (and, possibly, forced air systems).

I would add that, down the road, adding that kind of proportional control to an electric boiler would be no problem at all (anyone know -- does anyone do that? @Henry ?)

In the meantime -- whatever works!

EBEBRATT-Ed

Too much discussion about vapor stats and pressure controls. In a house those are safety high limits....not operating controls. The operating control is the thermostat. A house shouldn't build pressure while heating. If it does your wasting fuel.

If it take that long to get heat on a cold start it's probably one or a combination of 4 things:

VENTING
lousy water surging or priming
improper near boiler piping
underfired boiler
tripping on a safety (low water or pressure)

Jamie Hall

Ah... I beg to differ. Unless the boiler is set up to quite exactly match the condensing rate of the radiation -- which, unless it has a moderate modulation range and a way to measure the rate of condensation and the related controls is quite impossible to do except at one particular operating condition -- there will be times when the boiler is somewhat oversized for the radiation. At those times it is necessary to have some sort of mechanism to effectively modulate the heat output of the boiler, and that I think, is what we were discussing. Which, admittedly, was a bit of a tangent from the original thread.

Now as it happens I am entirely in agreement with the idea of a pressuretrol as a safety limit. However, that should not be the modulating controlling device. For example, on Cedric there is a vapourstat. That's the firing rate modulator. Then there is a pressuretrol, which is the main safety device. Then there is a manual reset pressuretrol (at a higher, but not unreasonable pressure) which is there in case the other two go west. There is also a Hoffman Differential Loop, which prevents damage to the traps if the vapourstat is misset or doesn't function for some reason.

mickeylewis123

It's been three years since I asked my question and I just re-read the thread from start to finish. Thanks to everyone for your responses. In summary, if operating efficiently, i should never hit the 2# limit.

motoguy128

With a larger header designed for vapor or vapor vacuum, and plenty of main vents, it might never get over 1-2oz.

Higher pressure means the system is heating up faster or that air can’t escape as fast is it’s heating up. A system that heats up faster will tend to overshoot when it reaches set point. Although the metal will store more heat. But overall you are better having longer on cycles and shorter off cycles.

This is one advantage of hot water with CI radiators, it has a lot more mass. So it radiates more consistently and evenly between on cycles. Water stores 8x more heat per lb than iron. SO a 500lb radiator will store 650 BTU’s with a 10F temp drop but if it holds 8 gallons of water, it’s energy storage is roughly doubled.