Would new radiator vents help my boiler hold pressure longer after it shuts down?

SteamStudent

I recently replaced my Pressuretrol with a Vaporstat and my boiler is far better regulated at low pressure. I also added a 0-3psi gage because my 0-30psi never seems to move. I left the 0-30psi gage on for code. Now, because I can see the pressure accurately on the low pressure gage, I notice that the pressure drops pretty quickly after the boiler shuts down. Then, if the thermostat isn't satisfied, the boiler comes back on until the Vaporstat shuts it back down again. Would replacing the radiator vents slow that pressure drop?

KC_Jones

You don't want pressure in steam heating. Pressure means you are slowing the steam down. The reason to try and control it at low pressure is because you are making steam to build pressure not heat the building.

Also, to be clear, using a vaprostat or pressuretrol as a control device is a band aid of sorts for an oversized boiler, inadequate venting, or combination of the 2. It's the easiest way to tame the best when there is nothing else left.

A vaporstat or pressuretrol are technically safety devices.

ethicalpaul

To add to KC's correct words about pressure, no, vents won't prevent you from losing pressure--the pressure loss is because steam in your system is condensing to 1/1700 its volume into a few drops of water

KC_Jones

ethicalpaul said:

To add to KC's correct words about pressure, no, vents won't prevent you from losing pressure--the pressure loss is because steam in your system is condensing to 1/1700 its volume into a few drops of water

Yeah, I kind of didn't exactly answer his direct question did I?! lol

Jamie Hall

Well... there's a debate. First, though -- a fast pressure drop, as the folks have said, is inevitable.

But on whether a vapourstat (or, in higher pressure systems, a pressuretrol) is a safety device or a control... If you should be so lucky as to have a system where the boiler is perfectly matched to the radiation, there is no need for a control and it is purely a safety device.

However, most folks aren't quite so fortunate, and in that case it is necessary to modulate the boiler output so that the average boiler output over time exactly equals the condensing capacity. How do you do that? Well, way back in the dark ages of coal, there were gadgets which varied the draught on the fire in accordance with pressure. Efficiency was horrible, but they worked. Then came oil and gas. There are two stage gas burners. If you have one of those, one needs to decide which stage to operate on -- and again, no one has yet come up with a simpler variable to measure than pressure, so you use a vapourstat to turn the high stage on (if the pressure is a bit low) or off (if it's creeping up). Most gas burners and almost all oil burners don't modulate at all -- they're either cranking full bore or they're off. So the only way to modulate the output to what is required is to control the relationship of on time to off time (technically you're looking at variable frequency pulse width modulation, but don't let that worry you). You do that with the vapourstat -- at which point, you can see, it is a control, not a safety at all.

Now it's not a bad idea at all to have a second pressure control -- and the cheapest would be a pressuretrol -- as a safety, set at a pressure which you really don't want the system to exceed -- such as around 2.5 psi cutout -- to protect the vents and traps and so on from overpressure. That one is a safety, not a control.

SteamStudent

Thank you both for your quick responses to my question. So it sounds like it is normal for the pressure to go down pretty quickly upon shutdown.

I have often wondered whether the boiler is oversized for this house. I will now measure my radiation and report back with that number and the BTU's of my boiler.

Jamie Hall

The boiler clearly isn't oversized for the radiation. Whether it is oversized for the house is another matter altogether.

At the risk of boring you to tears here, this is where you get into another completely different kind of control and modulation -- namely adjusting the heat output of the system as a whole to the heat load of the house as a whole. Again, the mod/con boys have us beat: they do this by sensing the outdoor temperature and modulating the boiler output to match what is needed. It works remarkably well for a tight house (it doesn't work well in a draughty one). But here we steam folks are at a disadvantage (for that matter, so are the hot water and hot air folks who can't modulate the output of the boiler or furnace). So again, the simplest procedure is variable frequency pulse width, only this time we vary the total on time of the system in relation to the total off time, and we do that with a thermostat. Voila!

I might note as an addition to my first comments that sometimes one is faced with a boiler which is really badly matched to the radiation, with the result that the on time becomes really short in relation to the off time. A very good way of controlling that is to keep the boiler off longer, after the pressure has dropped, allowing the system to cool some. One way to do that is with a fixed restart time delay, and that has worked well. Another, which I haven't heard of being used, would be to use the temperature of one of the radiators as the parameter, and allow the system to turn back on when that had dropped to some selected value.

SteamStudent

Thanks Jamie. The Vaporstat does seem to be an effective control because it shuts the boiler down (@1PSI) before the pressure gets high enough to blow out of the condensate tank (1.5PSI).
My Pressuretrol never shut down the flame until sometime after the condensate tank pressure released at 1.5PSI. There was always a very loud banging right at the tank before the condensate released which makes me wonder if there is some kind of bucket or F&T trap in the tank. I do not see any kind of trap outside the tank or elsewhere in the piping. So at least now it seems to be controlled. I would still like to find out what causes the loud banging and steam release. At the same time, it seems that the pressure release would never allow my pressure to rise above 1.5PSI.

Based on your advise, I will reinstall the Pressuretrol on a T to act as a backup safety control.

Dan_NJ

SteamStudent said:

Thank you both for your quick responses to my question. So it sounds like it is normal for the pressure to go down pretty quickly upon shutdown.
...

If you really want to see what's happening, you could add a vacuum gauge in addition to the low pressure gauge. I've had one on my one pipe system before - when that fast pressure drop is seen on the low pressure gauge the system goes into vacuum for about 5 minutes, going to maybe 10mm -10 " Hg, until some vent somewhere cracks open and I lose the vacuum.

Without a vacuum gauge on it I can still sit next to the boiler and listen to it continue to boil after firing is done and the pressure drops to 0 psi. I can also hear the boiling stop when the Hoffman 1a vent on the kitchen radiator cracks open and kills the vacuum.

SteamStudent

Thanks Dan. I am thinking that part of my problem is that not all of my radiator vents are closing when steam hits them. I don't hear some of them pop closed or open. I think I will start with some new vents first.

dabrakeman

If the vents weren't closing when steam hits them then you should with a mirror or light be able to see steam escaping.  Definitely try to clean or replace those vents.  Can lead to water loss.
Without trying to hijack the conversation I find it an interesting question of which vents tend to reopen soonest or latest after shutdown (assuming they had closed)and even which tend to be noisier or quieter when letting the air back in.  I would love to have my main vents reopen before my radiator vents but that doesn't seem to be the case.  Maybe just more thermal mass in the Gorton 2's.  I have mostly Hoffman 1a's and they can whistle like crazy as the vacuum breaks.

PMJ

Jamie Hall said:

The boiler clearly isn't oversized for the radiation. Whether it is oversized for the house is another matter altogether. At the risk of boring you to tears here, this is where you get into another completely different kind of control and modulation -- namely adjusting the heat output of the system as a whole to the heat load of the house as a whole. Again, the mod/con boys have us beat: they do this by sensing the outdoor temperature and modulating the boiler output to match what is needed. It works remarkably well for a tight house (it doesn't work well in a draughty one). But here we steam folks are at a disadvantage (for that matter, so are the hot water and hot air folks who can't modulate the output of the boiler or furnace). So again, the simplest procedure is variable frequency pulse width, only this time we vary the total on time of the system in relation to the total off time, and we do that with a thermostat. Voila! I might note as an addition to my first comments that sometimes one is faced with a boiler which is really badly matched to the radiation, with the result that the on time becomes really short in relation to the off time. A very good way of controlling that is to keep the boiler off longer, after the pressure has dropped, allowing the system to cool some. One way to do that is with a fixed restart time delay, and that has worked well. Another, which I haven't heard of being used, would be to use the temperature of one of the radiators as the parameter, and allow the system to turn back on when that had dropped to some selected value.

@Jamie Hall, your last paragraph - that is what I do and have for 3 seasons now. Explained it several times here. Remote temp sensor triggers end of burn and start of new burn (with fixed timers each side of it). Ends up with shorter burns and longer waits. Adjusts itself to conditions. When you add vacuum it is one fine result. 

Looks like maybe more interest is stirring.

Jamie Hall

I know it's what you do, @PMJ and -- believe it or not! -- I applaud it and endorse it, heartily!

Dan_NJ

dabrakeman said:

...
Without trying to hijack the conversation I find it an interesting question of which vents tend to reopen soonest or latest after shutdown (assuming they had closed)and even which tend to be noisier or quieter when letting the air back in.

The Hoffman 1a is definitely noisy when it opens back up and breaks the vacuum. But I can't say right now if that specific vent is opening because it's closest to the boiler or because it's a Hoffman 1a, or for no reason at all. My Big Mouth main vent at one time was the vacuum breaker on the system - always opened first after a cycle, but I can't recall if that was with the Hoffman 1a on that specific radiator or not. I'll be doing some more balancing in the near future and will try some others on the kitchen rad.

PMJ

Got to love that air. Wasteful, noisy, corrosive. The heat thief.

At least all you two pipers should be considering not breaking that vacuum at all. You need only one small opening to the atmosphere and that can be outside the living space. The loudest noise then is the water boiling in the boiler. A truly beautiful thing.