Boiler not building pressure

IMS47

It's early in the season, and the weather hasnt been too cold in my area yet, but I noticed that my boiler is only building pressure up to 4oz (2 pipe vapor system). Typically my vaporstat will cut out at 8 oz and back on at 4oz. But I'm barely getting 4oz and it runs until the thermostat kicks off. Any ideas why this might be happening? Or do I just need to wait for it to get colder?

Fred

That is nothing to complain about. That is exactly what you want. Did you do anything like put new/bigger vents on the system? add another or larger radiator? If not, I suspect it hit the 8 ounce cut-out as the temps drop and the boiler runs longer to satisfy the thermostat.

EBEBRATT-Ed

Unfortunately there is a lot of talk about vapor stats and low pressure gages....nothing wrong with that except on some posts the impression given is that a steam boiler must build pressure to operate properly.

Nothing is further from the truth. Pressure is not a problem unless it is too high. Nothing wrong with a steam system that heats the house and does not build pressure as @Fred pointed out

Koan

@IMS47 You probably have a similar system to mine, (Hoffman Vapor 2-pipe) and actually running at about 2 Oz pressure is preferable. What you describe means the system is functioning efficiently.

Both @Fred and @EBEBRATT-Ed are correct. I am chiming in because I have the same kind of system

As your boiler heats water and creates steam, the steam has to push air out of the system. This takes a little bit of pressure (maybe a couple oz/sq in) until the main vent sees steam and closes - then steam moves through the system. Ideally, you want just enough pressure to move the steam through the system and no more. The radiators will fill with steam until the trap on the return line sees steam and closes. Those with over-sized boilers (like mine) will keep creating steam once the traps are closed and hit the upper 8 Oz limit on the vaporstat. This is "cycling on pressure" and is very inefficient. It makes no sense to create more steam when it has nowhere to go. In that case all your boiler is doing is building steam pressure in the system that cannot be used. If you try to raise the temperature by a lot very quickly you may cycle on pressure, but this is not how you want the system to operate. If is is colder outside, your radiator traps will be open longer and you may even less likely to cycle on pressure unless you increase the thermostat a lot.

Now what I have to ask is why your system would typically cycle on pressure. What brand and size boiler do you have? How many square feet of radiator area do you have? You can figure this by seeing how many columns or tubes you have and how high they are - I attached a couple resources here to do it.

Next look on your boiler nameplate for EDR. Then see how close they are. Like @Fred mentioned, a bigger main vent can sometimes help.

IMS47

Thanks for the responses. I had asked this because my system is not working the same as it was last year, where it would hit 8oz and cut out. It was very consistent and now it's different. My system is an old style Tudor system that doesn't have traps on the radiators, just one in line trap at the end of the main.

Over this past summer, we had brown outs that were causing havoc on our electric. During this time, the auto fill valve on the boiler became stuck open, filling my pipes and coming out of my main vent! Not sure if this would damage that vent. I don't want to spend $100 just on a guess as I thought this might be why it's not building pressure.

ChrisJ

IMS47 said:

Thanks for the responses. I had asked this because my system is not working the same as it was last year, where it would hit 8oz and cut out. It was very consistent and now it's different. My system is an old style Tudor system that doesn't have traps on the radiators, just one in line trap at the end of the main.



Over this past summer, we had brown outs that were causing havoc on our electric. During this time, the auto fill valve on the boiler became stuck open, filling my pipes and coming out of my main vent! Not sure if this would damage that vent. I don't want to spend $100 just on a guess as I thought this might be why it's not building pressure.

If it's behaving differently for no apparently reason I would want to know why. I don't blame you.

I assume you checked for any obvious leaks and stuck vents. The next thing I would question is the burner. Is it producing the correct output?

Is this an oil fired system or gas?

Jamie Hall

Actually, if it was routinely hitting 8 ounces and cutting out... that wasn't working properly either. However, that makes it sound as though up to this summer your boiler was seriously oversize, or your main vent was stuck shut.

I too would want to know why. However, in trouble shooting you may be best served going back to first principles on these things -- and the very first is to resolve two questions: do the radiators get hot -- on a long run, hot across? and do the returns not get steam hot (they may get very warm indeed, but not steam hot -- you may need an IR thermometer or something of the sort to tell). If the answer to both those questions is "yes", then the boiler is making enough steam to fill the system -- regardless of the pressure -- and the pressure is low enough so the system can condense the steam which the boiler is making. Both good.

Water coming out of the main vent may well have altered its behavior. Indeed, if it is the type of vent -- as most are -- which are designed with a float to keep water from coming out, it may have been damaged. What type of vent is it? It may have been stuck closed, and is now failed open -- in many vapour systems, if they are really working right, no steam ever gets to the vent anyway and you might not be aware that it's not closing.

Then, as @ChrisJ noted, how about burner output? Is it correct? Have you had it tuned and checked?

EBEBRATT-Ed

I would remove the pressure gage, pressure control pigtails and connecting piping and make sure they are clear

the_donut

I'd let it run and meter the autofill. If you are taking on water, there's a leak you need to find. I would also check the returns for steam. Might have a trap stuck open from flood debris.