View Full Version : Boiler operating pressure

11-23-2010, 01:19 PM
15 year old multi zone hotwater hydo baseboard and radiator

Every year I would turn on my heat I normally bleed all valves until only water comes out. Make sure no air is in the system. This has never been a problem until now. Now there seems to never be enough pressure in the system. I used to do this without the zone being open.

I know I have a check valve from the supply that regulates the water and pressure entering the system. Can this be the problem? The pressure gage on the boiler is low. around 5 psi at idle. Never looked when running. But even when running I am not able to bleed easily. When I open and get little release but can just be thermal expansion.

Does this mean my check valve is bad? I expect to always have a constant supply pressure in the system. What is the normal expected pressure on the check valve.

11-23-2010, 05:24 PM
I think you are talking about your auto-fill valve. This would normally add water into the heat loop to maintain the preset water pressure (most are adjustable, I think, but come preset to about 15psi). Many boilers have a low-pressure cutout, and won't even fire if the pressure is at 5psi, mine won't run until it gets over 12, and the autofill is set to 15. If the pressure is too low, you can get steam in the boiler's heat exchanger (think the pressure cap on your car's radiator). To keep from potentially polluting your potable water, there's also a backflow preventor device before the autofill device. If you don't have an autofill device, you still would have the backflow preventor, but then you'd have a valve to manually fill the boiler.

11-25-2010, 07:08 PM
Well I do not have an autofill. The check valve acts as that by keeping a constant pressure on the check side.. And I do not have a backflow preventer. I believe the check valve does that too. I can not build up any pressure in the loop. It must be the check valve. Do these tend to fail? I can not see any other reason for the lack of pressure in the system. there seems to be a screw on the top of it. Is this a pressure sdjustment screw. If so which way is more pressure vs less pressure. Tight = ?.

11-26-2010, 05:26 AM
A "check valve" does NOT maintain a set pressure, that is the job for an automatic feed valve, but if yours has a screw on the top, then it IS an automatic feeder. Screw it in to increase the pressure. You need a minimum of 15 psi when the unit is not firing, but higher if you have three or more levels above the basement in the house.

11-27-2010, 05:39 PM
aaaa, I see. Then it is an auto feeder valve. Well I am not getting any pressure in the system. So these valves fail often? If I have no pressure in the system I can only assume it is not functioning. I plan to replace it this week but i just want to make sure it is the problem. As it cost about $70.

Currently the first floor has a full water loop. Second floor has a lot of air and bleeding the higehest point does not exhaust air at the rate i expect. Very slow and stops when the system equalizes. Next burn cycle on the unit builds more pressure and i get a little more out. In the past I would replace all the water in the loop with ease.

Thanks again.

11-28-2010, 05:41 AM
Most of them have a large nut on the bottom with a screen inside it. That may be full of debris and corrosion keeping the water from entering the boiler, but you could also try screwing the bolt in to try to raise the pressure setting.

11-28-2010, 08:35 AM
Many of the autofeed valves also have a manual override lever which you can use to open the valve. If you watch the pressure gauge while you do this, you should be able to set it manually until you can adjust, repair, or replace the one you have.

11-29-2010, 05:52 AM
Strangly just before I was going to take the existing feed valve off to get it replaced at my local plumbing shop... It opened up after one last adjustment to the adjustment screw. Seems it was just stuck. Water flew in as if the flood gates just opened. I not have 15 psi. There is still a need to replace it but it seems to be working now. Thanks for all the advice.