How to Adjust Boiler Pressure

On a closed system, how can one go about setting the pressure of a boiler? I would like to run my EFM at 12psi but the problem is it doesn't want to stay at 12psi. It will be 12 psi cold and then 25psi hot. I opened the PRV until there was no pressure in the system and then let some air out of the expansion tank and then pumped it up with a bicycle pump until it was 12 psi but it still creeps up. I shut off the water supply to the inlet just in case the valve was seeping but that didn't fix anything either. I'm thinking that maybe the expansion tank isn't working properly? The tank sounds hollow for about half or 2/3 of it, and it's hot about 1/2 way down. It doesn't overpressure and blow off the valve but I'd like it to run at a lower pressure if possible, I get nervous when I see the gauge at 25psi.

Sounds like you have an expansion tank that’s too little. If a expansion tank is correctly sized when you fill your system cold to 12 PSI and heat the boiler and all the zones you should only be 1-2 PSI over 12psi on initial fill, but if you do go to 25 no big deal just as long as you’re not running water out of the safety valve. if you don't mind me asking what make and model tank do you have, also do you know your total system volume?

Whats your temperature swing & how do you fill it ? I had problems with my boiler (oil - coal should be the same) since I switched to coal, the boiler is only used for hot water (superstor) & experiences a 70F swing in temp. What was happening was that the auto fill would kick in at the lower temp (120F) & bring it up to 15psi, then later there would be a call for hot water & the system would blow off at the higher temp (190F) To solve my problem, I shut off the auto fill & start out at 5psi, As U235a4 mentioned, you could use a larger expansion tank. Hope this helps. Which reminds me - I should go & check my water level !

To clarify my earlier post you should be able to fill the system say with 65F to 12 PSI, then heat up the entire system to your design temp say 180F and still have within 1-2psi so that would be 13-14psi although you can safely go to 25psi on most systems. If you have to relieve pressure after the system is heated your covering up the fact the expansion tank is too small. Now if the tank is losing pressure and water logged, then the tank needs replaced.

U235a4 wrote:Sounds like you have an expansion tank that’s too little.

Could be, you have two boilers and that is a significant mass of water. I have an expansion tank on each of my boilers.

U235a4 wrote:To clarify my earlier post you should be able to fill the system say with 65F to 12 PSI, then heat up the entire system to your design temp say 180F and still have within 1-2psi so that would be 13-14psi although you can safely go to 25psi on most systems. If you have to relieve pressure after the system is heated your covering up the fact the expansion tank is too small. Now if the tank is losing pressure and water logged, then the tank needs replaced.

I agree. You probably need a bigger tank or just add another to the system. Could be a bad tank also but if that is the case you will probably soon be blowing off the safety valve as the tank fills up. When that happens with an auto-fill valve you start a recurring cycle of dumping hot water out of the pressure relief only to refill the boiler with cold water which makes the stoker run which makes the water hot which raises the pressure which opens the relief valve...

Some homeowners, and even some techs, will open the drain valve on a ceiling hung expansion tank and drain off some water there. What some people don't know is that these tanks will drain a little water, seem to be empty, and are actually full of water because they get airbound.

To empty such a tank when the water is cool or cold, you need to remove the valve, and stick a piece of 1/4 tubing in through the valve outlet and up to the top of the tank. Have a bucket ready because it will pour out of there.

I know of ceiling tanks that are 50 years old or more. Some get water logged, some never do.

Unrelated to this topic, but if your pressure ever starts to build slowly and blow off the PRV, it's because you have unnecessary water entering the boiler, many times from a pinhole leak in the domestic water coil.

I agree that you must have an undersized expansion tank. I have a pressurized system and my pressure swing in only about 2-3psi hot or cold. Mt PRV pops at 20 psi So I set the boiler right around 12 psi.

You do have a pressure relief valve on the boiler don't you? You can adjust this pressure using this valve. That is how I adjust mine when needed. But if yours is swinging that much from cold to hot, I don't think this adjustment is going to help you. The problem is most likely expansion tank size. But I could be totally wrong, it wouldn't be the first time...

You say you shut the supply and it still rises? The Watts or watever PRV isn't doing it then. Does the boiler have a tankless water coil for heating domestic? A pinhole leak will cause the pressure to rise as that is at the house pressure. Shut the supply to the coil, open a hot water faucet to relieve the pressure and see if it repeats the increase in pressure on the boiler.

I solved my problem, I had too much water in the system. It's a long story but basically I screwed up by fiddling with the air in the expansion tank while it was filling. The cure was to remove the expansion tank,. drain it out, and then pressurize it to 12psi with no water in it. With the fill valve set at 12psi and the expansion tank set at 12 psi the system is at 12psi cold, it will go to 20psi at 200 degrees and then drops back down as the water cools, but normally it stays about 12-15 psi. The pressure relief valve is non adjustable 30psi, it works I tested it before firing the boiler. I wouldn't run a boiler without one, no way.