What is the safe pressure for a hot-water-heating system?

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What is the safe pressure for a hot-water-heating system?

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Boatbill

Retired now but always set the pressure slightly higher than necessary to expell all the air from the highest radiator. Many of the systems I worked on did not have basement expansion tanks but they were located on the top floor and had a sight glass just like a steam boiler. Back in the day!:-)

Jack Getkin

Boatbill: I have seen my share of those upper floor expansion tanks including most that were riveted together.

Another method for determining the correct pressure is to divide the height measured between the lowest and highest portions of the system by 2.31. 1 PSI of water pressure will lift a column of water 2.31 feet. To result of that calculation add 2 - 3 PSI to keep the upper portion of the system under pressure at all times. Also remember to pump away!

zwang1993

Hi, Dan,

Thank you for the article. It is very helpful.

My boiler has been having an issue with pressure. It can reach 30PSI and cause the relief valve to drip water.

I replaced relief valve, expansion tank and Water Fill Regulator. But every time about 3 days after the replacement, the pressure gradually reached 30PSI again and I see dripping water.

Do you know anything else which may cause this?

Thank you very much.

Jamie Hall

Is there a domestic hot water coil on this boiler?

lchmb

or an indirect storage tank...would also help to see how it's piped...

jamesonembo

I was attempting to remove air from a zone and neglected to place the zone valve in the proper position; as a result pressure doped to zero. I Additionally the quick fill valve seems to be inoperative. I have circulation but burner dose not come on.

Jack Getkin

The pressures and temperatures quoted in this article are very typical for residential systems. Commercial hot water heating systems can and often operate at dramatically different pressures and temperatures. The safest and best way to determine both maximum allowable working pressure (MAWP) and temperature is to verify the boiler manufacturers information. Check the boiler rating plate for MAWP and compare that with the rating of the actual relief valves installed on the boilers. Dan has provided a very excellent method for determining the system required fill pressure. Use that method to confirm the actual system pressure has been adjusted to meet the system needs. Maximum system water temperatures will vary according the design of the boiler with some being able to provide water temperatures of 220 Deg F. Condensing efficiency boilers generally top out at 180 - 190 Deg F but, here again, it is incumbent upon the technician to verify the boiler manufacture's limitations.

Jamie Hall

jamesonembo said:

I was attempting to remove air from a zone and neglected to place the zone valve in the proper position; as a result pressure doped to zero. I Additionally the quick fill valve seems to be inoperative. I have circulation but burner dose not come on.

Oops. Well, sounds like you now need to purge the whole system of air again and, most likely check the pressure in the expansion tank. Then fill to the required pressure -- for a two story house, 15 psi is ample -- then see if you can get circulation through the boiler -- then turn the boiler back on.

Duff

Please make sure your boiler is OFF AND COOL before introducing cold water into the system. The posting below by Jack Getkin is on the money. Read some of Dan's books, you will not be disappointed. There are different types of heating systems and how to rid them of air can be different. Monoflow / series loop / gravity system all have there own method to purge air. Do you start from the top floor or the bottom floor? Do you use the bypass on the fill valve or just let it work automatic? If your system is cold and now put it on did you allow enough room for the water to expand or will the safety valve start leaking? Keep reading and asking questions, good luck.

HotanCool

I was at a large Btu, Commercial Boiler, on the top,11th floor recently. Was questioning correct PSI. Couldn't get a definitive answer from the manufacturer,it was running at 30 psi. I assume it wouldn't need to be much higher, considering it pumped down mainly,relief was 100 psi. I'm not a design engineer,but always curious. Would be nice to know what the original design specs were.

Lance

Interesting question, what is a safe pressure? The better question should be what pressure does my design need to operate at and than make sure all the components meet that design with a safety factor involved for when things go wrong. Kinda what relief valve manufacturers do. We have a saying in electricity that the fuse has to be rated above the weakest circuit part it protects. Find the weakest and build it stronger. Our codes are designed for function but mostly for safety. I rarely set any pressure on a system above what is needed. Don't ever trust settings you haven't validated with proper engineering! Laws of physics play a large part in this industry. Don't trust bad instruments to be good. Verify all to be accurate & safe. If your relief valve blows, it is a clue something ain't safe. Its sad most installs never post the settings the build was commissioned with. Tags, settings, labels they do make life a little better.

Lance

AN Update to my first comment. I installed a properly sized boiler for a commercial job. The height requirement demanded a pressure that would exceed the manufacture's specifications. 30PSI relief valve. So, I called the manufacturer and asked if this boiler be rated for higher pressure. I needed to install a 50 PSI relief valve and run a pressure of around 30-35 PSI. They said yes and I did not need to modify anything on it. Warranty confirmed.

hot_rod

HotanCool said:

I was at a large Btu, Commercial Boiler, on the top,11th floor recently. Was questioning correct PSI. Couldn't get a definitive answer from the manufacturer,it was running at 30 psi. I assume it wouldn't need to be much higher, considering it pumped down mainly,relief was 100 psi. I'm not a design engineer,but always curious. Would be nice to know what the original design specs were.

At the very top point in the building? Really no need for more then 12- 15 psi.
110' below that it would be around 47 psi.

A 100 psi rated boiler??

EdTheHeaterMan

I am always impressed by Bob's quick response time. I guess that's why he is called @hot_rod

hot_rod

EdTheHeaterMan said:

I am always impressed by Bob's quick response time. I guess that's why he is called @hot_rod

better late then never