Boiler Pressure increases when heats up.

Users who are viewing this thread

Plumber69

In the Trades
Messages
2,380
Reaction score
214
Points
63
Location
Prince Rupert, British Columbia
After replacing a old boiler with a new boiler, I charged the system and removed all the air. I set the pressure at 18psi. The system was cold, but once heated up the pressure increased. I think this is normal. But how much will a the pressure increase going cold to hot? And do you set your pressure when the system is up to temp? Has (new feed valve set at 18psi) (new expansion tank)
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
First, why did you set it to 18psi? (Most systems won't need more than 12 psi.)

The expansion tank needs to be charged to the intended system pressure before pressurizing the system, which hopefully you did?

The pressure will always increase as the system comes up to temperature. With a properly sized & properly pre-charged expansion tank it will stay within reasonable bounds. If the expansion tank was pre-charged to 12psi and the cold-fill pressure of the system was 18psi the expansion tank may not have enough range to fully accomodate the expansion that occurs when the system is at operating temp without hitting some unusually high temp.
 

Plumber69

In the Trades
Messages
2,380
Reaction score
214
Points
63
Location
Prince Rupert, British Columbia
I did 18 cause, its in the basement and there two floors above. That to much? It has one of those amtrol 30 expansion tanks. I dont know to much on boiler pressures.
So what you are saying is you want your cold fill pressure to match the expansion tanks charged pressure?

From my understanding is you lose .433 psi for every foot you go up. I guess its only 25 feet rise.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
The idea is for the pressure at the top of the system to never go negative, which can potentially draw air into the system. If the highest point of the heating system plumbing is only 25' higher than the pressure gauge, it's normal to pressurize it to (25' x 0.43 psi/ft) + 3 psi= 13.75 psi, call it 14-15 psi, in your case. The + 3 psi it to give it some margin, to cover pressure deltas that crop up when the pumps are running.

But measure it- in most homes the pressure gauge on the boiler is well off the basement slab, with maybe 6-8' between it and the first-floor radiators, and maybe another 10-12' to the baseboards on the second floor, for maybe 20', not 25. That comes in at 11.6 psi , call it 12. With high-ceiling construction and tall radiators you might need to bump it to 15 psi, but most 2- story houses are just fine at 12.

You pre-charge the tank to the system pressure, not conversely. Many tanks come pre-charged to 12psi, but that has to be checked prior to pressurizing the system, and adjusted as-needed. If the tank is more than a few feet above/below the pressure gauge, you may want to run the math on it's pre-charge level to cover that pressure difference. An expansion tank located 10' above the pressure gauge should be set 4 psi below the system pressure measured at the gauge. If it's within a couple of feet (as would be typical), fuggedaboutit, it's not enough to really matter.
 
Last edited:
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks