Refilling a hot-water boiler

Steve_144

OK, after working on my heating system I find myself with a partially empty boiler. Attempts to fill it up trigger the pressure relief valve, and a glance at the pressure gauge confirm that the valve is working properly. So what's the trick in refilling the system without causing the pressure to increase (the valve is 30 PSI, and yes, I am filling it with the system shut off)?

Thanks,

Steve

Brad White_9

How

tall is your system in feet?

What pressure are you maintaining? (It may already BE full!)

Normally a relief valve trips at 30 PSIG (G for Gauge versus A for Absolute pressure). Your fill pressure would normally be between 12 and 18 PSIG for a 2 or 3 story house.

(If you live in Canada, this applies to a 2 or 3 storey hoose.)

You may have an isolated expansion tank too. Does the pressure climb when heated only or during cold fill?

Steve_144

The expansion tank is not isolated (I presume you mean "cut off from the rest of the system"). It is a two storey house. I assume you're asking about the height of the system from the highest radiator to the bottom of the boiler? If so, it's about 17 feet.

The pressure valve is currently reading 30 PSI, which is what it was when I shut off the water supply. If I turn the water back on, the pressure will climb until the pressure relief valve is triggered. This is with the system turned off.

I am filling it using street pressure (don't know how high that is, but it was fine before we started the work) through the auto-fill mechanism (sorry, I don't know what this is called). At first I thought the pressure was being caused by steam build up inside the boiler, due to there being air inside, but the water coming out of the pressure relief valve is only warm.

Thanks,

Steve

Brad White_9

Steve- Fill it to only 12-15 lbs.

Yes, you are correct in what I was saying about the expansion tank. It may yet be too small but that is down the road if need be.

Street pressure is fine but watch the gauge and shut it off when it gets to 12 PSIG. Your "reducing valve" should do this automatically but may not be adjusted correctly so check the set-screw. Personally I like to fill and isolate the CW line.

Your height is right-on for that pressure; it will be filled and have 4 PSIG residual on top of the system to force air out.

ALH_4

purge

The air needs a path to get out of the system. Generally you have to purge the air unless you have vents on each heating unit. There should be a drain on each zone. You have to purge air through each zone out of each drain into a sink or floor drain with a hose until you no longer get air. You may have to close valves on each zone in order to force the water through each zone depending on the piping arrangement and whether you have zone valves or zone pumps. Is your automatic air vent open? You want the final fill pressure in the 12-20 psi range generally. I would shut off the auto fill once it is purged.

Some auto-fill valves have a fast fill setting. You want to use this setting for purging the system, but it will allow domestic water pressure into the heating system which is much more than 30psi. Ideally there is a bypass around the auto-fill for purging.

I assume you have no antifreeze in the system.

Steve_144

I believe I have found the problem - the auto fill valve was in the fast-fill position (I must have hit it by accident). Thanks for bringing that up, Andrew, I might never have found it otherwise.

Steve