Ex gravity system questions - too much pressure!

Dale

I think you have a leaking manual infeed valve, Closing any valve doesn't necessarily mean the flow stops and as the expert says water is not compressible so a tiny bit raises pressure alot. This is a cheap fix/test, with the valve you have still closed shut the water off to the house and cut in another new valve upstream of the old one with an open tee in between, that way you can check for a leak. When your test is done plug the tee. The new valve could of course be downstream but since I think the old one leaks I would put the new one against higher pressure.

Daniel Nolan

Mystery pressure

Still troubleshooting my ex gravity FHW tankless boiler (from 1970) that is blowing off a couple of times a day.


I shut off the water to the infeed valve, and this dramatically reduced the blowoff. It has happened again, though, and the pressure went from 12 to over 20 psi somehow when it was blowing off.


After this I bled all the radiators, and got a lot of air out of one on the 3rd floor that had been shut off since we never use that room. (I opened it up all the way now, and made sure all radiator valves are completely open.) I will check them all daily for a few days and monitor the blowoff, but I have a few questions:


- how likely is it that a large amount of air in the system was heating up and expanding enough to drive the pressure way up in the system? the 3rd floor is pretty cold when the heat is off, so the air would cool quite a bit between heat cycles. Maybe it was pushing the pressure way up so it would blow off, then cooling way down and causing the pressure to drop so the infeed would let in more water, then repeating with each boiler cycle?? I did notice that with the infeed off the pressure in the system eventually went way down at one point.


- or is it more likely that there is a crack in the tankless hot water pipes inside the boiler and the pressure is backfeeding the system inside? and is this repairable? this boiler is built like a sherman tank and I'd hate to have to replace it with a new tin one just because one of the pipes inside is cracked.


I will monitor the system and bleed the radiators again to be sure. Any advice on this would be really appreciated.


Thanks,
-Dan

Mike T., Swampeast MO

- how likely is it that a large amount of air in the system was heating up and expanding enough to drive the pressure way up in the system?

Not likely. Air is easily compressible so the expansion from it heating would add very little to the pressure.

Water is nearly incompressible.

Probably the three most common causes of increasing pressure:

1) Improperly adjusted or defective auto-fill device. If you have one on your system shut off the water supply and monitor pressure. If only slight rise with increased temp adjust or replace.

2) Expansion tank problems. Old plain tanks often get waterlogged (filled with water). They're supposed to be about 50/50 water/air when the system is cool. If waterlogged you have to drain and re-fill. The problem will often reappear over time--solution: Bell & Gossett Air-Trol Fitting.

Modern bladder type tanks can develop a leaky bladder or loose their air charge. The air side shold be filled to your static fill pressure (usually 12 psi). To test the air charge you have to remove the pressure from the water side. Sometimes there's a shut-off valve and drain valve that allows you to do this with a full system--otherwise you have to drain, check/charge with air (keep a gauge on it for quite a while to verify the bladder is not leaking).

3) As you suggested, crack in a tankless coil (or indirect coil) allowing domestic pressure to overpressure the system. I have no idea if such a problem is repairable--the very few I've ever seen are long-abandoned. In that vein believe you can abandon the tankless by capping off and still use the boiler.

Steamhead (in transit)

Another possible cause

is that somehow the boiler is overheating and generating some steam.

One likely cause is over-firing, another is a bad high limit control. But a much more subtle cause can be an oversized circulator. If the water is moving so fast that it can't pick up the heat from the boiler's metal, the metal will overheat and whatever water makes contact with it can flash to steam. This could also cause the boiler to leak. Go here for a troubleshooting account of this:

http://www.heatinghelp.com/newsletter.cfm?Id=119

How much radiation (square feet EDR) is on that system? What make and model of boiler and circulator does it have?

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