Hydronic system drops pressure to zero within 24h

ignacj

Hydronic system drops pressure from 20PSI to 0 within 24. The systems heats ground floor, so I can partially see pipes in basement ceiling, partially they are covered with drywall (finished basement). I cannot find any signs of leaks on the ceiling drywall. I suspect expansion tank, but it seems to be making right kind of sound when dinged - bottom sound hollow, the top sounds water logged. I closed city supply valve to the system in order to prevent further leaks (if there is one). I purged the system of air without any improvement. Before system started losing pressure, pipes were making popping noise when water was circulating.
1. How much water would one-zone system need to lose in order to drop pressure to 0? My backflow preventer valve drips but very very slowly, maybe a 2 spoon-fulls in 24h.
2. Could bad expansion tank cause repeated pressure drops? I added city water and raised pressure 3-4 times just to see 0 PSI 24h later.
3. This is new/old house we moved in 6 months ago. The system has been operating past two months without any issues aside from annoying popping noise. I see most folks suggesting that Hydronic systems typically have about 12 PSI unlikely mine that has always been at 20SPI. Furnace and circulator pump are rated for much higher PSI. Is this 20 PSI excessive?
3. Anything else I need to look for before I call the experts?

Thank you
ig

EdTheHeaterMan

Most Hydronic heating systems are closed systems. That said, if your gauge reads 20 psi today and 0 psi tomorrow, there is a water or air leak somewhere. If you leave the city water fill valve open will the pressure stay at 20?

ignacj

Yes, if I leave city feed on, the pressure will remain at 20PSI. My concern is that if there is a leak, this will keep feeding it.
We were away for 3 weeks over the Christmas, I shut don city water during our absence. When I got back I opened faucet to check for leaks (expecting water pressure for 2-3 seconds). This lowered house pressure and backflow preventing safety valve discharged small amount of Hydronic pressure. This pulled crud into the valve that will not not properly seal and is continually dripping about 2 tablespoons per 24h. This is the reason I now keep water supply line OFF. For all practical purposes the system might have been "leaking" or loosing pressure before, but we would never find out due to open supply line refilling it.
At this point I suspect there must be a slow leak somewhere in the piping. This is an old rancher with sizable floor space and lots of pipes.
Would pinhole leak cause popping noise while water is being circulated? This noise intensified since we got back from 3 weeks absence.

Jamie Hall

In answer to one of your questions -- if the expansion tank is working properly, it would take a surprising amount of water to drop the system to 0. So... next check. When the system pressure is reading 0, get a good tire pressure gauge. On the end of the expansion tank opposite the pipe there is a Schrader valve -- just like a tire. Take the cap off and read the pressure. You get around 12 to 20 psi? Thank is probably OK. You get water? Tank is dead. You get some other pressure? Use a tire pump and bring it to 15 psi. Now fire up the system and see what happens to the pressure when it's nice and hot. The pressure should go up -- but not too much. If you started at 15 psi, up to 20 is fine. More isn't. Passed? Your tank is OK.

A backflow preventer shouldn't drip. Period. If it's dripping, it either needs to be overhauled (some can be) or replaced. If there are isolation valves for it, this isn't a big deal. If there aren't, it is a big deal...

If the tank is working properly, you have a significant leak somewhere -- more than the few drips you mention from your backflow preventer. If the tank isn't working properly, those few drips could do it.

The popping noise could be any of a variety of things. With the pressure dropping like that, it's quite possible that there is a lot of air in the system, which would need to be purged out. Could also be expansion noises.

ignacj

The nose in the pipe was fairly rapid, I'd say when circulator was running, it would make pop every second or so. This is definitely not expansion pop ever few minutes or so.
Thanks for the suggestion, I'll test the expansion tank.

ignacj

Tested expansion tank, 18 PSI on tire pressure gauge. When I push valve no water comes out.
Last two months the furnace pressure gauge was showing about 21-22 PSI.

ignacj

I do have shutoff valve right behind backflow preventer - on the city water side. It is currently cut off, so I am not terribly concerned with backflow preventer. I will replace that at some point.

HomerJSmith

What type of expansion tank do you have? Extrol or Filtrol type. In other words, what is your pressure regulating fill valve.

I always check a tridicator gauge with a separate gauge on the boiler drain connected to the drain bibb if I have suspicions that the tridicator may not be accurate.

Most commentators on this site recommend draining the EX tank before setting sys pressure in the tank and the tank pressure should match the pressure regulator pressure if you have an Extrol tank. Setting pressure in a Filtrol tank sets the sys pressure automatically. I like the tank pressure set at 15 psi.

It really doesn't take much water loss to take the tridicator pressure to zero, less than several cups. Air in the sys can do it. A defective EX tank can do it. A leaky back flow preventer can do it.

Popping noise can be the expansion of the pipes rubbing against the joisting.

MrScott

Not a pro, however I just hunted for leaks and would try first:
1.  Check the pressure relief valve.  Closing properly?  Operating at too low a pressure?  Gunk on the valve seat?
2.  Watch the system pressure during a boiler run cycle; it could be raising the pressure, pushing water out the relief valve, then cooling and reducing pressure.
Circulation loop expansion tank(s) are a suspect if the pressure swings much.
3.  Check manifolds and connections.
4.  Look for wet spots in ceilings, especially anywhere someone has put up a hanger or driven nails or screws.
5.  Save the hard-to-access pipes for last.