Hot water boiler/Hydronic Expansion

JOutterbridge

Seems like I've run across a boiler with a auto feed valve that is allowing flow into boiler when manual shut off is open. The feed (along with PRV) for the boiler comes in as a tee that is connected to the boiler return line and the other side of the tee goes to the tankless water coil on the unit. The system blows the relief valve only when the prv manual ball valve is open. Is it possible that the tankless coil when heating and cooling causes the PRV to think the system needs more water? If so does it need it's own expansion tank on the incoming cold line? The hot water supply has the main tank set to 14psi and verified while empty twice. The auto feed is set to 14 and hits that mark exactly when system is filling. What am I missing that causes the unit to overfill when the manual feed is open. It seems to hold 14 psi steady but over 24 hours it rises slowly to the relief point. I'm guessing it does this when the domestic water is in use. Any suggestions??

Zman

Usually a bad fill PRV or pinhole in the tankless coil causes this.
The domestic side should have an expansion tank especially if there is a backflow device on the house.

JOutterbridge

I know a pinhole leak could cause the issue but the unit is less than 5 years old. Also it seems not to happen when the manual feed is closed. I suppose a large leak would show up immediately within 24 hours like I'm seeing. Or a bad auto feed which is is new this year. I think I'll try an expansion tank first for the domestic. I couldn't find a backflow preventer. It is a well pump setup. I'll recheck the auto feed after that

Jamie Hall

If it's on a well pump, there is a check valve in that line from the well. You absolutely have to have an expansion tank on the domestic water -- and I expect there is one. You should check it for the proper charge and functioning as well (commonly, but by no means always, around 30 psi empty -- but check the pressure switch settings for the pump).

Zman

Maybe a picture of the well setup. If you have a well pressure tank, you likely do not need and expansion tank on the domestic side.

STEVEusaPA

If it doesn't happen when the manual feed is closed, why would you look at the expansion tank?
Take a stethoscope and put it on the feed valve while you open and close the manual shut off.
Try closing off the domestic lines, and feed line to the boiler. If the pressure doesn't rise over night, open the lines to the domestic side. If the pressure doesn't rise it's the feed valve. Could have something lodged in it preventing it from sealing.

JOutterbridge

Thanks Steve, great points. As for the well the tank is at 30psi. The cut in/out and pump operation is good. The reason why I asked is because the well tank is 50 yards away from the boiler. And the feed for the boiler and tankless coil come off the same tee within 4 feet of each other. (inside the boiler room) Didn't know if that would affect the boiler or siphoning or the feed valve etc.

Zman

Can you isolate the fill valve independent of the DHW coil? If isolating the fill valve and PRV fixes the problem, you have a bad fill PRV.

Are there any check valves downstream of the well tank?

JOutterbridge

No check valves downstream after the well pump tank/filter. Can't isolate the domestic coil from boiler fill valve unfortunately. I'll be looking into doing some ball valves to the home.

Zman

You are going to need to isolate them to narrow down this one.