Fill Valve with No Backflow Preventer: why no pressure drop?

CTYankee

Oil fired boiler has a standard Watts feed valve. The feed water comes from a well. I have a sediment filter next to the well pressure tank. When I periodically change this filter, I close a ball valve, shutting off the incoming water from the well, and open the kitchen faucet to bleed off pressure, so I can remove filter housing. I don't have a backflow preventer upstream of the feed valve on the boiler. My boiler pressure stays constant throughout this process. Why??? I would think that when the feed water pressure goes to zero, the boiler water would back feed into the domestic water and the boiler pressure would drop. What am I missing here? Thanks

Jamie Hall

The pressure control valve is acting like a rather bad check valve. They aren't meant for that service, but under some conditions they will check.

CTYankee

Interesting: I have been changing that sediment filter 3 or 4 times a year for over 30 years and I never noticed the boiler pressure drop. Thanks for the guidance.

hot_rod

Some fill valves have a check in the outlet. The Caleffi 573 do, for example.
I remember some brands had a little rubber flapper at the discharge.

Lyle {pheloa} Carter

Alot of boiler feed pressure reducing valves the rubber flapper check valve in them. This is used to prevent the backflow preventer from discharging every time the water is shut off to the building. 

Jamie Hall

All this is so. However... the built in check valve is not the code required backflow preventer, where the plumbing code requires one.

CTYankee

It's the standard Watts 1156 feed valve. I have to replace it every 3-5 years as it gets gummed up due to our well water conditions (I'm told it's some sort of clay that makes it past our 1 micron filter). Some pics:

hot_rod

If your system is leak free, no water should ever pass through a fill valve. Curious as to why you are replacing it occasionally? You really don't want to be adding fresh water to a closed hydronic system. You bring in minerals and air, with O2.


Most fill valve fail the opposite way, they do not open after years of sitting.

The cut away shows a flapper. Not intended to be a listed, approved BFD.

CTYankee

OK, that drawing made it much clearer: there does seem to be a check valve in there. I agree in a perfect world that valve should be feeding no additional water into the boiler loop.

hot_rod

CTYankee said:

OK, that drawing made it much clearer: there does seem to be a check valve in there. I agree in a perfect world that valve should be feeding no additional water into the boiler loop.

So does the system leak?

CTYankee

Not to my knowledge. The expansion tank will loose charge occasionally and I assume at that point when the expansion tank is not maintaining the 12 psi minimum pressure, the feed valve, also set to 12 psi, will allow flow into circuit.