Recurring air lock issue

mburg

I was called to this job due to no heat in one of 5 radiant zones. This system was installed about 12 years ago. The homeowner stated at least one of the zones will stop working every year. This year it was the living room/kitchen zone, but every zone in house has had a issue before with exception of the basement zone.

I was checking system pressures, about 15 psi in boiler. There was 0 psi at return from faulty zone. No water would drain out of hose in attempt to purge. I turned pump on still 0 psi at zone return, than I shut ball valve bellow boiler drain on return and pressure slowly increased to maybe 10 psi. I opened valve and heat was working, for now at least.

I’m not sure why the only way I could get any pressure/water in zone was to turn on pump? The water is being restricted someplace, but how?!

Every zone has a a check valve on return back to boiler past mixing valve, I suspect that removing these check valves will solve this recurring issue, but certainly not positive or entirely sure why we can’t maintain pressure in zones.

Does the mixing valve require a balanced pressure between hot and cold ports? Is circulator pulling zone pressure down somehow due to those check valves.

Am I way of base, I think I need some other opinions on this one.

Zman

It does sound to me like you have too many check valves and that when the water in the loop is not being circulated, it cools and is trapped and then pulls negative pressure.
I think you will need one check in the loop to prevent ghosting, is there one in the circulator? Does the mixing valve have checks?

EternalNoob

The answer to does the mixing valve require equal pressure on hot and cold ports is yes it does. I don't know that that's what's causing your issue, but since I'm not clear on how the system is plumbed, I'm mentioning it as a point of inquiry. Most mixing valves will have a listed specification for how much of imbalance between the H and C pressures is acceptable, 20% i think is common.

I once made the mistake of plumbing the cold radiant return to the cold side of MV from the upstream side of the pump, where the pressure is significantly lower, sometimes negative pressure. MV wasn't working worth a damn. When i re-plumbed with the downstream side of the pump Tee'd to the cold of the mixing valve, voila, problem solved.

EternalNoob

Also check the filters on the union connections of the MV, they can easily get clogged with debris.

hot_rod

It may be time to service the MVs, disassemble and clean them. If there is checks in the H&C tailpiece there are usually screens to protect them. Get some new fiber washers and remove one MV to see how they were installed. You should not need checks in the H&C port, maybe one downstream of them circ to prevent reverse floe from another circ running.

15 psi with nothing circulating? You should have 15 psi static pressure in the system, and that should be the same on both sides of them check. If it drops when a circ starts, it sounds like a flow restriction. The MV would be the suspect.

mburg

I leaning towards a pressure imbalance preventing flow from moving through mixing valve. Don’t overly think screens are plugged because this has been a issue since original install, and it only seems to be a issue at the start of the heating season. I will however check the screens in mixing valves assuming I can track down some gaskets. Planning on pulling check valve out of returns of each zone.

There was a 15 psi static pressure on the high temperature/boiler side of mixing valve. On the outlet side of mixing valve I had 0 psi with no circulators running. I turned zone circulator on, still 0 psi. Then I shut ball valve on return from zone and the pressure slowly increased in zone. Opened ball valve and pressure maintained.