Only one zone heating in a two-zone hot water based heating system (oil)

Question

I have two zones in my oil heating system, down and up -stairs with two separate thermostats one circulator and two zone valves.

For some reason the downstairs zone works perfectly well, but the upstairs zone isn't.

Troubleshooting steps I did so far with no success:

1. Purged the air from each zone

   Switch off electricity. Closed the valves so water doesn't flow back to the system. Opened the zone valves (both). Attached the hose first to the down stairs circle, then to the upstairs circle. Used multiple buckets until there were no bubbles. Sometimes I add some pressure from the water line to make sure all bubbles are getting purged.

2. Changed the upstairs zone valve power head to a new one

   I noticed that the upstairs line is hot until the zone valve and cold after the zone valve so I figured there might be an issue with the power head. Switched off electricity again. Took pictures and drawing of the wiring. Removed old head. Attached new head. Turned electricity back. Turned heating up. Waited but still pipe remained cold after the zone valve.

3. Changed the thermostats

   I figured maybe it is a faulty thermostat. So I switched of the electricity again. Switched the wiring from upstairs zone valve to the downstairs zone valve. So the downstairs thermostat controls the upstairs heating and vice-versa. Turned electricity back, turned on heating in both zones. Downstairs kept heating, but upstairs still no heat and pipe remained cold after zone valve.

4. Opened upstairs zone valve

   As a last try, I opened the upstairs zone valve and left it opened. Pipe still remained cold after several minutes.

Anybody has any advice where to continue troubleshooting? Thanks in advance!

Answer 1

Look for a check valve (flow check) in the upstairs zone piping, perhaps on the return line. See google images. Flow checks sometimes get stuck from corrosion or sediment if they go several months with no flow. You might get it working by calling for heat on that zone, then when the pump starts up, give the flow check a few sharp raps with a non-marring hammer.

I'm not recommending the following for general use, but I once got a stuck flow check to open by heating it with a torch until it started popping from steam production. The steam bubbles forced the swinging gate open within the valve. That's potentially dangerous, as it could cause the valve to explode if you don't know what you're doing.