I have a Navien combo-boiler system with two hydronic baseboard heating loops hooked up to it. Our on-demand water heating (showers, etc) are working fine. Both heating loops however, are not. I have both thermostats set to 70, but the main floor is currently 62 and the basement is 60.

I've checked the system display and I don't see any obvious errors. The main pipe headed to the loop splits is hot, but anything past the valves (?) is cold/warm at best. I've seen a few posts about a valve being stuck/wires needing to be re-wired...how can I narrow down the problem? I'm surprised both loops are affected if thats the case. I'd love to fix it myself if possible, but I've called an HVAC tech since my family does not handle cold well (though, I'm not too terribly perturbed).

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  • Can you edit to add a picture of the controller itself and the inside of one of the zone valves?
    – Chris O
    Jan 5, 2023 at 18:22
  • Should be updated now
    – Havegooda
    Jan 5, 2023 at 18:27
  • Edited my answer to reflect the new information
    – Chris O
    Jan 5, 2023 at 18:35
  • Everything seems to be working just not enough heat. Consider venting the air out of the hydronic baseboard heating loops
    – Traveler
    Jan 5, 2023 at 19:01
  • Your system looks like it is showing 9psi at 50C. I'd expect this close to 20psi. What is your pressure relief valve rating for the hydronic, is it 25? Rating should be on the relief tag. I'd add more water to the system and get the psi to 20, worse case your prv dumps a little water. Jan 5, 2023 at 19:20

1 Answer 1


Your boiler is obviously not the issue because your DHW is working. There are really only two other major components to a hydronic heating system: the circulator pump and the zone valves.

Circulator Pump

Is the circulator pump running?

  • If no, then check the circuit that the pump is plugged into. I see a GFCI. make sure that is not tripped and reset if necessary.

  • If yes, then move on to the control system and zone valves. If you have a multimeter, check voltage between the two terminals on the transformer. I suspect your transformer is OK because I see the LED on your relay is lit.

I will say that 9 out of 10 times when I've been called to troubleshoot a hydronic system, the circulator pump has been shut off, either by an electrical fault or control fault. Your controls appear (I'm just guessing because I see the lit LED on your pump relay) to be working OK so I would look closely at the pump and its electrical supply.

Zone Valves

The zone valves are commanded to open or close by applying voltage across a solenoid. This then opens the valve, allowing the circulating fluid to pass through that portion of the hydronic loop.

If the zone valves are not opening, these are the possible causes:

  • The control system isn't powered up. Check the transformer and control system.
  • The zone isn't calling for heat. Check the thermostat(s).
  • The controls are up and zone is calling for heat, but the zone valve is dead. Check for disconnected/loose wiring between control and valve, or check the valve itself.

Edit: Thanks for adding the pictures of the zone valves. I notice that there is what appears to be a fuse in the bottom corner with a rating of 125/250V which means that your zone valves could be line volt even though I suspect they are 24V. That just means be careful around them (although you already should be with any power, of course).

Those syncho motors should be audible when the open and close. Try powering the system off and on and see if you can hear one or both of those valves working.


Thanks for also adding a pic of the controls. I see that there is really only one signal wire (yellow) which suggests that the decision for which zone to open is being made at the thermostats themselves and the control board is just responsible for a general call for heating. If the zone valves are not working, I suggest looking at the thermostats themselves to verify they are working properly.

... One more thing...

There is one other potential issue and that is air bubbles. Since air compresses at a different rate than water or glycol or whatever is in the pipes, an air bubble at a high spot can block the flow enough that even though your zone valve is open and your pump is working, it can't push that air bubble back down and out of the system, effectively shutting off the zone.

If the pressure in the system was low due to a leak and any air got in, the air would make its way to the highest point and make that blocking bubble. Taking another look at the pressure gauge pictured below your boiler, that pressure looks low to me. I usually see at least 20-30 psi. There is an expansion tank that takes up some of the slack, but it can also leak, either externally or internally (the air bladder inside) so if everything on the control side tests out OK, that would be where I'd focus.

  • I think the pump is running. That's the Honeywell device thats hardwired into the block (relay?) below the GFCI outlet and light switch? I tried resetting the GFCI as well as turning off the switch for ~15 seconds and turned it back on. Boiler did some loud-ish noises that sounded normal. The Honeywell device seems to be vibrating, which I assume means it's running After turning the switch back on, the temp gauge below the boiler shot up to ~170f, though the loops past the valves never got hot in the 5 mins I was hanging down there watching
    – Havegooda
    Jan 5, 2023 at 18:00
  • I went and measured the transformer terminals (where red and white wires from valves connect?) and I'm seeing ~50v, unless I've misconfigured my multimeter. Currently set to 250 on the V~ section
    – Havegooda
    Jan 5, 2023 at 18:11
  • OK - we've verified that your pump is probably OK then. I'm looking at the zone valves now. Can you open one of them up and edit your question with a picture of what's under the cover? I want to make sure that the zone valves are working.
    – Chris O
    Jan 5, 2023 at 18:15
  • Updated to add pictures, tried to get all angles
    – Havegooda
    Jan 5, 2023 at 18:23
  • I'm thinking it's most likely the air bubble you theorized. Everything was working fine before I left for Christmas, and it's been chilly ever since I got back ~3-4 days ago. Didn't realize the thermostats were set to 70 and we were consistently at like 67-68 until just recently
    – Havegooda
    Jan 5, 2023 at 19:44

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