A couple of days ago, my hydronic heating system stopped working. Thankfully, the boiler is still working though, so I still have hot water.

The details:

  • When no one is using hot water, the boiler reports 0 GPM and the pipes are cold.
  • I checked the circuit breaker panel, and none of the circuits have been tripped
  • The pressure gauge above the expansion tank always shows 0.
  • I think I only have one zone, so there is no zone valve.
  • I don't think the two circulation pumps are running (I can't hear anything).
  • The thermostat still responds to temperature changes (it goes up if I turn on the fireplace).
  • FWIW, I can't see the wires that are supposed to go from the heating system up to the thermostat. Maybe it's hidden in some metal pipes.

[Update 1]:

I unscrewed the cap on the Webstone air separator a little, and only water came out (if air came out, it must have been very little). So I think that ruled out air being trapped. And as expected, nothing changed. Then, a few hours later, on a lark I changed the temperature on the thermostat, and magically it started working again! I'm really not sure why. If anyone has any ideas, please let me know. Oh, and the pressure gauge is still showing 0. Which also makes no sense.

Some images:

Whole System enter image description here

Boiler enter image description here

Circulation Pump 1 enter image description here

Circulation Pump 2 enter image description here

Expansion Tank and Pressure Gauge enter image description here

Bottom of Expansion Tank enter image description here

Does anyone have any ideas what could be wrong?

  • can you add a zoom out of your system and look on the circ and tell us direction of flow (there is an arrow). that pressure gauge should show ~25psi. normally you see a boiler fill valve instead of a standard pressure reduce valve. if you are going to fiddle with things you'll want to figure out which breaker is your circulator so you can pull the power before messing with bleeding the system. Mar 14, 2023 at 20:44
  • Ok, I've added a zoom out of the system. I see two arrows on the pump, one on each side towards the back. They both point down. Could you provide more detail about a boiler fill valve and standard pressure reduce valve? Mar 15, 2023 at 4:42
  • boiler fill valve combo pressure reduce valve looks like the following, the little lever allows you to override the pressure and allow more water to flow into the system. you just have standard prv which is fine but harder to verify if there is a problem with it. amazon.ca/WATTS-BRASS-TUBULAR-1156F-Regulator/dp/B000KLLW9M Mar 15, 2023 at 5:25
  • 1
    Maybe your thermostat battery is failing. Pressure gauge is likely faulty.... that was the point of using the hose bib as a test. You can still have air in your system, people don't typically bleed the system via the air eliminator so possibly the pump passed an air pocket that was preventing circulation. If there is air then you'll get cumulative damage to the circulator. Mar 20, 2023 at 17:00
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    Could be loose / corroded contacts in the thermostat or thermostat connections that results in intermittent working. your transformer could be dieing and not providing proper voltage. Yes air eliminator removes air, it is mechanical and can jam though so best to double check and run water out the bleed hose bibs to visually determine that no air is present. also lets you check water quality and verify there is pressure at the same time. Mar 21, 2023 at 0:35

1 Answer 1


Your boiler will only heat if water is flowing through it - if the water isn't moving, it heats up really quickly and the boiler will reach it's set temp and shut off.

Since your DHW is working (i.e. your boiler fires up and heats when the water is flowing through because you open a shower faucet, kitchen faucet, etc., then we can assume that your boiler is OK.

Your circulation pump appears to be the problem. You'll want to check any electrical cables feeding it for a breaker that might be tripped or a switch that might have been turned off.

If those are all OK and the pump still does not run, then you'll want to check the control system/thermostat, but I'd wager that since you say the display temp is up to date, the thermostat is probably OK. (I say probably because it's possible that the thermostat displays the right temp but still doesn't send control signals, but that is an unlikely failure mode.)

Your circulation pump might have shut off or been damaged due to cavitation as well. You mention this:

The pressure gauge above the expansion tank always shows 0.

If there is no pressure in the hydronic loop, you may have a leak that has caused air to enter the system. A circulation pump trying to push an air bubble won't last long because it relies on liquid flow for lubrication.

  • I've checked that none of the circuits tripped. I've also added some photos. I don't see any wires coming out of the pumps - each one has a metal tube coming out of it, which then goes into the wall. If the wires are in that tube, I don't know how to check them. I've read that you can get rid of air by bleeding the system. However, I'm not sure whether I do that at the Webstone thing above the expansion tank, or at those green valves, or at some other place. Mar 14, 2023 at 19:35
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    Chris O, covered most things adequately. A circulator pump works as follows. The pump itself is a 115 VAC motor. That 115 VAC to the pump is probably controlled by a relay. The relay in turn is controlled by a 24 VAC signal coming from your thermostat (the thermostat does NOT switch the 115 VAC itself). So if the pump is not working 1) the pump motor may be bad. 2) the 115 VAC relay may be bad, or 3) The thermostat is not sending the 24 VAC to the pump relay. There's a nice graphic showing this on one of the SE's Q&A posts.
    – SteveSh
    Mar 15, 2023 at 16:55

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