Background: This is a new house, with a boiler system I'm not familiar with. Instead of zone valves, each zone has its own pump (4 zones). There is a fifth pump on the other side of the boiler. Not 100% sure on the controller, but it's a Taco 4 zone.

Problem: Some zones are getting heat without calling for it. I could feel heat coming off the baseboards in the bedroom last night, and the room was extremely warm. The zone was indicating 72 at the thermostat, programmed to hold 66. Basement was 70, set to 66. Living room was 66, set to 66. Unsure on the 4th zone (didn't want to wake up the kiddo). Went down to boiler room, only the living room zone was calling for heat.

Where do I start with troubleshooting?

Edit: added diagram.Boiler diagram Each zone is using a circulation pump with an internal check valve. Possibly the check is bad? How to test that?

Additional issues identified: air vent is leaking and needs to be replaced. pressure tank reads 0 PSI and leaked water during read. Will try to recharge, but suspect bladder is shot.

Edit: Replaced air vent, so now I am free to start inspecting the zone circulation pumps. Also, the expansion tank actually has some pressure, but my pressure gauge can't read it. I thought it was leaking water, but it was just a few drips. I think it might be ok.

  • 1
    Any idea on the plumbing layout? Does each zone have it's own loop? If the loops share a return, does each loop have a check valve?
    – Tester101
    Commented Jan 26, 2016 at 17:35
  • Good question, @Tester101. I know I saw a check valve for the cold water inlet, but I don't remember seeing individual check valves for the other zones. A broken/disabled check valve could certainly explain the heating I'm seeing. I will make a full diagram tonight.
    – Mike Emery
    Commented Jan 26, 2016 at 21:23

5 Answers 5


For anyone curious: the issue ended up being the built-in check valve in the pump going to the master bedroom zone. It was almost completely disintegrated, allowing backflow when any other zone was running. I went ahead and replaced all check valves on all pumps while I was at it, and the overheating issue is now resolved. I also replaced the pressure tank and vent.


Assuming the thermostats are working, you may be getting residual heat once the thermostat turns off.

Do the pumps have speed controls? Many pumps have LOW, MEDIUM, and HIGH settings. A lower setting may avoid spikes in temperature.

Also, many pumps have mixing valves near them that control the temperature of the water being sent to the radiators in that zone. If you have selective zones that are too hot, you may be able to temper (in this case, lower) the water going to that zone.

  • I don't think it's residual. It appears to be active hot water flow in a zone that doesn't need heat. I'm going to put together a full diagram tonight. Thanks.
    – Mike Emery
    Commented Jan 26, 2016 at 21:25

I assume the helper pump on the return line (the fifth pump on the other side of the boiler) runs if any of the zone pumps are running. If the living room zone pump has failed, the helper pump will force hot water through all four zones. To test this, temporarily disconnect power to the helper pump.

Since the system is new, it seems more likely that the living room zone pump is not failing, but air has gotten into the zone radiators and is blocking circulation. 0 PSI at the pressure tank could be an indication of a blocked inlet valve. This could lead to air in the system.

In either case, going by your diagram, any zone calling for heat but with its flow somehow restricted is going to push hot water through the other zones. The effect appears very strange but is probably just the result of using separate zone pumps instead of one pump with zone valves.

  • My plan for now is to order new air vent and expansion tank. With those fixed, I can purge any air and hopefully restore sanity. The zone pumps have built-in check valves also, so those will need checking.
    – Mike Emery
    Commented Jan 27, 2016 at 17:24

If you haven't found the source of the problem allow me to add some pointers. First the fifth pump is a (run around) pump used to insure that all zones get the same temperature water. To check for the overheating problem, shut off the zones that are running when the rooms are overheating. If the problem goes away, re-energize one zone at a time to see if that zone is causing water to flow to the affected zone or zones. If another zone is causing the problem someone installed a crossover pipe by mistake.( may be impossible to find and fix). As for the expansion tank, if its a bladder tank find the air charging valve (same as a car tire fitting) push in the plunger to expel some air. If only air comes out the bladder is probably OK. Then check the pressure in the tank, ( system off - boiler cool) it should read the same as the boiler system. hope this helps


We were having trouble with a air handler fan running constantly for a new zone in a new addition to the house. I watched the boiler furnace repairman and he found that all the zones were wired in series and the new air handler zone controller was the last in the series. When any zone called for heat it would activate the air handler fan which would blow cold air when it's zone wasn't calling for heat.

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