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Question DetailsAsked on 11/22/2015

Will a thermostat make the furnace come on if the zone valve is defective?

The furnace comes on when 2 of the 3 thermostats in the house are turned up. The third zone is not circulating any hot water in the radiators , and the furnace does not come on when that thermostat is turned up.

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Generally no - thermostat generally controls the zone valve, not the boiler directly on hydronic (hot water) heating systems.

I suspose there might be some electronic controlled boiler out there that will, but all the ones I have ever seen the boiler turns on and off either totally based on the boiler water temperature (so after the zone comes on and the circulating cooled water hits the boiler), or with some with energy saving features can be based on the boiler temperature but will only turn on when one or more of the thermostats is simultaneously calling for heat too. Latter wiring is commonly also rigged so the circulating pump only runs when heat is called for also, former commonly has circulating pump running full time (which is my preference).

Simple test - since two zones are working fine, carefully (use wetted hand initially till you see how hot it is, or use baking/candy thermometer) check the pipe going out from the zone valve to see if it is hot or not, and also the return pipe on that zone versus the other two (check a few feet from boiler, not right at the manifold). Probably not more than luke warm, indicating the zone valve is not opening. Then, check the batteries in the thermostat for that zone to see if the thermostat is powered - and check it has not lost its programming if electronic. IF non-electronic old pop cover off or turn off room lights and see if there is a little flash in the mercury bulb mounted to the temperature adjustment lever when you turn it on and off, indicating power is flowing to it. Also, check (if electronic and it has one) if when you turn the thermostat up it shows (commonly a little flame icon or word ON or FURNACE ON) that it is calling for heat. If doing that, thermostat is likely OK.

Then, while one person watches the zone valve power head with flashlight, other can turn the thermostat up from below to above room temp, so you can see (with many brands) if the central piston in the zone valve moves down - commonly with a click or hum from the valve at the same time as the solenoid or wax heater activates the power head that opens the zone valve. May take a minute or two to open if heated wax type. If moving down about 1/4 inch, then water should be flowing through the pipe leading to the zone and heating it to about 160-200 degrees, depending on your unit. IF not, check if any shutoff/return valve on that zone got shut off by accident/

Past there, could be plumber or Heating and A/C contractor time for a normal homeowner.

IF a DIY person with nominal electrical skills, you could check for broken/disconnected wires to the thermostat (have to remove from wall to do that) and with a volt-ohm meter check if you have power in the leads to the thermostat - usually 24V, but sometimes 12, 18, 36, 48, 50, or 110/120V depending on system, so be careful not to short it out or zap yourself.

Also, check for broken wires at the zone valve, for wires that have come disconnected from the power head screws, or no power in the live (usually red) wire.

If all those check out OK and there is power to the power head (do NOT short out the power head - it will fry it quickly), then could be stuck zone valve (rare if you flush the system annually like you should), or dead power head (life can be from 5-40 years - highly variable. I have had ones last less than 5, other bought identical time lasted 30+ - I have one on my boiler 33 years old and still going strong).

If electrical savvy and you pay attention to what wires go where and power off before messing with it, changing a power head it usually not tough - most just twist off (power off, and let cool down first) and twist on, with only 2-3 wires to connect to it. MUST be done power off - doing life ruins many brands. Zone valve itself in plumbed in, so normally requires pipe soldering skills to change out.

Costs - power head typically $60-100, zone valve $25-125, combined factory assembled unit typically around $90-100 to as much as $200. Labor for Plumber or HVAC contractor to change out usually minimum trip charge assuming decent accessibility - about $75-150 labor charge usually except up to double that in a very few very high priced urban areas.

BTW - here is a link to more info on zone valve issues FYI - and Taco has good info on troubleshooting on their website too -

Answered 4 years ago by LCD

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