I replaced a digital non-programmable thermostat with a programmable thermostat and the new thermostat keeps the house around 60-62 no matter what temperature I set. When I contacted the manufacturer, he had us test the voltage and indicated our 29 volts was too much for 24 volt thermostat. Recommended getting a contractor out to resolve that - easier said than done. Are there other programmable thermostats that have a more flexible range? I haven't seen this range on packaging/specifications I'm looking at. This is for a combiboiler if that makes a difference.

Picture of wiring: https://imgur.com/a/qd9l4bF

Thermostat: Honeywell RTH2510, Boiler: Vesta VRC

  • Model # of thermostat and a picture of the wires going into the thermostat may be helpful in diagnosing this problem. – manassehkatz Mar 7 at 23:40
  • Can you give us the model # of both the thermostat and the boiler it's controlling? – ThreePhaseEel Mar 8 at 0:15
  • The thermostat is Honeywell's RTH2510 and the boiler is a Vesta VRC. imgur.com/a/qd9l4bF – Emily Mar 8 at 1:30
  • That's not that weird. AC voltage is listed in RMS (a modified average), so "24V" actually has a peak voltage much higher. Regardless, 29V is less than 20% over. Anything ought to be able to handle that. I think the service tech is shining you on. – Harper Mar 8 at 1:51
  • Sounds like a crappy thermostat. Did you check the voltage under load? – Ed Beal Mar 8 at 15:04

This seems like poor design of the thermostat: most household HVAC "24 VAC" transformers are designed to provide that voltage under load, i.e., while drawing the design current, perhaps one ampere. Open circuit (O.C.) voltage of 29 VAC is normal.

That said, it would be easy to reduce the voltage to 24 VAC. Two 6 VDC zener diodes such as the 1N5340BG (~US$0.50 each) soldered in series, with opposite polarity, would bring the supply down to 23 VAC, giving a 1 volt margin. However, I question the manufacturer's claim that the normal O.C. voltage is causing the issue. and doubt that lowering it by 6 volts would help. More likely, the connections are incorrect or the thermostat is defective.


I often get a 28-29 volt no load reading and have never had any issues like you're describing. They are actually designed to be 24v at 50% rated capacity so no load they should read like 24.8 or 25.1 Yes 29v is a little high it's probably a cheap transformer or a little breakdown of insulation in the secondary. Possibly high primary voltage could be adding a volt or so. Sometimes tech support spits out a pre conceived excuse for their junkie equipment. I'd say get a thermostat from a heating supplier. Home depot and the like buy cheaper versions of the stuff tradespeople get.

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