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