I have a 30 year old Burnham boiler heating system. I replaced the old thermostat with a Honeywell programmable. The new thermostat doesn’t keep up with the heat setting when the outside temps get really cold. Example: outside temp 5 degrees , thermostat setting 68, thermostat temp reading as low as 60. Any suggestions?
The thermostat just turns the boiler on and off as needed. Unless it’s cycling the boiler on and off even when below the set point, which seems unlikely, I would look elsewhere for the cause.
Since it’s programmable, is it possible that you’ve programmed it to allow the house to become cooler at certain periods? If so, it may be that the boiler was just able to keep a constant temperature with the old, constant thermostat, but that if the house is allowed to cool, it’s unable to bring it up to temperature when it’s cold outside.
Another possibility as that the new thermostat isn’t wired-in correctly. Is it just a programmable thermostat or is it an internet connected ‘smart thermostat’? These don’t just need batteries but actual AC (or converted, low voltage DC) power, which they need to get from a multi-wire (i.e. more than two wires) furnace thermostat system. So if you merely have a simple two-wire, on/off switch thermostat system, it isn’t going to provide the electrical power they require.
Programmable thermostats are also notoriously difficult to program because of their very limited user interface (just a tiny, monochrome LCD screen and only a few, multipurpose buttons). I’ve been a ‘tech guy’ nearly my whole life and even I find them a pain in the ass.
What KE said - it’s just a switch that turns the production of heat on and off as it perceives the need to do so. If it isn’t coming on at [set temp - 1 degree or so] and going off at [set temp + a bit] then there is something wrong with the thermostat - design, connection or programming.
It’s very likely that there is some problem Between Floor And Thermostat, too, if you’re used to the crude method of “spin it up to get warm, spin it down when I’m hot or remember to do so.” The more precise control might not be what you’re actually used to.
But if the unit is registering a temperature below the setting, and the heater’s not coming on… something’s wrong with it.
Ninja ETA: the more complex the thermostat, the more it’s likely to be a programming issue - again, it’s trying to be a lot smarter than you and you may not like a smart-ass uppity thermostat. 