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I recently purchased a house so this is my first winter in it. The house was built in 1939 so most of the house is on a single heating zone using hot water radiated heat. The radiators are cast iron and recessed into the walls. However, there is an addition to the house circa early 2000's that uses baseboard hot water radiators. The addition also seems to be less heat efficient, probably because it has more windows, and has drywall instead of plaster. The addition is on a separate heat zone from the rest of the house. They are controlled by two different thermostats.

I'm trying to figure out how to set the thermostats to maximize heat efficiency. So far I've been assuming that the baseboards are less efficient than the cast iron radiators, and so I've been setting the thermostat for the addition far lower (about 5 degrees) less than the main thermostat. This basically means that the baseboards in the addition never switch on, and the addition remains cooler than the rest of the house (which is okay - it is a kitchen / dining room which we are okay with being cooler). The addition just gets heated via heat transfer from the other rooms.

Is this the best approach? Should I set the addition thermostat to the same temperature? What other considerations are there in order to balance my zones correctly to maximize energy efficiency?

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  • Don't assume that 75 on one thermostat is the same as 75 on another. They all seem to have slight variations between them. Some models support "calibration" to put it in line with what you expect.
    – Steven
    Commented Jan 7, 2012 at 17:26
  • I found an interesting Q&A on this: finehomebuilding.com/how-to/qa/baseboard-vs-radiator-heat.aspx . Nothing about efficiency differences, but it is worth noting that a room heated with radiators may feel warmer than a room heater with baseboard convectors even when the air temperatures are the same.
    – Shimon Rura
    Commented Jan 9, 2012 at 3:27

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I think you are saying that you have two zones on one boiler, each fed by its own pump and thermostat (i.e. the radiators have been converted to hot water). My answer is based on this assumption.

Since they're both on the same boiler, the water is heated with equal fuel efficiency.

The two loops probably are different in how efficiently they transfer that heat to the room. Low efficiency in transferring heat to the room will decrease the heating capacity of your boiler, which is a problem if you can't get the house warm enough on a cold night. But as long as you're keeping up with heat demand, efficiency in transferring heat doesn't make a difference with fuel usage. (You can tell how efficient they are in transferring heat to the room by feeling the temperature at the end of the loop -- if it's pretty cold, the heat was transferred to the room very well)

So, the answer is that you should set the thermostats to the lowest temperature where you're comfortable. Your addition from 2000 likely has a lot more insulation than the original 1939 house (in 2000 the building code required a lot of insulation, 1939 not so much), so if you want to pick the more efficient one to heat, heat the addition not the main house. It will be more efficient because you'll lose less heat because of the greater insulation.

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  • +1 for "you should set the thermostats to the lowest temperature where you're comfortable". Pretty much anything else is overthinking the problem. Open new questions (or review existing ones) if you have questions about improving insulation, operation of your system, etc.
    – TomG
    Commented Jan 9, 2012 at 14:00

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