Heating for historic house
I own an early eighteenth century house (coastal Massachusetts) and I’m looking for guidance on what the most efficient and environmentally friendly heating system options are. Unfortunately, the construction style and historic nature of the house preclude insulation upgrades, which would seem to be the obvious first step in achieving heating efficiency. (The house has essentially no wall insulation – just brick infill or “nogging”, which was an early form of insulation used in Europe and America in the pre-Georgian period.) We currently have a gas boiler and slant fin baseboards, which I was initially contemplating replacing with a high-efficiency gas boiler and cast-iron radiators. I’d love to move to electric rather than fossil fuel if possible, but early/cursory research on heat pumps makes me think this may not be a good option. But, ultimately, I am a novice and feel the need for guidance beyond that which what my plumber and contractor are telling me. Any advice would be greatly appreciated.
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If you've made a final determination that insulation, window replacement, and air-sealing aren't in the cards (essentially, three of the largest opportunities to reduce heating demand), then the next step will be determining your heating (and cooling, if desired) load. This is a determination of the 'amount' of heating that will be required by any new system. This is not necessarily the same thing as the design capacity of your current system - systems are often over or undersized. You will probably know if your existing system is undersized, but it's more difficult to tell when a system is oversized. There are many ways to determine heating load, the one your contractor is probably most familiar with is the ACCA's Manual J. Your contractor can do this calculation, or you can pay a third party to do it.
Once you know your load requirements, you can have a more intelligent discussion about what systems can meet that load requirement. Efficiency and Environmentally Friendly are 'terms of art' - they mean different things to different people in different fields. For example, fuel oil system have a lot of emissions, but if you're burning exclusively recycled biodiesel it may meet your personal definition for environmentally friendly.
Following up on Sam's excellent answer, you may find these articles helpful: A Beginner’s Guide to HVAC Design and A Beginner’s Guide to Heating Systems.
Sam, Brian - thank you very much for these informative and thoughtful replies. I really appreciate the help. I'll do my research.