One of our objectives for this house is energy efficiency, and so far we're doing well on that score. The house is well situated on the site, with lots of glass on the south side and not so much on the north. There's no fireplace or chimney to waste heat, and no air conditioning to waste money. It's tight and well insulated, though not "super-insulated" like a true high-performance home.
So now we need to install the heating system for it, and the question is: what capacity is required? It's a simple question, with a simple answer, but it's very common for boilers and furnaces to be oversized and inefficient. Traditionally, this is because installers didn't want to risk dealing with unsatisfied cold customers in the middle of winter. There's also a tradition of engineering conservatism and "fudge-factors" that continues to this day.
We observed this phenomenon at work this week, in the initial proposals for installing our boiler. The heating engineer at MSI sized all the baseboards, according to heat loss projections for each room. They got the important part right, which is the relative balance of heat for each room. They didn't know what sort of a system we would drive the baseboards with, though, or how warm we want the house. So their analysis included baseboard that could be driven at high temperature by a traditional boiler, and also included a 40% "safety margin", which is the maximum margin recommended.
The first proposal was to install a boiler capable of driving all 130 feet of baseboard with 600 BTU/hour per foot, plus another 40% "safety margin". This led them to recommend a 155,000 BTUh condensing boiler. My own heat loss calculation concluded that the house needed 40,000 BTUh to maintain 65 degrees in 0 degree weather, so something was clearly amiss. In fact, at the lower temperatures of modern boilers, the 130 feet of baseboard only consumes 1/2 the heat capacity.
We needed a more credible analysis, so our architect got a pro to do a quick heat loss estimate. He used a whole-house method comparable to mine, and got about the same answer except he recommended a 40% margin on top of the whole-house loss.
So his recommendation was 67,000 btuh, my calculation was 64,000 btuh, and we'll go with a boiler that's rated for 80,000 btuh (what's called the I=B=R rating). That's still probably too big, but it's a modulating boiler which means it can turn down its fire automatically, so the efficiency won't suffer too much.