[dtgriscom]

Around here (NE USA) oversizing is common with furnaces; the attitude is "better safe than sorry" where "sorry" means "contractor gets called because furnace struggles in mid-winter".

I replaced my old (95kBTU/hour output) forced-hot-water furnace and separate hot water heater with a condensing furnace about eight years ago. I got a heat loss calculation done, which concluded I only needed 59kBTU/hr of heat. It was a struggle to get a small-enough furnace installed; one contractor said he couldn't in good conscience sell me anything smaller than 120kBTU/hr.

My final system was 89kBTU/hr. A couple of weeks ago, in record -13°F/-25°C cold and high winds, it was burbling along at 60% power. Could have been even smaller.

[bradfa]

We had an energy audit done a few years ago and the conclusion was that we needed about 40kBTU/h of furnace (upstate NY). We have an 80 (came with the house when we bought it). It’s actually quite hard to buy any gas furnace smaller than 60. But modulating gas furnaces seem to be becoming a thing now which is good and may make up for over sizing.