IME, a small portion of houses have severely over-sized furnaces. Of that small group, they would often be oversized at about 1.6-1.7 of what the house actually needs.
A larger group (and both groups don’t constitute “most”, IME) are over-sized by a factor of 1.1-1.25 of what’s needed.
Quite rarely have I a seen a furnace fully at 2.0, and never at 3.0. Over sizing a furnace doesn’t produce more profits for installing contractors as a practical matter. The wholesale price difference between the appropriately sized unit and the over-sized unit is less than $100, and the labor and materials are largely the same.
Over-sizing tracks the age of the house. Older homes are much more likely to be over-sized; the older the home the more likely it is to be over-sized. Newer homes----like from the 80’s forwards----are more likely to be slightly undersized.
While over sizing still happens in some retrofits, it’s usually due to installer inexperience, and unnecessary installer CYA. (which too is a sign of inexperience). Among new homes over sizing is really pretty rare.
I don’t know where you are getting your numbers. My point is pretty simple…in design day weather (the coldest weather for which a furnace is installed), a right-sized furnace will need to run 24 hrs/day, and this means that if it is off for a few hours, it will never be able to heat the house back up. If a furnace is able to recover after setback within a reasonable time (say 2 hours) on a design day, it is x2 oversized or worse.
Now right-sized according to a heat calc is a different matter because heat calcs have a huge amount of CYA. Indeed it is true that newer homes are right-sized according to the heat calc.
Eh…I don’t buy this argument. Yea, a Ferrari engine is less efficient than a Toyota. But those are heat engines, not furnaces. And just lugging around a larger engine in itself is part of the reason why they’re less efficient.
Furnaces are something different. All furnaces are typically 90-100% efficient REGARDLESS of what size they are. Even industrial size furnaces are going to be equivalently efficient. Otherwise, large buildings would be heated by many tiny furnaces instead of a single large one.
So it doesn’t matter if the furnace is oversized for your house. You’re getting the same fuel-to-heat efficiency factor regardless of the furnace size. In fact, as pointed out here, it’s a GOOD thing since it allows you to set back your furnace and recover quickly.
Um, no. All furnaces of the same type (NOT “all furnaces”) may have the same RATED efficiency, which is based on continuous operation. (90% is for condensing furnaces only.) But actual efficiency depends on operating conditions: a furnace that short-cycles is less efficient.
I did explain that earlier in this thread, when I said:
Indeed some of them are. By daisy-chaining (say) 10 furnaces, they get a 10-stage heating system. When it’s not very cold, maybe 3 of the furnaces will run; when it’s colder, maybe 7; and in extreme weather, all 10. This way, most of the furnaces are either always on or always off in given weather; only one of them cycles on and off multiple times an hour. Software achieves “wear leveling” by rotating the heating responsibilities evenly among the boilers over the season, so the first one doesn’t fail first.
For those playing at home, a mini version of this is a two-stage furnace, readily available if you pay a bit extra. Again, the idea of 2-stage is to extend cycle times.
And incidentally, there are no 100% efficient boilers.