Thanks for the benchmarks.Assume 50% for an annualized average for a single-glazed panel & reasonable DHW temps.
That seems unusually high monthly-average NG usage for simply DHW for two people. What are you using for a hot water heater, and is 75% your presumed efficiency, or the presumed efficiency of the solar/electric water heater?
Presumed; we use a total of 160 gal/day.
Actual use patterns make a HUGE difference in actual efficiency.
Didn't know that.
It's common to find furnaces 100-300% oversized, especially in older housing that has seen a number of upgrades to the building envelope (better insulation, better windows & doors, etc.).
Our 83% furnace runs 1/4th of the time in winter.
We did get better windows, but I should scrounge up the NG bills from before the windows were put in.
A properly sized or undersized 80%AFUE furnace can/will outperform a 300% oversized 90%AFUE furnace.
Maybe that's why people who switch to high eff. don't see it on their bill.
don't oversize the unit by more than 10% (in fact, undersizing it by 15% is usually "safe", in terms of actually ever feeling cold.)
I've heard -10%
AFUE numbers are based on about a 33% average duty cycle, which means on the coldest days of the year the thing will run almost constantly. If that's not happening, odds are it's significantly oversized, and you can subtract 10-15% (or more) off the rated efficiency, eaten up in cycling losses.
Good. That means my house is not as leaky as I thought, based on HDD and input BTU/hr.
The majority of replacement furnaces that get installed are either the same or one size larger in output (to be on the "safe" side, as in, "The customer will never call the installer screaming that they're too cold 'cuz it's not keeping up.") This is the WRONG thing to do, 100% of the time. Most of the time that "safe" factor was budgeted in by the original contractor, who only ball-parked it in the first place, and in the intervening decades the building envelope has been improved, lowering the heat load.