Hi, my natural gas forced air furnace in my small (900 sq feet) old 1935 home runs for 7-8 minutes and then shuts off and rests for 15 minutes then it starts up again and runs again for 7-8 minutes.
Its cold here 25 degrees.
What does this say about my furnace? It's normal? It's efficient? It's inefficient? You guys are the experts.
A 7-8 minute burn about hree times per hour is reasonably efficient on a low-mass heater like a hot air furnace. But a 33% duty-cycle at 25F isn't the best for comfort- ideally it would be a higher. The thing is oversized for comfort, but I've seen much worse.
The
99% outside design temps in KS are in positive single digits to low double digits F. A
right sized furnace would have an output capacity about 1.4x the heat loss of the house at the 99% condition. So if your 99% design temp is say, +7F (could be ~5F colder or ~5F warmer than that depending on where you are), when it's +7F the furnace would have a 1/1.4= ~71% duty cycle.
Assuming a 70F indoor temp, the load at +25F is about (70F -25F)/(7oF-7F)= 71% of what it is at +7F, and the duty cycle at 25F should be about 71% of 71%, or about 50%, not 33%. So if it were right sized it would be giving you 15 minute burns, 15 minutes off. Or with a tighter differential on the thermostat, 8 minutes on, 8 minutes off.
See Nate Adams' video & book chapter primers on
home comfort and how fundamental
HVAC sizing issues
affect comfort. The vast majority of hot air furnaces in the US are sub-0ptimally oversized for delivering comfort, most worse than yours. But your primary comfort issue is the low performance building envelope. The single pane glass and no insulation in the attic is an even bigger comfort (and efficiency) issue than your oversized furnace.
Take the time to first AIR SEAL the house and adding attic insulation will keep the indoor air more humid & comfortable since it isn't constantly convecting in much drier outdoor air. The most important air leaks in the house are those at the attic floor/upper floor ceiling plane, and at the foundation, since that's what defines the "stack effect" pressure drive. If it's on a vented crawlspace foundation the bottom of the house might be the bigger leak (and harder to deal with), but for most houses it's the attic plane first. If it's on a full basement, the foundation sill & band joist is often the biggest untreated leak in the house, and easier to get at. But air sealing is the first and most important (and cheapest) comfort improvement on an older barely insulated house, independent of the equipment size.
Exterior low-E storm windows are a fraction of the cost of replacement windows, and bring the performance of a wood-sash single pane up to nearly current code-min performance. The low-E glass is an upcharge over clear glass, but the higher performance boost makes it "pay back" in heating energy use about half the time of clear glass storms in a location with winters as cool as KS.
Spending the money on the building envelope efficiency pays back in comfort more quickly than just right-sizing the equipment. Your oversize factor isn't so crazy that it's a "must fix now", but once you've cut the heat load by 25-35% with air sealing, insulation, and better windows it's duty cycle will be even lower. The only time it will pay to right-size it is when the equipment is losing reliability or failing and needs to be replaced. But in the mean time it's pretty easy to track your actual heat load (and building efficiency improvements)
using the furnace as a measuring instrument, by logging fuel use against heating degree-days. which also gives you a good idea as to what size is right for the replacement.
