As the great Bald one said, its a matter of comfort, not efficiency.
The idea behind two stages is that your furnace is sized to heat the home even in the coldest of conditions. Anytime (which is 99%) it is is not the coldest of conditions the unit cycles on and off as a means of regulating the temperature in your home. A two stage system has the ability to run for longer, lower capacity run cycles to keep the temperature more even in the home.
Consider an accelorator on your car. You dont mash it to the floor, remove your foot, then mash it again to maintain a set speed. You find that place somewhere in the middle. A two stage is closer to the latter. Now a Modulating furnace will actually run like the gas pedal providing exact output to meet the need.
We just replaced an ancient single stage furnace with a modern two stage high efficiency one. From a comfort standpoint the two stage furnace is much, much better. It's quieter most of the time too. Well worth it in my opinion and I'm generally frugal.
Consider an accelorator on your car. You dont mash it to the floor, remove your foot, then mash it again to maintain a set speed.
You know, I had a friend who really did drive sort of like that. It was funny that she never seemed to notice, probably because she was doing it (like you mostly don't notice your heart beating) but for anyone else in the car it was torture. Your head lolls forward a few seconds, flops back against the seat a few seconds, lolls forward a few seconds... tell you what, I'll be happy to drive. No really, it's no problem....
Another benefit of the longer cycle on-times in low stage is the humdifier gets longer chances to do its job; the airflow is lower, but I think the net effect is still positive for humdification. Also air gets pulled through the filter longer; again since the flow rate is lower, this may come out a wash--but at least the air in rooms is in more constant motion.
There might be a slight energy benefit because the furnace will spend more of its time operating at its peak efficiency; starting up and shutting down are not peak efficiency. But in a properly sized modern furnace the difference will probably be very slight.
On the other hand, I had a hugely oversized old draft hood furnace where simply adding a stat with a five-minute minimum on-time (even though that made temp swings bigger and reduced comfort) cut heating fuel use 36%. Not chump change.
would the low stage running longer use more or less gas than a single stage at full blast for a shorter, but more frequent, period?
I meant to cover that earlier. The only difference would be that at low stage the furnace will be spending more time running at its top efficiency; heating up and cooling down are a bit wasteful, and would happen more frequently with the single stage. But the difference is likely to be slight.
At the end of the day, your house is losing X btu's of heat determined entirely by the weather,
and you're gonna burn X / 0.80 btu's worth of gas to get it back whether you burn that gas in long slow bursts or short fast ones. 1 or 2 stage won't make a big difference in gross gas consumption, it's the other benefits that make the 2 stage attractive.
Originally posted by jlk_250 We just replaced an ancient single stage furnace with a modern two stage high efficiency one. From a comfort standpoint the two stage furnace is much, much better. It's quieter most of the time too. Well worth it in my opinion and I'm generally frugal.
Couldn't have said it better myself! Same here... had a 1957 Lux Aire and replaced it with a 2 stage Carrier. LOVE it!! NO REGRETS! Runs on low flame 98% of the time even in 15 degree Michigan weather and is VERY quiet! Colder it is outside the longer the run time in low. Temp never fluctuates more than a degree from my set point.
Don't miss the Burning-Hot HIGH HEAT / High Blower 3 degree temp rise, then shut off and a 3 or 4 degree fall yoo-yoo thing. Being in a slab house is uncomfortable enough without those kind of temp swings in the Winter.