2 stage vs 1 stage furnace

Unless the Nest has a W2, second stage kicks in either 7 or 12 minutes into the cycle depending on a Dip switch setting.

To answer your question, if my heat loss was 66K, I wouldn’t install one capable of 88K.

Is there a reason to buy a single stage over a two stage? There sure is, $. Why would you install a two stage unit in an investment property? You don't get the savings. The tenant would. You pay more for a 2 stage unit.

If your unit is oversized your wasting money and ruining the controls by allowing short cycling. The ignition sequence, like everything else, has a life span cycle. The more it cycles the less time it will last. How long do you think your car starter would last if you shut the car off at every red light, and then start it up again.

Also long run cycles allow for the maximum efficiencies. i think you need to define short cycling.

i have an about 75,000 btu/hr 2 stage in an 800 ft^2 house which is probably quire oversized. It has a honewell 2 stage t-stat. It only runs 2nd stage when it is recovering from setback.

the 2 stage makes it run longer cycles on non design day conditions which keeps the temps more even especially if the ductwork is less than ideal.

I would like @EBEBRATT-Ed said do the heat loss and get a furnace about that size but i would do the 2 stage or a modulating and control it with a good 2 stage stat so the second stage doesn't kick on unless it needs it. i don't like the built in second stage runs on a timer.

Your short cycling timing indicaters taht you have an effective yield of no more than 66,000 BTUh if that is how your system is operating. With that information, does your home stay at the desired set point temperatures when the outdoor temperature is at its coldest? If you home stays comfortable even when the temperature drops below the normal coldest days, then you do not need an 88,000 BTU heater.

Your existing heated is already working like it is a 66,000 BTU Heater when the 88,000 gas burner can only operate for 7 minutes our of every 10 minutes. You would be better off putting less heat in your duct work if it is undersized. 50,000 BTU would be better for you and the undersized duct work. The only kick you need is the additional 16,000 BTU when it gets really cold.

EXACTLY RIGHT: That 43,000 BTU stage is insufficient for a very cold day. If you were in New Jersey, where I used to work, those coldest days could be below 0°F, and keeping the indoor temperature at 72°F was the job of the high fire stage. But we only saw temperatures below 0°F about once every four years or so, not every day of the winter. So to get the home to 72°F when the outdoor temperature is 0°F, you might need 66,000 BTU/h for about three or four hours every three or four years. Other than that, you don’t need 66,000 BTU/h.

The 43,000 BTU burner is not for those extreme days. That burner is for most of the rest of the winter.

For example, when it is 40°F outside, the house might only need about 22,000 BTU/h, and the 43,000 BTU stage is almost double what you need. When it is 30°F outside, you might need about 35,000 BTU/h, which is still less than 43,000 BTU/h.

Now when the temperature gets down to 20°F, that 43,000 BTU stage is too small. But guess what happens? The additional 23,000 BTU burners come on, and suddenly you have a furnace producing about 66,000 BTU/h, even though the house might only need about 48,000 BTU/h. So what happens? the 43,000 burner never shuts off, the most efficient use of fuel, no OFF and ON cycling. The additional 23,000 BTU burner cycles ON to make up the difference then cycles OFF when the temperature at the thermostat gets higher that the set point. The over sized 66,000 BTU burner does not shut off completely, only the 23,000 burner shuts off so the 43,000 burner can keep you comfortable. Much more efficient operation.

Does that make sense?

Your cycle description does not state the duration of how long is "everything off"? And is this cycle time typical of the coldest nights or design temps?

Try doing this calculation if you can, make sure it matches with numbers you are considering https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

did it not have a nest at some point, did run longer cycles before the nest was installed or was it already there when yo got it. the short cycling you describe seems to be a control problem although it could also be oversized in addition to the control problem. one common failure point of nests is that their battery doesn't get adequate power and it will die during a heat call causing the heat call to end then the battery charges and starts another heat call.

it could be incorrect anticipator settings/behavior in the thermostat or the thermostat being right at a heat vent too. maybe it could be a safety in the furnace shutting the burner down too.

Most homes are at design condition a small % of the year, less than 10%, you can look up temperature info for your location. As such a single stage is mostly oversized. The closer the furnace to the actual load the more efficient and comfortable the home. Below are some examples of temperature data for various cities.

Any air sealing upgrades will lower the load also. A load calc and blower door test can shed a lot of info on your needs. Most furnaces get over-sized.