I assume your current furnaces have about an 80% efficiency rating, so there outputs would be about 48K BTUs. If you buy 95% efficiency, which would make sense in your cold climate, then each furnace would have an output of about 57K BTUs. Therefore the new furnaces would generate more heat and warm up the house faster from a 15 degree set back without having to increase the input size.

If the current furnaces are able to maintain a 70-72 degree inside temperature on the coldest nights, then there is no need to increase the size. If you can tolerate the slow recovery by increasing the temperature remotely, then keep the sizes the same.

Correct on efficiency, but the furnace is actually 70K in (day&Night) - 56K out so similar to 60K high efficiency

Naturally faster recovery is desired as we get to the house before it is fully recovered if we start the heat when leaving

Increasing one furnace to 80K does not have much additional cost. However before you start buying bigger furnaces, you should have the HVAC contractor measure your return and supplies to see if they are properly sized. It could be an air flow and not furnace size issue.

Assuming the house has average insulation and tightness, I would think the recovery time would be shorter than you described. If the upper floor is taking much longer to recovery than the lower, then I suggest investigation if the attic insulation needs to be upgraded.

Thanks

It is a lot higher volume upstairs so I think it is ok - I can definitely have someone check, but it is a fairly new house.

Given that hot air rises, the downstairs furnace will be heating upstairs also, so you already have, in effect, a "larger" furnace upstairs. I would look into the attic insulation and increase that first. Also, look for other sources of heat loss (Windows, air I filtration, etc) on the upstairs floor.

And duct leakage.

In the "olden days", furnace sizing was done by inaccurate rules of thumb and then rounded up so what's there is more likely to be rather too large than too small already. The BTUs you have from your current furnaces should be more than plenty. Upsizing will decrease your comfort rather than adding to it because when you're there and the furnaces are in operation, they will run short on-cycles during which they will blow too much, too hot air and likely be noisy (because you have ducts sized for smaller furnaces).

A friend has a hunting/fishing place (hardly a cabin, it's >6000 sq feet) in a cold place (Montana) that's heated by two ground source heat pump systems. Its setback recovery in the winter takes something like 8 hours. It's hardly a problem because when he enters his trips to go up there on his calendar, he also enters a reminder for the appropriate number of hours before his expected arrival to turn up the heat. I think you could probably do the same.

And hopefully you have ceiling fans in the upstairs? It sounds like you're describing a layout where the public rooms are upstairs with higher ceilings and the lower floor has the bedrooms?

Larger furnace? It depends on how often you run the temps up and down and what kind of furnace. It does not seem like you are doing frequent set-backs, but I'll discuss that anyway. I think that modulating, condensing furnaces do not run at their peak efficiency when running at full fire. (I could be wrong.) Frequent deep setbacks burn a lot of fuel in a less efficient range and might not be worth it. A larger furnace might be able to be set up to run at a lower fire and still provide sufficient heat, essentially capping off output and keeping it in a more efficient range. The down side is that you will have more cycling of the furnace, rather then going to lower fire, at times of lower demand. That is also the drawback of installing a larger furnace of any type to enable faster recovery.

I am planning a two stage furnace (Bryant) - controlled by an ecobee thermostat (because of remote sensing capability). I am leaning to just keeping the current BTUs

The issue upstairs is not leaking - but high ceilings so more volume to heat and hot air going higher

Thanks

My first thought was duct issue with upstairs unit, or larger heat losses. Get an energy audit or buy a thermal imaging adapter for your smart phone. Also have airflow at registers and placement of registers and returns checked.

My main floor seemed colder than the basement after I finished my basement, because of many more windows. I installed removable interior storm windows and it solved the problem. I store them in my garage in summer and put them on when it starts feeling like fall is done. They pop on and off, and can be flush mount or in-jamb. Much more reasonable in price than window replacement. My windows arent even that old, they just arent that good.