100K BTUs doesn't sound like enough furnace for a Minnesota house that big or even half that big. .
The furnace is probably sized about right considering the gas usage so far
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I have a similar sized 2004 house with 4200 sq ft on 2 floors, an open floor plan and a finished basement. It heats fine with a 100,000 BTU forced air LP gas furnace in Northern Wis. The furnace is 94% efficient and runs about 30% duty cycle (on-time) on cold nights of 0F to
-15F. I saw 40% duty cycle on some -26F nights last year. This house has average 2x6 construction with fiberglass batts and a vapor barrier. It had a lot of air leaks that were fixed in 2011-2012 so current heating bills are about half the original numbers.
The info below is from before I fixed it.
15F outside temp needs about 20% furnace run time to maintain 66F inside.
94,000 btu's (94% x 100,000) can raise the temperature about 4F per hour in this size of home.
The furnace runs about 20 minutes and then shuts off for a few hours (it used to run once per hour).
If I turn it down 4F it will gradually cool and come on about 5 -8 hours later while it's 10-15F outside.
Looking at what ColdMN has now he could gather some info and figure out fairly close how much heat he needs regarding a wood stove.
- Record the existing furnace run time vs outside temperatures. This can be used to calculate approximate BTU's needed per heating degree day.
- Turn down the furnace and watch the rate of temp drop in the house per hour. Record the outside temp at the time and the date.
- Turn the furnace on and observe the rate of rise per hour. ie (15 minutes to go up 1F).
Looking at how fast it heats and cools with the original furnace tells us if the furnace is sized right and how efficient the house is.
Calculation example: If it runs 30% of the time at a certain temperature and is around 94% efficient then in this case it takes a steady input of 28,000 btu to keep up. (.30 x .94 x 100,000 capacity =28,000).
It's fairly easy to put a watt meter on the furnace blower or control lines and get exact run-time data but even just listening to the furnace run-time per hour would be useful.
I added some furnace run-time charts so you can see some actual furnace data for a similar sized house in a similar climate,