Furnace sizing and Goodman GMH95 questions
I'm in the process of buying a foreclosed home with a missing furnace and have an affordable quote for a Goodman GMH950703BX, but I have a few questions. Since it's February in Michigan, I'll need to be have the contractor do the furnace install as soon as I get possession. I do not want to pay for central air.
House is 30x40 ranch, built around 1980, with triple pane vinyl windows, which I assume are nitrogen filled. Insulation is unknown, doors insulated steel. There's a double pane sliding door, but it opens into an unheated 3 season room, so I assume it's no worse than a triple pane slider. Home has no discernible drafts on a windy day. The windows especially seem to be very high quality. It's in lower Michigan, 7000 degree days, can expect five to fifteen days a winter when temps drop below zero at night. -20 might happen every ten years or so.
In sizing, he used the 50 BTU / square foot rule and rounded up to 70k. I'll have to use propane, so it looks like actual output is 59k. The next size smaller is GMH950453BX, which puts out 39k on LP.
I've spent some time with a few online heat load calculators and with conservative assumptions like R10 walls and R20 ceiling and one air change per hour, I'm getting around 35-40k, worst number I can recall is 42k BTU at 92 degree temperature differential. I ignored the attached garage and the three season room, which together cover about 30% of the outside wall area, so I really think the heat loss should be less than I calculated.
Here's my main question:
Is there anything wrong with asking for the 45k furnace, if it might run 80-100% of the time on a really cold night? The vast majority of the time from October-May I would expect it to be running on the low stage. Seems like using a 70k would defeat the purpose of having a two stage burner.
With LP around $3 a gallon, I'm thinking that if a 45k furnace that burns 1/2 gallon an hour can't handle the load, I need to vigorously seal and insulate, not step up to the 70k furnace.
My next question, although the GMH95 is a two stage furnace, it appears you use it with a single stage thermostat.
And you can set it to run single stage, low stage for five minutes followed by high stage, or auto, which can vary from 1-12 minutes on low stage as needed? Does the furnace itself decide how long to run on low stage when you set it for "auto", and is this going to be the best setting to use?
Finally, is GoodCare extended warranty worth buying? None of the brochures on Goodman's web site describe it in any detail.
Edited to add: If I ask for a 20x25x4" media box on the air inlet and use MERV 12 filters to help with allergies, would this harm system performance?
Now I have a duct question.
Now I'm wondering about my ducts.
I'm assuming the rectangular trunk lines are adequate to supply the small round branches. The trunks look huge compared to the branches.
I have the following branches, all 13-15' long, straight runs with a round to rectangular 90° elbow connecting to the floor grids.
2 x 5" diameter
4 x 6" diameter
1 x 7" diameter
If I assume 75cfm per 5", 100cfm per 6", and 150cfm per 7", that's 800cfm, right? Are my assumptions too conservative?
The smallest GMH and GMVC furnaces come with 1200cfm blowers.
The trunk ducts have rectangular grids in them for basement heating, but I was thinking of blocking those off and not heating the basement.
Are my ducts adequate, or do I need to replace the entire system? Baseboard electric and 200A service is beginning to look better all the time. I can install that myself.