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.
GMH950703BX input rating is actually 69,000BTU and output is 66,400BTU per manufacturer spec. To maintain you would be using about 60% of the 66,400BTU. Propane burns @ 93,000BTU/1gal.
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.
Based on your geographical location and a house built in 1980 I calculate approx. 46,800BTU. Now, you also state that you have triple pane sliders. I say your right in stating that they are filled with nitrogen as every window manufacturer I know that offers the triple pane they fill them with nitrogen. Based on that I would be willing to bet that they didn't go cheap with insulation but for conversation sake we will say R-13 as that is the standard for a 2"x4" wall. We cannot ignore the two attached unconditioned spaces as there will be a loss there.
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.
Nothing wrong with asking, although remember you are hiring a professional in your geographical area. Personally I would go with his recommendation if it were my home.
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?
You can use single or two stage stats. This will be set up by the contractor. It should be setup to offer you the full benefit of the furnace you are installing.
Although others believe differently, I feel your better off with a 2-stage. The "controlled" studies by engineers show that aside from the comfort you truly have an energy savings. Some techs believe that a single stage is better because your using full resources instead of 60% to maintain temp. Personally and in my professional opinion I agree with the "controled" studies.
Finally, is GoodCare extended warranty worth buying? None of the brochures on Goodman's web site describe it in any detail.
This will extend your parts/labor to 10years and pays for all the labor, unless there is a call that you just needed to replace the batteries in stat, to include the call charge as well as the parts. However you need to keep up on your yearly PM's.
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?
I say no.