Our current 80% efficiency gas furnace is rated at 100k which means it puts out 80k BTU. Ok, so when upgrading to a 95% efficiency unit, do I get a 80k or 100k two-stage unit? The smaller unit will give us nearly 76k output which is slightly less than we have now, but the larger furnace will give us 95k which is well over - and from what I understand from a couple of HVAC contractors, oversizing theseunits will lead to problems down the road.
We live in the Mid-Atlantic so winters are reasonable.
You should probably have someone do a heat loss calculation for you it is possible that the original was not sized properly and if you have sealed your ducts or added insulation improved your windows you may be able to get an even smaller furnace and have it work well, I assume you are also looking at a new AC too, don't forget to invest in a new thermostat to take full advantage of your furnace.
Plus having an oversized unit is less efficient than a correctly sized unit of the same rated efficiency. An oversized furnace cycles on and off more often than a correctly sized furnace. The extra cycles reduce efficiency since it has to warm up the ducts every time/push out all the cold air from the ducts. Really if you wanted to be efficient you would set it so that it turns on when it hits 66, runs until it hits 74, then waits until 66 before it turns on again, unlike the standard way of on at 69 off at 71.
I've always gone by the line of thought that I would rather under size it than over size it. Say your under sized furnace can only keep the house 60F above the outside temperature, while the over sized one can keep it at 90F above. The over sized one would never run nonstop (the point when it is the most efficient) since it won't get to -20F, while with the undersized, when the temp drop down to 5F, it can only hold the house at 65F. But most of the time it is coldest during the night, when you have the heat down on 60F anyways, so who cares it it can only get up to 65F. Plus if it gets to cold during the day, put on a layer, or run some benchmark programs on your computer so that it is running full blast, or even bake some cookies, so that the oven puts out heat.
I've got to hit the phones again and see if I can find a contractor willing to take the time to do a proper load balancing.
You can calculate your own "Manual J" load with any number of available online calculators or worksheets. As mentioned previously, smaller is better than larger until you get to the point where the unit runs all the time and fails to heat or cool the living space. Going too big is not only inefficient (most of your electricity and gas goes to heating and cooling startup and shutdowns) it can cause strain on the blower fan if there is too much static pressure in the vents.
It also depends on the worst summer or winter weather you're likely to experience. We have hellaciously hot summers here but mild winters. I need a 5 ton system to give me the summer cooling capacity but my 2 stage heating never comes out of the first stage in the winter.
My advice is to buy a variable speed fan and two stage the part of the system that wont be taxed, and if your current system goes on and off frequently in the summer or winter (whichever is most extreme), then you can go smaller than what you have now and perhaps even a little smaller than a load calc. You may also get better dehumidification from a smaller unit that runs longer than a large unit that cycles frequently.
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