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Old 03-11-16, 09:04 PM   #6
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Originally Posted by gtojohn View Post
I agree, manual j can be off quite a bit. Our design temp here is 96F however in the past ten years we have long stints of 100f+ days. One year we had over 60 days straight of triple digits. HVACs sized too close to manual J couldn't keep up. Lots of unhappy customers. Heating obviously you need to plan for those extremes as well, just because you average 40f for a low doesn't mean you won't have a freak 20f week. Perhaps a good argument for multistage equipment that could ramp up when you need it.
I can't wrap my head around this post. ..at all.

I live in a house built in 1985. 2100 square feet. Before starting my insulation and air sealing retrofit efforts with about R30 tops cellulose in the attic, R13 in the walls with a 3/4" wrap of non-foil(plastic instead) polyiso wrap(R18, but not that much once considering thermal bridges with 2x4s 16" on center. No slab insulation. Not a pile of windows but enough on the northwest summer side to heat the place in the summer but not provide much benefit in the winter, low quality windows, most with failed seals that condense between the panes. 1500cfm50, roughly 5 ach50.

This house manual Js at about 30,000BTUhr at -11f design load. I haven't bothered calculating the manual J for air conitioning but I'd bet the 2 ton air conditioner was sized under manual J. The furnace has a 57,000BTUhr output.

Prior to doing any retrofit work, this standard code minimum 1985 construction had an actual measured heat load based on run time of 25kBTUhr during the middle of the night while the temperature was a steady -20f or lower for an 8 hour period. In the summer(after air sealing) under design conditions of 88f with the 2 ton air conditioner it more than keeps up. With 72f inside(I don't normally keep it this low but for measurements sake I did) and 95f outside I've extrapolated that the actual cooling load for 75f inside and 88f outside to be around 15kBTUhr. Since I'm plenty comfortable with the temperature at 75f in my house, with the smallest standard split system in my house I could have the system keep my house at 75f while there is full sun and 91f inside with a 1.5 ton system.

With that being said, if I kept the windows the same size and orientation, based on how the surface area of a space gets larger linearly at the perimeter in relation to the heat load, I could grow my house to 4624 sq keep and keep the same equipment that I have now. ..and if you don't buy the linear concept and multiple by square feet, I'd be at 3150 square feet before I actually run the furnace constantly at -20f temperatures and the air conditioner would then be properly sized for a load that I'm very comfortable living with.

After I got done with a basic level of only air sealing and resolving thermal bypasses into the attic for a cost of two $20 sheets of XPS and about 15 cans $60 of Great Stuff. I've brought a 25k BTUhr heat load at -20f down to 19kBTUhr and 17kBTUhr at a -11f outside, 70f inside design load.

I seriously think that if someone can't maintain heat with a 40k BTUhr furnace, that person either has a very large house, they have too much glass(or inefficient glass), and/or there is a need for additional insulation and air sealing.

Air conditioning is a little bit different, it's more dependent on window size, orientation, whether the glass rejects heat, air sealing, cooking loads(are you really running the oven for hours in the middle of a blazing sun day, and internal humidity loads). Even with there being different configurations, I think that a 1.5 ton and 2 ton system could manage in a well designed house where the cooling loads were in mind during its construction in my climate. You'd go up a little bit with temperature but solar and moisture loads on the hottest days are a larger factor. Windows, air leaks, and humidity loads are the largest contributors.

..with that being said, I'm thinking that if the central air even breaks down it's getting replaced with a 40k 90+% condensing furnace and a 1.5ton air conditioner. ..although the reality is that I'll probably toss in a super efficient inverter 12k mini-split heat pump upstairs instead to handle most cooling days and only use the 8.5SEER system I already have on days where I feel it needs a boost. I live in a higher cost electricity state where natural gas and geothermal has too high of an initial cost and the electric costs wash out the long term savings. An air source inverter mini-split has a lower initial cost way to get high performance heating but the cooling savings is where it's at in a moderately hot and lake humid state.

In short, especially for heating loads: I want to know the furnace size, square footage, outdoor temperature, insulation level, and air leakage of any home that a natural gas furnace heated home is experiencing a situation where it can't hold a steady state temperature. I'm having a really hard time, based on my own experience, that a home air sealed reasonably well and has at least R13 in the walls can't maintain its heat with the furnace that is installed to manual J spec. ...also keep in mind oversizing to some degree typically ends up being built into the incremental sizing of the equipment unless the manual J is sitting right on the numbers.
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