# Thread: Manual J and Furnace Efficiency

1. Regular Guest
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I just took my home plans to the county building department to begin the plan check process. I noticed (a little late) that the architect had indicated a 90% furnace. My plans had been to get an 80% furnace and a higher SEER AC given my area which has mild winters and hot dry summers.

My understanding by reading some posts here was that, unless money is no object, you should match the higher efficiency unit (AC or heating) to your most extreme weather condition. So if my winters are mild I would not necessarily benefit from a higher efficiency furnace as much as a higher SEER AC to deal with my hotter summers.

When I asked the building permit technician if the architect's specs for a 90% furnace could be changed he told me I would have to have another energy calculation.

Does this sound right to you? Is there a connection between an energy calculation (Manual J) and the efficiency ratings of the equipment?

I thought, perhaps in error, that the calculation had to do with capacity, not efficiency.

In any case, I left the plans and will ask the architect on Monday.

2. ct2
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just go wityh the 90% furnace. it will cost you more to change the design than the price difference in furnaces

3. I dont think the load changes but the furnaces are rated by input, not output. For instance a 100,000 btuh 90+ would have an output of say 92,000 btuh and a 100,000 80+ would have an output of something like 82,000.

The manual J wouldnt change, the requirement stays the same but the Btuh input may change as a result. The energy calculation may just be a calculation on how much they anticipate you spending on fuel which may indeed change.

Go with the 90+ anyway.

4. ## nathan

You are right, there is no technical difference from a 90% or an 80% as far as the amount of btu's needed to heat the home. However, due to losing 10% of your output, you may well have to use a larger furnace which might cause the need for ductwork changes, as well as changes in the exhaust.

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I guess it will all come down to money. I wanted to get a VS furnace as many on this site (including docholiday) has sold me on the comfort benefit. Combining 90% and VS will increase the cost of the system.

Ah, well, I'll just work a little harder.

Thanks for the advice.

6. You can get a VS in 80.

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Doc, that would involve getting the architect to do another calc, right?

I'm assuming (always dangerous) that there are national and maybe state minimum ebergy standards. I know that a 13 SEER requirement is coming up and 80% (or is ti 78%) is the current minimum on furnaces (is this right?)

I probably discussed equipment rating with the draftsman. I probably told him I would like a 90% furnace and a 14 SEER AC. I just didn't think the efficiency rating (beyond the legal minimums) would be specified in the plan and that I would then be required to use equipment that met the efficiency ratings specified in the plans.

8. I dont recall your situation. If its new contruction it will require a chimney, there are alot of considerations.

No need to recalculate but why mess with an 80 with gas prices being 20-30% higher than just last year? I was just pointing out your options.

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Doc, how does all this relate to the AC? Let's say I get a VS furnace. Does this have anything to do with the speed of the AC blower? I know this shows my ignorance, but some of the things I read on the Trane website seemed to indicate that in a split system (electric AC, gas furnace) that the furnace blower is involved in the AC fan or blower set up.

For example I might see a 2 speed AC unit and a 4 speed furnace on the Trane website. Could these work together?

10. Yeah 4 speed furnace fan is ok but it partially defeats the purpose of two speed cooling which is comfort, why cripple an upgraded system with a PSC motor?

You need to be asking the Trane guys this stuff, I always see the comments "Trane,..there is no subsitute" but I rarely find much in the way of support for this statement.

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I don't quite understand. If I have a VS furnace does that affect the fan set up of the AC?

Could you give a HVAC 101 lesson on how the fan set-up of the AC and furnace work? Are they independent of each other, or is there is relationship between the fan of the AC and the fan of the furnace?

12. The furnace is the blower for the AC. It knows how to speed the blower based on configuration and thermostat input.

13. Regular Guest
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OK, so if I get a VS furnace I also have a VS AC?

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