Below is my load calculation:

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Thank you!

Jeff

It is difficult to read your report. What did you use for indoor and outdoor winter and summer design temperatures? What are your wall and attic insulation values? Are the windows single or double pane?

The numbers you calculated are in the ball park. Let's see if your input data is reasonable.

I would say your downstairs furnace is oversized. Based on the model number, your AC condenser appears to be a 2 1/2 ton. Your heating calculation for upstairs may be a little light. If your AC sizing for upstairs is accurate, I would move up to 3 ton condenser. Do not like borderline for upstairs D.C. System.

IMO

mike_home,

I used indoor winter design temp of 78F and summer 70F; for outdoor I used winter 18F and summer 91F. For wall and attic insulation values, I used R11. All windows are double pane.

tigerdunes,

I gave the wrong AC model number, sorry, it should be 24ACC624A003. I think you are right about oversizing. Now I remember most of the time High-stage heat ran was when recovering from set back. In the few mornings when the outside was 10-15F, the furnace ran about half time High- and half time Low-stage. I was debating between 60K or 80K BTU. After getting a professional calculated Manual J of 120K BTU (as a part of energy audit), I picked the higher one:(

I like your suggestion of a 3-ton AC, which will be consistent with the 5-ton total cooling from the professional Manual J. Considering I have an 80K BTU furnace downstairs, do you think 40K BTU will be fine? Will the blower generate enough CFM for the 3-ton AC?

Thank you both for your advice!

Jeff

Jeff...if you decide on 3 ton AC condenser for upstairs system, you will need a matching furnace with a 3 ton rated blower. Probably not going to find a 40K furnace with a 3 ton rated blower. I suspect a 2 ton AC for downstairs system may be too small. Hope not. 2 1/2 ton would have been better.

IMO

Thanks, tigerdunes.

Is Bryant 926TA36040V17 rated for 3-ton AC? The "Cooling CFM @ 0.5 ESP" is only 1065, but I read somewhere that Carrier/Bryant only need 350/ton instead of 400. And there is the "36" in the model number...

Jeff

Jeff

nice furnace...looks like it can take a 3 ton AC condenser...however, you should confirm with your dealer. Just curious, where will the upstairs furnace be located?...since the 926T is a 2 stage var speed model, you will need a matching Bryant 2 stage thermostat. Also recommend a whole house media filter cabinet.

IMO

tigerdunes,

It'd be in the basement. Will definitely get the media filter cabinet.

For 40K furnace, would 2-stage and variable speed blower make a noticeable difference? Especially considering that I'm getting a single-stage AC. If not, I'd just get a single stage (e.g. Carrier 59SP5A040E17-12, Lennox EL195UH045XE36B, both rated 3-ton as well), if the saving is substantial.

Talking about T-stat, I really want one with remote sensor. Currently, the T-stat is located in the hallway, not enough feed back when the bedroom doors are closed. Do you know whether Carrier/Bryant/Lennox offer T-stat with remote sensor? My backup is Ecobee, but would prefer one from the same manufacturer if available.

Jeff

Jeff,

Your outdoor design temperatures match the document I use as a reference. The typical indoor design temperatures are 75 degrees for summer, and 72 degrees for winter. You can put in the actual thermostat settings you know you will use.

The R11 insulation for the walls is typical for a 1970s house. I would be concerned if you only have R11 insulation in the attic. At a minimum it should be R30. You should upgrade the insulation. It is not a big ticket item and the pay back should be relatively short.

Be aware that a condenser that is listed with a capacity of 2.5 tons will not produce 30,000 BTU of cooling. You will often see ratings of 27,000 - 28,000 BTU. That will means you will have to move up to a 3 ton condenser, which may produce 33,000 - 34,000 BTU of cooling.

mike_home,

Thank you very much for checking my calculation! I'll try the standard design temperatures to see how they change the results.

I actually don't know the R value for the Attic insulation, so I put in the lowest one. Will look into upgrading it.

Now I'm convinced a 3-ton AC is what I need. Just need to find a dealer who is willing to match it with a 40K BTU furnace.

Jeff