Cutting it close on furnace size

[kcb203]

I’m right at the border of choosing a 60k vs. 80k input furnace and need some advice. I’ve already asked a few questions here and appreciate the very helpful input.

I have a 58-year-old house in Arlington, Virginia, with a 20-year-old addition. The original house has an inch or two of rock wool batts in the walls, with the same in the attic with a layer of fiberglass over the top.

In my “worst case” load calculation, I factored in “poor infiltration,” no wall insulation because the old certainly isn’t the R-11 of new 3.5 inch fiberglass. I put R-4 on the crawlspace first floor duct even though only 33% of the house is over crawlspace, the rest is over a conditioned basement. (HVAC-Calc doesn’t seem to have a way to take partial crawlspaces into account for the ducting.) And I put in R-19 for the attic, even though I may upgrade to R-30 or R-38 in the near future. For my “best case” calculation, I factored in R-11 for the wall insulation and “average infiltration.”

I’m having trouble accounting for infiltration. I currently have original single-pane windows with leaky storms, but I’ve already signed a contract to upgrade to top-of-the-line Marvin windows. I plan to replace the attic insulation and seal any leaks through wires or piping holes. The three exterior doors are well weather-stripped or have a modern storm door. Would you rate this as average? Due to the age of the house, there is no Tyvek or even tar paper wrap. (The addition has a tar-paper wrap.) HVAC-Calc calculates an ACH of .7 for average, while the revised tables for the 8th Manual J for my size house at “average” put it at .32. This alone is a 5,600 BTU difference.
http://www.acca.org/tech/manualj/addenda/addendumD.pdf

Under my “worst case” scenario with “poor infiltration,” I get a heat loss of 56,629 BTU. (The worst case already accounts for the new windows.) The “best case” is 45,497 BTU. I figure that the insulation is probably about halfway between none and R-11, and the infiltration is halfway between poor and average. Averaging these results in a heat loss of 51,063 BTU.

My current furnace is 32 years old, with a 100k BTU input and a 80k BTU bonnet output. I’m considering a Carrier Infinity 96 as a replacement. The output of the 94.1 AFUE Carrier is 57K BTU. I assume the 32 year old furnace is about 60 AFUE, so the actual output is very close.

I don’t feel that the current furnace is undersized, but the new one could be if my guess at the AFUE of the old furnace is off and my load calculations are wrong. Would you feel comfortable going with the 60k input versus the 80k? Do you use a safety factor?

I figure that by replacing the windows and redoing the attic insulation (R-19 to R-38 would save 741 BTU), as well as by sealing the previously leaking attic trap door, putting gaskets on the exterior outlets and switches I could feel safe going without the 15-25% safety factor that the HVAC-Calc author recommends if my current furnace is really about 60k BTU output.

If I’m cold next winter, I could also add more insulation around the crawlspace ducting (1100 btu according to HVAC-Calc) or upgrade the fireplace door (1400 btu). But if I installed the 80k, I bet it would never kick into high except for setback recovery on a very cold day as the low stage is 49k BTU. Would you go with the 60?

One other question: When calculating wall heights, what about the 10” of joist, flooring, and drywall between floors? That is about 10% of the wall area, which for 25k BTUs for the whole house, is a difference of 2,500 BTU.
Thanks again for your help.

[dhvac]

looks to me like you have done your homework
and if you read your post you have answered you own question.

[beenthere]

If it were me, I'd go with the 60,000, and zone it, 1 stat in the old, and 1 in the addition.

[RoBoTeq]

I would personally rather go light then heavy on the heat. The worst case scenario is that you may get a little chilly the very few times the temps reach near zero.

A properly sized furnace to keep your home at 72º on a 0º day is basically oversized on any day over 0º.

Sweaters and throw covers are all the fashion in Virginia.

[johnl45]

The output capacity of the furnace or boiler must be equal to or greater the manual J heat load but preferably not more than 140% of the manual J design heating load.

[beenthere]

Although I'd go with the 60,000 @ 94% eff,(ir I did the load calc myself) the choice is yours.
But, don't forget that your old furnace has a steady state eff. of 80%, so you can't say that it only gave you 60,000btus of heat, it does 80,000 btus out when the burner runs for an hour continous.

If you want a safety factor, get an 80,000 2 stage 90+.

[casturbo]

Originally posted by RoBoTeq
I would personally rather go light then heavy on the heat. The worst case scenario is that you may get a little chilly the very few times the temps reach near zero.

A properly sized furnace to keep your home at 72º on a 0º day is basically oversized on any day over 0º.

Tell the HO that on very cold days they'll need sweaters on in their house to stay warm, and they'll throw you out ASAP. Come on, put the bigger furnace in and enjoy 100% comfort regardless of outside temperature. Just amagine if one of the grandparents end up living there and the house can't keep warm. The cost of the marginally bigger furnace is peanuts compared to ripping out the small furnace later on. A too small a furnace won't live long either if it never shuts off.

[hvacmd]

Have you considered the Rheem/Ruud variable output 90plus furnaces?Or that other manufacturers offer other outputs,like 75K.

[Big]

To put your mind at ease Get a @ stage system. That way if the ice age returns you are covered. Usually a 2 stage will give you 48k low and 8ok high good luck

[Milk man]

2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage 2 stage

Why sell anything else?

[kcb203]

I guess my post wasn't clear, but I am getting a two stage. But if I got the 80k, the second stage is 49K BTU output, so the second stage is approximately what the total capacity needs to be.

I'm getting the 60K. The installers will be here in an hour.

[beenthere]

Congrats, and how has the day gone for them.

[kcb203]

So far so good. They've done a very tidy job running lines, PVC, and cables.

I've read all the installation instructions myself, and my only concern is that the outdoor unit (38TDB037) is supposed to receive power for 24 hours before starting it up. I believe this is for the crankcase heater to ensure even oil distribution. The circuit breaker is off, and they're planning on finishing tomorrow, so it won't be receiving power for 24 hours. Is this a problem? Should I insist that they return on Monday rather than turn it on tomorrow? Or is this only a cold-weather problem--the forecast for tonight is for 58 and the high tomorrow is 65 and rainy.

Is this only a problem for brand-new units, or theoretically, if I have a summer power outage, should I not restart the A/C for 24 hours after power is restored. I don't believe anyone does that, though.

One other question--this unit is supposed to use 7/8 tubing for the vapor line. How is that measured--is it internal diameter, external diameter, or some strange nominal system like copper plumbing pipe? I assume they used the correct size because it slips into the fitting on the condensing unit without an adapter, but I want to be sure.

[Edited by kcb203 on 06-02-2005 at 09:34 PM]

[beenthere]

Refrigeration is measured by its OD.

We seldom have the power on for 24 hours.

We usually ,manage 4 to 6, in spring and fall, less in the summer.

On 2 stage a/c's, we have it on over night.

Have never had a problem.