Gas furnace size quandry

[dave davis]

You may have read my other post re oil furnace size, I've been given another option that sounds great but there's more questions. Got a great quote from a contractor, highly recommended, who (at one visit) appears very competent. Did a Manual J calculation and came up with 48.5k heat loss at 70 deg and avg infiltration (1.05 ac/hr), 42.7k with a tight house (.4 ac/hr). I feel the house is somewhere in between.

Also the comp is based on 0 deg that we get only rarely here in RI and we have alternate electric baseboard heating if necessary (though not desirable to use).

So, the issue is two choices: Amana AMV 9045 that goes 44.3k at high fire and 30.8k low, or AMV 9070 that yields 66.9k and 46.4k. We lived with a way oversized two stage gas furnace in a past house, that never ever went to high fire...doesn't make much sense to own high speed when you're never out of low.

On the other hand, the AMV 9045 might never catch up on the worst of days that happen kind of seldom. Am I incorrect in thinking that max efficiency comes from constant high fire at the worst expected conditions? I would imagine most of the time on cold days the furnace would run almost constant in low with occasional high fire...is this good to go?

The real issue is I'd love to have about a 50k furnace but none is being quoted. I have to make the decision here to go with this very efficient gas furnace, or to replace the existing oil with a Thermopride OH6 that provides 60k at low fire.

I really don't want to overfurnace but the spread between the 9045 and the 9070 have me forced to make a tough choice. Suggestions?

[Jultzya]

Originally posted by dave davis
Suggestions?

Tell your concerns to the contractor. See if he will offer another furnace that is closer to your heat loss.

They do exist. :)

[wyounger]

Best efficiency does come from sizing small and working them hard.

Unless there's somewhere you can easily add some insulation to bring the heat loss down a few thousand BTU, the 9045 looks a little too small. The 9070 wouldn't be a crying shame of oversizing, but not ideal. 70 is the indoor temperature you want, right? If you usually set to 68, then the 9045 may just make it.

Another brand might be a good option, if your contractor is willing. They are sizes out there that fill the gap between the two Amanas nicely. Trane has a two-stage 60k input, outputs are 37k and 56k. Rheem's modulating 94% furnace has a 60k input size, outputs are continuously (for all practical purposes) variable from 22,560 BTU to 55,800 BTU. This is an awesome choice because not only of the (nearly) stepless staging across different heat outputs, but also because the minimum is so much lower than anybody else's first stage. In my book that's the perfect choice for your application. It will run just about forever, mostly down very low, but it also has enough margin at the top end that it won't take a week to heat the place up again after a nor'easter knocks the power out, or if you set the temperature to 60 while you're gone to granny's for a few days at Christmas.