Lots of heat! It's a big house, and may very well warrant 150k btu's, but make sure you get a heat loss calculation done to determine sizing. The Carrier 58PHA might be worth looking into. 80% AFUE, 2 stage furnace with high efficiency blower (not variable speed). I could not find any furnaces over 80% AFUE efficiency in that size.

Here is a link that might be useful: Carrier 58PHA info

As an option most manufacturers allow twinning two smaller furnaces together with a kit or wire allowing any efficiency to be used in two equal models and sizes to meet total BTU requirement. The other benefit is the additional stages.

How did he arrive at that size?

If your 4100 sq ft house is a modern well insulated house, and at least partly two stories, I'd think you can get by fine with a 100K BTU..where do you live?

Here I'm thinking maybe the smaller furnaces are available in more efficient ratings. I understood years ago that some were even then efficient enough that the combustion products were cool enough to vent through a side wall, no chimney necessary. Don't know, that's what I recall before I decided to go geothermal.

Maybe two 75K BTU furnaces would be the best for efficiency and give you a survival strategy when one of them goes off-line during a cold streak.

Us heat pump folks would get by with 80K BTUs for your house. That may require full time running when it is zero outside.

Thank you for responding to this post. Basically, I am trying to educate myself before engaging some local contractors. The estimate of a 150k BTU furnace came from a friend who has recently retired from the HVAC industry after 30 years - he "eyed" the estimate. If I were try to recreate it -

225k BTU (current) * 60% efficient = 135k BTU
150k BTU (proposed) * 90% efficient = 135k BTU

To address some of the other questions - the house is in NE Ohio and I think we averaged below 32F in January with a week below 10F. And despite some efforts, I don't consider this house to be well insulated - something i hope to improve on.

With regards to twinning (two furnaces) - my duct work seems to split the house down the middle (left/right of a two story house). With two furnaces, is it typical to tie each furnace to a side?

Thanks again.

Your present furnace, yielding 135K input could still be oversized. On the coldest day this year, did it run full time to keep up, or was it cycling frequently. If the latter, a new 150 K unit could also be oversized.

The degree of cycling would depend on the tightness and construction features of your house, i.e., its heat loss.

V

Without any kind of heat loss calc, nobody has any idea whether the current furnace is right on or grossly oversized.

Next, the match is wrong. Look at the rating plate. An old furnace usually gave a "bonnet" capacity or OUTPUT. That was 80% of input. While the furnace may have an annual efficiency of 60% or less, in a steady state condition it would be putting out closer to 80% of input or 180,000 BTU.

We just did a house that had twin 108s or 216,000 BTU input and replaced with a single 120K 95%. Load calcs said we could. Other dealers may have gone back with much bigger equipment. In the old days of cheap gas, stuff WAS way oversized.

Apparently, you know what you're talking about....The Output Rating on the furnace is 171k BTU. With input at 225k, this pegs efficiency at about 76%. Regarding cycling - the furnace was running pretty good during the coldest days, but it would shut off at least once every 30 minutes (and come back on 5 minutes later for another 30 minutes).

There is no doubt that this house loses heat, but i hope to get that improved this spring/summer.

It appears that a heat loss calculation is in order. Thanks again.

Apparently, you know what you're talking about....The Output Rating on the furnace is 171k BTU. With input at 225k, this pegs efficiency at about 76%. Regarding cycling - the furnace was running pretty good during the coldest days, but it would shut off at least once every 30 minutes (and come back on 5 minutes later for another 30 minutes).

There is no doubt that this house loses heat, but i hope to get that improved this spring/summer.

It appears that a heat loss calculation is in order. Thanks again.

The idea of a split system for two furnaces is good and bad.

Most two zone, rather two furnaces or duct switching, splits the heating so that the bedrooms are on one circuit, the rest of the house on the other. Then, one can, say warm the bedrooms up in the morning to get showered, dressed and ready for work, while leaving the rest of the house at a set back temperature. There are other strategies I'm sure. For the benefit of two furnaces providing redundancy, one would like some cross-over so that heat can be to more/most rooms. This could be a manual duct switch. All this doesn't make a sense economically unless the system is being built, new, or can be easily modified. Most furnaces don't fail...but when they do it is always in the coldest time of year, when it is under the most stress. I use an wood airtight insert in my fireplace as my emergency backup.