efficiency of two stage LP furnace

[Joehvac25]

Best bet is to really look into if the 2nd stage is needed at all. In most cases a smaller furnace will do the job.

Yes and quite a bit less expensive of a job


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[Homeowner314]

Could also add a heat pump to it and save some propane

I thought about it when I bought the system back in February, but since I expect to be here 1-2 years, couldn't justify the extra money for the HP, GMVC furnace, and communicating thermostat I wanted. It would have just been a gift to whoever buys the place from me.
Next house will be built the way I want it, 2x10 walls, radiant floor heat, an air source HP sized to handle down to 10°F or so, and enough south facing glass that the HP won't run much when the sun is shining. Probably won't bother with an LP boiler, just use electric and/or wood for backup.

Best bet is to really look into if the 2nd stage is needed at all. In most cases a smaller furnace will do the job.

I bought the smaller furnace. First guy I brought in wanted to sell me 70,000 btu, based on 1200sq ft x 50 BTU/sq ft and rounding up.

My DIY Manual J came out around 39,000. I'm shooting for 33k after tripling the ceiling insulation and insulating the exposed part of the basement with 1" foam board.

I got a GMH950453BX, which puts out around 39k on propane.

If the smallest furnaces can handle a 1970s 30x40 ranch in Michigan, why don't they make smaller furnaces for people with modern airtight homes in Arkansas?

I saw a thread where a fellow with a 1400sqft new build in a warmer clime (Missouri?) thought his builder should put in a 90k instead of 60k furnace. I figured my 46k would be too big in his house.

Yes and quite a bit less expensive of a job

When I bought it last winter there was only about $25 difference between Goodman GMH950453BX and GMH950703BX. But if I'd bought the 69k, my burn times in 50° weather would be ridiculous. I've been getting 5-7 minute burns about an hour apart this morning with the 46k furnace on low stage.

[54regcab]

I thought about it when I bought the system back in February, but since I expect to be here 1-2 years, couldn't justify the extra money for the HP, GMVC furnace, and communicating thermostat I wanted. It would have just been a gift to whoever buys the place from me.
Next house will be built the way I want it, 2x10 walls, radiant floor heat, an air source HP sized to handle down to 10°F or so, and enough south facing glass that the HP won't run much when the sun is shining. Probably won't bother with an LP boiler, just use electric and/or wood for backup.



I bought the smaller furnace. First guy I brought in wanted to sell me 70,000 btu, based on 1200sq ft x 50 BTU/sq ft and rounding up.

My DIY Manual J came out around 39,000. I'm shooting for 33k after tripling the ceiling insulation and insulating the exposed part of the basement with 1" foam board.

I got a GMH950453BX, which puts out around 39k on propane.

If the smallest furnaces can handle a 1970s 30x40 ranch in Michigan, why don't they make smaller furnaces for people with modern airtight homes in Arkansas?

I saw a thread where a fellow with a 1400sqft new build in a warmer clime (Missouri?) thought his builder should put in a 90k instead of 60k furnace. I figured my 46k would be too big in his house.




When I bought it last winter there was only about $25 difference between Goodman GMH950453BX and GMH950703BX. But if I'd bought the 69k, my burn times in 50° weather would be ridiculous. I've been getting 5-7 minute burns about an hour apart this morning with the 46k furnace on low stage.

That's an excellent question. Typically cooling load dictates blower size which forces oversized heaters. The small price difference makes over sizing tempting to contractors who are afraid of the heat not keeping up and don't want to do a load calculation.

[motoguy128]

If the smallest furnaces can handle a 1970s 30x40 ranch in Michigan, why don't they make smaller furnaces for people with modern airtight homes in Arkansas?

.

Good question. Because as mentioend above, all too often furnaces are oversized. So there is little demand for 45k btu or even smaller furnaces. Additonally, at some point, the cost an complexity of a furnace no longer justifies the economics, so you start looking at a heat pump instead. Another option is hydronics. Either radiant floor heat or a hydronic air handler both using a combi boiler that makes both domestic hot water and hot water for heating. THese are usually tankless and high effciency.

IF I didn't already have a gas line going to my attic and had easy piping access, I might have considered a hydronic air handler and combi boiler for my upstairs in my home with a heat pump. But it made more sense to repalce hte furance. But not I have a 2 stage 60k BTU furnace where my heat loss is <30k BTU. SO low stage is still oversized at 37k BTU.