Need help in selecting Carrier furnace BTU size

[589HVAC]

I'm about to purchase a Carrier Infinity ICS gas furnace, model 58MVC. I've talked to three contractors:

Contractor #1 did a whole house heat load calculation, resulting in 42,300 BTUH, and recommended a 60 BTU furnace

Contractor #2 did a room by room heat load calculation, resulting in 54,400 BTUH, and recommended a 80 BTU furnace.

Contractor #3 did not do a heat load calculation, but recommended a 80 BTU furnace based on our current 100 BTU furnace and a similar house he did down the street.

My house is 55 years old and I'm not sure of the R-value in the walls. I have a better idea of what's in the attic, but not an exact R-value. I need advice in selecting a 60 or 80 BTU furnace.

1) Should I get a 3'rd heat load calculation?
2) How close to 60 BTU should I get before going to 80 BTU?
3) Is there any other advice?

I can e-mail the reports I received from contractor #1 & 2 if it's helpful.

[Joker]

What is the afue rating of the furnaces?

80% of 60K is 48K
80% of 80K is 64K

That must be one small house.

[drewski11]

What is the afue rating of the furnaces?

80% of 60K is 48K
80% of 80K is 64K

That must be one small house.

looks like all the quotes are for the Infinity ICS which is 95% AFUE

could be mild climate as well.

[589HVAC]

The Infinity ICS is 95% AFUE.
The house is approx 1500 square feet, located in Pittsfield MA.

[Joker]

The Infinity ICS is 95% AFUE.
The house is approx 1500 square feet, located in Pittsfield MA.

That is one tight house!

If this is a multi stage system I would go 80K. This is my opinion. Just in case of some freak weather you have enough that may never get used.

[Maikerum]

589, what you need to find out from the people who did the manual j calculation, is whether the number they are giving you is the actual number they calculated your btuh loss to be at the design temp, or if they have already added a margin to it.

In other words if at the design temp, which is the temp your climate only goes below 2.5% of the winter came out to be 40,000 btu. That is your heating design load. But since most people want to be warm even during the 2% of the time it is worse than that, those gusting blizzards, it is recommended to add to that a margin of 10 to 25%. So your calculated 40,000 btuh is used to recommend a furnace of 44,000 to 52,000.

If they have already added that margin in the numbers they gave you, that is one thing. If they have not, that is another.

Also, J-calcs are scientifically based and calculated, but they are still ESTIMATES, not guarantees. They don't know if your insulation has gaps at the top of walls, or is compressed around outlets, etc. So could be off by 5%.

So while it is not recommended to upsize for cooling, it is recommended to upsize somewhat for heating. Not double, but some, to cover for error and the coldest days.

So you just need to find out if any of the contractors already did that in the numbers they gave you, which could explain the discrepancies if some did and some did not.

[589HVAC]

I can't see how it could be that tight. After all it's 55 year old.

If I remember correctly, the Infinity ICS is a two stage modulating furnace.

I didn't realize that I should take the AFUE % of the furnace. So 95% of 60K is 57K. The higher of the two heat loads is 54.4K, so 57-54.4=2.6K. It's closer than I thought.

My main concern is that I don't want to end up with a furnace which cycles too much, isn't effecient, and breaks down more offten.

[Maikerum]

It could be that both contractors calculated the actual load to be around 43,000, and one added a 25% margin to get to 54.4 and the other did not. Or it could be that one of the load calcs is way off.

If the 54.4 k number has already had a margin put in, then you would be ok at the 60k furnace which puts out 57k.

If the 54.4 number has not had a margin put on it, you would need a larger furnace, since 57k is not as much as 54.4k times 1.2=65,280.

Then the output of an 80K unit--76k-- would be well more than enough, if there are no intermediate sizes. If you have one that big, you would want it to be 2-stage or modulating.

[drewski11]

If I remember correctly, the Infinity ICS is a two stage modulating furnace.

The ICS is 3 stage modulating. IIRC, the lowest stage is ~35% of capacity.

[BaldLoonie]

Around here, the average 1500 sq ft home would be 60K input 95% and still be oversized and we get below zero each winter. 3 stage or not, oversizing is not good!

Friend had a 1600 sq ft 2 story not counting basement. Foam insulation. It heated with 2 stage 60K on low only, so 36K input.

[drewski11]

Around here, the average 1500 sq ft home would be 60K input 95% and still be oversized and we get below zero each winter. 3 stage or not, oversizing is not good!

Friend had a 1600 sq ft 2 story not counting basement. Foam insulation. It heated with 2 stage 60K on low only, so 36K input.

seems like 60k input is the lowest many furnace models come in. what's up with that?

[Joker]

I can't see how it could be that tight. After all it's 55 year old.

If I remember correctly, the Infinity ICS is a two stage modulating furnace.

My main concern is that I don't want to end up with a furnace which cycles too much, isn't effecient, and breaks down more offten.

Its in Massachusetts and im in Texas and I have 80K(80%) and in the dead of winter its barely enough! LOL Texas gets bitter dry cold which may make the difference.

I also have a 1950 home with 1800sqft but pier and beam.

[Todd S 2]

I can't see how it could be that tight. After all it's 55 year old.

If I remember correctly, the Infinity ICS is a two stage modulating furnace.

I didn't realize that I should take the AFUE % of the furnace. So 95% of 60K is 57K. The higher of the two heat loads is 54.4K, so 57-54.4=2.6K. It's closer than I thought.

My main concern is that I don't want to end up with a furnace which cycles too much, isn't effecient, and breaks down more offten.

Is your house a 2 story?

Does it have a walk out basement?

How are the windows?