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# 80K BTU 92% furnace vs 90K BTU 80%

#1
12-24-17, 04:05 PM
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Join Date: Dec 2017
Location: usa
Posts: 3
80K BTU 92% furnace vs 90K BTU 80%

I need a new furnace and there are two proposals. One is an 90K BTU 80 % furnace for \$2800 and the other is a 80K BTU 92% furnace for \$3600. The way I see it the 90K 80% furnace puts out 72000 BTU and for \$ 800 more the 80K 92% puts out 73600 BTU. It seems I only get 1600 BTU for the additional \$ plus I have to worry about draining the condensation. The 1600 BTU seems insignificant.

The installer selling the 92% furnace says that furnace puts out a warmer heat than the 80% furnace and I will be warmer despite the similar BTU rating. Can this be true? I think a BTU is a BTU. Who can give me a straight story?

David

#2
12-24-17, 05:48 PM
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Join Date: Oct 2008
Location: New England
Posts: 9,457
Hi David and welcome to the forum.

The two may produce similar heat output, but the 80% furnace will require more fuel.

If you divide the net output by the efficiency you get the input.
72,000 divided by .8 = 90,000 BTUs input.
73,600 divided by .92 = 80,000 BTUs input.
Since they would be using the same fuel the 80% unit would be consuming approximately 10% more fuel, which basically just goes up the chimney.

Corrections welcome, but before you buy, how did they determine the size from the start? The best approach is to do, or have them do, a heat loss determination. But that should be done after you have reviewed some of the easy improvements to reduce the total. An energy audit would tell you where those improvements should be made or we can help.

Obviously I know nothing about your house, but it is common for heating guys to just replace with a similar size unit.

Let us know.

Bud

#3
12-24-17, 06:36 PM
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Join Date: Oct 2016
Posts: 2,137
Let’s back up a bit. Before we can answer, can you tell us if you really need an 80,000 BTU furnace in the first place?

#4
12-24-17, 09:29 PM
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Join Date: Dec 2017
Location: usa
Posts: 3
Wow 2 replies already. This is a good forum.

How did we decide I needed a 80K furnace. The 40 year old Fraser Johnson was 100K and they said it was a 60% unit. The gas company inspection detected carbon monoxide and they put a big red tag on it. The gas man thought he saw a cracked heater box. Fraser Johnson is out of business and there are no parts available and anyway the heater box seems to be welded in so I need a new furnace. The house being a little on the cold side I didn't want to go down in heat output. Its a 2300 sq ft house located in Souther Calif with high ceilings and lots of big windows which have lots of heat loss.

My heating bills for the coldest months of Jan Feb, and March were \$76, \$82, and \$62, thereafter not so much and mostly cooking and hot water. What other info should I provide?

David

#5
12-24-17, 10:58 PM
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Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 16,321
If your 2300 square foot home in southern California is cold with a 100k furnace, even one running about 50% AFUE, it shows that either your house has a HUGE heat loss or you have a lousy distribution system.

On the coldest days how long is a "burn cycle" on the present furnace and then how long an off cycle until it again lights?

#6
12-25-17, 04:55 AM
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Join Date: Jan 2017
Location: USA
Posts: 447
I would first have the energy audit done, as Bud9051 suggested to see just what your heat needs are. I would do the least costly changes to the outside house envelope to reduce my heating costs , like reducing air infiltration and adding insulation, etc. I would call the furnace company, tell them what size furnace you now need and ask for a price on a 2 stage condensing furnace. The new furnace will run on the reduced input the majority of the winter and still provide all the heat needed on the coldest days. This would give you the lowest operating costs with a not too expensive furnace and will also provide for lower temperature swings since the furnace will usually run for longer on cycles. This is what I did and it has worked for me. Just food for thought

#7
12-25-17, 05:10 AM
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Join Date: Oct 2016
Posts: 2,137
You need to do heat loss and gain calculations.
Manual J and Manual S first.
Manual D for the duct.
This is a free manual J calculator.

Bigger, oversized equipment is not better.

#8
12-25-17, 08:53 AM
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Join Date: Dec 2017
Location: usa
Posts: 3
OK I'll do a heat load calculation. It will be a lot of guessing because I have no idea what insulation if any is in the walls. I suspect none. I know there is a big heat loss because my house sits on a slab and in the winter the floor is cold. And there are these big (very big) windows that are single pane and I am replacing them one at a time with double pane which is a big job. There are also high ceilings with dead air space, Each window replacement costs more than my yearly gas bill. Home Depot made a bid on 3 windows and 2 months later decided their custom build couldn't do my size. It will be a slow process to reduce the heat loss.

So I still need a furnace right now cause its getting cold. The fact is that my old 100K 60% unit barely heated the house on cold days. Hope for some advice.

#9
12-25-17, 09:10 AM
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Join Date: Oct 2016
Posts: 2,137
If your going to guess then it’s pointless to even do. If you put inaccurate information in then you’ll get inaccurate information out.
The installation company should be doing a heat loss before determining size.
Your old furnace may have been malfunctioning, or you could have ductwork problems leading to the poor performance you’ve se n

#10
12-25-17, 10:37 AM
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Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 16,321
I can almost guarantee you have duct issues. As I pointed out yesterday in a different thread, most houses with forced air heating do NOT have the ductwork designed but simply toss in the ducts where they fit, Even when a system IS designed builders are notorious for moving ducts or changing the sizes to accommodate other parts of the building. Return air ducting is especially prone to being undersized with the absolute minimum number of grilles.

I also second Steamboy's recommendation for a two-stage furnace. Having two stages makes the heat loss calculation far less important. Firing at the lower rate the furnace will run longer and the temperatures inside will be much more even. I don't know about going with the 90+% AFUE (condensing) unit simply because you really don't have that much of a heating season. The additional fuel savings from a 90+% may not be an economic positive when weighed against the increased cost of the furnace. I would, however, strongly suggest a variable speed blower and setting it up for continuous air circulation during the heating season. The variable speed blower (not a four-speed which is standard) uses less power and during the furnace off cycle it will continue to run at a very slow speed but still circulating the air keeping the temperature fluctuations to a minimum.

#11
12-30-17, 07:18 PM
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Join Date: Mar 2004
Location: NA
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How did we decide I needed a 80K furnace. The 40 year old Fraser Johnson was 100K and they said it was a 60% unit. The gas company inspection detected carbon monoxide and they put a big red tag on it.
1. Do not size based on what's there.

2. you're confusing annual efficiency with steady state.

The 60% is like the city fuel economy and includes turning on and off, having the pilot light "idle" between cycles, having the draft hood vent indoor air up continuously.

The maximum output of your old furnace will be 70 to 80% of input.

80 000 btu/hr output is a lot. For a house of your size, assuming detached it better be in a climate where it hits 0F or lower and be poorly insulated.

The 80 000 btu/hr output, can be used to heat 2800 to 4000 sq ft houses in cold climates, provided house is insulated well.

Your gas bill amounts suggest that you don't even get real winters.

You may only need 60k at 92% and have a lot of surplus.

Being cold does not mean you have an undersized furnace. only if your furnace runs continuously and never satisfies the stat is it undersized.

Otherwise the problem is with distribution.