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Old 01-06-2011, 07:13 PM
 
36 posts, read 193,579 times
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Hi folks,

Just a general question for you. Is it true, that the 95% efficiency furnace will produce warmer heat coming out of the registery than a unit that only does 80% efficiency?

In my previous home, I had a 95% efficiency unit installed and the air coming out of the registery was very hot. Since we moved, this unit is only 80% efficiency, is air coming out is only luke warm....

please let me know

thanks
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Old 01-06-2011, 07:16 PM
 
Location: Eastern Washington
8,825 posts, read 23,242,151 times
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The temp coming out depends more on the air flow and the BTU of the furnace, rather than efficiency. The feeling of warmth or not from the registers is not a reliable indicator of system performance.
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Old 01-06-2011, 07:47 PM
 
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It's the opposite. Higher efficiency means lower temp in the output. It's physics. It takes more energy to raise the temp higher. Think of it this way. The flue temp on an 80% unit might be 140 F and the flue temp on a 95% unit might be 100 F. Which is putting out hotter air?
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Old 01-06-2011, 08:49 PM
 
36 posts, read 193,579 times
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Quote:
Originally Posted by wilson1010 View Post
It's the opposite. Higher efficiency means lower temp in the output. It's physics. It takes more energy to raise the temp higher. Think of it this way. The flue temp on an 80% unit might be 140 F and the flue temp on a 95% unit might be 100 F. Which is putting out hotter air?
So based on what you said is true. Then the 95% efficiency unit feels less comfortable then? Since the air at the supply registery only feels at 100 F (given your example) vs able to get at 140 F...

true?
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Old 01-06-2011, 08:53 PM
 
Location: Central Fl
2,901 posts, read 6,715,210 times
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One cannot compare the different houses because there are too many different factors to consider. In addition to the efficiencies, trunk ductwork layout, how many heat runs, etc, how many CFM's per room, amount of returns, type of filter, etc all come into play.

I generally hear that the newer furnaces feel cooler because they run more often with the fan at different speeds.

I really do not care what furnace feels hotter. It is more about constant comfort and having a well balanced airflow with the correct duct size runs, the correct CFM's for each size room, etc.

I also prefer the ability to have the exaust piped out of the house at a low temperature using PVC instead of a chimney flue and a liner. For that reason alone I'd go with a 95% furnace with a variable speed fan.

Frank
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Old 01-07-2011, 03:43 AM
 
23,271 posts, read 17,639,241 times
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If a hair dryer was used for heat it would be 100% efficient, it would neither heat your house or be cheap unless you already had electric heat in which case it's a wash.

Efficiency is based on BTU input and BTU output. When you burn a fuel each unit of fuel has a X amount of BTU's, this would be the BTU input. With natural gas it's 100,000 BTU per therm.

Your heating unit is going to lose X amount of BTU's out of the chimney. If you burn one therm and lose 15,000 BTU's out of the chimney you would get 85,000 BTU's of heat, this is the output.

In this case 85% efficiency.
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Old 01-07-2011, 07:09 AM
 
Location: Grosse Ile Michigan and Sometimes Orange County CA
15,819 posts, read 32,425,087 times
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A furnace will put out its rated output regardless of whether it is 80% or 95% efficient. Thus a 100,000 BTU furnace will put out up to 100,000 BTU regardless of whether it is 80% or 95% efficient. The distinction is how much gas or other fuel you use to get the 100,000 BTU. Both heat the same.

A 95% efficeint furnace has three main advantages that are important to me.

1. It uses less fuel and saves money in the long run.

2. It can vent through a side wall and does not reauire the installation of a vent stack all the way through the roof.

3. It creates somewhat less pollution and makes me feel better because I am taking a tiny step towards reducing my pollution output .
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Old 01-07-2011, 08:11 PM
 
23,271 posts, read 17,639,241 times
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Quote:
Originally Posted by Coldjensens View Post
A furnace will put out its rated output regardless of whether it is 80% or 95% efficient. Thus a 100,000 BTU furnace will put out up to 100,000 BTU regardless of whether it is 80% or 95% efficient. The distinction is how much gas or other fuel you use to get the 100,000 BTU. Both heat the same.
Be sure to look at what measurement the manufacturer is using, most will list both but I know it's common in the solid fuel industry (coal,wood and pellets) to advertise the input rating thus a unit listed at 100K BTU at 85% efficiency will only output 85K BTU.
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