Reducing BTU Output of Furnace


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Old 11-12-10, 07:53 PM
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Reducing BTU Output of Furnace

Is it possible to "turn down" the output of a furnace?

We have a 100,000 BTU furnace and have been told it is oversized for our house. Can the gas valve be adjusted to put out less gas? IE 75000 BTU rather than 100000?
 
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Old 11-13-10, 12:12 AM
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Originally Posted by phipps33 View Post
Is it possible to "turn down" the output of a furnace?

We have a 100,000 BTU furnace and have been told it is oversized for our house. Can the gas valve be adjusted to put out less gas? IE 75000 BTU rather than 100000?

Yes and No depending on what brandname furance and model number and other issue surround this.

It can able but you have to understand that once you " underfire " the furance this part have to be done by HVAC pros there is too many thing to cover and I rather let the HVAC pros deal with this matter for safety issue.

I know there is couple members may know more details so they will chime in one way or other.

Merci.
Marc
 
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Old 11-13-10, 10:36 AM
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Downfiring

Some equipment can be safely downfired & some cannot. Often the limiting factor is the temperature rise across the heat exchanger. All equipment has a temperature rise range which varies from furnace to furnace. If the furnace can be kept within this range after downfiring I see no reason why your's could not be downfired.
 
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Old 11-13-10, 01:44 PM
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Thanks for the info. I'm guessing this wouldn't be qualified as a DIY job.

How do you measure the temp rise across the heat exchangers? Is it as simple as measuring air temp in the inlet and outlet areas? Is a special thermometer required?
 
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Old 11-13-10, 01:59 PM
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Diy

No it is certainly not a DIY job. In addition to temperature rise an exhaust analysis needs to be done to ensure proper combustion.

To measure temperature rise you need two thermometers known to read the same. Place one in the air discharge of the furnace preferably after the first elbow & the second in the return at least a foot from the connection to the furnace. Allow the furnace to run a minimum of 5 minutes or until the difference in the themometers becomes stable.
This temperature rise is also known as "Delta T".
 
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Old 11-14-10, 05:25 PM
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If the furnace in question has an induced draft fan then it is highly unlikely that you can reduce the BTU output in the field.
 
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Old 11-14-10, 05:34 PM
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Originally Posted by Furd View Post
If the furnace in question has an induced draft fan then it is highly unlikely that you can reduce the BTU output in the field.
Thanks Furd for bringing that up with induced draft fan that will affect the whole thing.

Merci,
Marc
 
 

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