Tuning the temperature rise on a gas furnace


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Old 10-25-14, 04:01 AM
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Tuning the temperature rise on a gas furnace

Hello.

I recently had someone come out to give me a quote on a replacement furnace. He told me that my present furnace was WAY oversized for my house.
I presently have a two stage 90-140KBTU furnace installed. He said he would put in a 90KBTU and back it off to an 80KBTU.

This got me to thinking so I decided to decrease the gas supply to my furnace by slightly turning the gas valve on the pipe feeding the furnace. Now I want to know this: What is the tradeoff in a high temperature rise versus a low temperature rise? Is one more efficient? Is one more comfortable?

I have backed my fan speed down to the lowest settings and REALLY like how much quieter the unit is now too.

Any help would be greatly appreciated.

Thanks
 
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Old 10-25-14, 09:05 AM
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That's not how you adjust the gas pressure. You need a manometer to adjust gas pressure. By putting fan speed on low, you're making the temperature rise higher. There should be a range on the furnace stating what the temperature rise should be, such as 40-70. A high heat rise isn't good. If you had a 65 degree heat rise with a clean filter, once the filter gets dirty, the heat rise will get higher and start tripping the limit switch. I like to see the heat rise in the center of the range suggested.
 
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Old 10-25-14, 11:14 AM
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You cannot change the firing rate, either up or down, on a power burner (powered draft) by changing the gas pressure or even by changing the burner orifices. The reason is that the draft (almost always induced draft) is controlled by an orifice plate and THAT orifice is calculated with the proper sized burner orifices AND burner gas pressure to meet specific combustion gas analysis. Changing one, or even two of the three parameters WILL adversely affect the composition of the outlet gases, increasing pollutants such as nitrogen oxides, carbon monoxide and others.

On a two-stage furnace the induced draft blower has two distinct speeds to correspond to the two different firing rates. To meet the manufacturer's stack analysis ALL the variables; inlet gas pressure, manifold pressure at each firing rate, burner orifices, draft fan orifice and draft fan speed must be as stated by the manufacturer.
 
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Old 10-25-14, 04:04 PM
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Listen to furd.

Reducing the firing rate isn't an option unless there's an approved way to also reduce combustion air; on most furnaces, there isn't. (even if there was an option, you would have to find a tech who has a combustion analyzer to get the fuel to air mix correct.

Too much combustion air reduces efficiency and under-firing can rot out the primary heat exchanger.

A load calculation needs to be done to determine how much heat is needed.

The difference between 80 and 90k isn't that significant if you have a design heat loss of 70-78 000 BTU/hr. On the other hand, if the house only needs 65 000 btu/hr or less, the contractor should get a 70k two stage furnace*. (preferably one with a 4 ton drive/21" cabinet, running a low temperature rise - otherwise you may have airflow issues on a duct system designed for a large furnace)

*Goodman or amana gmvc95-70k comes to mind.
 
 

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