Furnace too big?

**jgriff02** · 10-17-2005, 04:44 PM

On a service call to check out a newer 125,000 BTU 90+ furnace. It is running a supply temperature of 146 degrees and the blower is on medium high speed. The homeowner is complaining that the fan noise is bothering them. The furnace seems somewhat oversized and is short cycling when on. Is there any reason why we shouldn't plug one or more of the burners, reducing capacity to 100K or less, and then reduce fan speed to get nameplate temperature rise?

**amb4081** · 10-17-2005, 04:58 PM

On a number of furnaces, the ignition takes place at one burner and the flame sensor proves flame at the last burner. If you plug one of the orifices, your flame isn't going to carry across and furnace will not remain lit.

**jgriff02** · 10-17-2005, 05:42 PM

How about either blocking a center burner or if the furnace has provision for two positions for the flame sensor (like some Lennox's do), block and reposition flame sensor.

**hvac hero** · 10-17-2005, 06:18 PM

I would not do this at all. Your right about the ignitor lighting the first burner & the sensor is on the last so if you blocked a middle one its gonna have a buildup of gas til it connects with the first 2 burners. It would likely blow out into the cabinet when it lit.

As far as moving the flame sensor, I would never do that. That gas valve is sending enough pressure to blow the fire through those 6 burners. If you plug one, its gonna blow a stronger flow through the other ones. the chambers might not be able to handle that much extra fire. If I was the contractor, i'd rather walk away than try something like this.

At the very least, ask the manufacturer. Even then I'd get em to give it to me in writing that its ok. I can guarantee you they wont do it though.

[Edited by hvac hero on 10-17-2005 at 09:04 PM]

**nutshell** · 10-17-2005, 06:19 PM

never block a burner. Most furnaces have derating kits you can buy to SAFELY derate them about 10%. Don't go anymore than that.

**bigtime** · 10-17-2005, 06:24 PM

Make sure the heat anticipator is set for longer cycles. If you plug a burner you are exposing yourself to a lot of liability. Dont do it.

**tinknocker service tech** · 10-17-2005, 06:32 PM

chances are the burner tubes not being use could start to condensate and cause prmature failure of the heat exchange
also to change the design and operation takes all the responsablitity on you and completly off the manufactor if something happens.
if the furnace is to big bring you thoughts to your bass and let him deside how to rectifie the problem

**beenthere** · 10-17-2005, 08:11 PM

See what size they REALLY need, and sell them a correct size furnace. And duct changes that may be needed.

**wallynut** · 10-17-2005, 08:34 PM

How do you figure? (If you plug one, its gonna blow a stronger flow through the other ones.)
Just wondering/

**wallynut** · 10-17-2005, 08:40 PM

" One idea I might try is partially closing the shutoff valve going into the furnace. Again I would want to ask the manufacturer, but this seems a lot less dangerous. All you would be doing is lowering the gas pressure going into the system."
Your not serious that this would lower pressure, are you?