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mike in bethel 12-06-2008 03:33 PM

Bryant 376 CAv
I have installed a Bryant 376 CAV furnace in my garage recently. The first time I used it (last weekend) I ran it for about an hour, during which time it cycled 3-4 times and all was good. Today I went to use it and it cycled one time, and now it doesn't fire. When the 110 is turned on to the furnace, the blower starts immediatley and stays running in spite of the fact the thermostat is set to "auto". When I installed the furnace, I installed a new Honeywell heat/cool thermostat so I could run the fan in the summer. Could this be a thermostat problem?

yuri 12-06-2008 04:15 PM

The limit control which is above the burners is probably open due to a lack of airflow/overheated. If it is blown/open the control board sees it open and runs the fan to cool off the furnace. You can check it with an ohm meter to see if it is stuck open/blown.

mike in bethel 12-06-2008 05:54 PM

Bryant 376 CAV
I will check it

mike in bethel 12-06-2008 05:57 PM

Bryant 376CAV
Thanks for your response - I just bought this furnace 3rd hand - do you think it is an installation issue?

yuri 12-06-2008 08:01 PM

Maybe. If the previous owners ran the unit too hot/never changed filters they would have excessively cycled the unit on the limit control until it was near failing. You may have to replace it and then check the temp rise thru the unit. Should have a rating on the model # sticker ie: 40-70 degF. Check the supply temp about a foot downstream of the bonnet (out of sight of the heat exchanger) and subtract the return temp. If it is too high then the fan needs speeding up and correct sized ductwork.

Good Luck

mike in bethel 12-07-2008 07:12 AM

Bryant 376 CAV
Thanks for your insight - I'll check it out this morning and see if the switch is open.

beenthere 12-07-2008 07:27 AM

What size BTU is the furnace.
And what size supply duct, and return duct did you install.
What size air filter are you using.

Is this a garage at your house.
Is the furnace on a stand/return box that raises the blower motor 18" off the floor(required by code).

mike in bethel 12-07-2008 09:06 AM

The furnace is 75,000 BTU
The lower plenum box measures 18" H X 14"W X 28" Deep. In this box are 2 holes 8"X12" with standard wall duct covers on them for the heat to come out.

The upper Plenum box 1s 24"T X 12 1/2"W X 20"D. About halfway up is a slot cut in the long axis with a 16"X20" filter installed at about a 35 degree angle. There are also 2 holes cut in this box for air inlet, one is 8"X14", and the other is 8"X12", again with standard wall duct covers on them,which when added together is about 100 Sq. Inches short of the filter area, but only 40 or so sq. inches short of the plenum size.

Yes, the garage is at my house - 3 1/2 Car

beenthere 12-07-2008 09:33 AM

The 8x14 and 8x12 grilles only have between 125 and 160 sq in free area.
Too small.

The 2-8x12 supply grilles only have about 150 sq in. Also too small.

If you have a thermometer that you can measure the return and supply air temps, I believe you'll find your exceeding allowable temp rise.

Need more return, and supply.

mike in bethel 12-07-2008 09:43 AM

How much area on either end do I need? It wouldn't be too difficult to add more area to the inlet side (top plenum), but the outlet side may be a bit more difficult. The furnace is installed such that the right side and the rear are approximatley 2" from walls. Unless you dont think that having a register against a wall matters - ???

beenthere 12-07-2008 10:08 AM

Put a 10, or 12 X 24 on the 28" long side of the supply plenum that faces the garage.

And you should have almost double the return that you have.

mike in bethel 12-07-2008 10:12 AM

Great! Thanks beenthere, I really appreciate your help. Is there anything that prohibits me removing the grates and installing screen instead (or nothing at all)?

beenthere 12-07-2008 10:18 AM

No, a " or larger mesh screen is fine.

Still need to make the openings bigger then they currently are though.

Never leave the openings with out a cover.
Just when you think nothing can happen.
Someone, will reach their hand in.
Or a dang cat crawls in.

mike in bethel 12-07-2008 10:23 AM

beenthere / Yuri
Excellent. Off to Menards for screen. Again, thank you very much for your help. Hopefully I can enlarge these openings, check the limit swich and replace as required, and be back in heat again. It's too cold in Minnnesota to not have heat in the garage!


mike in bethel 12-13-2008 02:08 PM

been there - the fun continues
After some investigation, I have concluded that one of the rollout switches directly above the burner is what is causing the no-lite problem. Per your siggestion, I have enlarged the square area of both the inlet and oulet to the sizes you recommended. Upon reset of the rollout switch, the furnace will lite the burner and run for 3-4 minutes and then it trips the switch and the fan stays running. My questions are this: Do these rollout switches tend to get weaker or trip earlier the more times they fail, thus needing replacement - or do I have another underlying problem here - or both?

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