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Jeakor

07:57AM | 04/23/01
Member Since: 05/12/01
2 lifetime posts
Bvhvac
I had a new blower installed on my furnace. When the heat stops, the blower stops with a loud bang. One minute later, the blower starts up again and runs for a minute. I suspect a timing problem, but the furnace guy said it is "normal". It was not normal with previous blowers. What can I adjust?

Juleous

07:53PM | 04/30/01
Member Since: 05/13/00
18 lifetime posts
This is not normal. Is it the actual blower or is the furnace itself. How old is the furnace? And does it have electronic control with control board.

Jay J

05:07PM | 05/04/01
Member Since: 10/26/00
782 lifetime posts
My blower always has a 2nd cycle that lasts anywhere from 20 seconds to, say, 30 seconds. I'm not sure why but it's done it for 8 years.

As far as the 'banging', could it be the ducts flexing back in place? To check this, the next time the unit comes on, go down there and put your hand on the plenum side of the unit (the output side.) WHen the unit shuts off, you'll know if it's the duct or not. I did have this problem and fixed it by bracing the 'soft spot'.

My best to ya and hope this helps.

Jay J -Moderator

PS: The new blower may have a greater 'push' than the old one. It's just a hunch I have.

Dodgefan

09:26AM | 05/08/01
Member Since: 05/02/01
17 lifetime posts
The higher fan speed makes your furnace more efficient. The banging is just what the other posters said...the ductwork is oil canning. I had the same problem a couple of years ago, I located the point of oil canning and wedged a piece of wood between the ductwork and the basement ceiling - the noise stopped! As for the blower stopping and restarting, that also has to do with the increased air flow. The plenum cools off quickly and when the fan stops, the latent heat in the furnace warms it back up and the blower restarts for a short time. The only way I know to stop that is to slow the fan speed.
Dodgefan

Juleous

04:49PM | 05/08/01
Member Since: 05/13/00
18 lifetime posts
Hey dodgefan where does the latent heat come from, I don't know of any state change occuring in a furnace unless it's a condensing furnace. Sounds like to me you need to brush up on your tech stuff there. Oilcanning could be likely, lot's of other reasons also. High speed fan more efficient not likely, the fan coming back-on is caused by residual heat in the heat-exchanger acting on the fan control in the off cycle. The best solution for that is adjust the fan control.

Dodgefan

09:46AM | 05/09/01
Member Since: 05/02/01
17 lifetime posts
Juleous,
You are exactly correct, sorry for the latent heat terminology, I should have used residual.
However, since an increase in fan speed results in a higher delta temp of the combustion gas through the heat exchanger, (the flue gas exits the heat exchanger at a lower temp) the net result is the furnace becomes more efficient.
Dodgefan

Juleous

08:15PM | 05/09/01
Member Since: 05/13/00
18 lifetime posts
Sounds close but I would not suggest lowering flue gas temp unless you want to replace your furnace in a couple of years. Or if you have condensing furnace then I don't know how much more temp you want to get out of the gas considering it's already 90 degrees. You have to be carefull what you recomend I have seen to many people do what you suggested and end replacing there entire furnace in a couple of years.

Juleous

08:17PM | 05/09/01
Member Since: 05/13/00
18 lifetime posts
Oh yea efficiency is measured over long haul so if you reduce your furnace life then you just through all that savings out the door with the furnace man.

Jeakor

04:05PM | 05/13/01
Member Since: 05/12/01
2 lifetime posts
Thank you for the suggestions. I would like to try adjusting the blower speed. There is a control called "fan limit" with pointers set at 100, 150, and 200. Is this where the fan speed gets adjusted?

Juleous

02:46PM | 05/14/01
Member Since: 05/13/00
18 lifetime posts
Actually that depends on your furnace most 7-% natural draft furnaces the primary limit control is set to 200 fahrenheit. What you do is follow the wires from the fan motor untill you find exactly where they attach. This is the fan control, this control may be adjustable take a look. Set it to turn off the blower at 90 degrees.
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