changed furnace & hot plenum

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Old 09-21-06, 07:40 PM
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Question changed furnace & hot plenum

I purchase this newly built house 5 years ago. After the first year the builder replaced an 80,000 with a 100,000 btu gas furnace because of an error by the installer. Unfortunately, the didn't replace the plenem and instead put in an adapter to fit between the plenum and the furnace to accomodate the larger opening. It should have been a 19.5" width (plenum) instead of it's present 16.5". After a couple of years, I've notice the plenem gets real hot when heating - to the point you can't touch it. I was told the pressure may be too great causing it to over heat. Also, it takes along time to cool the house down. They had added a little bubble to accomodate the larger coil. I was also told, it may be that the air passies too quickly over the coil not allowing the efficent transfer of heat to take place. Therefore, taking a longer amount of time to cool down the house in the summer. When I calculate the air space missing, it works out to over 1.5 cubic feet (3"x48"x19").

Should I replace the full plenum or extend it on one side to accomodate the extra 3"? Is this replacement a big job? any thoughts, suggestions, recommendations woudl be appreciated.

Thanks

B
 
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Old 09-21-06, 08:20 PM
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I would replace the whole plemum, and make sure the rest of the duct can handle the extra air flow.

Also, Why was the 100K put in?
 
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Old 09-22-06, 10:00 AM
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Wink

The original furnace was undersized at 80k. The house is 3100sq/ft and 100k was supposed to be installed. The duct work is 5" and shouldn't be a problem. I'm situated in Ottawa, ON, Canada. Anyone know someone in these parts who is reliable?

Thanks in advance.

B
 
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Old 09-22-06, 05:10 PM
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Ducts

At 5" you better have a lot of them.
 
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Old 09-22-06, 05:51 PM
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You might consider taking some photos and posting them here. It sounds to me like you have severely undersized ducts. Or maybe they are sized right but not enough of them. Not being critical but when the furnace goes off on high limit it is not a size of being undersized. It means you cannot get the heat it is making to the rooms. It could be a return air problem too but those couple of inches on the plenum are probably not causing you a problem.

Ken
 
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Old 09-22-06, 10:45 PM
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Wink

Did they add or enlarge the cold air grills or return duct any. For the bigger blower?

ED
 
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Old 10-24-06, 08:25 PM
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Just to update this issue. The problem was not the size of the plenum but rather the most obvious area...the air filter. One inch filters are the most useless filters available. I was using one of the 3m filters (1200) with a merv value of 11. As has been mentioned here before (Ed), the throughput is heavily restricted and the fan has to work extra hard especially a bigger plower fan that's in my high efficiency furnace. Also, these filters clogg up (daily) very quickly especially if you run your fan 24/7 and should be changed virtually monthly. Temporarily, I've changed to a low merv filter (6) which has made a big difference but I will soon install a 5" filter which are by far superior for throughput (more surface space). I did change my fan motor since the old one burnt out and added a low speed relay to run my 4 stage fan on low 24/7 since circulation and clean air are also important to me. The return plenum will also have to be enlarged to accommodate the filter and airflow requirements.

Just to answer some questions posed here; the house has 30 runs of 5" ducts with about 12 returns (16"x8"). This is adequate to support the HVAC system for basement, 1st, and 2nd floors.

B
 
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