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Old 11-09-2012, 08:15 AM   #1
Bill Kearney
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How hot should ducts get?

We're renting a house while ours is being torn down and a new one built. The gas furnace crapped out and the owner had it replaced with a Carrier 58STA135.

Trouble is the new one seems to be making the utility room that houses it a LOT hotter than seems appropriate. It's as if the furnace is making the air much hotter than necessary and that's transferring to the utility room via radiated heat from the duct work. I checked the duct seams and they're not leaking.

I don't have my IR thermometer gun handy so I can't take a direct measurement of the duct surface temp. But it sure feels quite a lot hotter than I remember the ducts in old house ever being. I've made sure the various registers around the house are fully opened. And that they, and the main filter, are clean.

Is there a setting for the air temp the furnace will create via the exchanger?

I usually wouldn't care but we've also got a fridge and freezer in the utility room (a whole half of a basement) and it's making the freezer run poorly. It gets to like 80F in that room when the furnace is running!

I ask here before I contact the landlord because I want to know what is or isn't correct. Rather than making him pay for a service call to fix something that isn't broken.


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Old 11-09-2012, 09:03 AM   #2
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How big is the house you are in? 135,000 btu's is a monster. Discharge temps for that furnace should be 35-65 degrees above return air temps, so if you have 70 F air entering the furnace you should have a maximum discharge temp of approx 135 F, so your ductwork shouldn't get hotter than that.
It may be that you could increase the fan speed to lower the discharge temps, but my guess is that you are probably way oversized on the furnace, and the ductwork is too small for the amount of heat you are generating.


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Old 11-09-2012, 09:07 AM   #3
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I totally agree. I can heat a 2000 sq ft home with that size of unit and it gets to -40F where I am. Probably way oversized.
"Cut it twice and it is still too short".
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Old 11-09-2012, 09:24 AM   #4
Bill Kearney
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It's a 4 bedroom split-level in the DC area. 50's era construction, so it's horrendously insulated (as in, next to none in the walls). I don't know the square footage. And since it's a rental I don't particularly care to learn 'too much' about it. I've no idea why that unit was chosen, I certainly would've put more thought into it than what the landlord appears to have. But saying that it's oversized does make some sense to me, as a friend's place some years ago had an AC system that was oversized and they had the opposite problem of too much cold air. So I get the gist of it.

If it's oversided is there a way to adjust it such that it doesn't crank out as much of a blast of heat? Or is there a way to retrofit it such that it wouldn't require replacing the whole unit (again)? There's a bit of a language barrier between me, the landlord and the asian guys that installed the furnace. As in, I have little hope of trying to get the problem sorted out properly. So knowing what can or can't be done might help me phrase my questions more effectively. Bad enough the landlord had to get stuck with a surprise furnace replacement bill, I don't want to give him more headaches. But also don't want to have to deal with the overheated space.

Thanks for the 135F number, I'll check the temp to see if it's close. By hand-feel it seemed to be at least that warm.
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