Implications of the air temperature at the discharge register

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Old 02-05-18, 03:32 PM
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Implications of the air temperature at the discharge register

I improved some HVAC duct insulation and decided to check my discharge temperatures. To my surprise, the temperature at the discharge register nearest to the furnace registered 147 degrees. The unit is in the basement where the temperature is 52 degrees and the intake ductwork is uninsulated so I assume the intake air is close to that.

While that's interesting information, I don't know what to make of it. The unit is house original so it is 30 years old. I could find no information on the unit showing what the temperature increase (discharge over intake) should be.

My questions are simple. What can be made of this data and what are the implications? If changes should be made what are they?
 
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Old 02-05-18, 03:47 PM
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Hi Tony,
I'm not an HVAC pro but I'll get you started. From what you posted it sounds like your intake air is all coming from that cool basement. Are there return ducts going up to the living space above?

The cool basement is an indication that it is really an unheated cold basement that is steeling heat from your system. Some simple insulation and air sealing would make the basement a lot warmer at no cost and help warm the floors above. Tell us more.

As for the delivered temperature there are guidelines but I won't try to guess. Let's see what the pros say.

Bud
 
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Old 02-05-18, 04:08 PM
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147f is fairly high. Your furnace specifies a heat rise. It's posted on the ID tag inside on the side wall near the burner. I doubt you are making 147f with 52f return temps. A typical heat rise is 50-70 degrees. Which means your return must be around 70f.

You can measure the return air near the furnace and subtract it from the supply temperature to get the rise. If the rise is too great the furnace will cycle on overheat.

52f is pretty cold for a basement and insulating the ducts may save a little on heating costs but the basement will become colder. Usually the furnace is in a conditioned space.
 
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Old 02-05-18, 04:42 PM
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Bud, sorry to confuse things. I have intake registers throughout the house but none in the basement. The intake ductwork is in the basement and uninsulated so I assume the intake air is something close to the basement temp.

PJ, I've thought anytime a furnace is located in a basement it is conditioned but I guess that's not the case. I have a walkout basement with 40+% of walls above ground and uninsulated. (I've looked into adding insulation but the cost of doing it properly is high.) All discharge ductwork is insulated at approx. R8. The intake ducts are not insulated but I don't believe that impacts basement temp.

I found the plate you referred to and included it here. In case it's hard to read, the important specs are "For temperature rise of 70F to 100F" and " Designed maximum outlet temp 200F or less"
 
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Old 02-05-18, 05:56 PM
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My basement runs around 65f. I haven't insulated my supply ducts as whatever leakage I get just rises into the house. Your basement is a little cooler because you have wrapped your supply ducts.

That is an extremely high rise furnace.
 
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Old 02-05-18, 06:19 PM
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Uninsulated basement walls have an r-value similar to a single pane of glass which everyone knows is terrible.

Tony, the wood framed above grade walls down there are easy to insulate and cover with drywall. A little more work is required for the foundation area but very well worth it. Currently you are losing a lot of heat to the basement which is quickly going directly outside. In addition, the floor above is losing heat through the floor to the cold surfaces down there. The loss occurs through radiant exchange, like standing in front of a cold window it is the radiant loss that you feel.

A simple way to insulate the foundation walls is to locate some Dow Thermax, it is approved to be installed without a covering of drywall. Just cut, glue it up, and tape the seams with foil tape.

Bud
 
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Old 02-06-18, 03:11 AM
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Uninsulated basement walls have an r-value similar to a single pane of glass which everyone knows is terrible.

Tony, the wood framed above grade walls down there are easy to insulate and cover with drywall.
Bud, is it just a matter of insulating the walkout walls like any above ground wall and adding sheetrock? Because of the additional moisture in a basement, my understanding is that 2" XPS board should be applied and sealed with spray foam followed by unfaced fiberglass insulation. I have 450 sft. of walkout wall, making this approach expensive and difficult.

I've read horror stories of basements insulated with fiberglass causing mold because of basement moisture issues. I'm concerned because once I'm finished I won't know what's happening to the insulation.
 
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Old 02-06-18, 04:12 AM
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There have been those horror stories about fiberglass insulation on basement walls but those are the below grade concrete areas that end up trapping the moisture from the concrete behind a the walls with the fiberglass. Above grade wood walls can be insulated and finished just like your house above. In CT, climate zone 5, you can meet the code requirement with just the 2" of rigid. But, if you add a stud wall you can drop to 1" of rigid and add 3.5" of Roxul. No Vapor barrier needed. Add some detail work to air seal and insulate the rim and your total heating cost will go down measurably while the house comfort goes up.

Note, if you currently have water/moisture problems down there they need to be addressed first. The actual approach you select needs to include what you intend to use the space for. Do you want to leave it as just a warm basement or turn it into living space?

Bud
Note, your thread here is about your furnace which is important and I'm drifting. Best you continue to resolve the temperature issue, and then start a new thread about finishing the basement.
 
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Old 02-06-18, 04:22 AM
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That is a pretty old furnace and yes they had that high a temperature rise. Old gravity furnaces had even higher temperature rire
 
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