Wednesday, 20 January 2021

Duct work questions

We're having a couple of problems that may be related to our duct work.

The first problem is that the registers farthest from the furnace are not able to heat their areas as those closer to the furnace.

The other problem is the thermal overload switch trips from time to time. We figured out that closing some registers closer to the furnace in an effort to get more air to the farther registers trips the overload switch.

The house is a 30x75 ranch with a full ICF basement with 9ft ceilings. Furnace is 100k BTU propane, forced air. The duct work is unsealed and uninsulated. There are 6 intakes 3.5" x 14.5" plus an outside air intake that has two 6 inch pipes to the main intake trunk.

There are two 16x8s coming off the furnace. Both are fed via a roughly 8x8 square hole in the furnace's main output shaft. This seems small to me, but I don't really understand duct sizing requirements and limitations.

One 16x8 is roughly 10ft long and has 6 registers fed by 6 inch round pipe. One blows into the basement, the rest into the house.

The other 16x8 is roughly 25ft and then steps down to a 12x8 for roughly 16ft. The 25ft section has 6 registers, one in the bottom of the 16x8 and two off 6 inch pipes blow into the basement, the other three up into the house.

The 12x8 section has 7 registers, one in the bottom of the 12x8 and another fed by a 6 inch pipe blow into the basement, the rest up into the house.

I was told that one register can be fed for every 2 inches of width on an 8" trunk. So the 12x8 is one over and the 16x8s are under by 2 each.

So if you got this far reading, thanks! And the questions are 1, are the registers farthest from the furnace not getting enough air because of problems in the duct work sizing/install? And 2, would the duct work be restricting air flow out of the furnace causing it to over heat?

Like I said, if we close too many registers in an effort to get more air out of the farthest registers, the furnace is more likely to over heat.

3, if the duct work is seriously wrong, what's the recommendation on a fix?

4, I assume that once fixed (or of it it ok as is), that air sealing and insulating the duct work will be money well spent?

5, will installing 6 inch round duct boosters on the four farthest registers help warm up those rooms farthest from the furnace?


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source https://hvac-talk.com/vbb/showthread.php?2225904-Duct-work-questions&goto=newpost

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