Thread: Heat loss

I was checking a furnace for rate of rise and I took my measurement at the return and also at the closest vent(8 feet away). Cold air was 70 degrees and vent in the slab was around 100 degrees. I went to all the other vents on the first floor(slab foundation) and got the same reading except for the living room(118 degrees) that was located in a direct line from the blower about 15 feet away. The second level had the same readings as the lower level vents of 100-102 degrees. The manufacturer label called for a 45-75 degree rate of rise. I called out the ROR as somewhat low and called for further evaluation. An HVAC tech came and said I was not taking the reading close enough to the exchanger and he drilled a hole in the rear of the unit and got 137 degrees or a rate of 67 degrees. Within spec. I then asked why I was only getting 100 degrees at all these vents on the interior. He did not go inside the house. He was actually surprised that we were getting such low readings at the vents. How could we be losing 30 degrees of heat to all these vents, let alone to the one only 8 feet away? His superior was called in as he was the designer of the system for the home. It is a 60,000 btu 80 some percent. The house was around 1650-1700 sq. feet two story. I also questioned the size of the furnace. I was told that the coolness of the slab was dropping the recorded air temp at the vents. The size of the furnace is correct for the square footage. I know you can lose a few degrees when checking the air temp at the register, but 30 degrees? I have never seen such a drastic drop except when there was water in the vent runs. No water seen. This seems to me that the furnace will run longer to maintain temp in the house and cost the buyer more than it should. Any ideas or comments are greatly appreciated and thanks in advance.

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The slab is a big heat sink.

It stands to reason that you would get a lower TD on ducts in a slab.

Of course my thought is that ducts in a slab is stupid to start with.

I don't see them much but the ones I have seen have had moisture issues.

I am not really seeing it as a big energy penalty though since the slab will radiate energy back to the home during the off cycle of the furnace.

Originally Posted by Jim Luttrall

The slab is a big heat sink.

It stands to reason that you would get a lower TD on ducts in a slab.

Of course my thought is that ducts in a slab is stupid to start with.

I don't see them much but the ones I have seen have had moisture issues.

I am not really seeing it as a big energy penalty though since the slab will radiate energy back to the home during the off cycle of the furnace.

I agree with Jim. Slab air ducts are problematic for several reasons with the largest being moisture problems.

When I find them I always recommend to my client that they should replace the slab ducts with either flex or metal ducts through the attic.

Another negative with having ducts below the slab is radon can easily enter the ducts and be distributed throughout the house every time the air handler runs.

Even though you may eventually "see" the heat radiated thru the slab the rooms or zones that are the farthest away from the heat source will always be cool. I have been called in to fix several of these installations.

The most recent was a custom home in Southern Utah. We changed the supply air ducts in the floor to return ducts then provided new ductwork overhead in a new dropped down ceiling. (The existing roof was TJI's with gyp directly on the bottom.)

The furnace size actually seems adequate to me. Just an improper delivery system.