High Efficiency gas furnace - return air temperature

[bubbabubbs]

The unit in question is Goodman GMV9509050XBA gas furnace (95% eff.,

90,000 BTU.) It says in the Installation Instructions (page 7, Location Requirements & Considerations) that the following must be observed:

"The temperature of the return air entering the furnace is between 55F and 100F when the furnace is heating."

I am curious why the 55F requirement. I mean, when I'm not in the house, I would like to set the temp as low as possible in order to save on my heating bill. I think I could otherwise set it as low as 45-50F and still keep the water pipes from freezing. But I wonder why I'm not supposed to go below 55F. What could happen? Could the unit get damaged and why?

Thanks

P.S. I'm in the Denver, CO area - 5,300 ft altitude, if that matters.

[Tony Hwang]

Hi, I am just guessing. If return air temp. is to low it may not produce warm enough air. Air is passing thru the heat exchanger at constant speed and think law of physics.

[Bubba]

Ertttttttt! Wrong answer Tony. Stick to what you do because it sure isnt this. Problem is possibility of condensation in the furnace.......................in the PRIMARY! Bubba

[Speedy Jim]

Yep, they spec that so that condensation is unlikely to occur in the main (pri) heat exchanger, which could cause early failure.

It can become a warranty issue (if you care).

Jim

[bubbabubbs]

Re: it becoming a warranty issue - how can they _prove_ I ever allowed the temp to drop below 55F?

[Speedy Jim]

Well, I don't rep the co., but I imagine they would say something like: "Your exchanger rusted out....you must have allowed the inlet air to get below 55 degrees, else it wouldn't happen."

Jim

[M Q]

They probably can't prove it. The spec just says the range over which it was designed to work correctly. It might also be that it is not designed to drain condensate from the primary. It may also depend upon the dew point of the combustion intake air. Ask the manufacturer what will happen.

[Tekkie®]

Tony Hwang posted for all of us...

That is ALL you are capable of; the subject does not matter. Go guess back in a.h.r you might find a sucker there.

[M Q]

...

Of course, unless you bought the extended warranty, the heat exchanger is likely to corrode through after your warranty expires.

[trader4]

I would also think that any concer over the return air temp being below

55 is likely predicated on it being under that temp for very long periods do to some unusual furnace application not seen in residential environments. It seems difficult to believe occassional operation like that for say a vacation home, where it will only run below 55 for short periods, followed by heating fully to normal temps, is going to cause problems.

[Tom The Great]

Now are you reading that right? Is that a statement of general operation, or an actual requirement?

tom @

[M Q]

...

In a vacation home, you are likely to be operating it below 55 for extended periods: as low as possible to keep the costs down, but above 32 to keep the pipes from freezing.

[Redcrosse]

It is my understanding that if the return air temperature is too low the heat exchanger can be "shocked"--that is to say it may expand and contract beyond design limits and fail.

[AKS]

Any metal will expand and contract with change of temperature but I do not believe that this problem occurs in low temp. residential furnaces however anything is possible Dido

[Edward R. Voytovich]

I am told by experts in the field that one of several ways that high-efficiency furnaces squeeze more BTU's from their fuels is by using thinner and thinner heat exchangers that naturally have tighter tolerances for expansion and contraction. Old coal fired furnaces had cast iron heat exchangers that could and did last for a very long time. Most "cracked heat exchangers" in those appliances seem to have been a salesman's way of getting a customer to buy . . . not an actual crack. Modern furnaces with crimped stainless steel heat exchangers are often projected to have a life of 15 years +/-.

AKS wrote:

[mm]

This seems like one of the false efficiencies, for the most part. Although it would take a few seconds, even a minute maybe longer to heat a thick heat exchanger wall, once it was heated all the way through, it would be just as efficient as a thin one.

Then at the end of the cycle, there would be more heat left over, which would disperse, some warming the circulating air which would continue to be circulated by the fan (until the low-limit thermostat switched the fan off) and the rest would eventually heat the basement a little bit, or wherever the furnace was. In the case of my basement, I need a bit of heat there in the winter, and there is a heating duct, and the furnace radiates is a small amount but probably needed for my comfort.

If the furnace were in the garage, well one normally goes to a garage even less than a basement, but doesn;t the whole furnace radiate heat, not just the rather small amount in even a thick heat exchanger wall.

Oy.

[trader4]

That simply isn't true. The heat is going to transfer more effectively across the thinner material. The thickness of the metal provides a resistance to heat flow, just as thicker insulation, wood, or anything else would.

[nicksanspam]

But metals are such good conductors that making the metal thinner won't help much, given high resistance air layers on both sides, and thicker metal will spread out hot spots and increase efficiency.

Nick

[trader4]

Wrong. Making the metal thinner does have a direct and significant impact on the heat transfer. Here's two references for you:

Theoretical, which from experience is the only type of source you recognize:

is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. If one end of a metal rod is at a higher temperature, then energy will be transferred down the rod toward the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones. For heat transfer between two plane surfaces, such as heat loss through the wall of a house, the rate of conduction heat transfer is:

Calculation

Q/t = kA(Thot-Tcold)/d

Q = heat transferred in time = t k = thermal conductivity of the barrier A = area T = temperature d = thickness of barrier

Clearly from the above, the conducted heat transfer is proportional to the thickness of the heat exchanger.

And sec "Plate thickness ranges from .024" for high efficiency to a heavy-duty and durable .050" thick plate"

Cearly they agree cutting the thickness in half makes a significant difference in efficiency.

[daytona°]