6 local shops can not answer this!!!

Arborist Forum

Help Support Arborist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

bassman

ArboristSite Operative
Joined
Jul 17, 2005
Messages
388
Reaction score
23
Location
canada
I have a mid efficency furnace that is mainly backup but whatever...here is my question.

does fan speed reduce stack tempatures on a regular gas furnace and if so would you get more out of your furnace if you ran a higher fan speed??

I know that you take more heat out of the water when you run a faster fan speed on a hydronic system so does this effect a gas fired furnace as well????
6 local shops have no idea what I mean and they say no .

I also think that a gas furnace should have a very low stack temp as there is nothing in gas exaust that would plug an exchanger like cresote would so you should be exchanging till there is not heat left I would think??
 
I have a mid efficency furnace that is mainly backup but whatever...here is my question.

does fan speed reduce stack tempatures on a regular gas furnace and if so would you get more out of your furnace if you ran a higher fan speed??

I know that you take more heat out of the water when you run a faster fan speed on a hydronic system so does this effect a gas fired furnace as well????
6 local shops have no idea what I mean and they say no .

I also think that a gas furnace should have a very low stack temp as there is nothing in gas exaust that would plug an exchanger like cresote would so you should be exchanging till there is not heat left I would think??

Yes, the more air you are blowing over the exchanger the more heat you are extracting from it.

Since all of the air is (mostly) staying in your home you should be able to get more of the heat that would otherwise be going up the stack into your living area by moving more air across the exchanger.

There is a point of diminishing returns, and then there is the problem of condensation forming in your flue pipe and exchanger from water as a product of combustion causing rust.

The outlet temps will seem cooler, the house will stay as warm and the furnace will use a little less fuel from greater efficiency.
 
Last edited:
You say you have a mid-efficiency furnace, but I'm not sure what that means. Does it have the traditional metal flue or is it vented by PVC pipe. If by PVC pipe, then you're already getting almost all the heat out anyway. If it's a traditional metal flue, then Kswoodman's point about condensation forming in the flue comes into play. You could also get to the point of the flue being too cool to act as a proper chimney-in which case you could get flue gases back into the house.

Another thing to keep in mind is the faster you run your fan, the more electricity you're drawing and the more wear and tear on the fan motor. Those costs have to be considered against the amount of reclaimed heat.
 
The heat exchangers designed for low exhaust temperature will likely be an all stainless design that can deal with the corrosion of the products of combustion in any condensation. The ones that are designed to operate hotter will be made of a lot less corrosion resistant alloy (likely 409) so you would not want to drop the flue gas temperature too much. It is possible too that the fan limit switch settings may just make the fan cycles quicker especially if it is a two speed fan. The manufacturers alway want as much bragging rights about efficiency so I would be doubtfull if they left much on the table there.
 
If you have a standard (78% with draft hood) or mid efficiency (80%) furnace you can't move too much air through it or the flue gases will condense in the chimney or pipe. This condensation is very corrosive and will destroy a furnace in a year or two. This is why you don't see many furnaces with efficiencies between 80 and 92%, because the high efficiency furnaces have to cool the gases below their dew point, and are constructed of stainless steel and use PVC to vent the exhaust. On an old-school oil or gas furnace you can squeeze a few more BTU's out of it by speeding up the fan a bit, but you had better make sure the stack temp stays at least 300 F.
 
Yes, the more air you are blowing over the exchanger the more heat you are extracting from it.

Since all of the air is (mostly) staying in your home you should be able to get more of the heat that would otherwise be going up the stack into your living area by moving more air across the exchanger.

There is a point of diminishing returns, and then there is the problem of condensation forming in your flue pipe and exchanger from water as a product of combustion causing rust.

The outlet temps will seem cooler, the house will stay as warm and the furnace will use a little less fuel from greater efficiency.

well...wait a sec.....faster air doesn't always mean more hot air.
 
Yes, the more air you are blowing over the exchanger the more heat you are extracting from it.

Since all of the air is (mostly) staying in your home you should be able to get more of the heat that would otherwise be going up the stack into your living area by moving more air across the exchanger.

There is a point of diminishing returns, and then there is the problem of condensation forming in your flue pipe and exchanger from water as a product of combustion causing rust.

The outlet temps will seem cooler, the house will stay as warm and the furnace will use a little less fuel from greater efficiency.

well...wait a sec.....faster air doesn't always mean more hot air.

MGA , you ARE right. Because there is more air moving across the exchanger it won't be as hot. It will be more air, it will be pulling more of the heat from the exchanger and it wont be as hot coming from his vents but the air will be pulling more of the BTU's from his furnace and delivering them to his living space.

I have a PU with a nearly plugged heater core. When I turn the heater on High fan the air is much cooler than what I want it to be to warm my hands. It is drawing in outside air and tryng to heat. Any warm air ir produces gets pushed out of the leaking seals around the doors. The temp inside the truck doesnt seem to change much.

On L-Low the heat from it doesnt go far but barely warms the cab. On Low or Med fan the air coming from it warms the inside of the cab and is hot enough to warm my hands in freezing weather.

He is recurculating the warm air inside his house and reheating it. That PU heater is drawing in cold outside air heating it and pushing any warm air out through the cracks.

As long as he is recurculating the warm air from inside his home he should be getting more of the heat from the furnace to his living space instead of letting it escape to his flue pipe. It wont be hotter but it will be moving more BTU's in the same period of time.

Maybe he is figuring on a 100% increase in the amount of air across the exchanger I was thinking of 10-20% more.

I'd clean the ducts to eliminate the drag created by any dust that settles there during the time the blower isn't running, too.
 
Last edited:

Latest posts

Back
Top