Reversing Furnace Fan

[GordonShumway]

I am new to this site and have tried finding an answer to a particular question. I am in the process of building an hearth for a wood stove. Our house is a large 2 story farmhouse with a basement. We are building a pad to put the wood stove in the center of the house on first floor and a fabricated double wall stainless steel chimney going through middle of house also. I have 2 questions that I am curious to know how things might work in theory.
1. Air return for furnace is located on floor about 8 ft from hearth pad and each room has registers installed on floor. My question would be, is it possible to reverse the furnace fan to pull the cold air from each room and have blow out the air return next to stove in hopes to circulate heat through house?
2. I am assuming this probably isn't safe due to carbon monoxide but is it possible to build ductwork around the chimney pipe and install a fan on second floor of ductwork to push the air inside the ductwork back down into basement? In hopes of trying to capture the heat from the chimney and use to warm basement and in return assist in heating the rest of the house.

These 2 question are mainly theories of ways to maybe distribute heat and I would like to know what folks would think of the ability for it to work? thanks

 

[begreen]

no and no.

 

[btuser]

You don't want to cool down the chimney. Forget about that idea.

Depending on your air handler it shouldn't be a problem.

 

[GordonShumway]

Ok thanks. I'm new to using a wood stove, but that makes sense not wanting to cool down the chimney.

 

[LLigetfa]

AFAIK code does not permit a return air vent within 10 feet of the stove. The chimney will give off as much heat as it should simply through convection. I put static vents both top and bottom on the chimney chase running through the second storey to cool the chase and reclaim some of the heat.

 

[Renovation]

no and no.

lol, there you go. Often a man of few words.

To offer a few more, first, I welcome you and admire your creativity and willingness to suggest an idea that will probably be shot down. I hope you'll continue doing it, so I'll have some company.

That said, your first idea is interesting, but probably inefficient. I'm in the process of replacing the ductwork in my house, and the requirements for return ducting, which suck air, is very different than supply ducts, which blow air. In brief, they have to be tuned for air direction, and supplies are much more sensitive to leaks, bends, constrictions etc. Returns are often much less streamlined, insulated, sealed, etc. Add to that the fact that supply ducts are installed so that the crimps are away from the furnace (the *opposite* of chimneys), so that air flows smoothly.

Reverse things, and you are totally throwing off the tuning of the system, and almost certainly destroying circulation efficiency. A better approach would be to tune your supplies and returns to work with your stove (hint #1: add lots more of both--that's the way heat-pump systems are ducted, for the same reason), and install one of the new high-efficiency DC furnace fans so you can circulate the air without a big electrical bill. That's what I'm doing.

Regarding your second idea, it is just really inefficient and dangerous. Modern stoves are designed to radiate heat from the stovebox, not the stovepipe or chimney, and need the chimney insulated as much as possible for a good draft and good stove performance. Sucking heat out of the chimney would produce minimal heat, and mess up draft. Your much better off getting an EPA stove, and working with the heat coming off the firebox--you'll burn less wood, and be warmer.

HTH, and thanks for the ideas and courage!

 

[LLigetfa]

Heating system duct designers often place returns where cold air would naturally pool to work with the natural air flow. This of course, doesn't work well for a combination central air and heat where the returns need to pull hot air down for cooling. Duct systems designed for both heating and cooling will often place some "high returns" where hot air tends to collect. The outlet registers however are almost always placed low to the floor exept in the case of a separate dedicated A/C system.

As for getting some heat down to the basement, consider drawing cold air up from the basement floor and blowing it up to where the stove is.

 

[certified106]

A squirrel cage blower will not work in reverse as they are only designed to push air in one direction.

 

[fdegree]

A squirrel cage blower will not work in reverse as they are only designed to push air in one direction.

This is 100% true, and to elaborate on this...

If you reverse the rotation of a squirrel cage fan, it will still move air in the same direction it did when it was rotating in the proper direction...although, the air volume will drop significantly.

On the other hand, if the rotation of a propeller type fan is reversed, the direction of it's air flow will reverse...just like a ceiling fan in your home.

 

[oldspark]

A squirrel cage blower will not work in reverse as they are only designed to push air in one direction.

This is 100% true, and to elaborate on this...

If you reverse the rotation of a squirrel cage fan, it will still move air in the same direction it did when it was rotating in the proper direction...although, the air volume will drop significantly.

On the other hand, if the rotation of a propeller type fan is reversed, the direction of it's air flow will reverse...just like a ceiling fan in your home.

I was hoping some one would point that out, been there done that on my old job.

 

[yooperdave]

this is why you should be careful of air movement in the duct system while the blower is shut down...if there is enough air movement to get the squirrel cage to rotate reversly, the motor may not be able to overcome it when it starts and would energize in a reverse rotation...still moving air...but not as well

 

[oldspark]

this is why you should be careful of air movement in the duct system while the blower is shut down...if there is enough air movement to get the squirrel cage to rotate reversly, the motor may not be able to overcome it when it starts and would energize in a reverse rotation...still moving air...but not as well

Never seen that happen, seen all sorts of fans running backwords do to wind and then power is put to the motor, draws a ton of amps for a short period of time and then its fine.

 

[Lumber-Jack]

A squirrel cage blower will not work in reverse as they are only designed to push air in one direction.

This is 100% true, and to elaborate on this...

If you reverse the rotation of a squirrel cage fan, it will still move air in the same direction it did when it was rotating in the proper direction...although, the air volume will drop significantly.

On the other hand, if the rotation of a propeller type fan is reversed, the direction of it's air flow will reverse...just like a ceiling fan in your home.

And don't forget about the furnace air filter, if you could get the furnace fan to blow backwards you'd be blowing all the trapped dust back out of the filter into the house, and you'd effectively have no air filter protecting your furnace component anymore.

I run my furnace fan (in the right direction) on low continuously to help distribute the heat through the house. The returns are not in the same room as my wood stove and they are located near the ground, but I find that by running the fan continuously it mixes and moderates the temperature throughout the house. The vents blow cool air, but that's ok because the 3 furnace vent in the stove room come down from the ceiling where the hot air is and mixes with it, the resultant air that leaves that room and flows into the rest of the house is a little warmer.

My furnace duct all run inside the insulated envelope of the house. I understand that in the US builders sometimes run ducting outside to save money??? If that's the case in your house this wouldn't work.

 

[Renovation]

And don't forget about the furnace air filter, if you could get the furnace fan to blow backwards you'd be blowing all the trapped dust back out of the filter into the house, and you'd effectively have no air filter protecting your furnace component anymore.

Excellent point.

I run my furnace fan (in the right direction) on low continuously to help distribute the heat through the house.

I think this is a great way to go, and makes getting an efficient DC fan worthwhile. A better way to go than reversing the existing fan.

My furnace duct all run inside the insulated envelope of the house. I understand that in the US builders sometimes run ducting outside to save money??? If that's the case in your house this wouldn't work.

Outside the insulated envelope, yes--either in the crawlspace or the attic. In that case I recommend insulating the ducts. That's a win all around: it makes recirculating your stove heat worthwhile, and will save you energy whenever you heat or cool.

 

[fdegree]

this is why you should be careful of air movement in the duct system while the blower is shut down...if there is enough air movement to get the squirrel cage to rotate reversly, the motor may not be able to overcome it when it starts and would energize in a reverse rotation...still moving air...but not as well

Never seen that happen, seen all sorts of fans running backwords do to wind and then power is put to the motor, draws a ton of amps for a short period of time and then its fine.

I have seen this happen too, but very rarely. It's been a long time since I learned about the different types of motors, so my next statement could be wrong, but...I think this can only happen with shaded pole motors. Someone please correct me if I'm wrong.

 

[certified106]

this is why you should be careful of air movement in the duct system while the blower is shut down...if there is enough air movement to get the squirrel cage to rotate reversly, the motor may not be able to overcome it when it starts and would energize in a reverse rotation...still moving air...but not as well

Never seen that happen, seen all sorts of fans running backwords do to wind and then power is put to the motor, draws a ton of amps for a short period of time and then its fine.

I have seen this happen too, but very rarely. It's been a long time since I learned about the different types of motors, so my next statement could be wrong, but...I think this can only happen with shaded pole motors. Someone please correct me if I'm wrong.

it can also happen on 220V Scroll compressors in your Heat Pump/AC unit if they are bleeding off while trying to restart/short cycling. It's an odd thing for sure.

 

[LLigetfa]

I've seen an air compressor with a bad centrifugal start winder switch unable to get over the hump on the compression stroke, bounce back, and then run backwards.

Anyway, to drag this thread back on topic, reversing the airflow in a duct system makes no sense.

 

[burntime]

Reversing Furnace Fan- boy that would suck!

Sorry late to the party but had to add that!

 

[yooperdave]

this is why you should be careful of air movement in the duct system while the blower is shut down...if there is enough air movement to get the squirrel cage to rotate reversly, the motor may not be able to overcome it when it starts and would energize in a reverse rotation...still moving air...but not as well

Never seen that happen, seen all sorts of fans running backwords do to wind and then power is put to the motor, draws a ton of amps for a short period of time and then its fine.

I have seen this happen too, but very rarely. It's been a long time since I learned about the different types of motors, so my next statement could be wrong, but...I think this can only happen with shaded pole motors. Someone please correct me if I'm wrong.

it can also happen on 220V Scroll compressors in your Heat Pump/AC unit if they are bleeding off while trying to restart/short cycling. It's an odd thing for sure.

i can't remember what type of motors and apologize to you sparky. i looked through all my info and can't remember where in my notes it is. however, i did find that when furnaces are twined and one motor starts slightly before the other, or the other squirrel cage is rotating reversely, it will stay energized in that rotation. i did find this on another site, however....
crab master
Controls Committee Join Date: Mar 2005
Location: Pacific Time Zone
Posts: 2,530

Both motors should come on at the same time with a twinning kit. Backdraft dampers are also nice because I've seen a few where a 'natural' draft causes a motor to spin backwards. Once a single phase motor is spinning backwards and power is applied it will stay running backwards, and therefore is likely your excessive vibration (one motor running backwards) that you are noticing. When it is running backwards the motor cannot cool itself as it is not moving air across it as it was designed and will overheat and eventually burn itself up, either electrically or mechanically (due to the bearings getting too hot).
Make sure the twinning kit is wired correctly, see if you can get some backdraft dampers installed and if the twinning kit is wired correctly you may have a faulty twinning kit.

I agree with checking the capacitors and amperages, if you have a motor running backwards the amps on that motor will be noticeably less than the one running correctly.

Also when you're replacing the motor are you also replacing the capacitor? I always do as a new capacitor is cheap insurance.
__________________
"How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." Freddy-B a.k.a Mongo/f.k.a IndyBlue
BIG Government = More Dependents
http://threedevilskennel.com/ or http://drahthaardog.com/ - not my websites.
Versatile Hunting Dog Federation - http://vhdf.org/

HVAC-TALK Site Rules Controls Forum Rules

 

[LLigetfa]

Wazzup with all the double posts? Must be the elevator button theory.

 

[burntime]

That is weird?

 

[begreen]

The site is in record territory. Over 20,000 folks logging on per day is slowing down the servers. People are getting impatient and hitting the Submit Post button more than once. Thus lots of double posts this week. Makes a lot more work for the mods, so please be patient and let your post get digested byte by byte.