fireplace insert heat exchanger?

[jasonblu]

Hi forum. First post. To the moderators, thank you in advance for having this site.

Onto my question.

I bought a clydesdale last year for my downstairs fireplace and noticed how much heat is lost into the fireplace cavity it sits in. I had an idea. I pulled the unit out as much as the play in the chimney would allow and set up a shopvac near it and rigged it to suck right above the ball looking collar and routed the exhaust to blow this hot air to the upstairs livingroom. It worked great in concept. I had nearly 200 degree air blowing out of the exhaust. it raised the temp in my livingroom by at least 5 degrees. naturally, not a permanent solution (would quickly burn out my motor not to mention ugly) but it proved a point.

I then set out to try and get this lost heat into my central ventilation system. The goal, to heat an upstairs bedroom on the opposite side of the house. This is where I am at a loss. I did another test. I rigged a 6" tube duct to a 600 cfm fan and tapped this into the central ventilation system to try and route hot air into the vent system. i dont get a single degree of heat coming out of even the hearest upstairs vent.

My next thought was to build an insulated box around the back of the unit to in effect capture all this heat and connect this to a vent that sits right above the fireplace. this is where the 600 cfm fan would be connected to create negative air pressure. Return air would be routed to blow right over that collar to balance the system. In effect, ambient air blows onto that colar, is heated as it swirls around in the insulated box, then is sucked up by the fan and distributed to the whole house via the central ducting system.

Before I got to all this expense, will it even work?

Any hvac engineers here? What physics am I up against?

Thank you for your time!

 

[fortydegnorth]

Not an expert, but I'd say by the time you get the hot air into duct work and then all the way to the room you are trying to heat you will lose most of the heat. You may have seen more heat from the shopvac due to the short run and also the shop vac may generate some of its own heat by the motor. If I run my shop vac for a long period it seems the exhaust air gets warm.

 

[jasonblu]

Not an expert, but I'd say by the time you get the hot air into duct work and then all the way to the room you are trying to heat you will lose most of the heat. You may have seen more heat from the shopvac due to the short run and also the shop vac may generate some of its own heat by the motor. If I run my shop vac for a long period it seems the exhaust air gets warm.

could very well be the case. As an added detail I do have all of the ducting in this immediate area insulated. I had the whole ceiling drywall ripped out for a different reason which afforded me the opportunity to do all this duct rerouting and insulating. Even that short distance, maybe 8 feet from the source, I got nothing by way of heat harvest when doing test no. 2. Granted, the room I am shooting for is nearly 80 feet away, and i wouldn't be able to insulate all this.
I am optimistic this nut can be cracked though, as my current configuration using heat pump/aux heat system blows 120 degree air out of all vents. Surely taping hot air off the backside of my insert that hits 400 to 600 degrees when fully stoked can compete with whatever heating elements are in my current heating system.

Thank you for your input though. I certainly want to hear the opinion of non-engineer types as well!

 

[jasonblu]

also wanted to point out. I thought of the shopvac motor issue and tested it out without the fire. The most I ever got out of it was maybe 80 degrees. Shooting my infrared gun at it, it was putting out consistent 200 degree temps, so at least 120 of it was harvested from the clydesdale.

BTW - I see you have one of these too. Are you happy with the fan? Mine seems underpowered...and i cant seem to keep my glass clean. The airwash system is a joke IMO. (and no, I dont use green wood.) but, not to hijack my own post...just need to chat with other 'dale owners sometime.

J

 

[boatboy63]

Just my opinion, but 600 cfm would be too much velocity to remove heat from the stove and put it into the air simply by blowing over it. Many wood furnaces work similarly, but the air is blown thru an outer cavity around the firebox which will pick up the heat. Sounds like your best choice would be to put a cold air return from a forced air furnace/heat pump near the stove and let it pull this heated air into it's blower. Don't expect a 30-40* differential from room air to air from forced air, but possibly a 10* differential. Your heat output would be similar to the output of a heat pump, given a hot stove and return nearby.

 

[fortydegnorth]

I'm happy with the Clydesdale but I would like a freestanding stove sometimes so I wouldn't need to rely on the blower. I have a freestanding steel stove in our shop and it cranks some serious heat even without a fan. I don't really think the fan is underpowered. I usually run it on medium unless I have some seriously good wood in that cranks the thermometer to 500 or so, then I crank the fan to take advantage of the heat and not hurt the stove. My fans can be a little noisy but its usually because the screws work a little loose over time. Once I tighten everything up its not too bad. The glass on mine will stay pretty clean except for the right lower corner area. I have had some funky wood here and there to blame for some of it but it still happens when I break out the good stuff. I just replaced my door gasket to make sure its all tightened up but I'm still getting haze in that corner. I don't worry about it too much and just clean it all up real nice if I'm having company over. I have a 1500 sq. ft. ranch thats pretty open and it heats it fairly well. I have brand new windows and doors but I haven't sealed the house up everywhere yet and I know I have some pretty big air leaks. I just bought 84 bales of blow in insulation for the attic that will take me over r-40 but it's still in the garage. I think once I get everything buttoned up tight the insert should keep my house around 70 easily. Right now it's usually 67-68 on average and I don't push it. I get long burns if I let it cool to 300 stove top or so and reload on a nice bed of coals, probably 7-9 hours depending on temps and wood. Also my Clydesdale is the older model, 2006 I believe, and I know the 2009 model and newer had some refinements.

 

[Renovation]

Hi SC, and welcome!

I love innovation myself, so I appreciate your ideas. I'll give you the benefit of my theoretical knowledge, and maybe someone who actually knows something will chime in.

I then set out to try and get this lost heat into my central ventilation system.
i dont get a single degree of heat coming out of even the hearest upstairs vent.

What's the insulation like on your ducting? Does it run through outside walls or uninsulated crawl spaces? I like the idea of using the furnace fan to move air around, but if ductwork is cold, it'll lose heat.

My next thought was to build an insulated box around the back of the unit to in effect capture all this heat and connect this to a vent that sits right above the fireplace. this is where the 600 cfm fan would be connected to create negative air pressure. Return air would be routed to blow right over that collar to balance the system. In effect, ambient air blows onto that colar, is heated as it swirls around in the insulated box, then is sucked up by the fan and distributed to the whole house via the central ducting system.

This sounds chancy and controversial. First off, it is illegal to plumb a wood stove into central heating, or have a return duct within 10 feet. One reason is that it is dangerous to create even the possibility of negative pressure at your stove--it could backdraft and leak fumes... you can imagine the rest.

But I believe positive pressure is a fine thing. It is counter-intuitive, but folks here have reported excellent results from sucking cold air from where the heat is desired, and blowing it on the stove--the heated air then migrates to where you want it. So I suggest some sort of system--part of your HVAC or separate--that blows air near your stove or in the cavity around it, and sucks where you want the heat to be. An insulated box around your stove would help keep losses down, as long as clearances to combustibles are maintained.

There's my suggestion, for comments from you and all.

HTH, and good luck!

 

[jasonblu]

I'm happy with the Clydesdale but I would like a freestanding stove sometimes so I wouldn't need to rely on the blower. I have a freestanding steel stove in our shop and it cranks some serious heat even without a fan. I don't really think the fan is underpowered. I usually run it on medium unless I have some seriously good wood in that cranks the thermometer to 500 or so, then I crank the fan to take advantage of the heat and not hurt the stove. My fans can be a little noisy but its usually because the screws work a little loose over time. Once I tighten everything up its not too bad. The glass on mine will stay pretty clean except for the right lower corner area. I have had some funky wood here and there to blame for some of it but it still happens when I break out the good stuff. I just replaced my door gasket to make sure its all tightened up but I'm still getting haze in that corner. I don't worry about it too much and just clean it all up real nice if I'm having company over. I have a 1500 sq. ft. ranch thats pretty open and it heats it fairly well. I have brand new windows and doors but I haven't sealed the house up everywhere yet and I know I have some pretty big air leaks. I just bought 84 bales of blow in insulation for the attic that will take me over r-40 but it's still in the garage. I think once I get everything buttoned up tight the insert should keep my house around 70 easily. Right now it's usually 67-68 on average and I don't push it. I get long burns if I let it cool to 300 stove top or so and reload on a nice bed of coals, probably 7-9 hours depending on temps and wood. Also my Clydesdale is the older model, 2006 I believe, and I know the 2009 model and newer had some refinements.

Thank you very much. Its great to hear from other users of this stove which I WANT to love. Maybe I am just too much a greenhorn and dont know how bad other lesser systems could get. I splurged big time on this so I suppose I expect the world out of it. Thanks again for sharing your experience!

 

[jasonblu]

Hi SC, and welcome!

I love innovation myself, so I appreciate your ideas. I'll give you the benefit of my theoretical knowledge, and maybe someone who actually knows something will chime in.

What's the insulation like on your ducting? Does it run through outside walls or uninsulated crawl spaces? I like the idea of using the furnace fan to move air around, but if ductwork is cold, it'll lose heat.

in the den where the fireplace is, the ductwork in this area, maybe 20 linear feet or so2 is just r9 or maybe r11 foil backed tube insulation fitted around the 6" ducting. the rest of the length that goes to the far upstairs bedroom is another 40' or so of just strait ductwork, with no insulation.

Great ideas. You have really encouraged me and caused a eureka moment there. Never thought of sucking cold air from our farthest room and blowing it on the stove, but it makes perfect sense. The house is, in fact, a closed loop, just like it wouldn't really matter where the fan was in a circle of duct work.

I cant help wanting to tinker and engineer through this problem. Its the Navy tech in me I suppose. Thanks for not just trying to shut this down as a fools errand (though if it is, I need someone to talk me down! but they better have some math ready! )

 

[Renovation]

Great ideas. You have really encouraged me and caused a eureka moment there. Never thought of sucking cold air from our farthest room and blowing it on the stove, but it makes perfect sense. The house is, in fact, a closed loop, just like it wouldn't really matter where the fan was in a circle of duct work.

You're very welcome, though I can't take credit for the idea--I learned it here. Folks have reported great results with moving *cold* air, whether with fans or ducting.

I cant help wanting to tinker and engineer through this problem. Its the Navy tech in me I suppose. Thanks for not just trying to shut this down as a fools errand (though if it is, I need someone to talk me down! but they better have some math ready! )

Well, I have to help a kindred soul. I'm the same way, and folks here have been kind enough to tolerate me and give me a head-up when I'm considering something particularly ill-advised. As long as you avoid safety issues, and have an idea what's involved, I don't find creativity a fool's errand, but a reward in itself, with the added benefit of possibly being useful.

You can give back by reporting back how it works out. Please feel free to keep asking questions.

HTH, and good luck!