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victordaniels

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Posts: 17
Reply with quote  #1 
Has anyone here channeled the hot air from their solar collector into the cold air return duct on their furnace? 

stmbtwle

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Reply with quote  #2 
I think that would work. It would add heat to the air, and the furnace blower might eliminate the need for a separate fan for the collector. You'd need a damper to shut off or adjust flow from the collector at night, though.

You would also need to feed the collector from the heated space, not outside.

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Willie, Tampa Bay
Bruce

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Reply with quote  #3 
I have thought about this a lot over the last couple of years.  I have a 3 foot wide gable roof running the length of my house above my basement garage doors that gets good sun. My forced air system return dead-ends right inside the band joist enclosed by the little hip roof.  The problem that I see is that when the sun is shining to heat the collector it would also be shining into the 11 windows on that side of the house, warming the house, resulting in the furnace system not running.  Your furnace (and the fan pulling air through the returns and the collector) would run more when the sun is not shining and pull cooler air from the collector. Worse, when the sun is shining and your furnace fan is not running, your collector may stagnate and overheat.  Secondary fans could be used to circulate the hot air into the house to prevent such overheating, but then why bother with the furnace return duct system?

Unless you have a large mass to store (and stabilize the temp) of the solar heat...
stmbtwle

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Reply with quote  #4 
I'll have to agree with Bruce. You'd have to be able to independently control the air to/from the collector. You might be able to do this if you have a "fan only" setting on the furnace, and a good damper on the collector. Automating this could be tricky. A separate system would probably be easier.

I've thought of doing this with a hot water collector, storage, and a heat exchanger in the HVAC duct. It would work at night when it's needed the most. However water collectors and insulated storage for a few hundred gallons of water are NOT easy or cheap. I decided against it.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
pianoman8020

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Posts: 34
Reply with quote  #5 
I would think you would loose half of your thermal production just for heating all those ducts and furnace in the basement which is typically cooler that main house interior.

That said, I have been looking at splicing in my 8" duct to the hot air side of the furnace near ducts leading directly to the main house area. My concerns are:
1) Difference in air pressure when the furnace and the collector are both on. Too much pressure will cause the collector to overheat and/or bulge the collector cover.
2) When both units are on the collector air may need to be dumped into the basement as a backup precaution. 
3) A better control system would be needed to be used. Maybe an Arduino control.
4) Damper, controls, and duct costs.

Jim from IL
victordaniels

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Posts: 17
Reply with quote  #6 
My idea was to blow the hot air from the collector into the cold air return. I see that when the furnace is on and the blower from the panels is off, the problem is that the furnace would suck cold air from the panels. I see the need for a valve control for this reason.

What about blowing the air from the panels into the ducts after the furnace blower. When the furnace is off, the air from the panels would go through the ducts. When the furnace is on and the blower from the panels is on they would both blow through the ducts into the house. When the furnace is on and the blower from the panels is off, there would just need to be a backflow valve to keep the furnace from blowing into the panels.
stmbtwle

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Reply with quote  #7 
The fan from the collector would have to be powerful enough to overcome the backpressure from the furnace. You'd probably need a second backflow valve in the furnace duct before the solar input.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
GOM

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Posts: 72
Reply with quote  #8 
A couple of houses ago (I moved a lot while in the Army), I grafted an outboard switch to the furnace's internal fan.  I didn't have access to a thermostat at the time so I would switch on only the furnace fan when the winter sun was shining brightly and circulate my solar heater's output throughout the house via the furnace fan.  Worked great.  Later, when I also began using the solar heater as a cooler at night, I would again switch on the furnace fan after nightfall and let it run on low speed until the next morning.  That also worked great and really helped with the heat/AC bills.  Just my $00.02.  Regards to all, GOM
victordaniels

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Posts: 17
Reply with quote  #9 
GOM,

How did it work as a cooler?
mclark999

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Posts: 183
Reply with quote  #10 
This is something I've thought about quite a bit too. All the good comments just gave me a new idea for my particular house. I have return air ducts on the main floor and in the basement. My solar furnace blows in hot air into one end of the basement, but only one room away from my cold air duct. I might be able to add an extension from the cold air duct into the basement room that gets most of the heat. 

Just want to add that I get a lot out of these discussions and everyone's open ideas.

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Denver, CO

Double screen hot air collector
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