Solar Thermal Hot Water and Cold Air Return Heat Exchanger

2 Posts
Mar 30, 2010 02:10 pm
Solar Thermal Hot Water and Cold Air Return Heat Exchanger

I have been wondering about a system that is talked about briefly, but hardly explained in much details in terms of pros/cons. 

I was thinking about running the hot water from my solar hot water system to a water to air heat exchanger located in the cold air return duct of my main furnace.  Anybody have any insight into this?  The idea would be to heat the air going into the furnace to offload it's work.  I guess something could be said also about using a solar air heating system to do the same thing, but directly sending the air to the cold air return.

In terms of the hot water system, I can foresee some questions. 

How much heat would I loose in the run to the ducts? 
What type of efficiency would I gain?
What is the sizing I would need on the hot water to run it in the winter?

and lastly
Would I need to isolate the duct with a separate blower to make it so the furnace intake isn't starved for air (the heat exchanger would slow down the air flow some).

Any help would be great.
9 Posts
Mar 30, 2010 04:41 pm
Re: Solar Thermal Hot Water and Cold Air Return Heat Exchanger

Hi Alan,
     I have hade some experience with that type of hydronic system, You are right in your consideration of slowing the air down.  The return air on most furnaces are sized about 10x25 and putting a coil in that small space would slow things down a bit.  Secondly the filtration system is usually right where the R/A drop hooks up to the furnace so if you put the coil upstream from that it will clog up in short order.  The best approach is to put a hydronic coil above the A coil in the plenum and set it up on a 2 stage temp. controller so the solar and furnace do not run at the same time.  you would also require zone valves to direct the hot water from the panels to the water heater and to the heating coil on a priorty basis, with hot water first then space heating after that has reached temperature.  first determine how long it takes to make the hot water, if it takes most of the day you are not going to benifit from the expense to do all of this unless you add the appropriate no. of panels.  another consideration is the blower size.  There is a lot more I need to know in order to design a system, furn make, model, or gas, elec., your location and solar zone, etc. e.mail me at oltoby58 @ and we can chat about this.
Jim C.
47 Posts
Apr 1, 2010 02:12 pm
Re: Solar Thermal Hot Water and Cold Air Return Heat Exchanger

Solar thermal can provide up to half of your space heating if designed correctly.  I have a simpler set-up at my house, where I am using solar water heating with a fan coil heater.  A thermostat is connected to the second relay on my differential temperature controller (Heliodyne Delta-T Pro), and if the storage tank is hot enough, and the thermostat is calling for heat, the relay turns on the fan and pumps to blow hot air into my living room.  It works amazingly well!  I'm putting data together later today showing the results. 

A bit more design work is needed to integrate it into an existing heating system like yours, be it forced hot air or radiant floors.  You need to get the timing right to have the original heat source turn off when you are sourcing from the solar, to make sure you don't inadvertently heat the solar with your furnace.  You would set up a primary and secondary loop, where the solar would provide the heat until it runs out, then your backup heat would take over.

Sizing the solar system depends on a lot of variables, like how tight of an envelope you have in the house (is it well insulated, good windows, etc?)and where you are located.  My house is in New England, so we get long, cold winters.  The house is 760 sqft, one story, fairly well insulated with new windows.  I used four 4'x8' collectors, 128sqft, or about 20% of the footprint.  I used a 120 gallon storage tank (1:1 ratio sqft of collectors to gallons of water), but 2 would be better (unless it was being used for concrete radiant floors, in which case the floor can act as storage).  I haven't received my latest electric bill since turning it on last month to supplement my electric heat, but I suspect it is easily providing half my heat.  If you want to see details and pix of my system, you can check it out at
2 Posts
Apr 1, 2010 04:49 pm
Re: Solar Thermal Hot Water and Cold Air Return Heat Exchanger

Interesting to hear your results.  I had read about your system, though I didn't realize exactly what you were doing.  I have a 3200 sqft house with two furnaces.  We're in lower michigan so we have a pretty cold winter. 

it hards to know what to expect by integrating the solar hot water (contemplating solar air also) into the furnace.  But that's why I was asking to see what others were able to do and how well it worked. 

Complicating my installation is the long run from the water heater to the furnace.  I also have long duct runs in the house, so I'm not sure how hot the air needs to be coming out of the furnace yet (will measure my existing temp shortly) to replace or supplement the current function.

Both yours and the previous poster mentioned using a bypass.  I had initially thought of pre-heating the air going into the furnace to offload some of it's work.  I have a high efficiency furnace and so it should smartly reduce gas consumption to heat the air.  I am not that much of a heat wizard though.  I'm more of a electrical engineer so I'm a little worried about how best to set it up. 

I also have the option of using an HRV to reclaim heat from the inside.  They installed a Make-Up air control but it goes directly to the outside of the house.  So in the winter I get some cold air entering and in the summer I get some hot air entering.. which makes the systems work hotter.  I hear the HRV could help a bit by warming/cooler that air.


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