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Posts: 3
Reply with quote  #1 
I am new to this forum but first, before picking your brains, I would like to thank you all collectively as I have learned much from your sharing of information and experience.
I recently built a solar hot air system consisting of two 4x8 double screen collectors.  In each collector I installed in the upper corner  (probably out of the optimum airflow) a temperature sensor and a snap disc (110/90).  My ducting is 6inch  R6 flexible ducting with a total of about 25 feet for both output and return.  My blower is rated for 250 cfm free air.  I realize that this run of flexible ducting could cause a considerable pressure drop.
In any event I am seeing about a 25 to 30 degree temperature differential between the temperatures sensed in the upper corners of the collectors, and the temperature of air being delivered where the duct enters the house.  I have tried using the blower both pulling and pushing with essentially the same result.  I believe my construction and sealing to be relatively tight.  Being October outdoor temps  are relatively mild so I would expect losses between collection and distribution to be minimal.  Anyway, my question is:   Is a temperature differential of 25 to 30 degrees between the collector temp and the delivery temp to be expected.  I am interested in what differential temps others have experienced.  I would appreciate your thoughts on the variables possibly contributing to this differential.  I have thus far hypothesized the following:  1) Placement position of the temp sensor position in a somewhat dead air location.  2)  Insufficient airflow.  3)  Insufficient transfer of heat from the absorber to the airflow.    Your thoughts?   Also I have noticed my 110/90 snap discs do not click on until approximately 120 degrees but shut off at the expected 90 degrees.  (Before painting the snap discs flat black they weren't clicking on until 125 to 130 degrees.)  I am interested in others experience regarding snap discs, as I am considering swapping them out for some that close around 100 and open at 85.


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Posts: 2,321
Reply with quote  #2 
Hi Coaster,

Welcome! Were are you located and do you have a pics you would like to share?

You are asking the right questions. If monitoring temps you want both probes to be in or very near to the collector itself. The duct adapter that transitions to  your flex duct is a good location. With 25' duct runs, you will see some temp loss between the collector and when it enters the house. I had 7-8 degree temp loss with a 25' return duct last winter, but I didn't protect the ducts from the wind. With such long duct runs, So you may want to shield the ducts from the wind as this will help maintain the heat. Also, when using flex ducts, make sure the ducts are pulled tight to minimize the restriction. Since your fan is rated for 250CFM in free air, crank it up to full speed and see just how much of that 250CFM you are losing to static pressure. A few adjustments in the ducting can make a big difference. I used flex last year, but this year I'm pulling out the flex part and inserting solid ducts into the insulation. This will go a long way in easing the duct restriction.

Are you one fan for BOTH heaters? If so, that will put a strain on your fan and you may want to look for a 2nd fan in the meantime. 

As far as the temp rise, a big portion of the temp rise is directly related to the speed at which the air passes through your system. Higher flow is more efficient than low speeds. Do you know your flow rate when you were getting the 25-30 degree temp increase?

Last winter my snap switches were flipping on and off within 5 degrees or so of the rated temp. they were usually a little slow as any changes in temp have to transfer through the metal cover. I purchased adjustable snap switches for this winter. I plan to have my system turn on at 90 and off at 70. But I also like to have my fan speeds rather high compared to others. The air doesn't feel as warm as at lower speeds, but I'm running more efficiently as I extract more BTU's.

Greg in MN[wave]


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Posts: 98
Reply with quote  #3 
When I used a 400cfm free air fan for my 19ftx9ft collector the temperature difference from collector to the outlet in the living room at 30ft distance was also fairly high, I think 20 to 30 degree celsius.

I then installed a 740cfm fan and now they are almost equal.

I suspect it's not just the free air cfm that matters but more how the fan can handle duct resistance. The new fan is much more powerful than just double (400cfm to 740cfm)

I suspect your fan could also be stronger.

Posts: 3
Reply with quote  #4 
Thanks Greg,
I am located in Northwestern New Jersey  (approx. 6400 heating degree days).
I may not have been clear or complete in my description.  I am getting a temperature decrease of 25 to 30 degrees
from what I measure inside the top corners of my collectors compared to the temp of the air entering the house.
My output ducts on the collectors are located on the top back center of the collectors.  The ducting run is a total of 
25 feet, or approximately 12 or 13 feet out and 12 or 13 back.  Each duct runs out to between the collectors and then 
is split with a "T" to connect to each collectors output or return respectively.  After posting I added another 250 cfm 
fan, so that now one is pushing and one is pulling.  Airflow is improved (guesstimate 10 to 20%)  It is later in the day 
so the sun is hitting the collectors less optimally than earlier so it will have to wait to another day to see to what extent
the temp decrease from collector to house has changed.
Thanks for your advice and experience concerning ducting and snap switches.  After reading your reply I am planning
on swapping out my snap switches so my system can cut in and out at lower more efficient temperatures.
I will add pictures when I get a chance.
Thanks again,


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Posts: 1,845
Reply with quote  #5 
High coaster,
A 25 degree drop seems like a lot but if your sensor is in the corner, out of the flow it may be unrepresentative.  I always put mine near the adaptor like Greg says.  I am more concerned about the Ts in the input and output.  That would be bad enough on the input but I can't imagine what would happen if both outputs crash into each other at a tee then combine and go out the single leg of a T to the house.  Plus if your just using a T is a an overall 6" T or does it go from a 6" at the two outputs to an 8" or 10" duct to the house?  An 8 inch duct would have about 1/2 the resistance of a 6" duct and once they are combined that's a lot of heat and flow through a 6" pipe. A double six inch Y to a 8 inch adaptor would be a much better choice both ways.  Another option would be to do this for the inputs and run the existing 25 foot lines to each collector then you can feed them into different rooms or areas.
One 8" pipe is very similar in area to two 6" pipes though the single 8" will have less resistance so it may be better to do that on the output.

Move your sensor first and see how it reads before you change any ducting.  As Paul said a bigger fan may help also.

Posts: 720
Reply with quote  #6 
I had that much of a loss with my 1st hot air panel. I also had about the same length (12') of ductwork between the hot air exit on the panel & the air temp entering the house. I had a remote thermometer placed in the airflow, exiting the panel.

My ductwork was poliso foam, with a small section of flex between the panel & ductwork. My loss was 25-30% between the panel & house.

I used the panel (16'L x 5'T) for 2 years &  couldn't get a better ratio for the heat loss. I even insulated around the the foam ductwork & it didn't help. I don't remember the fan CFM rating, too many years ago.

Sorry I don't have a good solution.

Good luck

Central IL
Solar air & water

Posts: 3
Reply with quote  #7 
Many thanks to all who have contributed their experiences and advise in regards to this topic.

     Initially I thought my adding another fan  (now push/ pull 250 cfm free air each)  would reduce the temperature drop.  As the airflow appears to be significantly increased I know my efficiency has to have improved.  

     In very clear direct sun I have seen a 40 degree drop (145 degree on sensors in collectors and 105 degree delivery.  In that instance I also took measures off the glazing with an infrared thermometer and was getting measures of around 72 near the bottom and 94 near the top of the collectors.  I think what I might be seeing is that my sensors are showing more like an absorber temperature, while the infrared is giving me a glazing temperature.  At this point the ambient temperature was 50 degrees.
    As the day had thus far had about 50%  cloud cover I also witnessed that when the collectors are indicating a temperature of about 100, I am getting a delivery temp of about 90 degrees, for a temperature drop of only about 10 degrees.  

     In both cases my return air to the collectors was approximately 65 degrees.   In the first case the temp rise from the return air temp to the delivery air temp was about 40 degrees.  In the second instance it was 25 degrees.

     I have ordered adjustable snap discs to enable the blowers to cut in at a lower temperature.  Of interest,  neither of my snap discs (110/90)  were cutting in until at least 120 degrees.  I spray painted them black and at least one of them is cutting in at 116 degrees (not sure about the other as I have them wired in parallel so that cut in starts when at least one has hit the rise temp, and cut out isn't until both have hit the drop temperature.)

Posts: 46
Reply with quote  #8 
Before getting too discouraged with your duct loss you need to measure the air temp leaving the collector with the probe in the beginning of the exit duct and OUT of the sunlight. This will give you a true reading of the duct loss.


Posts: 7
Reply with quote  #9 
1) Your duct should be isolated at least R15. Your current isolation is there only to ease your conscience.
2) Y are better than T in ductwork
3) your main duct should be 8''. 6'' would be good for 2x4'', not 2x6''
4) smooth ducts are better than flexible.
5) check yor cfm under static pressure from solar heater and duct- just put a bag on outlet and count when it fills

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Posts: 2,321
Reply with quote  #10 

Welcome! Good advise, but you should note that you were responding to a post that is nearly 4 years old. You can check when the member last visited the site by hovering your cursor over their name.

Greg in MN
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