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gregkn73

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Reply with quote  #1 
Hi,

I couldn't find a guideline for sizing fans for different systems. I am particularly interested in sizing fans for solar hot air collector and low thermal mass sunspace. Thanks in advance Greg.

gbwillson

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Reply with quote  #2 
Greg-

As you mentioned, different systems have different air requirements, but 3CFM output per square foot of collector space is the rule of thumb often used. In my opinion, that is far too low. Most collectors run far more efficiently at higher CFM's. Higher CFM's capture more of the BTU's collected. If the CFM's are too low, heat is lost through the cold glazing, which is the biggest point of heat loss. If the CFM's are too high, the output air might feel like a lukewarm or cool breeze. But this is where the collector is capturing the maximum amount of the BTU's. 

Sure, warm or hot air coming out of the exhaust feels really nice, but it IS less efficient. Any output temp over the ambient temperature in the room is adding BTU's. But if the exhaust is not blowing an undesirable breeze, I would suggest the highest CFM's you are comfortable with. 

My collectors output 400+ CFM minimum, but my exhaust is in the basement, so no coolish breeze is felt on the main floor. I also suggest a way to control the fan output so you can adjust the output temp to your preferred comfort/efficiency level. 


Greg in MN

 
gregkn73

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Reply with quote  #3 
Hi Greg, thanks for your quick and with many details answer. Solar hot air collectors and ltm sunspaces have the same rule of thumb, for fan sizing?. The first ones achieve increased T but low volume of air while the second ones lower T but a lot bigger volume of air. So I thought sunspaces require higher fan capacities. Particularly I have the proper space and location to built a 75f2 collector or and - depending on wife's approval - 120f2 sunspaces. What will be your choice of fan in either case? The plan is the fans outlets to be placed at the stairs from basement to the main house, so no problem from breeze either.
gbwillson

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Reply with quote  #4 
Having the ability to adjust the airflow with a fan speed control would certainly be an advantage. So go with the bigger fan. All of my fans so far have been scrounged from garage sales, thrift shops, alley trash, and the Habitat ReStore. So you don't always have the perfect fan.

Living in Minnesota, I would LOVE to have a sunspace. I thought of enclosing my deck, but it faces East. Assuming your desired sunspace would be on the South side of the house, couldn't you have both? Mount on air heater on the South wall of the house and enclose the area of the sunspace. Plenty of sun would still get to the wall mounted heater. It might even work a bit better since it would be protected from the winter winds. Just a thought...

Greg in MN
gregkn73

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Reply with quote  #5 
Hi Greg,

Thanks again for suggestion, very interesting.

I have found also this http://simplysolar.supporttopics.com/post/airflow-targets-and-measuring-in-a-hot-air-collector-6086773?highlight=measuring+airflow&pid=1275723842&full_version=1
where Scott make the same suggestion as you, as far fan sizing.

I have taken wife's approval for sunspace . the orientation is southwest, which is not optimal, but for my relative mild climate in nnortheast Greece, I think, it is not such an issue. Do you or anybody elseelse think, in sunspaces where T areare not as high as in hot air collectors, Oversizing the fan is a good solutionsolution as in the latter ones?
gbwillson

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Reply with quote  #6 
I kinda figured the sunspace/wife would win out. Build the sunspace, but perhaps consider adding a collector at some point. As far as oversizing the fan, I think if you can find one cheap or free, regardless of size. You can always switch to a different size fan later. But a larger fan will always have it's CFM output easily and cheaply reduced. Depending on your fan needs you might look into a solar powered fan, such as an attic fan. The fan speed would be self-adjusting depending on the intensity of the sun.

Here is a link to Dan's sunspace. It's well thought out and works well in a frigid climate, so it should give you some ideas to ponder.
http://simplysolar.supporttopics.com/post/solar-dans-sunspace-6481871?highlight=sunspace&trail=10

Greg in MN
gregkn73

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Reply with quote  #7 
Greg the truth is I try harder to persuade her for the sunspace, not only because I can make a little bigger, but also because I like it more than just a collector.
I haven't see any experimentation comparing sunspaces with solar hot air collectors, do you know any or by your experience which will be performing better?

How attic fan can handle the duct pressure? I was thinking as better solution handlimg the pressure,an inline ducting fan. Unfortunately in Greece rarely we use fans for heat or fresh air exchange, so I can't find easily cheap fans to try.
SolarInterested

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Reply with quote  #8 
Quote:
Originally Posted by gregkn73
... I haven't see any experimentation comparing sunspaces with solar hot air collectors, do you know any or by your experience which will be performing better?
Not sure if there's a comparison but the BuildItSolar site has a section on "Low Thermal Mass Sunspaces for Home Space Heating"

http://www.builditsolar.com/Projects/Sunspace/sunspaces.htm

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Both temperature rise and airflow are integral to comparing hot air collectors
gbwillson

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Reply with quote  #9 
I've never seen any comparison between a sunspace versus a collector. But you have to think the collector would perform better as far as BTU's per SF. After all, the collector is specifically designed to capture the sun's rays and transfer them to the air. Whereas the sunspace has to have many performance compromises such as being a certain size, or looking good on the side of your home. 

Inline fans that are designed as "booster fans" are normally not very effective at handling static pressure. They are designed to assist air that is already moving. There are inline fans that perform very well with higher levels of static pressure, such as this:
https://www.amazon.com/gp/product/B073S4CPT2/ref=crt_ewc_title_gw_1?ie=UTF8&psc=1&smid=A1OIIZJKTXNTOC

This above fan is a mixed-flow fan which means it has the advantages of an axial fan and a centrifugal fan. I only mentioned attic fans because many now seem to have solar panel to power them. But attic fans tend to move a lot of air and tend to be quite loud, which is not a problem in an attic, but would be in a sunroom.

So how large were you thinking of making your sun space?

Greg in MN
gregkn73

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Reply with quote  #10 
I have seen Gary's sunspace and how well it performed, following some strict guidelines, but it is not a side by side comparison with an hot air collector. It seems that if someone follow Gary's guidelines, it has the same efficiency as an air collector.

Greg the bigger I can make it is 3.5l*2w*3h, with a glazing area of 3.5m*3.2m
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