Recommended temps for snap disc thermostat?

[skizman]

Is there a recommended on/off temp for a snap disc thermostat for a solar air heater box? The one I have in there now is a 80/95 temps. When the sun hit's the box (4'x6') the fan will run for hours. I thought with a lower temp switch, the box wouldn't bake inside all day and would stay cooler. Now I'm wondering if I should go up in temps, like a 95/110 temp switch. Thoughts???
Thanks!!!!!!!!

[Mike90250]

I'd see what a site called buildItSolar has to say, lots o DIY thermal stuff there

[LucMan]

Maybe you need to move more air ?

[skizman]

There's plenty of air moving. The fan is 120 cfm and actually bulges out the clear panels on the front.

[J.P.M.]

There's plenty of air moving. The fan is 120 cfm and actually bulges out the clear panels on the front.

Unless you've got some way to measure it, that 120 CFM is probably the unrestricted (rated) blower flow rate. If the actual flow through the box is 120 CFM (~ 9.1 lbm/min.), and the net solar gain is, say 150 *24/60 = 60 BTU/min., the temp. rise of the air through the box will be ~ 25 - 30 deg. F. Much higher delta T. than that might be an indication that the flow rate is less than 120 CFM.

The fact that the glazing is bulging would tell me that there's a fair amount of pressure drop downstream of the collector. That would result in a lower flow rate. Is the outlet throttled ? Any restrictions in the line ? What are the line sizes ?

My guess is you've got small line sizes or other pressure drop inducing restrictions that are increasing pressure drop and thus decreasing the flow rate.

[Wy_White_Wolf]

80/95 Would be plenty high for air heating a home. Is the snap switch mounted closer to the panels inlet or outlet? Also what is the temperature differential between the inlet and outlet?

If the fan is bulging the glazing you have too much air flow or too restrictive of a panel/outlet.

WWW

[J.P.M.]

80/95 Would be plenty high for air heating a home. Is the snap switch mounted closer to the panels inlet or outlet? Also what is the temperature differential between the inlet and outlet?

If the fan is bulging the glazing you have too much air flow or too restrictive of a panel/outlet.

WWW

With a 120 CFM blower and bulging glazing, my money would be on pressure drop inducing flow restrictions downstream reducing the flow rate --->>> increasing the temp. differential inlet to outlet.

If the OP is still with us: To reiterate WWW's question, what's the temp. differential under a sunny sky, inlet to outlet ? Also, what's the temp. diff., outside air to collector air inlet temp. ?

[skizman]

Unless you've got some way to measure it, that 120 CFM is probably the unrestricted (rated) blower flow rate. If the actual flow through the box is 120 CFM (~ 9.1 lbm/min.), and the net solar gain is, say 150 *24/60 = 60 BTU/min., the temp. rise of the air through the box will be ~ 25 - 30 deg. F. Much higher delta T. than that might be an indication that the flow rate is less than 120 CFM.

The fact that the glazing is bulging would tell me that there's a fair amount of pressure drop downstream of the collector. That would result in a lower flow rate. Is the outlet throttled ? Any restrictions in the line ? What are the line sizes ?

My guess is you've got small line sizes or other pressure drop inducing restrictions that are increasing pressure drop and thus decreasing the flow rate.

Outlet isn't throttled. Both ducts are round 4". The hot exhaust outlet is 6' long with two elbows. The suction is 12' with three elbows because it comes from the bottom of the box...up to top and then goes right over to the window along side the exhaust . Had to keep the box 6' from the window where it gets sun cause of trees.
Thanks!

[skizman]

80/95 Would be plenty high for air heating a home. Is the snap switch mounted closer to the panels inlet or outlet? Also what is the temperature differential between the inlet and outlet?

If the fan is bulging the glazing you have too much air flow or too restrictive of a panel/outlet.

WWW

Snap switch is in the hot outlet couple inches from the back of the box. I know that without the fan running, the inside of the box will get up to 160 degrees. When it's running, the exhaust stays about 95 degrees but that's when the snap switch cuts on so it never gets warmer than that. Cold weather coming over the weekend so I'll know more about the temp differences.
Thanks!!!!

[skizman]

With a 120 CFM blower and bugging glazing, my money would be on pressure drop inducing flow restrictions downstream reducing the flow rate --->>> increasing the temp. differential inlet to outlet.

If the OP is still with us: To reiterate WWW's question, what's the temp. differential under a sunny sky, inlet to outlet ? Also, what's the temp. diff., outside air to collector air inlet temp. ?

I'll know more about the temp differences when the cold snap comes in over the weekend. thanks.

[J.P.M.]

Snap switch is in the hot outlet couple inches from the back of the box. I know that without the fan running, the inside of the box will get up to 160 degrees. When it's running, the exhaust stays about 95 degrees but that's when the snap switch cuts on so it never gets warmer than that. Cold weather coming over the weekend so I'll know more about the temp differences.
Thanks!!!!

What color is the switch and is it exposed to sunlight ?

A long time ago, I made a sunshine switch for a thermal shutter by painting a switch flat black and putting it in a test tube and then allowing the test tube to be exposed to the sky. When the sun shone, the switch heated up and activated. Sun went away, the process reversed. Maybe your switch is behaving in a somewhat similar way. Just a thought.

[skizman]

What color is the switch and is it exposed to sunlight ?

A long time ago, I made a sunshine switch for a thermal shutter by painting a switch flat black and putting it in a test tube and then allowing the test tube to be exposed to the sky. When the sun shone, the switch heated up and activated. Sun went away, the process reversed. Maybe your switch is behaving in a somewhat similar way. Just a thought.

I drilled a half inch hole in the duct, attached the switch with screws and then it's covered by almost two inches of insulation. Your switch in the sun sounds interesting. Thanks!!!!!

[nearboston]

Have you thought about changing to 2 fans with lower cfm and a second snap switch with a lower activation temperature??

One fan turns on @ say..75 deg, and the second at 90.