You covered up the sensor? What do you mean by this?

If you keep turning the sensor up does it eventually stay off?

Nathan - Thanks for your reply. My husband made a sheet metal umbrella over the sensor. And yes if I keep turning it up, it will shut off.

Then it could just be a bit inaccurate, they're often not actually calibrated. Just put an accurate thermometer next to it and turn it up slowly until the fan switches off. (or turn it down until the fan switches on)

I would shade the thermostat with a wood covering tha keeps all sun off of it. Metal will conduct heat and radiate it onto the thermostat. Make sure that there is at least one inch of air space around the thermostat on the sides and top. The bottom and front should be left open. If you do have to cover the front then cut some slots in the top of the cover to allow air to flow from the bottom through the top and out. Good luck.

A fan aspirated wooden box is the way to go. This one is made of MDF with a Plexi-glas door so I can read the gauges. It has holes on the left end and a 12VDC computer box fan on the right end to pull air through the box. The fan runs on a 12VDC "wall-wart" power supply located at the breaker panel. The advantage of this is that it can be mounted on the south wall in the "plant zone" for greater accuracy.

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Thanks for all the suggestions. I agree that a wood shade might work better then the sheet metal one.
We played around with it this weekend and found out it is 10 degreees off. So if I want the fan to come on when the greenhouse is 75 then I set the fan thermostat at 85. I had 3 thermometers set at different locations to get an accurate reading of the temperature. That is how we found it was 10 degrees off.

cottagefarmer - we must have been posting at the same time. I really like your set up. I will have to show it to my husband. I am sure he will say, 'oh boy another project and I thought I was done'. It looks like you have the same digital thermometer as I do. thanks for the picture.

Here is another angle that shows the fan itself. Also, paint the box glossy white.

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cottagefarmer - thanks. I will copy those pictures to show to my hubby if you don't mind.

I thought we'd agreed that shiny things reflect heat better? Wood is poor insulator so I would stick with the metal umbrella, making sure that it is shiny on top and bottom.

I'm not convinced that forced air is required here, the metal umbrell should be adequate if the metal is reflective and connected to the main frame.