Here is what I got back from Bayliss:
"You are quite correct in your assumption that the direct sun on the
autovents is warming the power tubes, causing them to open earlier than
you would have desired. Do you have the autovent set to open at the
maximum end of the temperature range? If not, then you could adjust the
units in the sun to open at a higher temperature than the other vents.
As you suggest you could cover the tube. Rather than using tin foil,
you could cover the tube with insulating foil. The insulating foil
would delay the time it takes for the heat to reach the wax inside the
tube. On days which are not so sunny though, you would find that the
unit is taking much longer to react to the ambient than the ones you
have not covered.
I hope that this information is of help to you."
So I'm thinking that rather than wrap them with an insulating material, I will make an insulated shade for them. That way they will be exposed to the ambient temperature as well and work as they are supposed to.
