At 85 F. my top univent opens about 12 in. and the bottom vent opens about 8 inches (since it is cooler at the bottom. One thing to try is to put the pistons in a refrigerator for a few hours before installing. That fully retracts the piston so you can get maximum extension after installing. Passive ventilation can only help reduce inside temperatures somewhat. It will still be warmer inside than outside, sometimes by a considerable amount. Exhaust fans, shade cloth, misting, etc. are all necessary in my area once the outside temps. get above 85-90 F.

My greenhouse has both Bayless and Univent openers. Both brands work well. Replaced one power tube after five years use on one, with the others still working fine after 8 years.

burt4750, I have a HFGH but have not installed automatic openers. I do recall when we first built the greenhouse (over six years ago) I had to fiddle quite a bit with the vent windows to make sure they worked smoothly in the openings. There is quite a bit of flex in the Harbor Freight frame, and I recall having to loosen some of the bolts around the window frames, then "wiggling" things around until the windows fit better. Not a very scientific sounding solution, but it might be enough to resolve a slight drag along the edges. So far I've not come across anyone whose Harbor Freight roof windows were manufactured to an incorrectly large size for the openings, so I'm kinda suspecting it's just that the flexible frame needs to be "wiggled and tweaked" to make sure the windows fall into place more easily. Perhaps, worth a try? Here is a link that might be useful: Building our Harbor Freight 10x12 Greenhouse This post was edited by mudhouse on Mon, Nov 4, 13 at 21:09

alan50, arrange things so that you can be near the valve with your ear near to the solenoid when the current is switched on. You should hear a 'click' followed by the buzz or vibration. The click indicates that the plunger inside the solenoid has been pulled upwards to open a port which does the same thing as the manual bleed screw. I suspect that the plunger is stuck in the solenoid. If that is true you can unscrew the solenoid and free it up. If it is too rusted out or something, you can easily replace it. Shut off the water first so that you can keep from losing the O-ring seal.