Where are you measuring the temperature?
What is the room temperature?

dcarch

Room temps are mid 70's. I have the thermometer laying on top of the trays, near the stems of the plants.

Not good.

You are measuring the top layer air temperature directly above the soil only. Because the dark soil aborpts the heat from the lights and may read much higher.

Try measuring at about leaf level and put a piece of white cover over the thermometer so that the light is not heating the thermometer directly.

dcarch

You can mount the ballast almost wherever you want, but longer wiring runs may cause problems. The ballast is only producing around 10% of the heat anyway. The heat trapped inside the fixture shouldn't be the main problem, although I have found that using a fan which is too small can actually make the plants warmer because it simply moves warm air high up in the fixture to low down near the plants but doesn't move enough completely fresh air in.

If the room is at 70F, the temperature at the plants is always going to be a little high. Find a colder room, use a stronger fan, or as a last resort mount the lights further away.

On a different tack, you could just use 4 tubes instead of two overdriven tubes, efficiency will improve and so you should have less heating problems. Of course this would give you a lot more light but more heat also from each fixture, but spread over a larger area, which may or may not be useful to you.

Moving the shelf into another room isn't possible. I'm going to try bigger fans and possibly mounting the ballast on the outside to help release the heat.

Any more ideas welcomed!

Make sure the grounding is OK if you move the ballast. Designs vary, but a common technique is a metal ballast case and metal fittings all connected to eachother and the earth. You can do it how you like, but the ballast and the fitting should both be grounded.

shrubs n bulbs wrote:
"a fan which is too small can actually make the plants warmer because it simply moves warm air high up in the fixture to low down near the plants but doesn't move enough completely fresh air in."

I think this is your problem. An 80mm fan or two isn't going to do a lot, especially if they are aimed at each other from opposite ends of the shelf.

A couple small oscillating fans aimed at the shelves from a few feet away do the job better for me.