I just purchased a balboa spa pack. It comes with an in the tub thermowell fitting, which my tub does not have a hole for. Should I drill the fiberglass or is there somewhere else I should locate this?

Yes, very carefully... it's the best of all worlds when it comes to detecting the water temperature.

I normally get a large wood bit, the width is slightly greater than the diameter of the threads. Then I drill from INSIDE to the OUTSIDE of the tub. Never do this backwards, (from underneath the tub). You run the risk of fracturing the acrylic shell on the other side.

It takes about 5 minutes total to install. Once the hole is drilled, then with the gasket on the thermowell, I put some clear silicone on the threads, and through the hole. Push the thermowell thru, then tighten down the lockring BY HAND in the back. (Many of these use left hand threads from Balboa!). Then let it dry... good to go.

(Sorry for the late reply).

I wanted to avoid drilling any new holes in a 25-year old tub shell, so I tried installing the thermowell into a tee in-line on the intake side of the pump, but it is not registering correctly (it's about 6 degrees below thermometer reading in tub). So I am going to drill a hole in the spa and insert the thermowell directly as suggested. You mentioned using a wood drill bit. Are you talking about a regular spiraling drill bit, or a hole saw?

I used a hole saw to insert the spa light that came with my new pack, as the original spa had no light either. It seemed to work well, but that was a much bigger diameter than what I need for the thermowell.

If I can use a standard large wood bit, that would be better.

I use a flat blade type of wood bit drilling directly into the acrylic (the inside) and thru the fiberglass and it works great. Just don't do this backwards (from the back of the shell) or else you'll shatter the acrylic when the bit travels thru the fiberglass.

I used a flat-type wood bit and it worked fine.

However, for drilling out the larger hole for the underwater light, I needed to use a hole saw blade, which worked okay. Except now I have a very slight leak even though I've cauled it with a significant amount of silicon. All the silicon is on the thread and on the back side of the tub. I was considering putting a small amount of caulk on the inside the next time I drain the tub.