I'm adding a hot water trench heater into my floor as part of a renovation and I need to cut a rectangular opening into my floor to install it. The floor is 2 1/4" wide 1950's oak with most of the pieces around 12-18" long. The room is 30 feet long. The trench opening is approx 10" wide and 8' long with the long side butting up against the ends of the floor.

I'd like to have the metal grill of the heater with as little gap as possible but the general rule is 3/8" gap. If i do need a gap what could I use to fill that gap?

Thanks in advance!

if the trench is dug perpendicular to the flooring you will not need an expansion gap since oak does not expand lengthwise.
Some grills have a lip that can cover an expansion gap. You may be able to use multiple grills to make up the full length.

When it comes to adding that hot water trench heater, it's a good idea to stick with the general rule of a 3/8" gap. This gap accounts for any expansion that might happen due to temperature changes and keeps your floor from buckling or warping over time. It might not be the tight fit you're aiming for, but it's a smart move for the long run. If you do need to fill the gap, some flexible filler or sealant could work well. One option could be a flexible silicone-based filler or caulk. Silicone sealants are known for their flexibility, water resistance, and ability to adhere well to various surfaces, including wood.