condenser coil HX
The best fix for what you are trying to do is to recircuit the coil. You need three or four parallel paths for the water to follow. Look at the coil and divide it into sections, usually the easiest is to use each row of tubes as one circuit. That way you can make a manifold out of 7/8 or 1 1/8 OD copper tube by drilling holes in it to match the diameter of the small tubes with the same spacing as the coil. Then silver solder the manifold in place, do the same on the bottom and walla you have a coil with larger flow capacity. Done it a few times with Refrigerant 22 condensor coils with 200-300 PSI pressure no problem. Copper silver solders well with out flux, just clean well and use phos copper or low silver content solder, it fills bad joints real good. I made a heat reclaim coil to heat domestic hot water out of 7/8 manifold tubes and 1/2 OD tubes in between, mounted it about 2 inches above the wood stove and piped it into the element holes on an old electric water heater tank. Worked for almost 20 yrs till tank went bad. Looking for a tank now to replace it. Coil however is still like new.
