Was receiving no heat, so checked the monovalve as noted in this forum. Found the rubber seal on the valve broken, so ordered and replaced the valve. Now I have heat all the time. A/C works great (yes, I'm in Florida where we need it in February) until the engine heats up, the heat overpowers the A/C. Removed the monovalve and reinstalled again, just in case there was some binding. No difference.

Any suggestions on how I can cut off the hot water from flowing into the heat exchanger?

Thanks
Brian