Thanks, I had the monovalve out early on. Nothing showed any damage. I'm not sure where the diaphram or seal are. I blew through it and it was open. I then applied current to the solenoid and it closed and opened with the current on or off. I thought it might be sticking sometimes so removed the connector so it could not close and we still lost heat after prolonged cruising. Just got the water pump out and it looks like new on the inside. Think I'll order a monovalve too like you suggest.