No heater output at idle

There is no heater output on my maxum v drive at idle. Above idle at say 1400rpm there is plenty of water flow and heat.

The engine is the mercruiser mag EFI. I've checked the raw water impellor and its fine, Ive back flused the heat hoses and there was no scale or restrictions.

I've ordered a new thermostat and impellor anyway and will see how that goes but has anyone had any experience of this?

Now no heater at idle really isn't that much of an issue but the heater water circuit also supplies the regulator for my propane conversion (the boat is dual fuel and runs on either gasoline or propane) and without a decent flow of hot water the regulator freezes up and I have to switch back to expensive gasoline which is a bummer!