Hydronic system to preheat engine

[SoggyHalyard]

Hi all -- I'm planning a hydronic heat install for cruising in AK. I'm planning to plumb my engine coolant through a heat exchanger with my hydronic loop which will include my water heater and cabin fans.

When the engine on and coolant is flowing, I'll be able to turn my hydronic circ pump on and use waste heat from my engine -- great. I'm trying to determine how to preheat my engine when running the hydronic system.

It seems to me that I need a similar pump on the engine side to circulate coolant when the engine is not running. Also, maybe this could also be a great emergency limp-home feature if my engine driven coolant pump ever gave up the ghost.

Can someone advise if my assumption of this setup acting as a backup coolant system for the engine is realistic and what type and size pump I should be thinking about to put in the engine coolant loop for my 65hp yanmar?

[Lepke]

I have Detroits so having enough heat or circulation isn't a problem. I have an insulated supply and return lines that run almost the boat length. Cabin heaters are isolated by a solenoid valve that only opens when that cabin calls for heat. With a heat call from any cabin, the boiler pump turns on. The heaters are marine forced air with one or two in each area. Fan comes on when coolant in heater reaches 90°, so it doesn't blow cold air.
My boiler, cabin heaters, and main engines use the same coolant. The boiler stores about 10 gallons. To preheat the engines, I open manual valves to the engine and manually turn on the boiler circulation pump. When the temp of the boiler tank drops the burner ignites, but I leave the circulation pump on manual until the engines are running.
When I use one or both engines for heat, I open the engine valves and the engine coolant pump is enough to circulate. I have the burner shut off. If the temp drops enough the boiler pump will come on. In really cold weather I use both mains for heat and easily heat the whole boat..
Your hydronic pump should be enough.