We have an everhot hot water heat exchanger on our Webasto based hydronic system on our boat.
THere is a set of valves that select winter loop or summer loop. In winter mode the hotwater from the diesel heater passes through the everhot and thus on to the heaters in cabins and back to the webasto heater (and an expansion tank and a filter and a pump etc).
In summer mode the hot heating water just goes to the everhot heat exchanger and thus does not heat up the cabins.
THe time from start of system to heating the cabins is around 10 to 15 minutes depending on how cold everything is.
I've never timed how long it takes to come up to full operating temp in summer mode. Usually we just heat water on the stove for dishes (in the summer) rather than start the hydronic system. In the winter if the hydronic system is running of course hot water is ready all the time.
The everhot system is a heat exchanger that works quite well. Heat is transferred from the hydronic loop to the fresh water line as soon as the hydronic loop is hotter. In fact it can over heat your hot fresh water and scald you so be sure to put in a mixer to keep the hot water to something reasonable.
If you are tied to a dock
with shore power
and city water you can take showers all day long with this system. At least till your diesel runs out.
Off the dock
and your limits are battery
and fresh water.
You can get a second heat exchanger that is placed one side on the engine coolant
and the other on the hydronic "heatant". THis allows the engine to keep the boat warm and make hot water using the hydronic system.
Our system was installed with little thought to where the cabin
heat exchangers were placed. This hurt storage
space because they were placed in the bottom of lockers just where you might want to place things. So do plan well if your system is not installed yet. I'll be moving all of them sooner or later. I've done 2 out of 8.
Use an AC pump to the hydronic water. Lasts longer than a DC pump. I installed a small (300 watt?) inverter
to run the AC pump. THere is a master heat switch which powers the inverter
as well as the webasto system.
is 90 % PEX on our system. THis is OK but does present some routing issues that could have been handled better. Rubber hose designed for hydronic heating works well (Don't use radiatior hose). It should be insulated to that heat goes where you want it.
It took me a while to understand all that was going on. This system came with the boat without documentation
Oh, on last thing. Make sure that your webasto exhaust
does not blow on the Genoa sheets
PS I'm in the Portland
area if you want to come over and have a look at it.