Fantino
It would be perfectly possible to spilt into separate loop but you’ll lose heating underway of course. More common practice is to hook up heater on same loop and by arrangement of check valves or manual valves be able to utilize both heat from engine and from diesel water heater.
Personal preference would be to keep the air heater on one engine as the effect is instant and very high typically, and then hook up diesel water heater on other loop which is connected to hot water tank. If you T-on the diesel heater exit on the cooling water exit from engine with a check valve on both exits, and inlet on engine T-ed on inlet to diesel heater, flow will go in complete loop when running engine, and in reduced loop omitting engine when running diesel heater. Ensure clean and particle free coolant for trouble free operation of check valves. Manual valves are of course possible but more cumbersome..
There’s a multitude of heater brands available, Webasto, Eberspächer (which may be identical to Espar?), Planar, Autotherm (latter two are Russian I believe), Mikuni etc, as well as very cheap no-name copies on AliExpress. Based on what I’ve seen here over the years most of them seem to work well. Installation of exhaust and fuel lines vs heat and leakage is critical. Some also produce both heated water and heated air. Trumo has been a popular brand in caravans, with integrated water tank and air heating, but I think they are discontinued and/or not recommended for marine installation. You may also find heating water to run through radiator to heat cabin is inefficient. Personally I would prefer separate air heater. For your size of boat I’d say minimum 5kW nominal effect. Have less preferences on water heater but if water is also going to be used to heat cabin I believe you need quite a bit more effect and what you may experience is then that the hot water tank is heated to max very quickly making the heater stop.. Thus my gut feeling is separate systems. I have a diesel air heater that I run liberally because it keeps moisture away, preferring to adjust inside temperature by opening the door as the heaters burn cleaner on full. No experience with water heater but a lot of people on this forum has:
There are a lot of threads on this forum on the subject, in Norwegian.. Try a Google search with brand name and ‘site:baatplassen.no’ and then hopefully Google Translate is your friend ?
