Iíve been planning my hydronic system for a little while now. currently glassing and setting up the new water heater cabinet, which is the heart of my system.
for what your describing you have a couple options
1) use a heat exchanger for one of the loops, and/or between the loops.
probably your easiest way forward. id run the engine to the water heater and a heat exchanger on the inlet side of the hydronic heater. another heat exchanger from the output of the hydronic heater to the inlet of the water heater. if i am thinking on this correctly, the engine heats the water heater directly and hydronic loop via heat exchanger, the hydronic heater heats the hydronic loop directly and the water heater via heat exchanger.
2)get a dual coil water heater.
this is more expensive, unless you need a new water heater anyway. harder to find in the US. i have found several euro brands where these are more popular. i believe European canal
boats use engine or diesel
hydronic heaters regularly. researching these units they may be be a higher quality than US brands, jury is still out on that but evidence is pointing that way. the euro models are cheaper than the US brands, but then you have to have it shipped.
Regardless of where youíre getting it, connect one loop to the engine, and the other loop to the hydronic heater, easy peasy. The water heater is the heat exchanger for both loops/systems.
Both ways you can add a 3 way valve and check valve just after the water heater to shorten the system in summer. This just keeps the engine and hydronic heater heat concentrated into the water heater.
you may have to replace the hydronic water pump
with a better one and put a pair of relays in there to turn on the hydronic circulating pump when the engine or hydronic pump is on. Been seeing that the water pumps that come with the hydronic heaters are not that good. it would also be a benefit to add a 10w hot water circulating pump and pipe insulation
to your hot water system.
Another alternative option is to use a ďOpen Hydronic SystemĒ. There are several different piping diagrams at https://www.radiantcompany.com/system/opensystem/
. Essentially the hydronic heat and domestic hot water are connected. There is some controversy here with similar house systems built the system works well with boats. This requires the use of a dual coil hot water heater or exchangers. Some of the diagrams/systems are complicated, but there arenít a lot of use cases to get a realistic comparison of performance.
If you do a search here there is another system, that probably doesnít work
for you since you already have your system. Essentially itís a reverse hydronic system. The engine and hydronic heater both heat an insulated header tank. There is a 50ft+ coil of copper water pipe inside that domestic hot water goes through. The tank essentially stores hot coolant and you ae only heating domestic hot water that you use, kinda like an instant water heater. You would run a separate loop and pump out of this tank for the hydronic heat system. People have made their coolant tank out of metal and with fiberglass
is good to 300 degrees, which is good enough for engine and hydronic coolant, had been in use for several years with no issues.
Tank and pipe insulation
is an important element to all of these systems.