If having 200 degree water in the tank and mixing it down to 130 is the correct solution, then why is my electric thermostat limited to 140, not 200?
Because it is electric and is easy to adjust and limit by simply switching ON & OFF the electric element. By turning the electric aquastat up to 140 and blending the output down to 120F with a thermostatic mixing valve you can create more
usable hot water capacity from a small tank. Run the water heater temp to engine temp and you'll get even more usable hot water, if you have a thermostatic mixing valve installed.
In order to do this on engine fired hot water heaters, with a "thermostat" gets very complicated. It uses 12V DC power, lacks reliability, and the water heaters are simply not designed for it because they lack the ability to properly install a second
aquastat..
In a home heating system, with an "indirect fired" hot water heater, which is pretty much the same thing as a marine water heater but without eh electric element, there is an
aquastat well in the tank.
This
aquastat tells the boilers
circulator to start circulating heated boiler water through the coil. Many installers of indirect fired hot water heaters run the
aquastat temp as high as possible then mix it down with a thermostatic mixing valve on the tanks output. This yields more
usable hot water by blending the hot with cold at the outlet. Instead of 40 gallons of 120F hot water you run it to 140F, 150F, 160F etc. then mix that with 55F street/well water and get more
usable hot water capacity. It is pretty standard practice in both residential and commercial applications. The mixing valves are no different.
On a boat you simply can't tell your engine to stop or start circulating water to the water heater by means of an
aquastat, like you can with a boiler. You also lack the "well" or tapping to insert an
aquastat into in the water heater in order to control and sense temp.
Could this be done? Sure, anything
could be done, but it would require some pretty fancy plumbing, more components, more money and a custom modified water heater. It woudl also result in less reliability for more money...... You would also get less effective hot water from the small tank....
Also any interruption of the flow of the water heater circuit on the engine can result in an overheat of some engines because this is a
bypass circuit on some engines. This would need to be considered in the plumbing which means bypass loops and zone valve type devices, which are not very reliable to begin with, even on land..
This is why the industry, world wide, has settled on thermostatic mixing valves to control the temp of engine fired marine water heaters.. Thermostatic mixing valves have been standard equipment in Europe for years. They also allow you to get more
usable capacity out of a small tank by blending tank output water with cold domestic water and "mixing it" for a lower net output temp..
Thermostatic mixing is extremely reliable and is mandated in the US for such things as shower valves to maintain a steady hot water temp. This is not new technology and has been used on in plumbing systems to control water temps since the early 1900's...... The US marine industry was about 40 years behind our land based plumbing standards, now we are caught up...
