I really appreciated your article in Backwoods Home Magazine on Hydronic Wood Stoves, as well as many others.
I have been interested in the practicality of setting up a propane on-demand water heater in a small off-grid cabin to provide both domestic hot water, as well as hydronic space heating. I have birds that do not do well with wood stoves or with un-vented propane, but need a somewhat constant minimum temperature of 60 degrees F. I was thinking to be able to dump hot water into a water tank (say, an old water heater with insulating jacket removed) in the space, which would act as a thermal mass slow radiator. My only quandary is how to pressurize the system to return water to the on-demand water heater.
Do you have any thoughts on this, or criticisms in general of this setup?
Thank you for your consideration.
Please read this very carefully – it is possible to do what you want to do, but there are many design issues that most do-it-yourself type installations do not resolve and usually end up destroying the instant hot water heater.
First, any heated hot water also used for space heating must first heat a pressurized storage tank. This does not need to be large, but it must use a second pump to circulate the heated water through any radiation or in floor heating loops.
Second, if you route the same “drinking” water through the heating system and do not use a heat exchanger to keep them separate, then your pumps will need to be stainless steel, the tank will need to be glass lined (use an electric hot water tank), and all piping in the heating circuit will need to be copper or brass, and no lead solder used anywhere.
Almost all instant in-line heaters will have a disclaimer that voids the warranty if you re-circulate hot water that was already heated back around and through the heater again. This is because their heating coils and flow rates are based on cold ground water entering the heater. If you re-circulate hot water, the heater will over-heat and damage some of the parts. Also, the built-in thermostat will not know what to do if it “sees” hot water entering the heater.
The storage tank and related valves and pumps allows the water in the heating loop to keep re-circulating at the desired flow rate back to the tank, and the separate pump only cycles flow through the heater at a different flow rate when the tank temperature drops. Even then you will most likely need to operate the domestic hot water temperature much hotter than desired, and install an automatic tempering mixing valve to limit the water temperature of the water going to the bathroom and kitchen faucets.
I have designed and installed a few of these systems, but found its much less complicated and will take up less space if you have an instant gas-fired domestic hot water heater for sink and shower hot water, and a separate packaged gas or wood fired hydronic hot water boiler. These are very small and very efficient.
Finally, remember that many of these components have tiny orifices that will freeze and break if this home will not be heated during cold winters.