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Get Powered Up! Certified Energy Manager Jeff Yago answers your alternative energy questions

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Archive for the ‘On-demand’ Category


On demand water heater used for hydronic heating

Monday, September 8th, 2008

Hello Jeff.

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.

Good luck,

Jeff Yago


Remote cabin water heater

Sunday, August 24th, 2008


My cabin in Utah is almost complete. Solar equipment is on the way. (2) panels, (8) batteries, outback inverter, etc. Outside is done. Basically trim work and plumbing left on the interior. My question is what would you recommend for water heat. I am looking at tankless heaters. Do they pull a lot of electricity or should I go propane? Also, what is your opinion on composting toilets?


Brent Toft


Don’t even think of using electricity to heat water. We rarely do this with large systems, much less a system the size you described.

You have 3 choices depending on how much you use this cabin and access.

My first choice is an instant tank-less hot water heater as some brands like a Infinion does not require any electricity and uses just propane. The problem with any tank-less water heater is there piping will quickly freeze if the home is not heated while you are away and should be drained each time.

My second choice is a solar hot water heater, which you can buy with an optional DC pump powered pump which uses very little battery power but also can freeze when away for long periods if not properly designed and installed.

If you are not lazy, several companies still make a wood fueled water heater that is an old style insulated water tank with an internal flue pipe and a fire-box below. most hold less then 30 gallons, so they heat up fairly quickly when you build a fire. However, their fire-box is small and will not hold much wood so don’t expect to have hot water all day after building one small fire. There are also hot water heating coil options for many standard wood cook stoves that can heat a separate insulated water storage tank.

I own 2 composting toilets, one in a garage and one in a trailer. They are great if you do not have a large family. For one or two people, you may be able to go many months without emptying the recycled soil and you only need to add a little compost material each time it is used. They also make larger models with a separate holding composting tank located under the floor and separate from the seat part. These usually requier an electric motor and some even have electric heaters so keep it simple.

Good Luck,

Jeff Yago



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