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heat coil and water temperature Options
#1 Posted : Saturday, August 22, 2009 3:30:45 AM
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PEX and plastic tanks that cannot take really HOT water, would be best to NOT use in a woodstove coil system. Theres plenty of materials that can take the heat. Such as copper, galvanized steel, and even the orange PEX-AL-PEX (aluminum tubing that has the cross linked polyethylene coating inside and out, and a much higher temp rating than PEX). 

 For low electric consumption consider a thermosiphon system from the woodstove coils to your hot water storage tank. Such systems use no pumps or electricity. (the limitation is that your woodstove coils MUST be lower than the close by storage tank and the tank must be large enough to handle the output of the coils you are using...).

A storage tank lets you use the heat when and as you need it, even for heating awhile after your woodstove fire has gone out. The storage tank is like a buffer that compensates for the inconsistent output of wood stoves. It should be able to handle the almost boiling water that your woodstove coils will sometimes build up to. The higher it can handle the more BTUs it can store. A common electric water heater tank makes a good inexpensive heat storage tank (even if you will never be able to connect its electric heating coils). They are so common they are inexpensive to buy. Get the type with 2 side fittings, one drain and one relief valve (high and low) on the tank's same side, for easy plumbing to your stove coils.

Trying to control your coils' output relying on a pump could be a catastrophy if your power or pump goes out while a hot fire is going. It is not a very fail safe design.

If you want the "lower-temp-capable" PEX on your heating loops, such as in a slab,  use a tempering valve to mix the hot (from the tank) and cooler (return loop) water as it is circulated. Those are commonly used for solar potable water applications, but could work as well in hydronic service.

Making your overall system able to handle the hotter water would allow for conserving the electricity used for circulation. Less gallons would have to be pumped.

#2 Posted : Wednesday, August 26, 2009 4:32:16 PM
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Posts: 134,494

I didn't mention that another plus of using the water heater tank for storage is that you could have the electric heating coils for a backup heat source if you're away, or just let the fire die out.

The woodstove coils are usually intended to be used for potable water heating. Your storage tank should double for that also. Generally, the capacity of the coils used for heating potable water would not be large enough for hydronic heating applications. It could be a great supplemental heat source, though, and good for heating some remote place in your house that is hard to get the woodstove heat to. Adding more or larger coils (in a big enough woodstove) could give you some hydronic heating capacity.

I make my own stove coils, welding together stainless steel pipe or boiler tubing, but most of the time now I refer folks to a manufacturer's website for info on stove coils. They have installation diagrams, and offer many kits as well as custom made coils service. Welding, I cannot compete with their mandrel bent coils systems, except for making cheap ones for myself now.


#3 Posted : Wednesday, August 26, 2009 4:32:16 PM
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Posts: 134,494

Given that plastic plumbing components, such as pex and plastic tanks, have a maximum temperature rating: Would it be possible to add a simple temperature control to a heating coil mounted in a wood stove to prevent the outgoing water from exceeding the maximum temperature?

I picture a DC circulator pump, a temperature sensor for the output water and some sort of controller to vary the voltage to the DC pump to maintain the water at a specified temperature. But what would that controller be ?

This might also provide greater electrical efficiency for the off-gridders. For a simplistic example: instead of pumping water at 10gpm and 24vdc and getting output temp of 150F, one might pump at 5gpm and 12vdc getting a desired output temp of 180F. Why circulate the water more often that necessary when your electrical resources may be limited ?





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