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Can I heat Water in a Masonry Heater?

Dear Eric,

We've talked about adding water heating to our Maine Masonry Heater, but I'm concerned about it effecting combustion efficiency, because I see coils in fireboxes. What do you think?

Dear friend,

My goal isn’t to tell you what you should do, but to educate you so that you can decide what you want to do.

Whatever you do with a water coil, a professional plumber should be involved, I think, both to make sure everything is done right, and to make code enforcement happy.

Before we talk about water coils, I want to share how I envision your firing experience to be. Given the nature of the home, you may need morning fires only sporadically. I envision an evening fire heating the stove for the night, and the stove cooling into the morning or early afternoon for the next evening (after dinner?) fire. This is important because the heater and it’s firing schedule will become a somewhat fixed quantity based on the house and your comfort level and how you fire the stove. Because the heater fires once a day, some of these coil placements will give you batches of heated water also. This is one reason that having the heater operate in series with a conventional water heater is attractive, because you’ll never experience a sensible shortfall of water, and at best, might have to take a long hot shower or heat the basement up if there’s too much hot water.

I’m sure a lot of what I’m going to say here you already know, but I want to start at the beginning:

How much heat is driven into the water depends on:

1 – temperature gradient (this is why some put their coils in the firebox).

2 – surface area of coil in the high heat area

3 – surface area in contact with the high heat area.

I make a distinction between #2 and #3 because there are options in which the coil is surrounded by air and options where the coil is embedded in the masonry.

Coil Materials I have seen used include Stainless and copper. Others may work, but I've not seen them used.

1) Flexible copper pipe –

2) Straight copper pipe

3) Stainless steel pipe -

4) Stainless “box” water jacket

Notes on Material:

1) Flexible copper pipe – Has been used in the past by other builders, lifespan is unknown (maybe 20 years?) – smaller diameter will slow water flow through system compared with a larger diameter pipe.

2) Straight copper pipe – Possible to have a manifold at the base of the stove and another at the top of the stove, with a series of straight pipes running through clay between liner and skin.

3) Stainless steel pipe – Can be purchased in almost any imaginable shape, because there’s a company in NY that custom bends it. Could put it behind a firebox (or completely around a firebox) in a void in the masonry that’s accessible from below. Is heavier gauge and will last a long time.

4) Stainless “box” water jacket – easy to install and a flat surface against a brick in an air filled void makes this an easy unit to service also. Limited surface area requires placement in the thermal mass near the highest heat areas.

NOTE – options 3 and 4 could be made to encircle the firebox exit on three sides for more heated water.

Coil can be placed:

1) In firebox

2) Near firebox in masonry work

This can be done either in an airspace or embedded in masonry

3) In between the inner channel liners and outer skin material

This can be in an airspace or embedded in clay mortar.

4) Embedded in the plaster work on the outside of the stove

Notes on placement

5) In firebox – will rob heat from burn, effecting efficiency at startup, which is the most important time from an emissions standpoint, but will provide most hot water.

6) In gas-path past combustion – High heat – requires penetration through skin of heater, which leaves us relying on gaskets to keep room and channels separate.

7) Near firebox in masonry work

Nice balance in terms of high Delta-T but not robbing firebox performance. Depending on how the coil is constructed can be challenging to replace. (this is a discussion that we can have if you’re interested in it). In a lot of ways this is the most reliable way to do it, in the sense that it’s been done this way a lot. It’s a reliable way to get a “decent” amount of water to a “decent” temperature. This is also the way I'm most comfortable with because it has been done so much.

8) In between the inner channel liners and outer skin material

Moving into the area of the stove with less temperature fluctuation, but lower Delta-T. This placement starts to intercept heat from getting to the surface so we can start potentially to moderate output from the heater and shunt / hot water to other uses (a concern for this client due to a super efficient house -Eric). Because there’s thermal mass between the exhaust gas and this point, the “batch” effect would be less here. Also, one could place a much larger surface area of coil. Will coil surface area mitigate lower delta-T? I just don't know, I haven't done it this way yet.

9) Embedded in the plaster work on the outside of the stove.

This will arguably get the least amount of heat energy as a result of the lower delta-T, but will do the best job of moving heat away from the heater as desired. Possibility of coil expansion effecting plaster. There are things that can be done during plastering to mitigate this, and I don’t know how much of a concern it is.

In any placement it’s important to think about what happens when the power goes out. I recommend designing the system so that there’s a thermosiphon to draw heated water away from the water jacket no matter what. Having the tank above the heater and within about 4’ horizontally is recommended to achieve this.

Let me know what you think and we'll keep the conversation going!

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