[Greenbuilding] heating water with wood stove - heat transfer/efficiency calculations?

[Frank Tettemer]

Hi Reuben, please see my comments, below:

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Frank,

Thanks so much for those great examples. I really appreciate your 
insights here, especially on the custom SS stove pipe water jacket. Very 
cool project—tempting to try a fully instrumented version.

A couple of questions.

(1) when you say closed loop are we talking a storage tank with a coil 
inside?

In most of my set-ups, over the years, I've used the actual water that 
comes to the fixtures in the closed loop to the stoves. In a standard 
hot water tank, the bottom port, (intended as a drain port), is the port 
to use, heading towards the lowest part of the loop at the wood stove. 
The flow out of the wood stove, back up to the tank can/should enter the 
tank at the top of the tank. No 'dip tube' on this port ... leave the 
dip tube for the cold water, coming from the plumbing pressure tank. The 
port that is commonly used from the tank to the fixtures can still be 
used for it's intended purpose,
and the hot water coming from the stove can enter the tank through the 
port that is commonly used for the P/T release valve, on the side of the 
tank, up near the top.
If you can't obtain a tank with three ports near the top, for whatever 
reason, I've had success using the wider, threaded port near the top. 
This port is most commonly used for the electrical heating element. 
Simply unscrew the element, remove it, and find fittings and reducers to 
use this opening as a port.
All these instances use a common electrical hot water tank out of 
necessity/economy for our rather isolated county here in Ontario.
Now-a-days, one can purchase insulated storage tanks, with coils inside. 
I think this would be better, namely, when there's problems to solve, 
the actual water going to the fixtures is not involved in the 
circulation loops. I think we've mentioned this before, but here is a 
good one that I've used twice:
http://www.viessmann.ca/en/products/speicher-wassererwaermer/Vitocell-B_100.html
You can click on the thumbnail on that page, to see a cross section 
showing the two loops, for solar thermal and for boilers.

(2) since ten gallons of hot water would be plenty for each of the 
households I have in mind here, I’m tempted to think that any 
temperature rise achieved would be (roughly) 4x as quick/take ¼ as much 
wood to achieve with a 10 gal tank vs your 40 gallons. Does that sound 
reasonable?

I think you're right about this. But my first reaction is to think that 
a small tank would be more difficult in actual practice. Consider how 
easy it would be to overheat the water in a small tank.
At home, I have a cheap thermometer, (designed for indoor/outdoor with a 
remote sensor), which is mounted in the kitchen, next to the wood stove. 
The sensor is taped to the copper pipe, coming from the tank's drain 
port, and heading to the wood stove. This tells me the temperature of 
the coolest portion of the water tank. When this reading becomes too 
alarming, (on cold winter days, firing the stove hard), we usually act 
by filling the bathtub with hot water. This releases the heat back into 
the house, where it does some good, before the automatic dumping of this 
water, by the P/T valve, down the drain. This is not a common problem, 
but does occur a dozen times each Winter. So overheating should be 
planned for, and does require the installation of an automatic dump by 
an 80 degree C P/T valve.

(3) On the Stanley cook stove, they do make a water jacket—or rather two 
different sized ones for that stove, which is installed at the back (and 
sides) of the firebox. My mother’s Stanley cook stove came with the 
larger water jacket factory installed, and she loved how much hot water 
it produced—too much really. When the water jacket sprang a leak (almost 
certainly because we foolishly fired the stove for 18 years with no 
water in the jacket) we tried to have it welded, and when that didn’t 
work we went with the Lehman’s-supplied much smaller SS water jacket 
Waterford sells for this stove. This one has much less heat transfer 
area and consequently doesn’t make hot water as quickly. A compounding 
problem in her situation is that the storage tank is 80 gallons(!) 
because that was the only tank the solar installed could find with an 
internal coil…..

My observations over the years concur. The engineers have erred on the 
conservative side for safety.
In other words, to prevent overheating and/or steam explosions, they 
make sure that the internal water jackets/pipes are not too efficient.
That is why it is so darned hard to find an approved water system that 
actually makes hot water, not tepid water.

But overall I’m in agreement with you and Corwyn that with a short daily 
burn cycle in a wood stove this DHW effort may not be well matched to 
the fuel input. Which leaves me wondering if the smartest thing would be 
to use the woodstove as a preheater and top things off with a tunable 
propane (or biogas if I can source any) pilot light in the winter, and 
use a coil of black hose inside a glass box on the roof for the summer 
hot water?

Almost all my installations have assumed that the water tank 
temperatures would fluctuate wildly. And they do. At home, after heavy 
use, the temperature of the coolest stratification of the tank can be as 
low as 16 degrees C, and slowly climb up to 65 degrees C over a six to 
eight hour burn time. So thinking of the tank simply as preheated water, 
and then routing it through a modulating-flame in-line hot water heater 
yields the most consistent results for the home's occupants. That way, 
no one is disappointed in the shower.

-- 
Frank Tettemer
Living Sol ~ Building and Design
www.livingsol.com
613 756 3884