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Home > Archive > Alternative Power sources > September 2005 > Wood stove water heating options
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Wood stove water heating options
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| aberr8te@yahoo.com 2005-09-18, 11:21 pm |
| Hi,
I know this topic has been discussed occasionally, but I would like to
bring it up again in a little more detail. I would like to heat water
by using a wood stove. Specifically, I will be heating a hot tub. Most
systems I've read about online are just to supplement a domestic hot
water heater, which requires considerably less BTU than my application.
I'd like to have the cabability of heating 600 gallons of water from 50
F to 104 F in 4-6 hours.
In the future, I might use the same system for hydronic baseboard
heating (to supplement my existing living-space heating). But the hot
tub is the main priority.
The systems I've read about are:
1- Entire firebox immersed in water
(Snorkel / Scuba / Chofu stoves)
2- Heat exchanger inside flue
(coil of piping suspended in flue)
3- Heat exchanger outside flue
(coil of piping wrapped around outside)
4- Flue passing through water
(stove pipe just passes through a container of water; one-pass or
two-pass versions exist)
5- Heat exchanger inside fire box
(radiator-type piping mounted to the top of the fire box)
6- Heat exchanger outside fire box
(radiator-type piping, possibly enclosed in a box filled with cement,
mounted to the back of the fire box)
#1 is a bit expensive, and the Snorkel stoves I've seen for sale online
seem to be for rather small hot tubs. Besides, I already have a nice
wood stove that I'd like to use.
#3 is bad because the stove pipe expands/contracts and a tight fit is
hard to get.
I don't think that #5 will collect enough heat.
Which of the remaining options is best, and what are the
upsides/downsides of each? I'm leaning towards running the stove pipe
through a small water tank. (I could run it directly through the hot
tub water, except I want the wood stove set back from the tub by a few
feet). It seems like the easiest solution as far as labor goes, and the
big outdoor wood boilers use that method (though I've heard they make
lots of smoke, which would annoy the neighbors).
I am aware that soot will build up more rapidly than usual with any of
these methods, as the fire will be burning colder. Maybe pre-heating
the air or adding a blower would help to make a hotter fire. I could
combine the stove blower with a hot tub bubbler by pulling air from the
firebox and bubbling it through the water. I'd need a filter, of
course, to avoid dirtying the tub up. This would be a nice symbiotic
system, because normally a bubbler cools down the tub *fast*, since it
uses cold outdoor air.
Thanks for your advice and input.
Shamus
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| Steve Spence 2005-09-19, 12:21 am |
| aberr8te@yahoo.com wrote:
> Hi,
>
> I know this topic has been discussed occasionally, but I would like to
> bring it up again in a little more detail. I would like to heat water
> by using a wood stove. Specifically, I will be heating a hot tub. Most
> systems I've read about online are just to supplement a domestic hot
> water heater, which requires considerably less BTU than my application.
> I'd like to have the cabability of heating 600 gallons of water from 50
> F to 104 F in 4-6 hours.
>
> In the future, I might use the same system for hydronic baseboard
> heating (to supplement my existing living-space heating). But the hot
> tub is the main priority.
>
> The systems I've read about are:
>
> 1- Entire firebox immersed in water
> (Snorkel / Scuba / Chofu stoves)
>
> Thanks for your advice and input.
> Shamus
>
have you seen www.cowboyhottubs.com ?
--
Steve Spence
Dir., Green Trust, http://www.green-trust.org
Contributing Editor, http://www.off-grid.net
http://www.rebelwolf.com/essn.html
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| aberr8te@yahoo.com 2005-09-19, 12:21 am |
| Hi Steve,
Yes I did take a look at their stove. It looks nice, but I think it
would be a bit undersized. In their Q&A page, they even recommend using
two stoves for a 300-gallon tank, and suggest using a snorkel stove for
anything larger. I think whether I used multiple CowboyHotTub stoves,
or one snorkel stove, it would cost a little over a grand. Considering
I already have a nice wood stove, I'd prefer to use it. Plus it
wouldn't take up any space inside the tub, and I wouldn't have to worry
about getting burned by it. I appreciate the help though.
Shamus
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| Solar Flare 2005-09-19, 12:21 am |
| Cannabalism comes to mind here....LOL
"Steve Spence" <sspence@green-trust.org> wrote in message
news:depXe.9141$gE7.6369@fe08.lga...
aberr8te@yahoo.com wrote:
>
have you seen www.cowboyhottubs.com ?
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| aberr8te@yahoo.com 2005-09-19, 11:21 am |
| Hi,
Just noticed a typo. I wrote:
I don't think that #5 will collect enough heat.
I meant #6 (exchanger outside of fire box) won't collect enough heat.
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| barry@sme-online.com 2005-09-19, 12:21 pm |
| You're really asking a lot. Raising temp of 5000lb of water by 54 deg F
in 4 hrs. requires 67.5KB/h avg. input, assuming no loss from heated
water.
Clean combustion, for one thing, requires high firebox temps (> 1000
deg F), so to maintain heat output (control fireside deposits), you
need a large insulated firebox to burn something like 15 lb./hr., and a
large heat-exchanger. Even with clean-burning, lots of ash will be
heading toward the stack, with a stopover on heating surface, reducing
output.
Overall, you're talking "boiler" here, not "stove." Not to mention a
significant potential safety hazard. Interesting idea, though.
HTH,
J
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| aberr8te@yahoo.com 2005-09-19, 1:21 pm |
| Please excuse me if this is a double-post, I'm not sure if it went
through.
In my original post, there was a typo. It should have read:
I don't think that #6 will collect enough heat.
(leaving #2, #4, and #5)
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| Loren Amelang 2005-09-19, 4:21 pm |
| On 18 Sep 2005 18:57:33 -0700, aberr8te@yahoo.com wrote:
>I know this topic has been discussed occasionally, but I would like to
>bring it up again in a little more detail. I would like to heat water
>by using a wood stove. Specifically, I will be heating a hot tub. Most
>systems I've read about online are just to supplement a domestic hot
>water heater, which requires considerably less BTU than my application.
>I'd like to have the cabability of heating 600 gallons of water from 50
>F to 104 F in 4-6 hours.
BTDT.
I use an 18" (H) x 18" (W) x 36" (front door to back end) welded steel
box, with fourteen three-foot lengths of 3/4 copper zigzagging across
the inside top of the box. Originally it was in a bunker below the 600
gallon hot tub, with about 5' of rise to create a thermosyphon. That
worked, but when fired up high enough to actually provide 10 degrees F
rise per hour it tended to begin boiling occasionally and eject a lot
of hot water in a very short time. You had to be careful not to sit
near the outlet!
Now it is mounted farther from the tub and uses a small DC circulator
pump (El-SID 10B24) and three exchanger loops of soft copper tubing
inside the bottom of the tub to provide steady heat. Less exciting,
but safer. The exchanger is required because the woodstove, a gas
boiler, and the solar DHW panels are all interconnected with the hot
tub, DHW exchange tank, and hydronic tubing.
Loren
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| aberr8te@yahoo.com 2005-09-19, 10:21 pm |
| Loren Amelang wrote:
> BTDT.
>
> I use an 18" (H) x 18" (W) x 36" (front door to back end) welded steel
> box, with fourteen three-foot lengths of 3/4 copper zigzagging across
> the inside top of the box. Originally it was in a bunker below the 600
> gallon hot tub, with about 5' of rise to create a thermosyphon. That
> worked, but when fired up high enough to actually provide 10 degrees F
> rise per hour it tended to begin boiling occasionally and eject a lot
> of hot water in a very short time. You had to be careful not to sit
> near the outlet!
That sounds exactly like what I want. Do you have any pictures or a web
site that shows your setup? BTW, did you also make the hot tub DIY
style? Mine is 2x4 framed with fiberglass-coated ply. It holds water
fine, but I haven't hooked a heat source up yet. I plan on making 2
benches out of fiberglass, and installing 4 or 8 jets per bench.
> Now it is mounted farther from the tub and uses a small DC circulator
> pump (El-SID 10B24) and three exchanger loops of soft copper tubing
> inside the bottom of the tub to provide steady heat. Less exciting,
> but safer. The exchanger is required because the woodstove, a gas
> boiler, and the solar DHW panels are all interconnected with the hot
> tub, DHW exchange tank, and hydronic tubing.
Are you still able to get near 10 degrees F rise per hour with this new
setup? I think I will connect the hot tub directly to the stove (no
tub-side exchanger). If I want hydronic heating or DHW, then I will put
an exchanger in the tub (seperate loop, not directly connected to
stove).
> Loren
Thanks for the info.
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| Loren Amelang 2005-09-20, 12:21 pm |
| On 19 Sep 2005 17:26:31 -0700, aberr8te@yahoo.com wrote:
>Loren Amelang wrote:
>
>
>That sounds exactly like what I want. Do you have any pictures or a web
>site that shows your setup?
Sorry, no.
>BTW, did you also make the hot tub DIY
>style? Mine is 2x4 framed with fiberglass-coated ply. It holds water
>fine, but I haven't hooked a heat source up yet. I plan on making 2
>benches out of fiberglass, and installing 4 or 8 jets per bench.
I did something even simpler once many years ago - one of those huge
wooden fruit bins with a waterbed liner inside. Lasted several years,
until I made a real wood tub.
>
>
>Are you still able to get near 10 degrees F rise per hour with this new
>setup?
Actually, I never just heat the tub anymore, as the DHW indirect tank
is always in the loop, but I suspect the same output is available.
>I think I will connect the hot tub directly to the stove (no
>tub-side exchanger). If I want hydronic heating or DHW, then I will put
>an exchanger in the tub (seperate loop, not directly connected to
>stove).
Unless you have a very low-temp hydronic system (thin, very conductive
floor over the tubing) or like _very_ hot hot tubs, that isn't going
to work. I run my DHW at 130F minimum, and my thick wood floor doesn't
get up to temp until 150F water is available.
Loren
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| aberr8te@yahoo.com 2005-09-20, 1:21 pm |
|
Loren Amelang wrote:
> Actually, I never just heat the tub anymore, as the DHW indirect tank
> is always in the loop, but I suspect the same output is available.
>
>
> Unless you have a very low-temp hydronic system (thin, very conductive
> floor over the tubing) or like _very_ hot hot tubs, that isn't going
> to work. I run my DHW at 130F minimum, and my thick wood floor doesn't
> get up to temp until 150F water is available.
My plans would be to use baseboard hydronic heating (I don't have pipes
buried under the floor). But you're right, 104 degree water wouldn't be
that great. I suppose I could let the tub water get extra-hot, and pour
50 gallons of cold water in when I want to use the tub (3 times a week
maybe).
I'm assuming that to do it the way you did, I would need to put a 3-way
junction at the pipe coming out of the stove, and then a few zone
valves? Then I would need 3 seperate thermostats (104 for the tub, 130
for DHW, 150 for hydronic heating)? And probably some way to kill the
pump if all 3 zone valves were closed (plus a pressure relief valve).
The cost and complexity of that type of setup would probably hold me
back, but I'm very impressed if you did all that.
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| Loren Amelang 2005-09-22, 3:21 pm |
| On 20 Sep 2005 09:06:20 -0700, aberr8te@yahoo.com wrote:
>I'm assuming that to do it the way you did, I would need to put a 3-way
>junction at the pipe coming out of the stove, and then a few zone
>valves? Then I would need 3 seperate thermostats (104 for the tub, 130
>for DHW, 150 for hydronic heating)? And probably some way to kill the
>pump if all 3 zone valves were closed (plus a pressure relief valve).
I guess if you have an open system (the hot tub water runs through the
hydronic and DHW loops), all you stand to lose is water and heat. But
when my stove was plumbed open system, stopping the pump when the
stove was hot would blow 8' jets of steaming hot water out of the TP
valves at both ends of the stove coil. Awesome and dangerous, and not
something you want to have happen in normal operation.
That's one reason I have the 60-gallon DHW indirect exchanger tank in
the loop at all times now - if the heat is not needed anywhere else,
that tank can absorb the heat from the fuel left in the stove
gracefully. (Be sure to have a tempering valve on the DHW output so
users don't get surprised by the extra-hot water.) Still, if the pump
fails at the wrong moment, there will be serious steam ejected from
the TP valves!
Loren
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