| 
  • If you are citizen of an European Union member nation, you may not use this service unless you are at least 16 years old.

  • You already know Dokkio is an AI-powered assistant to organize & manage your digital files & messages. Very soon, Dokkio will support Outlook as well as One Drive. Check it out today!

View
 

Radiant floor design conversation

Page history last edited by Deanne Bednar 14 years, 4 months ago

10/29/09

christina writes:

I would really suggest you get the (about $6) "Heating Water with Wood"

manual from Lehmans.com (Amish general store) and get it into the hands

of your designers if they haven't heated water with wood before.

Temperatures in the firebox easily exceed 500 degF - they can be double

that, which is why that manual advises Stainless Steel piping in the

firebox and for a few feet beyond the firebox because copper piping

could melt. You get the most heat from the wood with a heat exchange

pipe that is within the stove firebox, but this is what makes it

difficult to have antifreeze in the pipes because of the high

temperatures. The Amish don't use antifreeze, they just never let the

room the wood-fired waterheater is in ever drop below freezing in

winter, thus they don't put this system in buildings that are only

occasionally heated and they don't leave home for long in winter unless

they get a housesitter. The manual covers the numerous safety features

such a system should have when you are dealing with temperatures higher

than 212 degF so the water can become steam unexpectedly, with

potentially explosive force.

I'm not sure where 2" pipe diameters came into the stove to tank loop,

since the Amish have done successful thermosiphon systems with much

smaller (and cheaper) diameter pipes. Also, I don't know what to make of

"corn" glycol, other than corn is the source material, it doesn't tell

me what the end product is chemically. I'm only aware of two types of

antifreeze: propylene glycol (food grade, used in solar DHW systems) and

ethylene glycol (poisonous, used in cars). There may be other chemical

formulations of antifreeze than those, but I don't know anything about

its properties. If you think you might want to heat potable water with

the same system as you are using for heating the building, code would

require you to use a food-grade antifreeze. If it is not going to be

food-grade, you should take precautions to ensure that ignorant folks

like children can't get into contact with it at the tank (warnings,

locks, etc).

There is the possibility of putting your heat exchanger piping on the

outside of the woodstove box, which causes a dramatic loss of efficiency

of getting the energy contained into the wood into the watertank. But if

you absolutely need to have antifreeze in the system, this may be your

only option. There are off the shelf external heat exchangers for

woodstoves (I believe Lehmans sells some), which typically have flat

metal panels with piping welded or brazed onto it. These are mounted

about a 1/4 inch from the surface of the woodstove, and can only capture

some of the radiant heat coming off of the surface of the woodstove,

which is a lot less than the amount of heat available in the firebox.

That is why these external heat exchangers typically have the pipes

doubling back and forth over most of the external surface area of the

stove, which also means that the thermosiphon loop is much longer, and

therefore the resistance to flowing in the pipe is greater in the

external heat exchanger. These off the shelf units are flat panels, so

maybe one could replace the heatsheild at the back of your stove, but

they may not work well (or aesthetically) at the sides your existing

woodstove, which is slightly curved, isn't it? I still think that many

of the safety features in "Heating Water with Wood" might be wise to

incorporate into this system, too.

These form incompatibilities may cause you to decide that you want to

"make your own" heat exchanger, embedding it in cob like your intern

suggests, but I can't predict how this will work. Even if you were to

replicate the length and configuration of the tubing in the

off-the-shelf model that has been time tested for successful flow & temp

range, you could still have unpleasantly different results. A heat

exchanger & thermosiphon loop should always have the path of the heated

water be slightly uphill, thus roughly horizontal pipe runs between the

switch-backs, so there are no places where an air bubble could form and

be trapped and block the flow (as could happen if the pipes run up and

down w/ u-returns - if flow stops, possibility of steam flash

explosion). The store bought heat exchangers don't have a conductive

material between the stove and the heat exchanger; this air gap may keep

the pipes cool enough that the antifreeze can't get cooked or the slowly

moving water flash to steam. Also, the pipes will expand and contract a

lot over the operational temperature swing of the stove, and this may

cause the cob to crack and break up. Finally you should definitely test

to see how your particular cob mix reacts with both the metal of the

water pipes and the shell of the stove at high temperatures over time

before you cover large portions of the stove or pipes with the cob. If

the materials in the cob aren't PH neutral (unlikely) or are the metals'

polar opposite on the galvanic spectrum, there could be a lot of

corrosion to either or both. But it may be worth experimenting, since

rocket stoves work and they involve putting cob over a metal firebox &

flue, though its likely different metals.

While running solar system pumps with solar panels is a workable idea

(at least theoretically), since you shouldn't have much solar heat to

move if you don't also have the solar electricity, too, the same logic

is not applicable to heating water with wood. The wood heat is much more

intense and is more likely to be in effect when the sun is not shining

than when you have sun to make electricity. Thus you'd need to have a

big battery bank to store solar electricity for the pump for your

woodstove loop. Even with a battery bank, it should still be backed up

with a grid connection, because if the pump loses electricity while the

woodstove is still running, the water stops moving because it can't get

past the pump and the results can be catastrophic. That is why a

well-designed, time-tested thermosiphon loop system is safer than a

pumped loop, since the water is moved by the heat itself, and the hotter

the stove gets, the faster it moves, and the flow won't stop until the

stove is cold again. The Amish system uses a closed loop thermosiphon

but at atmospheric pressure with substantial air space at the top of the

tank that permits expansion of the fluid. This arrangement could

probably work for the external heat exchanger you likely need just the

same as the Amish internal heat exchanger. I wouldn't recommend a sealed

pressurized closed loop typically seen in a pumped system, because even

if the pump is fully functioning, the high temps of a wood stove could

cause pressure in the system to rise beyond the capacity of the

expansion tank to compensate for.

I know all this technical stuff is intimidating to you, so feel free to

forward my remarks to the Harlows directly, and to have them call me on

my cell phone (734-355-2528). In short synopsis of the above, w/out all

the explanations, for a system with antifreeze in it, I think the

off-the-shelf external woodstove heat exchanger replacing the heat

shield on the back of your stove (and possible side mounted exchangers

too?) might be your best bet, paired with a closed loop unpressurized

thermosiphon design with no pump, similar to what is described in

"Heating Water with Wood" from Lehmans.com. It will mean that you have

to burn lots more wood to get the tank topped off with heat, but the

anti-freeze in the system will allow you to walk away from the building

for weeks or months in the winter with no worse consequence than the

temperature dropping to close to the outside temperature.

Best of luck with your project!

Christina

Comments (0)

You don't have permission to comment on this page.