radiator heat question...

[MomOf4]

DH is in an invention mode right now (no complaints here) - he's working on some solar and wind stuff, but he had this idea for a radiator, and I think it's a good one, but want some input...

He wants to wrap the stovepipe on the woodstove with 30-40' of flexible copper tubing. He wants to hook up some sort of pump, run by either solar or wind power with battery backup. He'll run water through the copper tubing to be heated up by the stovepipe. Then, the hot water from the copper tube will run under the floor, insulated, and run it into the corner of the kitchen or dining room to a radiator to add heat in those rooms. In the dining room, the furnace would pull the heated air throughout the house.

Thoughts, ideas, etc?

[suitcase_sally]

Solder the pipe from the woodstove to a finned baseboard radiator. Loop the radiator pipe back to the woodstove. You really don't need any motor to circulate the water in the pipe. Natural convection will do that. You will have to have the water leaving the woodstove up higher than the return line as water will cool and sink and that's what moves the water thru the line. I have hot water baseboard heat and when the power goes off I still have heat based on the convection principal.

[ericjeeper]

pressure relief valve and an expansion tank. Otherwise he has invented a bomb

[MomOf4]

Thanks suitcase sally!!

Thanks eric - gave me a good laugh...he probably knows that (being a specialist in automotive AC and condensors), and I think he mentioned it, but I had no idea!

[Nevada]

Originally Posted by ericjeeper

pressure relief valve and an expansion tank. Otherwise he has invented a bomb

Of course there are heat transfer fliuds available. If the conditions are likely to boil water then water isn't a good idea.

[suitcase_sally]

Originally Posted by MomOf4

Thanks suitcase sally!!

Thanks eric - gave me a good laugh...he probably knows that (being a specialist in automotive AC and condensors), and I think he mentioned it, but I had no idea!

When I originally answered, I was thinking you were referring to a car radiator. Then I thought, "no, can't be". I guess a car radiator would work, but you would get better heat distribution from the finned baseboard strips.

If there is the possibilty of the water ever freezing, put antifreeze in the lines. Don't use automotive anti-freeze.

[Cabin Fever]

I would think that 30 to 40 feet of water-filled copper tubing might be too much weight for simple stovepipe to hold up. I'd test if for a while without a fire in the woodstove to see whether it all comes falling down or not.

And remember, it's not a great idea to extract too much heat from the stovepipe. This may cause excessive cooling of the rising smoke which will promote creosote condensation in the stovepipe and chimney and increase the potential for a chimney fire.

[Beeman]

Originally Posted by ericjeeper

pressure relief valve and an expansion tank. Otherwise he has invented a bomb

Very important. All fluids will expand under heat and a safety pressure relief valve is necessary.

The price of the copper should be enough to scare you, especially for an experiment. I have seen the copper coil around the stove pipe done before and it will work. Could possibly make a heat exchanger that attaches to the stove itself using a coil/condenser assy from an industrial AC or something from the scrap yard.

[MomOf4]

Cabin fever - good point on the cooling of the pipe. But, we have double wall pipe, so I would think it wouldn't affect it too much.

Beeman - ya, we priced the copper. It would cost us about $60 for 40' - it is expensive. Glad to hear this has been done before - I'll pass that idea along to DH.

Thanks!

[Beeman]

With double wall pipe you might not get as much heat or enough to make it worthwhile. How hot does the outside of the pipe get?

[ET1 SS]

Originally Posted by MomOf4

DH is in an invention mode right now (no complaints here) - he's working on some solar and wind stuff, but he had this idea for a radiator, and I think it's a good one, but want some input...

He wants to wrap the stovepipe on the woodstove with 30-40' of flexible copper tubing. He wants to hook up some sort of pump, run by either solar or wind power with battery backup. He'll run water through the copper tubing to be heated up by the stovepipe. Then, the hot water from the copper tube will run under the floor, insulated, and run it into the corner of the kitchen or dining room to a radiator to add heat in those rooms. In the dining room, the furnace would pull the heated air throughout the house.

Thoughts, ideas, etc?

We did that once.

If you do it in stages, you're fine.

I had the water in a drum so it was an open loop, pumped up through the coils and back into the drum. It heated the drum's water very nicely too.

Then later I added a second loop to take the heated water from the drum to an automotive radiator with a fan to heat the far end of our home.

As open loops you do not need pressure relief valves.

Have fun


Our home now uses 50' of copper tubing wrapped around our wood-stove's upper chamber.

[MomOf4]

Originally Posted by Beeman

With double wall pipe you might not get as much heat or enough to make it worthwhile. How hot does the outside of the pipe get?

We gauge both inside and outside temps to avoid overburn and causing a chimney fire...the outside reaches 200*F...

[haypoint]

The copper pipe won't hold much water, so the pump would have to move it very slowly to get the water hot enough to be of much use. You'd need over 100 feet of 3/4 inch pipe to have it get hot enough by the time it begins the coil until it heads to the radiator. Water tem would have to be at least 150 degrees to produce a noticable heat from the radiator. Neighbor tried to use a radiator and fan to heat his clothes dryer. Didn't work, needed lots higher heat, above 200 to create enough hot air to dry clothes. Heating a room would be easier, but you need a lot of very hot water to make it work.

[PyroDon]

Rather than coil around the flue why not make your coils and embed them in plaster paris made to fit the side of the stove . it will retain more heat around the coil .
if your using an open system dont use iron or steel pipe as the water will pick up oxygen from the air and cause corrosion.
If you have a salvage yard you may be able to pick up a stainless steel tubing coils

[Jeff54321]

The pressure safety valve and expansion tank are essential. The advice to take heat from the stove itself rather than the pipe is also very good.

Have you considered what it will look like if you wrap the stove pipe with copper tubing. Imagine the tin man meets an empty milk truck in a head on collision. I've seen what you are contemplating and "pleasing to look at" does not come to mind....Okay in a work shop not so good in the living room.

[ET1 SS]

Originally Posted by Jeff54321

The pressure safety valve and expansion tank are essential. ...

Only in a pressurized closed loop.

[Cabin Fever]

Originally Posted by MomOf4

Cabin fever - good point on the cooling of the pipe. But, we have double wall pipe, so I would think it wouldn't affect it too much....

We have doublewall pipe coming out of our woodburning stove and most of the time I can put my hand on it. Your idea would work much better with singlewall pipe.

[Nevada]

Originally Posted by Beeman

Very important. All fluids will expand under heat and a safety pressure relief valve is necessary.

Only if the heat transfer fluid can boil at the conditions in the system, and (as ET1 SS pointed out) only if the system is closed so it can pressurize. If you use water for the heat transfer fluid in the contemplated system it will most likely boil at times, so a pressure relief valve should be installed.

However, an inexpensive heat transfer fluid could be selected that won't boil at the anticipated system conditions. While some thermal expansion will still occur, fashioning a small vertical expansion leg should be sufficient to protect the system.

Moreover, a constant liquid phase heat transfer fluid will be more pleasant than listening to water bump in the heating coil.