# Thermal Siphoning



## VonWolfen (May 24, 2004)

This one has us locals laughing at ourselves, but it has stirred an interesting debate. I am building a wood burning furnace in the basement and I'm using water through a radiator to blow hot air through the duct work. As I was figuring this out, I have come to a bit of a problem, and frankly I don't have a lot of time to build a mock-up. At any rate, for simplicity, lets say that I have a big vat of water sitting on a hot wood furnace...next to it is a big radiator. The two outlets on the water vat (bottom and top) and the radiator basicly line up. If I plumbed the top to top and bottom to bottom, which direction would the thermal siphoning cause the water to flow? If I plumbed top to bottom and bottom to top...then what? Mind you it doesn't really matter in some ways, since travel is all I want, but it would help if I knew which way the water would travel through the radiator. We have SEVERAL opinions locally and not much agreement! Wondering if someone could help!


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## SolarGary (Sep 8, 2005)

Hi,
I'd say that if you hook top to top and bottom to bottom the water will flow out the top of the tank into the top of the radiator.

If you think of all the water in the tank and radiator starting at the same temperature, then the water in the radiator will be cooled by the air flowing through it -- this will make the water in the radiator more dense, and it will want to sink down and out the bottom radiator hose into the tank. This will pull hot water in from the tank, which will cool and sink -- and so on.

I think you might be disappointed with the flow you will get with just the thermosyphoning, but I could be wrong. I'd consider at least leaving provisions for installing a circulation pump -- maybe a Grundfos or Taco HVAC pump?
You will get more thermosyphon flow if the radiator is above the tank and the pipe sizes are large with gentle curves. You won't get any thermosyphon flow if the radiator is below the tank (the cold water will be happy to just sit in the radiator as its the lowest point in the system).

The radiator and pipes will need to be completely full of water (I think) for it to thermosyphon -- do radiators have an air bubble at the top?

Gary


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## artificer (Feb 26, 2007)

I agree with SolarGary. The higher the radiator is above the tank, the more flow you'll get. The hotter the water in the tank is, the more flow you'll have. To maximize the heat transfer to the air, you want the radiator to be as hot as possible. Either hot water with thermosiphon, or warm water with a pump.

To get any flow, the bottom of the radiator should be higher than the upper outlet of the tank.

If you google on lister engines and thermosiphon, you'll see some examples of similar setups.

Michael


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## VonWolfen (May 24, 2004)

Thanks....I agree. The problem seems to come from the fact that there is so much intense heat at the bottom of the tank that some feel the "push" will be exerted on the small diameter outlet/inlet at the bottom of the water tank. Others feel the system will go static because of the localized heat. The radiator cap is another interesting concept...some here feel that the system will pressurize with a cap...not really a good thing. Others think that it is a good thing for temperature stabilization. I think it has to be open or I'll have an unsafe situation. I will have provision for a pump, I just havent decided on an auto water pump, which has the capacity for 160+ degree water or the electric mentioned by solargary. I do have an extra 80 watt solar panel mounted on the house that I can use..it is not being used right now since the exotic composting toilet system I bought was a total failure and I had it for the dual fans. Right now I'm trying to get the massive FTW truck radiator soldered up! We'll see what happens!


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## wy_white_wolf (Oct 14, 2004)

I do agree with Solar Gary, but something to think about.

Consider the old prec type coffey makers. Have both outlets for the tank on top. But one of them is hooked to a pipe extending down to the bottom where it makes a cone. When it "percs" it should send water to the top of the radiator where it can drain back down to the tank. This might even work better with a cross flow radiator than a down flow.

The radiator cap will work as a saftey valve to prvent over pressurizing. I don't think leaving it loose would achieve anything.


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## OntarioMan (Feb 11, 2007)

How close is the radiator to the wood furnace? 

If the furnace is inside the house, and you're going to move air through ducts to remote parts of the house - what is the purpose of the water and the radiator?


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## VonWolfen (May 24, 2004)

wy-white-wolf: I have thought about the perc concept and even a couple other possible mysteries. The perc concept better approximates what is happening in this application, since we are so used to engine applications..it is confusing at best. The draw back, I'm thinking, is that a perculation may in fact be a boil or approaching a boil, which I hope doesn't happen.

ontarioman: The use of water and a radiator does a few things better than just a hot furnace. The water volume is fairly large and therefore provides a large storage of heat. The fire can then be in an airtight furnace and a thermostatically contolled blower comes on when the water temp drops to 150 degrees...which fires up the wood and heats the water back up to 160. The radiator is a heat exchanger, like your car, only the temp is pretty predictable, and there are economies and comfort factors such as household thermostats and clean heat and such. That is my opinion. There is a similar system locally and I am fond of its operation, although there are some changes I'm making. In my application the radiator is about 6 to 7 feet from the furnace.


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## Al. Countryboy (Oct 2, 2004)

I bought one of those wood furnaces that was used in a basement with alot of the housing that went up to vents in different rooms of the house. The heater has a blower on the of it. It is in my attached green house at the back of the house. I put vents in the floors in a number of my rooms. It pulls the cool air from the floors and circulates it around the heater box and back into the house throught a window. It have a thermostat that kicks on the blower when the heater is hot and off as the fire goes down real low. Most of the time it will run till about day light before the fan starts cutting on and off. It has worked well for years. Not sure why your are wanting to go in the direction that you are going? Hot air will rise from the heater. You might try just putting some vents in your floors that will open and close and adjust to meet the temp. needs in different rooms. May could use a blower to circulate it even more. Maybe build a hood that will fit close to the top of the heater if you do not want to heat up the basement alot. We have a large home and other that having to replace the blower motor last year and opening and closing a few doors that go up stairs to regualte the heat down stairs which seems to get too warm before it get really cold, it has worked great. Just and idea. Good luck.


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## VonWolfen (May 24, 2004)

Thanks Al. Countryboy....I'm all for keeping things as simple as possible, and in many ways your system is better. There are a few reasons why I have set this up in this manner.....one is that the radiator will also provide cooling in the summer from water pumped from underground storage.....I also have a couple other issues that seem to make this workable, but I may end up with something like yours if this gets too complex.


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## Al. Countryboy (Oct 2, 2004)

I actually have had two systems simular to what your are doing. One I took 50ft.1/2 inch copper tubing and wraped it around the stove pipe of the heater and used a small pump to circulate it into a talapia tank in our sunroom. I was disappointed with the results, but the other which has worked very well for us is that I put a piece of about 1 1/2 or maybe 2 inch inch gal. pipe about 4 ft. long with elbos reduced down on each end to 3/4. This was installed in my chimney when it was built. I bought an electric hot water heater took out one the elements and hooked cold water coming in to the bottom of the tank where you drain the tank from and where the bottom element was I hooked up the 3/4 line going into the chimney and up and out of the chimney into the top of the hot water heater. The stove pipe from the heater comes out into the chimney at about where the pipe inside starts up. The hot water rises going into the top of the tank pushing the cold back up through the pipe in the heater. There is a pipe going off the top of the heater where the preheated water then goes into our elctric hot water heater on the opposite side of the chinmey. This simple set up has worked for at least 5 years now with no problems other that a little dripping from the gal. pipe until rust sealed them off. There are days when it is cold and we keep a good hot fire that our electric heater rarely comes on because the water in the other tank has gotten so warm that it does not cause the other heater to kick on. Not all of my projects work out, but this one has proven to more than off set the costs of the fan 110v that circulates the air into the house from the wood heater by not having our hot water heater 220v kick on very much.


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## Al. Countryboy (Oct 2, 2004)

How hot does a solar hot water get? I am not using it to circulate the water into my fish tank during the summer so it does not have water in it. It is about 32 inches wide and about 12 feet long. This is also something that I made last year. Not sure how hot the thing is getting up on top of my green house, but know that it smelled like wood burning while I was up there for the last couple of days and noticed that the 2x4's around the edges were chard from the heat. I have decided to cover it up to keep it from getting so hot when not in use or I am afraid it may burst into flames.


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## VonWolfen (May 24, 2004)

I am not quite certain as to what you are referring to when you say "how hot can solar get". Just so you know, my furnace is large enough to place two 55 gal drum on top of it, which will be the fluid for transfer through the radiator, probably using a pump like the ones on outdoor wood furnaces. I'm having a bit of an issue with the placement of the squirrel cage fan (a 18 incher). Some local experts say that there must be a return system to make the system viable with a fan that large. It is possible they are correct, even though this is an old Amish farmhouse and pretty "loose"


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## paperboy-7 (Feb 15, 2005)

hi always put a pop-off valve in any system this is a must. i have put in these systems for 40yrs. there is always a chance of stalling or backing up. you must plumb your hot supply rising to where your heatexchanger is. there are many sights that give plumbed systems, think circle of heat hot rises, w/no up or down in plumbing,,,then cold falls back to heating. your return, after you have used the heat must return to the LOWEST point or your flow will stall. as you start your system up feel along your pipes as your water heats it should heat going up be cooler back to heater.. BUT always use POP_OFF, that is plumbed to a safe place. be carefull, just my 2cents would hate to see anyone get hurt. later butch


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## VonWolfen (May 24, 2004)

Wow...40 years! You certainly are the most experienced. In that vein, I did happen to find some old engineering data from around 1920...it actually stated that the plumbing is top to top and bottom to bottom and that the inlets/outlets must match in height exactly for optimum flow. I just don't have the time for experimentation, so I've elected to use a pump specifically made for this type application. I have also elected an "open" system so that I don't have to face the "pressure" issue. Did you find that there was ample movement of the water to effect a decent heat transfer from the radiator or was it brutally slow? The pump takes care of that issue, but I'm all for reduction of parts and simplification. Thanks


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## paperboy-7 (Feb 15, 2005)

hi its so easy to run piping right but w/pump it will work anyway. your pump will always move more water. my outside taylor is an open system w/heatexchanger for hot water but cost 7000.00. i wouldnt have bought that heater because its not all ss but was here when bought house.. most people run a closed system so they can take house hotwater w/out 2 systems running w/one heater. good luck butch


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