Thermosyphons - do others have experience to share?

I was intrigued to see the thermosyphon idea introduced for the Wofati designs mentioned here.  I've only been lurking but am interested in passive heating and cooling.  

This thermosyphon idea was to put a tube (copper I think) into a deep hole for an enclosed greenhouse type situation (the Wofati is something entirely different but I'm simplifying-- and like the idea of it possibly existing in a greenhouse).  The top of the pipe is exposed to the sun and as it warms it draws up air from the depths of the hole (which could itself be a cold sink).  This will circulate the air, both heating the cold air as it rises and cooling the ceiling as it exits the pipe).

I'm interested in using this idea for multiple situations.  Currently I'm installing a stove to heat my home although I want it in my main floor.  My basement is always very cold so I was thinking of utilizing a thermosyphon in a way similar to how this person does with his stove.  

The idea is maybe sucking up the cold air by installing a pipe drilled through my floor, I could passively replace that cold air with the stove on my main floor.

My questions are.  Regarding thermosyphons, operating in the normal comfort levels a person may use them, are there formulas that are helpful to know?  Like how big a pipe should be for a given length?  Or what materials are best?  Copper vs Aluminum vs a combination of materials to include something insulating (for instance, below the heat source).  

Thanks for posting this question Seth! I too have been looking at the greenhouse posts to better understand passive approaches to moderating indoor temperature swings using ideas related to passive heat exchange via convection: low spots, deep pipes, and risers.
My experience is unfolding:
A few years back, caliche prevented the extreme runoff from a 100-year flood from percolating into the soil. Excess runoff over-flowed water catchment ponds and flowed into my semi-subterranean living space. To remediate, I took a garden hose with PVC extension and pressure drilled a 9 foot deep 6” diameter hole in the earthen floor, past the caliche layer, and into coarse sand to drain the room. The water percolated through the sand fairly quickly and the room drained. To keep the drain for an unlikely future event, I lined the hole with clay pipe.
So, now I have a cover on a 9 foot deep (plus sand below) emergency drainage system that is unused. Several questions have emerged: could that “cold sink” or earth’s heat rising through this dry pipe help heat the room in winter or cool it in summer? Would a raised platform over this clay pipe provide more warmth than sitting over the uninsulated earthen floor? Shall I insulate the platform seating at ground level or extend the pipe upward?
Like you, I have looked at the subterranean greenhouse threads to better understand passive convection approaches to normalizing wild temperature swings and use less fuel to heat the space. There seems to be some connection to Wofati but I’m not finding optimal depths and straightforward applications for greenhouses or dwellings.
Anyone else experimenting with thermosiphons / passive heat exchange / natural convection?

Thanks for adding to this, Amy. I’d say your clay pipe is at least a cold drain considering that size. Have you taken temperature measurements inside it?  

In my situation I hope to replace cool air in my basement with warmer air. In the greenhouses I believe this deep hole would gather the cold air to then also be replaced by warmer air using this thermosyphon. Perhaps it would serve as a climate battery of sorts.

In your case I believe you may need the pipe to contain a smaller diameter pipe which extends much further up where it can be heated by something to create an updraft.

Hey Seth,
My response here will bump up your question and I hope someone with experience will read you question and give you some ideas.
By the way, your idea of taking some temperature readings at various depths is an obviously really important data gathering step. I appreciate your question about that. I just ordered a thermometer with a long probe so that I can find out the temperatures at various depths as you suggested. Thank you for your help!
Amy

Bumping this thread again.
Interesting news: the temperature below ground is downright balmy! The thermometer with a 10’ long probe (Taylor brand) arrived yesterday. The reading at 10’ (the hole is actually 12’ deep and the sand is bone dry) is steady at 61 F (checking regularly). The ambient room temperature without a fire fluctuates between 35 and 45 degrees F. A thermosyphon would be amazing if I could bring that warmth up into the room.

Sort of yes.  I did the solar collector on the outside of the building and was sucking off the basement floor inside and running it outside thru the collector and back in thru a clerestory window upstairs.

First thing while things are cool it works okay.  But as the air upstairs warms up it comes to a stop as the upstairs warms up.  Your stack effect begins to taper off and basically it plugs off with the warm air upstairs.  The lower the draw the warmer the air gets.  The warmer the air gets the harder it is to get to go down.  If you could stand enough upstairs heat it would probably keep working.  But I can tell you that the house temperature at nearly 100 was not enough to let it function decently on convection

I then added fans that do roughly 6 air changes hour in the basement to it.(that is doing roughly 4 air changes an hour upstairs)  That almost does it.  The simple problem is that if I want to heat tons of concrete down stairs then I have to haul tons of air down stairs to do it with.  But this is still not enough.  I need both more air changes and the ability to carry air down after I quite getting gain from the collector while the upstairs is still warm.  If you study the greenhouse info they are suggesting 5 or 6 air changes an hour minimum.  From what I have seen here that is still not enough.  Ceres information from one of their early videos that I can no longer find suggested that it should be a minimum of 10 air changes per hour.  From what I have seen guessing they are closer to right.

Now one other catch on this is that you can warm the walls of the basement up nicely but making a real change in floor temperature is nearly impossible.  I had counted on being able to warm both the walls and floor up to be my  thermal "battery".  The walls warm up nicely as expected.  But even having gotten my intake down to a strip 1 inch high and 12 feet long I can't get the floor to change temperature much.   1 inch off the floor the temperature was 76 degrees but put the sensor down on the floor and it was 60 degrees this fall even after weeks of the basement climbing into the mid 70's every day for weeks.  After a bunch of study I see it is a combination of stratification and boundary layer protecting it.  Either I need to put heat under the floor or I need to physically scrub the air flow across it to see real change.  I balanced a 2 foot x 2 foot box fan on (3) 5 gallon buckets pointed straight down and ran it on high for 24 hours.  That got me about a 6 foot diameter circle where the concrete was nearly room temperature.  I could feel that circle for about 3 days in the temperature of the floor vs 10 feet away on either side of that spot.

Here is the write up on the collector in case it contains anything else that might help you.

https://permies.com/t/106115/Thermal-solar-collector-panel

DC fans are way cheaper per cubic foot of air in the surplus stuff.  Now if you are using it in a house suggest both quiet and power use should be on the shopping criteria.  44 dB is satisfactory but the 68 dB that I got to be the second circulation path is loud.  I got lucky on the first set of 6 and got both quiet and efficient.  Got 2 more with twice the flow rating.  The new fans move twice as much air but take 4X the power to do it and are very noticeable from their noise running.  Now I need to get them installed to see if doing 10 air changes an hour with the basement will help.

Interesting thread. We are in a sprawling two-story off-grid on 3' concrete pylons in NE Ontario (-15F/-26C this morning), heated by a three-plate cookstove with an over-the-burners warmer and side-on oven with a diverter.

The stove gets external air from a partially enclosed tool room built on to the original structure, and has a very high, very well-insulated chimney that feeds through the second floor for direct radiant heat.

The original building also has large extensions to the front and back that are really too big to heat with the stove but employ clever use of levels to sink some of the cold air coming off the front door/dance floor area. On the other end of the house there is a cold room that holds the pressure tank for our well -- a trap door leads to the area which is directly underneath the tub.

The last tenant here was leaning on the crazy-old-coot button pretty hard but did some sort of small appliance repair, so he had skillz. One of the more rational modifications he made was to install 6" ducting along the ceilings leading from just above the stove to a filter box and then out to the coldest parts of the house, including the area under the always-cold tub and adjoining bathroom area. There is also a shorter duct just through the ceiling above the stove that goes up to the second floor again and bends around a bit to spread some more hot air around. That area gets a lot of draw because the second floor above the forward addition has a 20' loft ceiling with optional venting near the front roof.

There is a big DC fan on the filter box above the stove, but instead of pushing the warm air out, I believe he has it set to draw the cold air back to the stove area. Presumably the convection is thus improved to heat the house better, but it is hard to measure how well it works. It does seem to help with air quality and keep mould down in the cold room. And certainly the cold air intakes seem to be cold...

Because there is so much space under the house I was wondering if it would be useful to build a simple rocket stove *outside* and vent the exhaust through ducting directly under the floor. With some effort we could run one long pipe directly down the center of the building right beneath the central corridor and under the bathroom floor just short of the tub/cold-room foundation.

Most of the building is skirted in the winter so we might even be able to contrive a defractionalization effect. (Is that the word? i.e., A relatively open space to let the cold air settle out and the hot air rise to floor level.)

Many years ago I attempted to bring fresh air down an unused flue of a 2 flue chimney.  The chimney was 40 feet high in the center of the building and the used flue was around 150 degrees warm.   It was just about impossible....

Now "hot" and "cold" are relative and I have been intrigued by the wofati concept of digging a trench to let the "cold" floor air fall down and push up the "warm" deep trench air.  I would love to  know what actually happened in that situation.  Assuming the "cold" floor air was anywhere from 32 degrees to 0 degrees F. and the year round temperature below 4 feet was 40 degrees F., here are my guesses:
1)  At 32 degrees; with equal volume air in the trench and in the wofati, maybe 1 or 2 degrees warmer....
2)  At say 24 degrees or lower; probably no measurable effect....

Please someone who did this; let us know!

Razer

I have read the "white paper" from Verge Permaculture on solar greenhouse construction, which uses a thermal battery concept and they have worked out which factors contribute the most to its successful operation.  The one guy Rob is an engineer and its technical information we could all use in designing our structures.   https://vergepermaculture.ca/  Somehow I can't figure out how to paste a url into a thread, sorry folks.

Any way if I remember correctly the type of pipe doesn't mater and the battery can be uninsulated and just plain old dirt.  I'm pretty sure plain air doesn't work as well and that's part of the problem with using a basement to store heat.  I am curious does it heat he basement air well?  I could use something like that in my basement for when I am working down there that could be turned on and off when needed.  I can live with a cold cement floor.

C. Letellier wrote:


Now one other catch on this is that you can warm the walls of the basement up nicely but making a real change in floor temperature is nearly impossible.  I had counted on being able to warm both the walls and floor up to be my  thermal "battery".  The walls warm up nicely as expected.  But even having gotten my intake down to a strip 1 inch high and 12 feet long I can't get the floor to change temperature much.   1 inch off the floor the temperature was 76 degrees but put the sensor down on the floor and it was 60 degrees this fall even after weeks of the basement climbing into the mid 70's every day for weeks.  After a bunch of study I see it is a combination of stratification and boundary layer protecting it.  Either I need to put heat under the floor or I need to physically scrub the air flow across it to see real change.

You could do both by building a raised platform that covers the entire room like a second floor, but is only an inch or two tall. Make it so the hollow spaces channel the air underneath, acting as an extension of the system.