radiant heat

Not everyone wants to play with rocket mass heaters for a variety of reasons.    This past weekend I was was watching a very interesting video about radiant heating with warm to hot water.  What I found interesting was that I have always seen it done with in floor radiant piping.  Because many of us are trying to build with homes that are passively heated the reality of in floor heating seems to be that there actually are some better ways.  What they were talking about is low temperature radiant emitters.  Think of cast iron vs light weight radiators.. Most of these are pretty much available from Europe  But site built was the ultimate low mass heater.  They were installing them low on the wall and on the ceiling ... but in places on the wall where they are not blocked by furniture such as sofa..  

So what they used was a layer of OSN then 2 inches of poly foam board with aluminum coating both sides. then most important aluminum heat spreader sheets http://www.builditsolar.com/Experimental/RadFloor  http://sc01.alicdn.com/kf/HTB1vqGnIXXXXXbtXXXXq6xXFXXXt/PEX-ALUMINIUM-RADIANT-HEAT-TRANSFER-PLATES-Aluminium.jpg_350x350.jpg/HeatSpreaders.htm  a couple of links to show the heat spreader plates.  Apparently this is the way to do this kind of install other wise you have spent a lot of time and money and you will be upset that your effort was a total waste of time. It is moving the heat away from the pex piping that makes this radiate. I looked at the FLIR pictures and it was super obvious  you NEED the heat spreaders.  

The reason for going this route for heating is that with modern well insulated houses the actual btu heat loss per square foot is tiny.   18 BTU per hour per square foot.  Consider this .. forced air heating might be between 75000 to 100000 btu per hour for an average size home that was built before the idea of well sealed and insulated homes became the norm.

Quick question, is "OSN" a typo for "OSB" or am I missing something here?
I use a Dvorak keyboard, don't recall where the B and N keys are in Qwerty layouts...

I'm going to assume it's OSB...
The European heater panel things are neat, aren't they? I tripped over them a while back and got ideas off them. I'd love to see them for sale cheap at Home Depot, wouldn't that be cool?

I'm doing PEX radiant in my house, most of it in slab, some below a floor, and if I need more, making panels somewhat like what you described. The stuff below the floor I'm using metal heat spreaders, but I'm going cheap, I'm a dumpster diving recycling sort, and am using second hand roofing metal. Compare that to the specialty aluminum spreaders, I'm thinking a couple layers of it will work as well, and be a LOT cheaper. The panel design I'll use if I need more heat is (looking at layers in order) foam, PEX, metal spreader, cement board (durock etc) and paint or something to make it pretty. The cement board makes excellent thermal mass to radiate the heat the PEX is spreading on the metal. I think thermal mass is an important part of the equation. The metal spreader moves the heat more evenly, but having it heat thermal mass up will take the output to an even higher point.

You are correct, a well sealed house does not need a lot of heat input. I have crunched numbers till I'm blue, and the house I'm building is more sq footage than the rental we are in (due to the basement in ours) the furnace here in the rental that doesn't do well at all against the crappy construction and bad design is a 32 ton unit. My numbers say I need a 1.6 ton system. Even if I'm very wrong, and I don't think I am, say a 3 ton system, that's a FAR cry from that 32. And the difference is due to lack of uncontrolled air infiltration and tight insulation. I'm putting in cheap windows for now, if I get more money I'll put in better ones, but I suspect that I can pull off cheap ones since there's not a lot of other cold air infiltration. The air intake will be temperature controlled immediately.

Something else to consider is the way humans feel warm or cool is not just air temperature.  From some notes I took (don't recall where)

The Mean Radiant Temperature (MRT) dictates what most of the other parameters need to be (excluding humidity) in order to deliver good comfort. A higher MRT requires a lower air temperature in order to deliver the same degree of comfort

Environmental factors:
   Air temperature
   Radiant temperature
   Air velocity
   Humidity

Personal factors:
   Clothing Insulation
   Metabolic heat

So temperature is only part of it, radiant is a major factor in it, as well as humidity and air velocity, but all most houses do is adjust air temperature. I'm designing for all of those factors for both heating and cooling. I'd love to see more homes take into account all of the factors that influence how we feel, and design to make them most efficient. The more I learned about how badly houses are designed, the more disgusted I got with it all. In my world the world would be moving towards the right in the picture...


Good topic to start! Great thing to discuss! I wish I knew how to make the whole world notice it.

the b and n are right next to each other.. should have been OSB.. oriented strand board.    

Have been looking at problems with super tight houses.. they are so tight that now we have to add mechanical ventilation to keep them from being sick houses. sigh..   32 tons of ac?? does your bill run you 5000 a month? that is massive.  consider for a moment.. those big ac units on the roof of places like walmart are usually only 7.5 tons each.. so you are talking 4 of those monster package units.

in an average house  with 8 ft ceilings one ton will cool 600 -800 sq ft of area I think someone gave you wrong info on the size of your hvac where you are. that is kind of funny.    anyway 1.6 tons is going to be 1.5 tons since that would be the nearest size.  heating is a bit different in the way it is calculated but close enough.  

As I watched that hydronic video I had always kind of thought that it did not always make sense to run the heat in the floor.  The biggest reason is the lag time and the reality that during the day when you are getting heat gain from the sun one would tend to over shoot the target temperature by many degrees after the system stops pumping.  This could actually make for floors that feel to warm.  What the presenter was saying was that low mass emitters respond quickly and the overshot in the temp is greatly reduced.  Walls because the emitters can also be on the wall actually are a bit more efficient since they have both radiant heating and convection from bottom to the top.  

As to the roofing metal great Idea I think. Though I think I would try to make the ridges deeper so as have much more of the pex in contact to transfer the heat.    Wish I had stolen the pictures LOL

they are so tight that now we have to add mechanical ventilation to keep them from being sick houses. sigh

 Yup, that would be the middle picture in the drawings I posted. I agree, that SO doesn't work. But neither does assuming your makeup air is going to be leaking in the cracks. I have been reading a lot of codes and construction stuff, and things like bathroom vents, the older codes assumed enough air leaking into a house randomly to allow you to run bathroom or kitchen vent fans with no makeup air. That's a LOT of leakage assumed! That's just bad design in my eyes. No wonder they need massive furnaces.

I tried to look up my notes on whether the units on my math came out in tons, the computer they are on is shut down for lightning storm, can't access them right now. The system I was working with calculated heating load in ranges of 10-20, 20-30, 30-40, 40-50, if you were above 50 it wanted you to buy two furnaces. On that scale I required 1.6. The one in this house (a heat pump heat/ac system) was a 32. I may be wrong about it being tons, might be thousands of BTUs, I did this math quite some time ago, and kept the concept in my head, but not the exact units, since they were irrelevant to my design, as I'm not buying that kind of system. (Edit, looking at Lowe's website, looks like it's thousands of BTU's, my apologies!)

 I would try to make the ridges deeper so as have much more of the pex in contact to transfer the heat.

Hmm, good thought. The metal I scrounged has ridges about the same size as my PEX, I'll check. Might be worth putting some clay/cement type stuff in there to tighten the contact up, easier than tweaking the metal, I think. If I were going to tweak the metal, I'd probably start with flat sheets, and bend them to fit. It was looking at prices of flat sheet stock that made me start scrounging old roofing metal. The flat sheets cost less than the ones they sell for PEX, but still quite a lot, to me. I'm paying for the things in my house that matter most to me to be exact, and scrounging everything I can so I can afford the expensive parts.

I put a lot of thought into my house design, I think I have the best I can come up with that fits the rest of my parameters (link is in my signature on my posts, Maison du Bricolage, you might find it interesting reading.) It's multiple systems, that work together, good airflow with the windows and design, good circulation within the house, makeup air controlled well, radiant keeping the thermal mass warm, and small amounts of heating/cooling to change air temperatures I need to change. It's not a fast twitch response system, can't just hit the thermostat and bring it up 5 degrees, it's a slow change constant mass system, that will get to temperature in a few days and hold there for months. I had a builder tell me "no one would tolerate a system like that, if you come in cold you can't turn up the heat!" Well, perhaps the problem is in their expectations, in all of human history no one could walk in, hit a switch, and expect the heat to slam on and heat the whole house to the exact temperature desired, in the last 50 years or so that's become required? I think it's a problem in human heads, not in heating systems.

I had always kind of thought that it did not always make sense to run the heat in the floor.

Not if the floor is in your sunny area, no, it does not. You would have two slow heat gain systems running concurrently and they would not agree on temperature. In my eyes, the radiant floors is for where you don't have solar gain. Mine is going into my basement slab, and under the bathroom floors, neither one which are solar gain areas. Because my climate is both heating and cooling territory, I'm not putting my thermal mass in a sunny area, it would just run amok in summer and require shading etc. Instead, my thermal mass is basement floors and walls, and a central core wall that has the main temperature control output air piped though it to keep it whatever temperature we are going for (heated or cooled) as the air gets circulated. The core wall will get a bit of sun in winter, but the main thermal mass temperature adjustment will be via the PEX (heat or cooled.)

Walls because the emitters can also be on the wall actually are a bit more efficient since they have both radiant heating and convection from bottom to the top.

Walls are also more efficient due to how bodies perceive radiant heat, it's in a line of sight pattern, so if you have only the floor being warm (with no convective heat involved, just the radiant) only your feet feel warm. If you have the wall being warm, your whole side facing the wall is warm. Radiant heat off the ceiling (especially a sloped ceiling) feels very natural to humans since it mimics the sun. Adding the convective to the radiant is why I'm a big fan of thermal mass, because it holds and disperses the heat smoothly. That's why if I need to add panels, they will have a chunk of thermal mass on them and good air flow around them. The rental we are in has one room we like for hanging out in, that has terrible heat issues, air leaks, insulation lacking, bad furnace ducting issues, air return in a bad place etc. I am not going to fix this place, so I closed off most of their system, put in a freestanding small electric radiant heater, piled some bricks in front of it and added a small fan in the right position. We call it Stonehenge In The House, the temperature in this room stabilized really nicely, and the electric bill went down.

Wouldn't be AWESOME if more people were looking at houses this way? This is, I think, the future of home design. I still look forward to the day I can walk into Lowe's and buy a good radiant panel heater. (Although me being me, I'd rather build my own system!) (How about: I look forward to the day OTHER people can walk into Lowe's and buy a good radiant panel heater!) The lack of knowledge that other options exist and expectations that have been trained by a "I expect it NOW!" mentality in society are the major obstacles to changing the way we heat our homes. I am glad to see the knowledge slowing starting to spread about radiant heat, I have no idea how to change people's expectations. The Rocket Mass Heater work that is being done on this site is awesome, changes minds toward radiant heating. But I agree with you, not everyone wants to build one, deal with wood, or fight with codes to make it legal. I am leaving in my house design the option to add a RHM in if I need it, but I'm hoping that I will end up not needing that much heat added to my house in the first place, if I have good insulation, controlled air inputs, and good design for my climate. To me it's a Permaculture thing, design the solution that fits the needs and reality, instead of assuming that off the shelf solutions are the best just because they sell them. When you design land, you start with water control, swales etc, not sprinkler systems, when you design a home, you need to start with air infiltration, thermal mass and radiant heat, not with furnaces. Sprinklers correct small issues with water in the yard after your swales, and I think home heat needs to be only correcting the small issues after the the main problems have been addressed.

Quite a rant, one of these days I need to quit posting while I'm drinking my morning caffeine :) Anyways, awesome to spread the word about radiant, and hopefully one of these days we can buy good panel heaters at Lowe's ....

I built radiant floor heating into my house, and lived with it for 15 years before building my RMH two years ago.

The beauty of warm floors is that if your feet feel warm, the rest of you does not need to be so warm to feel comfortable. I could be comfortable at 62F air temp and barefoot. Heated air rises, so the air temperature tends to be fairly uniform throughout the occupied zone. Heat from above puts your feet in the coolest region. My basement has a PEX-embedded concrete slab with south-facing windows, and I never experienced the floor being uncomfortably warm.

The RMH I have used the last two winters works excellently for no money and not too much work, but I do need 64F and slippers to feel comfortable in the wood-floored main level. (The basement still depends on the radiant system.)

Finally, I made the aluminum spreaders for the main wood-framed floor tubing from a roll of flashing cut to convenient size pieces, heated to anneal and soften it, and formed to shape with a simple home-made jig. Close contact between PEX and spreader is critical for best function.

loved your comment about the bricks.. made me laugh.. if you want thermal mass that is a bit more portable and actually more efficient.. collect 2 liter soda bottles fill them with water. they will absorb and release the heat more efficiently than the bricks.  Good places to add them are as high up near the ceiling as you can get them.. like on top of kitchen cabinets and the top of book cases. talk about scrounging

As to you hvac likely you have a 2.5 ton unit  which is 30000 btu but could be tweaked to 32000 btu depending upon the unit and the compressor installed along with the sizing of the a coil.  Yea I used to sell hvac wholesale so always had time to look at these things..  

I am a believer in thermal mass and passive heating and cooling.  One only has to walk in a cave . ultimate thermal mass.   I figure even using mechanical means  driving temps from 58 degrees to 70 does not take much .  I have been looking at pahs as a good way to imitate the mass of a cave.  So with that thought and to keep from over heating.. Overhangs make sense in my design. I am looking at a variation of the earth ship only without the junk tires.. ( I worry about off gassing even though some stay they are inert)  I like the idea of earth berming on 3 sides.  having a larger atrium space  Deeper at 12 ft.  I am not in the desert so having sloped windows makes no sense to me as they will cause severe over heating during the summer. During the winter the sun is low enough to provide daylighting and heat gain.  

What I have been considering is pre building panels with the osb insulation heat spreaders and pex.  The connectors are good enough to not really worry about leaks.  working on the flat at ground level is a lot easier than laying up on a wall or ceiling.  Gravity becomes my friend.

Commentary on radiant in bathroom. People go to electric because it has quick response. One normally is only in there short periods of time.  Do NOT put any near your toilet.. Why you ask.. Because the seal on the toilet is wax. Install radiant and you can easily melt this and just flush it along with the seal between the toilet the floor and allow sewer gas in.  Seriously OH YUCK.  The other thing when installing this sort of system  put them on a 15 minute timer.  No need to heat when you are not in their.. and it is not going to really change your energy usage in a meaningful way by heating or not heating the bathroom.  

the simplest jig I have seen for doing pex spreader plate is to just use two pieces of plywood  mounted with a gap just big enough to insert the pex tubing that you desire to use.. get a pipe with the same outside diameter as the pex tubing. and use aluminum flashing already cut to width.. This stuff is super soft so no need to anneal to make it soft.  it is also thin so there is little mass this allows it to spread the heat very quickly.    Anyway just align the flashing on the jig and step down on the pipe.. instant 'u' shape to drop the pex into.. gosh flashing is cheap ... cut into 2 -4 foot lengths and use a stapler manual or air powered to install them over the pex.

I chose to go with pex al pex because there is less problem with oxygen getting into the system causing corrosion in the pump.  I also went with larger piping for the simple reason that it has a larger surface area and a single loop can go 500 ft vs 250 or less with 1/2 pex.  Lots of folks put in many zones and a pump on each. Looks great but it is horridly inefficient.  Add up the total amps needed to run those pumps vs a single larger pump and the efficiency becomes very apparent.  One can still run all the zones as desired  there will only be a marginal back pressure difference happening in the larger pump.  Remember turning on pumps to cycle . Every time you turn a pump on it draws 2-3 times the current as when it is actually running. So  single larger pump that runs slow and does not cycle is going to give you more bang for the buck.  

Good morning! Quick reply while I eat before I get out the door...
Bricks were what I had expediently when I got annoyed one cold morning, I grabbed 3 landscape blocks off the tree ring in the front yard of the rental. Put them on a thermal break platform to keep heat and dirt off the carpet, and placed them in front of the radiant heater, put a small fan on low that blows upwards diagonal between the heater and the bricks. I leave the fan on 24/7, and the heater on low, this room is nice now, and it was really bad before. Took the bricks a day or two to dry out and warm up, but then the thermal mass effect kicked in, and it works well.

I, too, have seen the lots of pumps type bad designs. I am using for my reference book Book on  Hydronic Heating  (not a book for people who don't like college level textbooks) and I found the author by a series he does on a site called The Glitch and the Fix  The Glitch and the Fix I liked his style and his knowledge. One of his things is don't overdo the pumps.

I appreciate your comment about too close to the toilet melts the wax ring. Wondering I can thermal break it, I would love to end up with an air flow duct that blows on the toilet to warm up the cold porcelain, as thermal mass in the bathroom, as well as decadence, but the wax ring could be an issue... Need to look up wax ring melt temps...

Out the door....

Steve - that is exactly what I did for a jig. I screwed an actual piece of PEX onto the jig for inner form. I made the sides of the upper form with a taper so I could squeeze it after pushing down, and crimp the flashing tighter around the tubing. The more contact between PEX and spreader the better.

there is much to be said about keeping things simple.  both for the shooter board to make the flashing and reading about what people who actually have been successfully installing radiant heating are doing.   Some things work  and somethings work a LOT better for the same efforts and inputs

Laughing you gave me the link to the book I had been looking at.. the video was by the author of this book. how funny is that?

Heh, cool! He is a wise man, I like his style. He thinks like I do, I can understand why he does things. I despise people who do things "because this is how we have always done it." with no good logical reason they can explain.

I don't watch video if I don't have to, I read, that book is the current textbook at the colleges, new edition, I couldn't get it used, one of the few books I have paid a lot of money for (Mollison's big black Design book was another) some things are worth it to me. The local library gets me a lot on interlibrary loan, the lady who does it says after seeing what I have borrowed that I should get a degree when I'm done. She says she flips through them, doesn't understand much at all. She was invited to see the house when it's done, and is looking forward to it.

Am not looking to to much permaculture.. but am looking forward to doing a lot of sheet mulching.   Hope to get fresh mulch from tree services  I am more interested in building swales to slow the flow of water.

Our house has radiant floor heat. We love it. I designed and built it. The PEX was pretty cheap. It is a very inexpensive system, and extremely efficient.

We are adding a Solar-Thermal loop onto our existing system. Yes it gains a lot of heat during the day, the idea is to put that heat into a thermal-bank somewhere, until you need it [like at night].

I live in a cave and can keep 18`c 64`F all winter wih 10`c 30`F outside. I try my best to up the temps in summer and keep the rock free of plants, for heat accumulation.

I was thinking to add some mass heating by having a hot water tank, and heat the water 10meters above the cliff, where I have more sun. Then send the water to the cave tank at the end of the day...
Can it work?

I have talked with a natural builder from Northern Germany that has used radiant wall heating in many projects. His company has lots of construction photos on their website, here are a couple of the projects where the  PEX tubing can be seen.

http://www.naturbau-angeln.de/stamer/
http://www.naturbau-angeln.de/kaehler-friedrichsstadt/

The plaster the PEX with clay so there's no need for the heat spreading fins.

Worried about wax melting???

Try a WAXLESS SEAL!

Now you can heat your bowl with pex run very close to the base.
the only worry I would have is water evaporating faster, and running dry if you are not home for awhile.  

Perhaps a shutoff at the toilet area for when you are away?

David would be interesting to see flir pictures of the heat in those walls.  The point was heat spread along with insulation on the back side so the heat goes where you want it to.  plaster is fine but I do not know how well it spreads the heat.. Much like people doing straight staple up with no heat spreading plates.  There seems to be a monster difference in how well the heat is pushed out to where one wants it.  When I first started my research it was just staple up and some were doing the heat spreading plates..   Years later seeing the feed back on one vs the other..

One can play games and hope for the best or one can just get it right the first time Actually paying attention to those who are experts.  Personally I prefer an expert over a jack leg even if they are in Europe.  Have fun with whatever you choose..   Personally I only want to do it once and not have to retro to fix what I already knew would not work as well.  

Galan   I have also considered how to store the heat.  Big insulated plywood tank, with a foam cover.  solar panels that are drain back and into a long loop of poly just immersed in the tank. then another 2 loop of poly to supply both heat for the radiant and domestic water.  Thinking my tank needs to be 4 x 4 x8 feet nominal.  Once the tank is heated should not be a need for any external heating.  But I think a small water heater would not be a bad thing to add for the domestic  30 gallons for back up.   I am building passively and insulated so I am fairly sure I wont need to play games with the heating loop

xisca  with a big tank and PANELs that actually put the heat into the tank you can get the tank of to 120 degree easily.. adding radiant panels in your living space should allow you to heat just where you want the heat.  Being in a cave you will need to insulate the back of those radiators  and reflect the heat out where you want it.. remember radiant heats everything not just the air..   the rock is going to absorb the heat

Ben I think those waxless rings are some sort of foam I do not know how they do with heat  but am guessing that if one is only using 105-110 degree water should not be a problem with them.

Brad Hengen:
Added to my shopping list! Thank you! I knew they existed, had not looked up where to find them. :D

steve pailet II wrote:xisca  with a big tank and PANELs that actually put the heat into the tank you can get the tank of to 120 degree easily.. adding radiant panels in your living space should allow you to heat just where you want the heat.  Being in a cave you will need to insulate the back of those radiators  and reflect the heat out where you want it.. remember radiant heats everything not just the air..   the rock is going to absorb the heat

I think I want the radiation in the rock, so it would add to the summer heat, that is for sure what heats me right now...
Also, I would put it in a place that is outside the cave, with some thickness of rock but not much, and by going through, it will reach the living room! As it is small, I cannot take any place from the living space. The tank would be closed from the outside too, no need to heat the terrasse...

It would also be in the kitchen place. I wa thinking of buying a wine container in stainless steel... Maybe get some hot water from it at the same time?

Xisca Nicolas  when I was talking about the tank I never said it had to be inside .. pipe in the water and insulate the pipe and pump

steve pailet II wrote:Galan   I have also considered how to store the heat.  Big insulated plywood tank, with a foam cover.  solar panels that are drain back and into a long loop of poly just immersed in the tank. then another 2 loop of poly to supply both heat for the radiant and domestic water.  Thinking my tank needs to be 4 x 4 x8 feet nominal.  Once the tank is heated should not be a need for any external heating.  But I think a small water heater would not be a bad thing to add for the domestic  30 gallons for back up.   I am building passively and insulated so I am fairly sure I wont need to play games with the heating loop

I have previously built large tanks, I found that it can be a much larger project than what I had expected.

My thermal-bank that we have been using the past 10 years is a series of dead water heater tanks.

As we shift over to using Solar-Thermal, I have a new thermal-bank setup. It is a Horse trough, 8foot in diameter and rated to hold 750 gallons.

steve pailet II wrote:Xisca Nicolas  when I was talking about the tank I never said it had to be inside .. pipe in the water and insulate the pipe and pump

My idea is with no pump, no use of electricity but of gravity only.

I want to heat the water above a cliff, send it down to the tank, and that the tank act as a termal mass, heat by radiation and conduction to the rock, and then I move the water to the garden...
And pour down more hot water from above! I was thinking to heat this with black water PE pipes.  

sounds like you might think about doing a thermo siphon heater, but place the heat tank and place it below the cave   This would be totally without pumps or electricity

steve pailet II wrote: This past weekend I was was watching a very interesting video about radiant heating with warm to hot water.

Care to share a link?

wish I could but honestly it was from a link and I did not look it up.. I just clicked that link

steve pailet II wrote:sounds like you might think about doing a thermo siphon heater, but place the heat tank and place it below the cave   This would be totally without pumps or electricity

I would not have enough sun... that is why I want to heat the water above, and send down the water. Of course it is possible to make hot water go up, but I am limited by room and sun below the cave...

that being the case  get a solar panel and battery.. let it run a small circulating pump

steve pailet II wrote:that being the case  get a solar panel and battery.. let it run a small circulating pump

Do you mean it is impossible to heat water and send it hot downward to the tank where I want the heat? And then dispose of the cool water again downward?
I really have cliffs and very vertical lines!

Yeah, the radiant heat is the cheap way to maintain the temperature of our rooms and floors. It is sustainable way of using the resources.

Xisca Nicolas wrote:
Do you mean it is impossible to heat water and send it hot downward to the tank where I want the heat? And then dispose of the cool water again downward?

Xisca: Yes, it is possible to heat the water up top and dispose of it downward.
The problem you are having is just terminology, most "radiant systems" are a closed loop, to one extent or another, the water is reheated and recirculated many times, so that's what he is expecting you want. What you are describing is called an open loop: heat the water once, and that's all. So the problem isn't with your idea, it's just with the words being used. Basically, the pump is to take the water back up to be heated again and used again. If you don't need the water to be used more than once, a pump is not required. But if you wanted to reuse it, pump it back up the hill and heat it up again.

I would suggest if you are only heating the water once, heat it as hot as you can make it, so you get the most effect from it. Most of the radiant systems use fairly low temperature water (under 100 F) and just keep it circulating. If are doing as hot as you can and you use a wine container (I don't know what a wine container looks like) make sure your domestic hot water use line comes off the bottom so you are using the cooler water off the bottom of the tank and the hotter water on the top of the tank is heating the cave up. I'd insulate the front of it, so the heat goes back into the rock, but make it so I could could take that front insulation off easily if the room was just too cold and I wanted a bit more heat.

In a closed loop where you are recirculating water your circulator pump can be a pump with a small motor. A 1/25hp motor is fairly standard.

But if you wanted to have an open system, and to pump water up hill, you may require a much larger motor.

Xisca Nicolas wrote:

steve pailet II wrote:that being the case  get a solar panel and battery.. let it run a small circulating pump

Do you mean it is impossible to heat water and send it hot downward to the tank where I want the heat? And then dispose of the cool water again downward?
I really have cliffs and very vertical lines!

if you think heat runs down hill do this.  boil water  in a pan.. put your hand over the top of it. see if you feel heat.  now put your hand below and off to the side.. you tell me which is hotter  hear rises  this is called convection.  Radiation the heat will go all around.  You are not radiating heat till it gets where you are going.. you have a choice you can pump or you can forget it.   Unless you can figure out how to ignore basic thermodynamics.  I doubt that any human has ever figured out how to do this.

What you are asking cannot be done without a real way to move the water and hence to move the heat.  

A comment about falling water.. that and dumping is the only way to get the water to go downhill.    A note about efficiency.  The hotter you get water and store said water in general because the water with a much higher deltaT   that is the heat in the tank vs the heat in the air trying to cool it down  the more inefficient per same volume of water.    When you have a much larger tank at a lower temperature you are going to have much more efficient storage.  All I know is this is a physical reality.  When there is a lower ratio of temp to temp per volume you are going to have that lower temp water stick around.  Most of us have little use for water at 160 unless you want a nice cup of tea.. But.. at 120 you can put your hand in it and wash that tea cup and heat your home

Pearl Sutton wrote: terminology, most "radiant systems" are a closed loop, to one extent or another, the water is reheated and recirculated many times, so that's what he is expecting you want. What you are describing is called an open loop: heat the water once, and that's all.

Yes I want to dispose of the water, of course I will need a little water tank before watering the plants! Maybe I can find a way to wash clothes or take a bath!

steve pailet II wrote:

if you think heat runs down hill do this.  
What you are asking cannot be done without a real way to move the water and hence to move the heat.  
When there is a lower ratio of temp to temp per volume you are going to have that lower temp water stick around.  
I think of course I will loose some of the heat in the pipes that will send the water downward. I want to heat the water, and then in the evening open so that the water comes down to the cave tank and fill it with the hot water.
I was thinking of big volume like 300 liters... and get the heat by conduction through the rock.
Some examples of sainless steel containers...I even know somebody who did a rocket stove with one of them!

http://www.kegsmanufacturing.com/tanques_acero_inoxidable_alimentos_despacho_inox_bebidas.htm

all I can say is keep us up to speed on what you decide