Internal hydronic convection walls!

Bit of a mouthful maybe.

Designing a single story long house with a solid fuel stove - looking for a way of getting heat to the rest of the building. There's an internal, non-structural wall that cuts through the centre. It could be double stud, with heated pex piping inside and adjustable top and bottom vents to create a convection current in adjoining rooms.

...any thoughts as to why this might be a bad idea?

I think its cool.

i wonder what the best way to drain the system if it needs maintenance or to winterize the home if going away for a while.

Perhaps the in-wall piping can be two horozontal runs interconnected by many vertical connections,  the verticals allow convection, the bottom run allows drainage.  

I also think you might want a way to regulate the system pressure, to keep it at ambient pressure, a long "chimney run" open,on top,could be,made.  If a cloaed system is more desireable a tank with a bladder.  Both the liquid and the pipes will change volume with the temperature but not at the same pace and to the same extent.  I'd guess system pressure would rise deapite pipe expansion.

How would you set a maximum temperature on the convection fluid (water or anti-freeze or,what not).  Is this fluid heated directly on the stove and at risk of boiling? or is,it,moving heat from hot,room?

obviously my fat fingers like hitting comma instead of the spacebar and i have a hard time hitting the "s".  My apologies to the type-editors who had to read my post above :)

Nice idea!
I would consider aluminum or steel panels, so you would get radiant heat as well as convection.
They could double as access panels.
Keep in mind, any hollow  wall could be poor at blocking noise.

I do not think the idea will work because this is radiant heat.

Radiant floor heat does not heat the air, it heats only the objects in the room that thereby heats the air. This is a huge difference because you must get heat to the objects. Typically this is done through the floor. If you put radiant heat in the wall cavity, it will not "project the heat out", all it will do is heat the wall so your effective radiator is 6 inches wide and x amount of feet long. That is not enough radiator to heat a home no matter how high of a water temp is running through the wall. Even then, high temps is not how radiant heat gets its efficiency.

But I am not all bad news. I had the same problem as you in one of my houses; half of it had radiant floor heat with hydronic tubes in the concrete slab, but the older part did not. So to get 100% radiant floor heat, I hilti gunned 1 x 3 strapping down to the concrete floor a sleepers, every 16 inches, than ran pex between the strapping. Then I filled in between the strapping with sand to get me the same heat sink ability as concrete, but without all the mixing, then I laid flooring on top of my sleepers and screwed them down.

It worked extrmely well and that house has had 100% radiant floor heat for several years now.

It may work like a Trombe wall
Also radiant heaaters are ofren used as small panels in European homes.

i wonder if your design would have a cooling effect too?
(would be nice for us folks in the tropics)

Thanks for your comments, lots to think about.

I would run the loop from a thermal store - heated by a stove and other inputs.

I had imagined this approach would function like a trombe wall too - but perhaps I'm missing something. The wall is lacking the thermal mass in a trombe - but wouldn't the cavity heat up like a "bell" - and create a convection current?

the differences between your wooden wall with hot pipes in it and a trombe wall are:
-the surface area which is heated is huge compared to the very small surface area of your pipes;
-the sun heats the trombe wall for free while what is heating your water in the pipes for how much?;
-the surface area air  travels through is smooth in a trombe wall aiding convection flow whereas your wooden surface area would need alot of work to get that kind of smoothness ...pure convection flow doesn't need a lot of friction to over come;
-the trombe wall's heat delta on its face where it draws air in is higher than your wooden wall since your heat source is so area limited ...it is possible once your wooden wall gets hot, convection flow on its outside face would impede convection flow to its inner chamber;
-the trombe wall doesn't need the cost of pipe and fittings;
-I suspect, don't know, you'll need a pump to get enough heat into your wall and a fan to force air to get enough air to move enough air mass, a trombe wall doesn't even need a fan if it is constructed correctly

based on these facts, your wooden wall is clearly not as efficient as a trombe wall is in creating convection flow or in using solar energy (trombe wall doesn't need pipes and water which implies cost and extra labor therefore drops its overall life efficiency)

however, I love creative thinking  -keep it up!   just because one of your brain childern isn't viable doesn't mean the next one won't be!

good luck!

Thanks for the breakdown Orin! Great points.

The water would indeed be pumped through the pipe.

I will keep on pondering

The European radiant wall panels work very nicely, but it needs to have thermal mass so it holds the heat and disperses it. Have you looked at how those are made? Basically the piping is bedded well into a metal or other thermal mass panel. What I plan to do in my bathrooms is PEX off my radiant heat flooring embedded behind cement board, so it's thermal mass that works as a radiator. So, in your case, I'd not try to move the air, I'd make the wall into a free-standing radiant panel. Bed your pipes into something very heat conductive (grout, cob, cement, etc, not sand) and make the wall surfaces something similar (sheetrock is not the best option, but it's doable) (I like cement board.)  Doing it with just air flow you will find it takes a LOT of PEX to heat air, but not a lot to heat a thermal mass.

You are on the right track! :D

The man who wrote the current textbook in use for teaching Hydronic Systems Design classes is John Siegenthaler. An article by him you might find useful is  John Siegenthaler: Radiant panel systems  I bought the textbook, first semester the updated version was in use, it was 80.00. The older versions are cheaper and available used, but he has a lot of writing on the net, look him up, it's VERY eye opening!! He really knows what he's talking about.

Dear Sean

Take a look at this pdf, it’s the installation guide for viega’s Climate panel system.  It clearly shows installation on vertical walls.  I’ve installed this product myself and love it, because it has much faster response time than tubes imbedded in gypcrete and its only 1/2”, so doors and other transitions are a breeze.  

https://www.viega.us/content/dam/viega/aem_online_assets/download_assets/us/impr_1218_climate_panel.pdf

Its expensive, so I have played around with making my own, it is for 5/16” tubing, so a router with T slot cutter and straight edge will get the grooves done.  There is a thin aluminum panel stamped to the back to reflect heat out or up towards where you want the heat to go.  There are rolls of radiant barriers avail to use for the backing.  

I’ve not had to use this on walls yet, but have known it’s doable and always keep it in mind.

Yes, you are heating objects, not air, but we still use thermostat that measures air temp to control the system, the air gets warmed by the objects.

For this to work, I believe you need to have the tubes against the back of the wall material, be it wood or drywall, that’s why the climate panel idea.  

If you do the convection approach, I’d be concerned that you are putting the warmed air up at ceiling height, not where you want it.  I volunteer at a facility that has hydronic radiators behind wall panels very similar to what you detailed, we have never seen this system give any useful benefit to the hall.  Instead we only use the part of the system with tubes Imbedded in the concrete floor.  This system is massive, the convective part has over 40 hydronic radiators behind the walls, and we never use it because when it’s cold, the heat ends up at ceiling level, and you freeze unless you use the infloor system.

I have John Siegenthaler’s book and highly recommend reading it, a very valuable reference, has helped me figure out how to do several out of the box installations.

A friend in Provo, Utah bought a house with a 'water wall'.  It is an internal, non-structural wall that is about a foot wide.  Internally it has a heavy plastic reservoir that holds about 10k gallons of water (on a concrete slab).  As a thermal mass it was pretty awesome, the open convection spread heat throughout the house, but it didn't shift temperature quickly. :). It was also valuable for moderating temperature seasonally (annualized thermal inertia).  It didn't have air conditioning or swamp coolers, and it gets stunningly hot in the summer and downright frigid in the winter.

Drainage was handled at one end with a small unobtrusive door to get access to a threaded spigot to put a hose on.