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Thermosiphon/ natural Convection using wood stove hot water loop to heat radiant in floor system

 
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Hi,  So I have everything set up for my in slab heat. Tubing, manifold, circulator pump expansion tank, etc.. The only thing I am still working on is the heating source.  So what I plan to do is this. I have an indoor wood burning stove that I have installed a 1" Stainless steel hot water loop inside the fire box.  Beside my stove, I have a 55 gallon drum I plan to have sitting on some blocks to make it a little higher than the loop. From the bottom of the tank the "cold" line will go into the bottom of the loop, from there the top side of the loop will angle back over and come back into to tank at the top of the tank, but still remain under the water level.  This will hopefully heat up the entire tank once the stove has been running, via thermosiphon/natural convection.  Now, inside of the 55 gallon tank. I have 2 Radiators submerged that will work as a heat exchanger. Once the tank is hot enough, the pump will kick on and pump the closed loop fluid thru the two submerged radiators, pulling the heat from the "hot water storage tank" through my floor and returning back to the radiators.  I am confident this will work although I am sure I will have some tweaking to do once everything us up and running.  I welcome any comments or advice on this setup!  Has anyone attempted anything like this before?  If this works I plan to replace the 55 gallon steel drum with a Stainless steel one In the future so rusting out won't be a problem.   So if this all works the idea is that I will only be running a small circulator pump to heat my entire 3,000 sq ft home!  
 
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That should work to heat the home, but I did not see many controls for overtemp situations. That is my biggest concern.

If your thermos siphon does not work as planned (they often do not) you could just add another circulator pump and force a hot water loop. But try it and see first...

There are a few details I would be interested in, like individual pex loop lengths in your floor, but if all of them are exactly the same length, then you should be fine. If they are different lengths, then you might need flow control valves (quarter turn ball valves will also work as flow controls by varying the degree of closing), otherwise you will get uneven heating in individual pex flooring loops. If your pex loop lengths vary in your floor, then you would need a flow control in each pex loop, of some sort, to balance out the system. It sucks, because once set, it is set for life, but with varied loop lengths (like my house) it is required to balance the floor.

You also might need a flow control valve on your pex return manifolds to ensure your flow is proper through the pex tubing, this ensures you get your delta T, or 15 drop in temperature to ensure you are getting proper heating of your floor. It is simple, if your water temperature entering your floor in 90 degrees, you have to control the flow coming out of the return manifold so that it is 75 degrees. If it comes back as 80 degrees, you are not slowing down the flow enough to let that hot water properly heat your floor, and if it comes back as 70 degrees, then your flow is too slow. It has to be 15 degrees...also called Delta T, and is critical.

My real question is, what are your controls for over-temp water?

If you have concrete floors, it is critical that you do not get above 110 degrees or it will degrade your concrete. There is only two ways to knock down the heat, but the cheapest (and less effecient way) is to inject cold water from domestic hot water as a regulating device. I am not sure how you would control this temp though inexpensively without a PLC.

If you have wood floors, the concern of over-temp is not that bad, but I did not see anything listed for thermostats, relays, or zone valves. Without these, you will not get very consistent heat, but you need that for concrete floors too, and you did say "slab" leading me to conclude you have concrete.

I make the assumption that on your expansion tank you have your vent, relief valve, air eliminating valve and make up water? It typically comes as a kit with your expansion tank? You will need all that because while your woodstove/55 gallon drum loop is not presurized, your radiant floor heating system is. If your woodstove ever was to boil the water, that means the water in your radiant floor system would be over 212 grees and flash to steam, you would need a relief valve to insure it did not go bang. Always remember with radiant floor heat, there are two systems: the primary loop and stove, and then the radiant floor heating system. They are two systems woking in unison.

 
Jess Hutch
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So most of my lines in the concrete are vary close to the same length. @ around 250 ft. and I have 10 loops.  I have a Stainless steel manifold that has a control valve and shows the gpm on each line.  So I just plan to adjust these valves accordingly.  As far as the overheating issue. I was just going to put a thermostat/controller to run the circulator pump. If the temp gets too hot it will shut off. And as far as my hot water storage tank, it is not pressurized. I will have a port at the top of the tank where I can fill it as it will eventually evaporate  So the tank can just continue to cycle hot water if I am not running the in floor pump.  I am sure I will have hiccups along the way that will need addressed. But cross your fingers. I hope it will work eventually.  And I will have temp gauges on the tank, before going into the floor and the return. So thank you for all of the information you shared.  Good info to know if I need to speed up or slow down the flow.  And yes I am aware that overheating the concrete is very bad!  Thanks again!
 
Jess Hutch
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And yes, I have the air eliminator, pressure relief valves with expansion tank that are needed to correctly run the system!  Just waiting on someone to help me fill the system with Glycol.
 
Travis Johnson
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Jess Hutch wrote:And yes, I have the air eliminator, pressure relief valves with expansion tank that are needed to correctly run the system!  Just waiting on someone to help me fill the system with Glycol.



I would not bother with the glycol, it will make your system 10% less efficient, and costs a lot of money. Just run straight water, and take the money you would spend on glycol, and buy a back up generator from Harbor Freight for $299. If the power goes out, plug in your generator, and keep heating your home. I have run mine on straight water for 13 years without issue. You have nothing in your system that will corrode, (other than the 55 gallon test drum).

Also, you can run your circulating pump for your floor, as flowing water does not freeze. Glycol is really a waste of money no matter how you look at it.
 
Travis Johnson
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I think you are all set by the sounds. I was not sure if you had a bunch of stuff missing, or you just did not mention all the details. I am glad it was the last one.

How does your circulating pump work with your thermostat? Is it just on or off with a simple circulating pump, or do you have it tied into a PLC so that it acts as a metering valve adjusting the temperature of the water for the ambient conditions outside?

That is how mine works instead of a tempering valve. I never did see the point of heating water, just to cool it back down! With my plc controlling my variable speed circulating pump, I do have zone relays with priority for zone valves, but they never come into play. The PLC is smarter then the thermostats of the zone valves and just adjusts the water as ambient conditions exist.

How many zones are you running?

I got three zones in my 2200 sq ft home, but I tied my thermostats together so that (2) out of the (3) have to call for heat before the system will even fire. That way if one thermostat is in a cool spot, my system does not run for just 1/3 of the house. It will take 2/3 calling for heat, or it just will not come on. That saves me quite a bit of money right there, as well as adding a 2 minute delay before the system fires up for any dual call for heat, all that so that false calls for heat are eliminated.

I also adjusted my default temperature to 0 degrees so that if ambient tempearture gets to that point, my plc stops trying to adjust the heat, sends the heat to 100 degrees running through my floor. That is where zone valves come into play in n overheat situation for the space, but not having them will save you $400, so I can see why you did not add them in.

It is nice talking to someone that knows a bit about heating systems. In another life I might have been a boiler tech.


 
Travis Johnson
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By the way, if the Department of Energy ever gets off its duff and starts giving out forecasts in digital code, our plc's could read the upcoming forecast and adjust the settings for the predicted weather coming at it, and adjust its temperatures. Can you imagine the savings? I have people looking at buying my other home and they can not believe how little fuel it takes to heat that home, but if my boiler knew it was cold NOW, but in 12 hours a warm front was moving in, it could dial back on the heat and have the slab match the warmer predicted temperatures  and REALLY save me some money.

That is almost Jetson's heat right there, but its here, now, and I put my system in 13 years ago. You just cannot beat a PLC because it is thinking and adjusting every minute just so you are always at the set temperature, in my case 70 degrees. It is always within 1 degree of whatever I set it at.
 
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