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Underfloor heating.... outside!

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solarsimon

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Oct 9, 2020
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Hi,
I'll have 30kW of PV fairly soon, with restricted permission to export, so excess power. Once the batteries are full & the car is charged & the hot water tank is hot, I'm considering heating water to pump in an underfloor area outdoors - goal is to have warm feet in the evenings. In the UK, where LPG patio heaters are commonly used in summer evenings.
I'm thinking about burying the pipes deep and certainly won't be using any insulation - the bigger the thermal mass the better. I might get a used hot water tank for more extra storage/ time shifting of the heating.

Keen to her what people's thoughts are on this and any suggestions, from an audience that should appreciate that it's not going to be any worse for the planet!
 
rodrick said:
Unless you insulate the slab your wasting you time you see little to no difference
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The theory in not insulating was to maximise the thermal mass. I don't care about how much is lost downwards if it gives me a larger lump of heat
 
CRenner said:
Would you run it at night on batteries or would it hold the heat from daytime?
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I suppose I might heat from batteries but typically not. It's all about storing daytime excess power through the evening, with minimal cost
 
If you don't insulate below your patio then I guess you get much more thermal mass, but the problem is that the energy you put into it will heat that area - below the patio - and so you'll not see the benefit yourself. Be better to insulate under the patio and heat the patio and it'll then radiate and conduct the heat in the evenings so you can be toasty outdoors in typically cool UK weather..... It's a nice idea if you can get it right.

If you want other ideas, what about warming the soil in flowerbeds for better growth and veg production, heating greenhouse, heating garage for car etc.
 
Why not view the insulation between the slab and the earth as reducing heat loss and also channelling the heat to where you want it ...
 
MisterB1959 said:
Why not view the insulation between the slab and the earth as reducing heat loss and also channelling the heat to where you want it ...
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I appreciate I can get a higher temperature if I insulate between slab & earth, but I'm not too worried about the heat loss into the ground. As long as it doesn't rain that heat's not really going away in a hurry. The main reason though is that I don't want to dig any deeper than necessary, nor pay for insulation. If I put any insulation in I'll consider a vertical sheet of insulation around the perimeter, so the heat can only go up (good) or down (good for preventing the upper part of the slab from cooling so quickly, but also good for tomorrow)
 
xcentric said:
If you don't insulate below your patio then I guess you get much more thermal mass, but the problem is that the energy you put into it will heat that area - below the patio - and so you'll not see the benefit yourself. Be better to insulate under the patio and heat the patio and it'll then radiate and conduct the heat in the evenings so you can be toasty outdoors in typically cool UK weather..... It's a nice idea if you can get it right.

If you want other ideas, what about warming the soil in flowerbeds for better growth and veg production, heating greenhouse, heating garage for car etc.
Click to expand...
My gut reaction is that the downward heat loss won't be huge and that it'll also have some benefit for subsequent days. Gut reaction though, and my gut sometimes gets things badly wrong!
 
If water flows through, I think that would carry the heat away.
Without water, I think the dirt would serve as both thermal mass and insulation, infinite capacity.

Some years my foundation filled with water, so wouldn't work for me. But I have thought of pipes pounded into gravel fill under garage slap for that purpose.
 
I've installed quite a few heated slabs/sidewalks over the years. If you don't fully insulate below your heat source, your system will not be very efficient...
You may also want to consider a glycol based antifreeze instead of the water.
 
I did something similar to this about 20 years ago. I used a large wood boiler to heat water to about 160F. I circulated this water into a greenhouse that was built with a sand floor. The sand floor was covered by a heavy duty landscape fabric with a woven fabric to allow for water to pass through. In the 6" to 8" of sand, I placed a grid work of cement reinforcing wire over the whole floor. I then used sip ties and PEX tubing at 6" spacing. I had 2" of rigid foam insulation under the sand. All of this was inside a 22'wide x 60' long x 10' side walls. It was controlled by a thermostat and a circulator. It has worked very well and still serves me well. I would suggest using the insulation because I think it will work much better. I don't think the heat will migrate down through the soil well at all.
Have you ever built a huge fire on top of frozen ground? It always amazes me how little the ground will thaw even after 10 or 12 hours of a raging fire. I am talking about fires that consume whole trees, fed with excavator or large grapple loader.
 
solarsimon said:
My gut reaction is that the downward heat loss won't be huge and that it'll also have some benefit for subsequent days. Gut reaction though, and my gut sometimes gets things badly wrong!
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It will suck all the heat you can throw at it, so you will get rid of your extra energy anyway.
 
Ron Asbury said:
I've installed quite a few heated slabs/sidewalks over the years. If you don't fully insulate below your heat source, your system will not be very efficient...
You may also want to consider a glycol based antifreeze instead of the water.
Click to expand...
Efficiency doesn't matter in my case - not at all. IT'll only ever get utilised on sunny days when there's nothing else I can do with the power
 
solarsimon said:
Efficiency doesn't matter in my case - not at all. IT'll only ever get utilised on sunny days when there's nothing else I can do with the power
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Waste of time and money, there is a point when the mass gets so big that your energy imput does little to nothing because it can dissipate all the heat. You will need to isolate the mass so the heat will build up. I have been in hydronic/water heating for over 30 years and am a certified geothermal installer, I have seen this tried before.
 
Last edited:
solarsimon said:
Efficiency doesn't matter in my case - not at all. IT'll only ever get utilised on sunny days when there's nothing else I can do with the power
Click to expand...
You've received mountains of great, and consistent, advice here. You seem to not like what you're hearing, so good luck with your idea as described. Please do report back on results. But fair warning - luck almost always loses to physics. Trying to heat a slab without insulation is essentially the same as trying to heat the entire planet. That's one big heat sink!
 
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