Underfloor heating.... outside!

[time2roll]

Much of the heat lost to the soil may never come back as thermal mass. As long as the patio is dug up I would have at least some minimal insulation.

 

[xcentric]

think about it like this - why not just heat the air around your house? answer - cos it'll dissipate and most won't be in the places you want it. Same applies to the ground. If you don't insulate then it'll simply dissipate away and make no difference. But if you heat an insulated (or part-insulated) slab, then it'll get warmer than the surroundings and keep the heat where you want it. But you do what you want - if you want cheap, just heat the ground. If you want effective, insulate around the area you're heating.

 

[solarsimon]

think about it like this - why not just heat the air around your house? answer - cos it'll dissipate and most won't be in the places you want it. Same applies to the ground. If you don't insulate then it'll simply dissipate away and make no difference. But if you heat an insulated (or part-insulated) slab, then it'll get warmer than the surroundings and keep the heat where you want it. But you do what you want - if you want cheap, just heat the ground. If you want effective, insulate around the area you're heating.

Thanks for your feedback. I believe there's a difference though... the air has almost zero specific heat capacity, or thermal mass.
I do want cheap - I can't get a digger in there. It's 5m x 5m approx, stony ground, so digging that out will be a nightmare, and costly.

 

[solarsimon]

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!

I sometimes look at threads & wonder how many more times someone has to be told something before it sinks in! This time I'm the recipient!
But I'm slightly defiant, because as you say, physics will determine it. So here are some sums to help me better visualize it & for people to steer me.

BTW It would need to all be dug by hand because it's in a walled area with no digger access. That's one huge reason for not wanting to put insulation in!

5m x 5m soil at a foot deep = 12 tons
Using SHC of 800 J/kg.k if I can put in 60kWh over a day it could give me a 22 deg C rise in that big lump. (lossless). But there are losses for sure...

Using a thermal conductivity of soil of 0.5W/m.k I only get about 1.5kW of conduction downwards for 0.3m of soil over 25m2. Plus the same again going out horizontally so that's 3kW being lost in soil conduction.

Meanwhile, on the surface I might expect 150W/m2 of underfloor heating 150x25m2 = ~4kW of heat from floor. Over a 6 hour evening that could be 24kWh of useful heating provided.

Summary -
Input: 60kWh heat in over 6 hrs of sunshine.
Losses: 3kW conduction loss sides & underneath for 6 hrs sunshine + 6 hrs evening = 12x3=36kWh
Heating 4kW of heating over 6 hrs = 24kWh

This is all steady state & doesn't include ramping up or down, and assumes I can get all that heat in there.

We all now energy can't be destroyed, so please help me understand how I'm going to

 

[solarsimon]

Would you run it at night on batteries or would it hold the heat from daytime?

Just daytime heat typically, but I *could* use some spare battery energy I guess.