solar floor heating - Part 1

Premise and purpose

Queensland winter often has sunny days which are warm outside. The paradox is we build houses which are often cold inside. I can only blame our colonial heritage.

Sitting around inside the house it is often still at 15°C at lunch while outside has risen to 20°C in the sunshine. (Its fcuking stupid isn't it!). I had been thinking about a roof mounted solution, but as it turns out the northern side of the house is actually in full sunlight most of the day. When doing any gardening on that side you have to strip back to a T-shirt or you'll start getting hot. So my goal was to work out how to get the heat of the winter northern sun into my house without spending a lot of money.

We have a reasonably typical old house here, raised 40cm or so from the ground on stumps with hardwood floors. They get mighty chilly in the winter. The solution to that from the 70's was carpet, but I think carpet is filthy stinky stuff which people don't quite get how impossible it is to clean and rarely think about it (or if they do think, quickly realize that "its stinky stuff which is impossible to clean").

discussion

So I got to thinking that the nice warm sun on the north side of our house would be able to heat my floor making the inside of the house warmer.

So for less than $200 I've bought:

• 100m of 13mm black poly pipe for both collector and floor heating section
• a solar powered water pump to circulate the water
• some cable ties and staples to attach the pipe

and after a bit of crawling around under the house I have it fitted



so ... next is to add some insulation under the floor (was going to use builders paper but a better idea is some thin foam) and make a more effective collector.

Right now, with just a single circuit, its just slightly cool to the feet while the unheated areas are uncomfortably cold on my feet (read need slipers). Not bad for no extra cost as time goes by...

For my next stage I reckon that I'll change this test collector to 4 collectors of about 500x500mm with clear plastic covers (to reduce wind induced heat losses) and black backs (to capture more heat), this should significantly increase the efficiency (reduce the losses) of the heat absorber and allow me to run solar 4 pumps to shunt the water around.

Right now I'm just running a single circuit of water (say the blue line in the figure to the left) so with the extra heat collector, I'll double up and reverse the direction.

If you look at the figure left, the blue line comes in and zig zags up the floor then come straight back to the heat collector. If I run the other circuit in the opposite direction (like the green line) then any heating change (as the water cools) of one circuit will be countered by the other which will have any temperature gradient in the other direction. This way I'll get 2 pipes between each floor joist (where as now I only have one) which should further increase the transfer of solar energy to my floor.

When choosing the pumps, there were 2 types, one was 200L/hour for $80 and the other was 140L/hour for $40. I reckon that 4 pumps covering the floor area under the house will (in combination with the insulation layer under the floor) actually make the floor warm to walk on in winter!!

Another point about the solar water pump is that it only turns on when the panel gets enough energy (which means the heat collector is now hot and ready) and turn off when the sun goes down (or when it goes behind a cloud) meaning that I do not need any other regulating system in this (to prevent it pumping cold water under my floor).

Simple, effective and cheap.

Further the system is then easily adapted to have GAS heat the water (using a simple and low cost instant heat gas system and a regular electric pump) if we want to keep the floor heat in the nights or to use on rainy or cloudy days (which aren't actually as cold in the evenings as the clear days are).

Water temperature on the outlet end of the pipe (after its gone around under the house) is coming out at 34°C
For reference:

•   Part 2 with some extra details at
•   Part 3 and
•   some measurement here and
•   here about the gas heating of the water in the junction tank and
•   here about windy rainy days observations