Sunday, 21 August 2011

solar floor heating - why solar will only be part of it

I've been quite happy with my solar floor heating home improvement project, but I am under no illusions that it is a complete solution in itself (see all related posts for the complete project rundown).

Why not?

Firstly it doesn't work well at night ;-)

Secondly on days like today for instance, it started out raining, with the rain moving in late in the evening. This allowed the temperature to drop (no cloud cover keeping the place warm over night) and then little or no effective solar energy to heat the water or drive the pump. The graph below was obtained from the BOM site just this morning and sums up nicely why an auxiliary power supply is needed on some days.

So you can see from the graph that not only is it cooler its also more humid and (dangerously from the perspective of mold and fungus as I have already blogged about) the dew point is VERY close to the ambient temperature. This is reflected in the relative humidity (which is sitting at about 87% outside as I write this).

By heating the interior of the house at this time we get some significant benefits (apart from being warm) by effectively lowering the relative humidity inside the house and thus preventing everything becoming sticky and damp. Also as the sun comes out now and then and starts to heat up the air outside we don't have the problem of the interior of the house being cooler than air outside and thus causing some condensation of moisture happening as the warmer moist are comes inside the house and touches cooler things.

That green line of dew point means that water will start to condense out of the air when things are at (in the case above) 13 or so degrees C. That's why glasses with cold water in them start to look frosty and why polished wood starts to feel tacky.

Insulation also works best when its got some thermal mass to insulate. An empty thermos bottle won't stay warm long with no water in it (try heating the interior with a hair dryer and see for your self). Since our house has no thermal mass heated up to a higher temperature for the walls, floor and ceiling to insulate it will loose heat quickly at night and over time the next day without the addition of heat.

So by modifying my design to have the common pool:

I can now incorporate something like a gas heating system to heat the water and a small auxiliary power supply for the solar pump (on the floor circuit) to heat the water in the common pool, and then move that warm water around the floor pipes; without wasting that heat in the solar heat collector section.

In my view gas provides the most effective way to store and purchase energy for heating. It gives excellent efficiency and burns very cleanly (no nasty toxins such as sulphur). However I don't want to burn gas inside my house, as it will produce micro-soot; CO2 as well as potentially leak gas in the house.

This is of course not news to Canadians who have been burning their furnaces in a way that keeps house air separated from combustion and puts combustion gasses back outside where they belong. Even open fire places have a chimney right?

My solution to this is to get a small low flow gas hot water systems such as this one. These are designed for caravans and other low flow outdoor situations. They are rated at about 6Liters per minute which is a bit more than I'll need. I can turn the water temperature (meaning gas consumption rate) down as I need only have the water at about 35°C

By using a water heater like this I then get to keep my combustion outside and not mix it with my internal house air.

I can run it of a 9Kg cylinder and pump the water through it to the common water bus with a low power 12v pump.

My intention is to set this up on a timer which will turn it on at about 6pm, run it for busts (say 20min on 20min off) till about 10pm then turn it on again at about 5am to start heating the floor.

This in turn will make my solar system more effective as the water will already be warm in the morning when it starts up giving it a head start on warming it up.

Of course this adds to the complexity and cost of the system, these systems sell for about AU$200 and I'll probably need another $100 for the pump, timer and circuitry.

I'm estimating that the gas use will be something like 18Kg of gas for the winter months (or about $50 today).

We get to have polished wooden floors that are slightly warm underfoot and that heat the house. Just like the large surface area radiators are not too hot to burn you (unlike smaller bar radiators) the floor heating has an even larger surface area and so keeps the house warm too.

Polished floors are cleaner than carpet (got any pets? Cat hair and carpet ... mmmm my favorite for allergy, oh and do you see carpet in many hospital rooms?) and feel lovely under foot when not cold. So now that we're heating them there is even less reason to cover them up with festy filthy carpet.

Low cost, low energy and effective ... not bad I'd say


Heating Under Floor said...

Hi all,

Nice blog! The benefits of using solar floor heat during the day are outweighed by the disadvantages of using the conventional system at night. Although some early solar air heating systems used rocks as a heat-storage medium. Thank you...

obakesan said...

Heading Under Floor seems to be simply spaming, but in case I'm wrong I've left it.

The statement is however a little misleading as you can actually still save on the energy not spent when heating during the day ... :-)