Wednesday, 17 August 2011

Temperature and heat of the slab

So we're getting all this temperature data from the thermometers in the slab. Not sure exactly what to do with it, or exactly what it all means yet!

There are ten thermometers in the slab: two in each corner, and two in the middle. One at the bottom in the foundation slab, and one in the screed floor.  They are numbered from 1 and 2 in the middle, 3 and 4 in the north-east corner, then clockwise until 9 and 10 in the north west corner. Odd numbers are at the bottom and even numbers at the top. 

You can see eight of these on the graph. The software from T&D will only show eight bits of data at a time. If you look at the graph you can see the lines at the top moving up and down rapidly, fluctuating with the temperature in the house, in turn affected by the outside temperature. The bottom lines, at the bottom of the slab, are much more sedate. 

One highlight is 6th July when the windows were installed and the fluctuations at floor level were quelled. 

The weather changed after the middle of July and it got a significantly cooler. Luckily this was just after we got back from a camping trip. You can see the peak of the temperature at the top around 20:00 on 16th July, which didn't reach the bottom of the slab until 05:00 on 19th July, two and a half days later. 

As a thermal system, I think there are nine ways in which heat can move:
Going in:
1. From the sun to the screed
2. From the boiler to the screed through the under floor heating pipes
3. From the air in the room to the screed (when the room temperature is above floor temperature)

Going out:
4. From the screed into the room (when the floor is warmer than the room)
5. From the screed through the walls around the foundation to the external air.
6. From the bottom of the foundation to the ground under the house.

Within the slab:
7. Up and down (depending on temperature difference between top and bottom)
8. North-south (especially when the sun is heating the floor in the winter.)
9. Through the underfloor heating pipes (when there is a big difference between north and south).

This heat will all pass according to the second law of thermodynamics, that you can hear more about here on you tube from Flanders and Swann.

According to the calculations in the Passive House software, for the heating season between October and April, 5,195 kWh of heat are going to come in through the windows on the south. January will get the most heat. This is a combination of fine weather and a low angle of the sun. On average there will be 31 kWh per day. January also has the coldest temperatures. 

I'm not sure how much of this heat is going to go straight into the slab. Some will hit walls or furniture, some will be reflected from the slab and the heat that does reach the slab may leave it quickly.

Also according to the Passive house software, most heat will be lost through the slab in February, and the figure it gives is 4.6 kWh per day. 

The slab is like a box, representing the structural foundation, filled with some gravel and topped with a screed floor.  According to the builder's invoice, there is around 40 cubic metres of concrete; 16.5 at the bottom, 4 standing up around the edges and 20 on the floor. At a density of 1600 kg per cubic metre, that's 65,000 kg. 

There's around 22 cubic metres of gravel in it, which amounts to 26 tonnes, if the density is 1200 kg per cubic metre. 

It's difficult to be sure of the specific heat capacity, but 0.8 kJ/kg seems a reasonable estimate. 1kWh is 3,600 kJ, so the whole slab holds around 73 kilowatt hours per Kelvin. In other words, if it drops one degree it will release 73 kilowatt hours.

In the very worst winter weather, the whole house will lose 55 kilowatt hours in one day, so even with no sunlight or heating, the temperature of the slab should drop by less than one degree.