Wiring the floor

I have to backtrack a bit now. Those who know me well know I can talk about thermal mass and energy efficiency at length, but I'm not going to go much into that just now. Let me summarise my position here: I am interested in good design and energy efficiency and I am happy to invest my time and effort doing some modelling to investigate worthwhile options, but unfortunately (since I am not a millionaire) I am largely limited to implementing ideas that are cost effective and can pay for themselves in a reasonable time.

I originally wanted to put hydronic heating through the slab -- ie. pipes through which water is pumped -- and the primary reason was to enable access to the huge thermal mass tied up in the slab and footings, in a kind of semi-passive cooling system. Unfortunately this turns out not to be cost effective, and that's primarily because of the huge amount of pipe (they use plastic plumbing pipe) that is required. In addition the engineers want the slab to be an inch thicker, and use thicker reinforcing mesh.

So, in a not-particularly-clear segway, I decided instead to put resistive heating (ie. electric heating elements) in the slab. The reason I decided to do this is because it costs about 30% what hydronic would cost. Of course it only does heating, which isn't the main reason I wanted hydronic, but since you can run the heating off-peak it still seemed like an attractive option. And I can save half by installing it myself. And by 'myself' I mean, I can get my friends to help me :-). It's a good thing they did too because I never would have managed to do it in one weekend by myself.

There are a few simple rules to follow when laying electric floor heating. The most important one is DON'T CROSS THE WIRES. I got great instructions from Eden Comfort Conditioning about how to install the wires. They advise to generally try to run the cables parallel with the top bars of the mesh, use plenty of cable ties, don't go under walls (might end up with a dynabolt through it), and when you get near the end of the roll, unroll it to see how you're going. (This is even more important for the in-screed heating -- see later.) That's because each heating cable is made to order and can't be cut. It has a 'cold tail' which joins onto the heating element, and the entire heating element must be embedded in concrete.

I also prepositioned some plastic conduits for digital temperature sensors. Long-time readers will be familiar with the pain I've since been through to make sure (belatedly) that I can actually push the sensors into the pipes as far as I need to (the longest one is 8m). You can see the plastic conduits under the mesh here. They all come up in one central location (in what will be a cupboard) so I can connect them to a digital controller.

I decided to heat our large open-plan living area (kitchen, meals, family), the hallway and bathrooms. The hallway because it runs the length of the house and I figure it will help to keep the whole house warm. For reasons I don't completely understand Eden Comfort usually put higher power output in the wet areas. They were very helpful so I really didn't want to argue too much, but I really would have preferred to put all the heating cables in the slab because then they would all be done and out of the way now. And I wouldn't have to learn how to install cables in-screed as well, which is what Steve and I have just done last weekend (I think I must owe Steve a whole house of help by now).

You see the wet areas are normally set-down a little during the main pour to allow the plumber to get the slope to the drain just right by pouring a screed before tiling. So in-screed means we attach a light mesh to the slab and then attach the heating cable, much like as we did to the mesh in the slab, but this time at 100 mm centres (vs. 200 mm centres in the slab). It's much easier to see the kind of patterns you end up with. The blue part is a waterproofing paint where the shower cubicle will be. I have marked out the vanity and toilet and you can see we have not heated in those areas. You might notice it gets a bit wiggly in the shower cubicle. That's what happens when you get near the end and find you have an extra metre or so to use up. This was the second bathroom and we had already learned our lesson. We had to redo the first one because I got to the end and had another 2 m to use in a much smaller room than this one. You can see what I mean when you see how wiggly this is!