Monday, 2 January 2012

A hot slab of concrete

Actually it was a rather cold slab, and what we really want is a warm slab of concrete, not a hot one. Well, perhaps a luke warm slab. In fact what we really want is a room temperature slab. 

To be honest, the temperature of the slab is not a direct concern, but we want the temperature inside to be warm in the winter and cool in the summer. Because the concepts of warm and cool are relative, we may reasonably get away with cool being hotter than warm. 18 degrees may be warm in the winter when it's below freezing outside, and 25 degrees cool in the summer when it's over 35 outside.

So we want the slab to be slightly above room temperature in winter, and slightly below in Summer. If we can keep it somewhere between 20 and 25 degrees for the whole year, we should be comfortable.

The slab itself, with fifteen centimetres of concrete at the bottom, ten at the top and 70 centimetres of aggregate between, is mostly going to work as thermal mass, maintaining such a steady temperature,as I wrote before.

We have a 460 litre tank of hot water, which will be heated by an atmospheric heat pump, using cheap nighttime electricity to elevate the abundant but cool heat in the nighttime air to piping hot water. Actually, it may make more sense for us to modify this to take heat from the hot air under the solar panels in the day time, but that needs to be dealt with in a whole new blog. 

The heat pump, known as an Eco cute, is capable of controlling four heating circuits as well as providing domestic hot water and reheating the bath. But, the heating circuits send water at around 60 degrees, and keep sending it until the temperature sensor reads something like 50 degrees in the return pipes. As a 25 degree slab is going to be sufficiently warm, the heating circuits, as they are, are not much use to us. Instead they pass through a thermostatic mixing valve, which can be set to some temperature between 30 and 60 degrees, that will mix a suitable amount of hot water from the boiler to the water coming back from the slab. I suspect we will usually set this as low as possible, although as soon as we move in, we may want to get the slab up from 10 degrees as quickly as possible. 

They filled the underslab water pipes with antifreeze. It will be a very cold day when anywhere near these pipes gets anywhere near freezing, but I suppose there are advantages with protection from rusting, and there may be an increase in heat capacity. The water may need changing every couple of years, but we will see.