Friday, 27 May 2011

Exhaust air and heat pumps

As well as being a highly insulated, highly airtight and wind-tight structure, the house has an active ventilation system with a heat exchanger. It's a good idea to have active ventilation with an
airtight house as it will stop us from suffocating. It's a good idea having a heat exchanger because this will mean we lose less heat in the winter, and gain less heat in the summer. It's a good idea having
an airtight house with active ventilation because this means the air goes in and out through the heat exchanger. Try sucking through a straw with holes in it, and you'll see what I mean.

The heat exchanger is over 90% efficient, so most of the heat will be recovered from the exhaust heat, and transferred to the fresh air coming in. This means if it's 20 degrees inside, and zero outside, the air coming into the house will be 18 degrees and the air going out will be 2 degrees above freezing. In the summer, if it's 20 degrees inside and 30 degrees outside, air will come in at 21 degrees. There is an over-ride so, for example on a summer night, if it's 25 degrees inside and 20 degrees outside, rather than trying to exchange heat, it will just get rid of the hot air and bring in the cool air.

There is a heat pump on the roof which is used by the "Eco Cute" water heating system. This takes heat out of the cold air and pumps it into hot water in a way that is worthy of another post, if you're not careful. The ventilation system is in the loft and will be sucking air in from the East wall and blowing out of the north wall. I was quite seriously suggesting that the exhaust air should be directed straight towards the heat pump. In the winter, exhaust air is going to be a couple of degrees above ambient, which will make it slightly more efficient, and less likely to be below freezing. The heat pump is set to run at night time, using cheap electricity, and in the summer, when the ventilation system is in over-ride, the air being pumped out is also going to be hotter than ambient. Even though it can exceed 35 degrees in the day time, it's usually below 25 degrees at night. In 1983 there were two days when it stayed above 25 degrees all night, and that was a record.

The only time it is likely to be warmer than ambient is in summer daytime, when we're least likely to be making hot water.