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 transfered 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 worth 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 thought it can excede 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.