Airsource heatpump with underground duct.

Do-more

I've been searching but can't find any info on this.

I first saw it mentioned on "Grand Designs" this season but they didn't go into much detail.

What they did was to lay a duct up the side of the driveway about a metre underground to warm the incoming air to the heatpump helping efficiency in cold weather and to prevent icing of the evaporator.

Has anyone come across this before?

I'd like to know if it works and what length of duct is optimal, too short and the air wouldn't be warmed enough and too long and the resistance of the pipe could cause problems. They seemed to be using a terracotta pipe but I guess a thin walled plastic pipe would do as well.

I suppose it would be wise to have a two temp. sensors one in the pipe and one outdoors so that if the outdoor temp. was higher than in the pipe a bypass could be operated for direct air intake.

The much lower capital costs of an air source heatpump are attractive but I have always been put off by the possibilities of high electricity consumption due to de-icing because of our relatively high humidity in winter, if this works well it could be a good solution.

sinnerboy

the duct to pre warm air supply to the air to water heat pump was a fairly simple idea . don't know how you would "optimise" this . perhaps others here would . but i gues if you are going to form a duct - dig the longest one you can within reason .

you can "look again" at grand designs here - which one was it ?

http://en.wikipedia.org/wiki/Grand_Designs#Series_Seven.2C_2007

Do a google on "earthtube" plenty of green sites have info, here's one for starters http://www.thenaturalhome.com/earthtube.htm.

I did consider using one myself but too much effort involved!

Do-more

Thanks folks, now that I know the name of what I'm searching for it's a lot easier!

Seems earth tubes are widely used in Germany and Austria to pre warm the incoming air for MHRV units in passive houses for the reasons I outlined above.