2007-10-25

Heat from the Earth

Now a little something about the furnace we're looking to get.

When we bought our home in 2005, the previous owner put in the cheapest furnace he could get - he was just flipping the house. At the time (nothing's changed I understand) the efficacy of a furnace did not have an effect on the value of a house.

One cold winter day the price of natural gas jumped in sympathy of high crude oil prices. Yay capitalism. The good old CBC morning radio was interviewing someone who heated their home using the stored heat in the ground. What bent my brain was this statement:
"The ground in Ottawa stays 10C year round and that's plenty to heat my house, yeehaw."
A house @ 10C just isn't comfy. I knew of geothermal heating using hot geysers in Iceland, but heating your house using something that's colder than what you would find comfortable didn't make sense.

Then a web site beat me with a clue stick (Images in this post are from that link, more info from CanRen, Wikipedia, and GeoExchange.org). Let me summarize it for you.


Air Conditioners
An air conditioner compresses a gas (refrigerant) to make it hotter by grace of thermodynamics. That hot gas is then cooled to air temperature using that big loud thing outside your house you identify as your air conditioner. The gas is then decompressed inside your ductwork - usually near a fan - where it cools down. The cold gas then cools the circulating air in your house and the cycle repeats. An air conditioner is moving heat out of your house - it doesn't "make cold" (I will not get started on how wrong that statement is). Your refrigerator and basement freezer operate on the same principal - cool down the inside while heating up the outside.

Air Sourced Head Pumps
One can reverse this and heat your house while cooling the outside air. This is commonly called an air source heat pump. Problem is the thermodynamics of refrigerants can't move much heat from -40C air to heat your house to +25C. That's 65C difference in temperature. In the summer you're only trying to cool your house by 10C (35C outside, 25C inside). Think about it.

Ground and Water Sourced Heat Pumps
A ground source heat pump tries to refrigerate the ground to keep your house warm. Consider that the density of air at STP is 1.2kg per cubic meter while rock averages 2,400kg per cubic meter. 2000x more dense. Not taking into account the greater specific heat capacity - you'll have a very very hard time freezing the ground over the course of a Canadian winter before the summer sun warms it up again. Now you're heating your home to 25C from ground that is 10C - a manageable 15C temperature differential.

And as a bonus - it's much easier to cool your home in the summer.
And as a second bonus - domestic hot water heating is taken care of as an after thought.

How does it work? Well, there are a few flavours of heat pumps.
  1. Air Source
  2. Water Source
  3. Ground Source
Air source I've mentioned above. Water source uses well water - or a lake, pond, spring, etc). Ground water itself comes in a few sub-flavours.


  1. Closed loop horizontal
  2. Closed loop vertical
  3. Direct Expansion (DX) vertical
Horizontal is just as it sounds. Loops of plastic coil containing a water-ethanol anti-freeze solution circulate a few feet below the surface of your property. This requires you tear up at least a 1/2 acre of lawn.



Vertical requires drilling 30 to100 metres down into the ground far from the colder surface. In these bore holes, plastic piping containing water-ethanol is put in for heat exchange.



DX is a vertical system but copper piping containing refrigerant is put into 30 metre drilled holes. The copper piping is a better heat exchanger and the refrigerant is now circulated in the ground where the heat is. Heat exchange is amplified.

2 comments:

Dan said...

How goes the hunt to find an installer?

... Smells Like Fries said...

We've found one whose price is appropriate.

Saving that for an other post. :P