Cohousing-L Mailing List: Ground Source Heat Pumps

[Thomas Lofft (tlofft]

A couple extra points:

One of the key differences between through the wall (or ceiling) air source 
heat pumps and ground source (geothermal) heat pump systems is 'where is the 
energy coming from?'


The air to air heat pumps are seeking to extract energy from the prevailing 
outdoor air. As temperatures outdoors drop, the outdoor air becomes cooler and 
the demand indoors increases, requiring the heat pump to work harder and harder 
to extract heat energy from the cooler air. If the thermostat is raised more 
than two degrees in an attempt to boost heat production, all that does is kick 
in the electric resistance heating elements which are your most expensive heat 
source possible.  Similarly, as summer heat increases and A/C switches on to 
cool the indoors, the unit works harder to cool itself as outdoor temps 
continue to rise.


The ground source heat pumps are extracting heat energy from the ground which 
has a fairly constant year round temperature of 45-50 degrees. In winter, this 
is still a constant source of energy; in summer it is still a reliable heat 
sink for absorbing the heat energy being pumped away from the household and 
cooled by the ground temperature.


I have not done any direct field research, but I fully believe that the 
constancy of the heat pump makes it very reliable for year round interior 
climate control as well as necessary dehumidification. Keep the thermostat at a 
constant temperature. Avoid kicking into electric resistance heat. This is not 
your parents' old oil furnace.


The single most important feature of either type, air source or ground source, 
is that neither have any local airborne combustion gas emissions. Your 
community will unquestionably have much cleaner air than by attempting to rely 
on any local fossil fuel combustion system.


If you are considering a multi-unit ground source system, please seriously 
consider all the comments offered by Sharon Villines and Katie Henry who have 
managed these systems and analyzed the operation and cost factors. Especially, 
research the case history of any installation contractor giving you a 
design-build proposal for providing and installing a new system.


As to cost, that also depends upon the local contractor as much as the 
manufacturer. Both should consider the economy of scale of installing multiple 
units and give appropriate discounts.


Cheers, best wishes for year round economy and comfort.


Tom Lofft

Liberty Village, MD

Where 18 homes are all living with ground source HVAC systems