Re: Re: Heating systems
From: Roger Diggle (digglemacline.com)
Date: Thu, 13 Oct 94 23:42 CDT
Ground-source heat pumps are not really new technology, but young enough
technology that only in the last 5 years or so have they become reliable
enough to be a viable choice.  The typical Ground-source heat pump contains a
compressor, a refrigerant/air heat exchange coil, a refrigerant/water heat
exchange coil, and a "reversing valve," which makes the refrigerant flow one
direction to move heat from the water to the air (heating mode), and the
other direction to move heat from the air to the water (cooling mode).  There
are also some other types of equipment that may be appropriate, but I'll take
that up later.

The typical installation includes a heat exchanger made of plastic pipe and
buried underground to extract heat from the soil or to put it back there.  If
a large enough pond or river is available, the heat exchanger can also be
submerged in the water.  A pump moves water through the ground-souce heat
exchanger, then through the water/refrigerant heat exchanger where heat is
added or removed, finally back to the ground-source heat exchanger where the
water is brought back to ground temperature.  The size and geology of your
site can make a big difference in the feasibility of an installation because
of varying ground-source heat exchanger costs.  Sometimes a large enough heat
exchanger can be installed on a small site by boring holes and installing
long U-tubes in the holes.

(Older installations sometimes used lake, river, or ground water directly,
without using a heat exchanger, pumping water out of a well, for instance,
and returning it to the ground elsewhere with an injection well.  I work
regularly in a 28 unit apartment building that's set up this way.  Many
states now prohibit such installations, and there are other problems even
where they are allowed.  I would recommend against such an installation, even
if it is available to you.)

> Nancy Wight                                   wight [at] world.std.com
> New View Neighborhood Development             Acton, MA

> 1) If we install electric geothermal heat pumps, we will not be
> having gas  piped to our site, which means we have to use
> electricity (where solar is  not feasible) for heating water.  Given
> that gas is much cheaper than  electricity, I'm wondering if we will
> *really* see an overall energy  savings.

Although they are not common, water-to-water heat pumps are also available. 
Such a heat pump can be set up for heating a floor slab, for perimiter
baseboard heating, and/or for heating domestic water.  I don't believe you
indicated whether you'd be air conditioning or not, but in NC it seems
likely.  Water-to-water units would preclude air conditioning in most cases,
but consider it for domestic water heating.

Therma-Stor Products of Madison, WI, also makes a line of heat pumped
domestic water heaters in residential and commercial sizes.  The typical unit
extracts heat from the air and pumps it into the water, a particularly good
system for restaurant (or co-housing?) kitchens which have a lot of waste
heat in the air.  McDonalds (gasp) was trying them out in their Madison
restaurants, and was seriously thinking of using them nation-wide.  They
basically heat water for free if you have an air conditioning load.  They
have standard resistance electric elements for back-up.  I see no reason why
one of these units could not be operated as a water-source heat pump, but I
can't tell you if that configuration is available ready-made.  Therma-stor
might be interested in a guinea pig, though.

If you are in a position to experiment, there is a lot that could be done
with heat pumps in conjuction with solar panels.  A solar-assisted
water-source heat pump (gad) could greatly increase the efficiency of your
solar panels, and allow them to be used in lousier weather than with a
standard installation.  This technology has been talked about in engineering
circles, and perhaps actually used somewhere, though I've never seen an
actual installation.  Modern computerized electronic controls make it more
feasible, but that's starting to get pretty high-tech.  It would be an
expensive, custom installation.

> 2) I'm worried about the "extreme weather", which we have a lot of. 
> Would  we also have to have a furnace (and air-conditioning system)
> for backup? 

If your installation is not troublesome, you will seldom need backup heat in
NC, but I'd install it anyway, or be prepared with some portable electric
heaters.  The amount of backup heat use in a standard ground-source
installation makes the choice of a gas furnace prohibitively expensive.  In
the apartment building I care for, the backup system is baseboard heaters
with inexpensive integral thermostats.  First cost is very low and it's very
reliable.  Most of the apartments only have one exposed cold wall, and we
generally don't need back-up heat intil its 10f or lower outside, and then
only on the north side of the building.  However, the equipment does bust
occasionally, and that's when the backup is a real blessing.

If, as I suspect, you are going the forced air route and air conditioning
with your heat pumps, there's one more product you might want to check into. 
Therma-stor makes a product that's a combination of heat recovery ventilator,
water heater, and heat pump.  It would replace kitchen and bath fans, the
standard water heater, dehumidifier, and a small air conditioner.  In heating
mode, Therma-stor says that the unit recovers heat from the "stale" air
stream and that the recovered heat can be used to heat the water and provide
part of the space heat.  It appears to be a well thought out product, but I
have not seen one in operation.

Roger Diggle    diggle [at] macline.com
Porchlight CoHousing, Madison, WI

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