On Sun, 11 May 2008 21:34:55 -0500, "Pete C."
wrote:

Hi Pete,

Thanks for the clarification. I hadn't heard of this technique before
so I appreciate you taking the time to describe it to me.

From what I understand, the cost of a ground source heat pump is
typically two to three times that of a conventional air source system
so if, for argument sake, we peg the cost of our standard system at
$7.5K the equivalent GSHP would be $15K or more. Obviously, the exact
cost of either system would depend upon a host of factors, but for now
I'm going to assume the premium runs in the range of $7,500.00.

As a quick, back-of-the-envelope exercise, an average new home with a
space heating demand of 15,000 kWh/year, if equipped with an
air-source heat pump (8.5 HSPF/Zone 4) would use roughly 6,000
kWh/year. That same home heated by a GSHP (seasonal COP of 3.75)
might come in closer to 4,000 kWh/year. The difference of 2,000 kWh
at $0.12 per kWh works out to be $240.00 a year; we might reasonably
assume DHW and air conditioning savings kick in another $360.00, in
which case our combined savings total $600.00/year -- on the other
hand, an air source heat pump with a desuperheater might claw that
back to less than $400.00. Nonetheless, if we assume an incremental
savings of $600.00, the simple payback here is 12.5 years, with
various cash discount rates and utility rate assumptions moving the
exact position one or more years in either direction. If the
breakeven point, as in this example, is more than ten years, it's fair
to say your money would be better spent elsewhere; for a lot of
consumers, even a five year breakeven point is a dicey proposition.

Again, these are rough numbers and we can fine-tune them further, but
I wanted you to understand my base assumptions.

Cheers,
Paul

Interesting. How do you figure that would work out in North Texas (north
of Dallas) where there is a lot of A/C load in 100 degree weather and a
surprising amount of heating demand in the winter as well? Another non
monetary factor to consider is the lack of an outdoor unit with a
somewhat noisy fan, and the need to clean it regularly of dirt, pollen,
leaves, etc. This is of particular interest since I expect to replace
the full HVAC here, probably next year. I also expect to be here quite a
few years longer.

Hi Pete,

I can explore this in more detail later on when time permits, if you
so wish, but right now it's getting late and I'll soon have to call it
a night.

Anyway, in terms of operating performance, SEER ratings are suppose to
be reasonably representative of what we can be expect to encounter
over the course of the entire cooling season. However, if it is
helpful, EER is based on steady-state operation at a 95F ambient
temperature (not far from the 100F mark you mention). To get a rough
sense of how these two numbers compare, you can convert SEER to EER by
multiplying the former by either 0.9 (relatively low humidity) or 0.8
(high humidity). So, if we assume you live in a hot and humid area,
an ultra high efficiency heat pump with a 21 SEER rating would have an
EER of 16.8 and a high efficiency model with an 18 SEER rating would
have an EER of 14.4. To then convert EER to COP -- a more useful
measure when comparing these products to GSHPs -- take this second
number and divide it by 3.4. Thus, our 21 SEER unit would have a COP
in the range of 4.9 (@ 95F) and the 18 SEER version would clock in at
about 4.2. Again, this is a rough approximation, but the results seem
to be more or less in line with what we could expect from a typical
GSHP.

Your concerns related to noise are hard for me to address. The CAC in
my Toronto home is a two-stage high efficiency model and on the low
setting the outside compressor is surprisingly quiet (a light and not
unobjectionable hum). I don't ever recall it kicking on high so I
can't honestly tell you how loud it is when operating at full
capacity. I don't believe I had any issues with dirt, leaves or
pollen -- I may have washed down the outer cabinet with soap and water
once or twice but that's basically it.

A GSHP may very well prove your best choice given your particular
needs, utility costs, operating environment and so on -- I really
can't say. But I wouldn't automatically rule out a high-efficiency
air source heat pump without investigating this option further; these
products have really come a long way in recent years. Whatever you
ultimately decide, good luck!

Cheers,
Paul