Hi John,
On Thu, 05 Apr 2007 14:07:56 GMT, Paul M. Eldridge
I'm going to suggest that the numbers don't tell the whole story.
Selecting a cut-off temperature isn't a simple one-dimensional matter.
There are several issues. At what point does the quality of the heat
degrade too much? 75 degree air blowing out the duct work is
unsatisfactory.
When is the compressor exhaust valve temperature excessive? The very
low flow of the refrigerant when all the heat is essentially coming
from the electricity powering the compressor (COP approaching 1) can
allow valve overheating.
What is the humidity in the environment? Even if it's below freezing
if the dew point is close to the dry bulb then the evaporator will ice
quickly and more frequent defrost cycles will be necessary. Defrost
cycles both impact energy efficiency and interrupt the heating.
Compressor exhaust valve heating can be addressed with the proper
design and materials specs. It can also be "addressed" by the mfr
simply accepting a shorter compressor life. Which manufacturer does
what?
There is a major difference between what is technically feasible and
what makes sense. It seems to me that pushing the heat pump envelope
just doesn't make sense. Why deploy equipment that provides
marginally satisfactory heat (at best), is expensive and short lived
when the alternatives are so much better.
I don't work in the HVAC field, so I can't speak to the issues you
raise, but I can tell you that my own experience has been rather
positive and in my conversations with friends and neighbours, my
general sense is that the overall level of satisfaction is quite high.
Just to provide you with some background, here in Nova Scotia, fewer
than 500 homes have access to natural gas at this time, so just about
everyone heats with oil or electric. Nova Scotia Power's residential
rate, as of April 1st, is $0.1076 per kWh and heating oil currently
sells in the range of $0.80 to $0.85 per litre ($3.20 per US gallon).
One litre of heating oil contains 36,500 BTUs and assuming an AFUE of
82% (typical of most new boilers), the cost of oil heat comes in at
$0.97 per kWh(e). Of course, homes with older, less efficient boilers
might pay something closer to $0.12 per kWh(e).
Last winter, my heat pump's seasonal COP was 2.45, so after this most
recent rate increase, my average cost per kWh of heat is 4.4 cents. A
heat pump with a HSPF rating of 9.0 might provide heat for as little
as 3.5 cents. On that basis, the operating cost of a high efficiency
heat pump is roughly one-third that of electric resistance or oil.
Macroscopically, it makes little sense to burn fuel at a power plant,
suffer the inefficiency of conversion to electricity, only to turn it
back into heat again, even if the process has a little gain. Only
with nuclear or hydro generated power would that be logical. If one
is going to burn fuel for comfort heat, why not burn it on-site?
I don't have access to the latest numbers, but NSP's heat rate is in
the neighbourhood of 10,000 BTU/kWh. That would place generation
efficiency at about 34 per cent, with additional T&D losses perhaps
pulling that down towards 31 to 32 per cent. My COP of 2.45 brings
that back up to 77 or 78 per cent and a high efficiency heat pump
might kick it to between 90 to 95 per cent. By comparison, my
five-year-old Slant/Fin boiler has an AFUE of 82 per cent.
Bear in mind not all of our electricity is thermal generated -- about
twenty per cent is renewable and this percentage will grow over time
due to provincial mandates. I've opted to purchase 100% green power
(an additional $4.00 per 125 kWh), so my net emissions are
theoretically zero.
To me, some utility's obsessive promotion of heat pumps for the sole
purpose of taking market share from the other energy sources is at
best unseemly.
I'm not in a position to judge Nova Scotia Power's motives, but I made
a conscious decision to limit my heating oil consumption for several
reasons, not the least of which my fear that a portion of the proceeds
will be used to fund terrorist organizations. Last year, I used a
total of 828 litres of heating oil (220 gallons), the bulk of this
being for the production of DHW. I'm now trying to decide if I will
install a heat pump water heater (a NyleTherm-1), which would cut my
oil consumption by another 500 litres or more; it would also help
minimize my use of the dehumidifier during the summer months (a double
bonus).
I have a modern (<3 years old) heat pump in my mountain home. (Special
circumstance - I found a surplus heat pump cheaper than I could get
the same capacity AC-only package unit. Apparently nobody else wants
'em either.) I hate it. The quality of heat is lousy unless the
resistance heat is triggered and it takes forever to make the place
comfortable even with the ambient as warm as the mid 20s. It's my
third tier heat source, after the wood stove and the propane space
heater, in that order. My impression hasn't changed a bit regarding
heat pumps from my first one >30 years ago.
This is pure speculation on my part, but I can think of a couple
reasons why our experience here in Canada would be more favourable
than your own. One is that our homes are generally better insulated
and less leaky than our American counterparts -- don't quote me, but I
believe our provincial code requires electrically heated homes to be
insulated to R28 / R60 (walls and ceiling respectively) and R20 / R40
in the case of oil. And because our homes have so little natural
leakage, heat recovery ventilators are required by law. The upshot is
that we typically require fewer BTUs to stay warm (well within the
heating capacity of most standard size heat pumps) and there should be
no uncomfortable drafts or cold exterior surfaces that would
compromise occupant comfort. Put another way, the performance bar for
our heating systems can be set that much lower.
Secondly, and perhaps not surprisingly, heat pumps in Canada are sized
according to our heating requirements, whereas I understand yours are
tilted more towards cooling (at least in cooling dominated climates).
My sense is that this "next size up" rule helps to minimize the single
most common complaint with heat pumps; that of the so-called "cold
blow".
Cheers,
Paul