On 21 Feb 2005 14:28:33 -0800, wrote:

p j m@see _my _sig _for_address.com wrote:

Oh, yes, absolutely.

The TBP is the point where the output of the heat pump is
sufficient to maintain target temps inside ( regardless of cost ).

The ECB is the point where the cost of getting a BTU out of
the HP exceeds the cost of getting it from some other source ( gas /
oil ).

In theory, the ECB of a HP CAN NOT fall below unity ** as
compared to electric strip heat **, IE, it will ALWAYS return at
least
1 KW worth of heat for 1 KW of power to run it ( COP of 1 ).

Well agreed on the last point. However if the alternative source is
gas/oil, the EBP will be reached well before the COP falls close to 1.
Right?

Very likely, but of course depending on the cost of each fuel
source, and the efficiency of the furnace in question. At a practical
'real world' level - 'yes'.

And if the HP is sized so that it can maintain design temp even below
the EBP, then it seems clear that as outdoor temperature drops, further
increase in heat loss simultaneous with reduction in capacity to reach
the TBP, could only result in the cost of electric HP heat exceeding
gas/oil at the TBP.

Yes, exactly. More importantly, proper heat pump sizing is
mainly oriented towards COOLING needs, where SHR ( sensible heat ratio
), IE, latent heat removal, is so important.

By general concept, heat pumps are most effective where it is
warmer. Thus, in context, cooling season is going to be a dominant
concern. Thus, moisture removal is critical to proper system design.
Thus, oversizing ( such as to provide more heat during the lesser
heating season, at the expense of proper moisture removal in summer )
is not advised.

In an area that is so dominantly cold rather than hot, heat
pumps make a lot less sense, if any. In areas where it is dominantly
hot, heat pumps should be sized as AC units, with AC needs in mind.


So I'd say that the statement "below the 'thermal balance point', the
heat pump is STILL putting out economical heat into the house" depends
on

1) alternate heat is resistive strips, such that the EBP cannot ever
realistically be reached - OR -

2) the TBP is above the EBP, and the presence of an EBP implies fossil
fuel alternate heat source; in this case, the HP would never be run
below the EBP because the fossil source would have taken over.

Close, not quite cigar material.

The HP can still be contributing economically effective heat
below it's TBP, even if the TBP is below the EBP. Then again, this is
likely to be splitting hairs - if it's *****ing* cold out, you need
some *real* heat ! :-)

My own system as case in point - when it gets ~ 35 outside,
the HP can't keep up, and the strips kick in as stage 2. But I let
the HP continue to run, because THOSE BTU's are still costing me less
than the ones coming from the strip heat. IE, below TBP, above EBP.
At much less than ~ 25, I get tired of hearing the fan run all the
time and the cost difference is so minor, so I just go to plain strip
heat. Below ~ 17, I need BOTH to maintain temps - neither one by
itself is enough. The HP is STILL adding BTU's, very very noticabley
and measurably, but not at as much savings if any below the strip
heat. But the strip heat by itself won't keep up.

When I was at my neighbor's diagnosing an (unrelated) electrical
problem, I discovered his HP can maintain 67F indoors with 17F outdoors
with the outdoor unit fan not running (burnt out).

That's rough :-) The outdoor fan is how the HP accumualtes
heat to be moved inside ...... without it - you got squat.

His COP can't be
that great in that situation, certainly not as high as 2. It was
installed in 1978, oil backup. He's on his 2nd tank of oil since
installation.

Tell him to invest in a fixed fan :-)



Paul ( pjm @ pobox . com ) - remove spaces to email me
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