DIYbanter - View Single Post

wrote:

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.

I agree fully.

My neighbor provides a counterpoint. Our climate would dictate fossil
fuel for a significant part of the heating season, but he wants no gas
in the house and even hates to use the oil backup heat. He'd be
all-electric except for fear of freezing during a blackout. Thus his
emergency heat is oil while for some reason he apparently has no
strips. So to him, his setup "makes sense" and inasmuch as he places a
dollar value on avoiding natural gas, it may even be "economical" for
him.

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 ! :-)

Maybe it comes down to what we mean by "economically effective." To me,
it means the most economical (least costly) choice of heat source,
given the physical design of the system. Therefore, operating the HP
anywhere below EBP is not economically effective, by the very
definition of the EBP.

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.

You have electric resistance strip heaters as backup, so you fall into
category (1) above. By your own words, you're also in category (2)
where TBP is above EBP (except you don't have fossil). So you don't
have a case of operating below both TBP and EBP while still trying to
call it "economical." Thus I think I do get that cigar!

To clarify, I posit that an EBP doesn't really apply for an
all-electric system, because at the worst the choice is between HP with
COP of 1 or strips with COP of 1. But either of these choices would
generally be more costly than fossil fuel at today's electric and fuel
prices, in $ per BTU of just the fuel or watts-hours. So the EBP
applies most directly to HP/fossil systems. In some sense, there is an
EBP with HP/strip systems, in that the COP falls close enough to 1 that
the extra wear of running the compressor and fan can't be justified
simply to avoid using the strips.

As a side note, even in climates where fossil would seem to make sense,
the expense of getting gas or oil service installed in certain
locations, coupled to the additional expense of purchasing and
maintaining the furnace relative to just an air handler, could make
all-electric more "economical" in a total cost of ownership sense.

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.

Right, you're using the compressor work as additional electric heat,
even though you might not be pulling much additional heat from the
outdoor coil.

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.

Without the od fan running, you get at least the heat from the
compressor. And that was apparently enough to keep his house 50F warmer
than the outdoor temperature. I was surprised to say the least. It must
be *grossly* oversized for the cooling load, where a 20 degree td would
be more typical in these parts.

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 :-)

I did and he did. Big old York "coffeetable" outdoor unit.

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