Another heat pump question

[Caleb Hess]

Does anyone here know the inside details of heat pump defrost controllers?
Are they smart enough to tell when the coils actually have ice forming, or
do they just do it preventively whether there's frost accumulating or not?
Is the energy cost of the defrost cycle included in the SEER/COP ratings?

 

[Gymmie Bob]

I believe they can measue it with pressure.

 

[daestrom]

Does anyone here know the inside details of heat pump defrost controllers?
Are they smart enough to tell when the coils actually have ice forming, or
do they just do it preventively whether there's frost accumulating or not?
Is the energy cost of the defrost cycle included in the SEER/COP ratings?

It varies. Some older units are just timers. But a low suction pressure
can be an indication of excess frost.

SEER ratings are for air conditioning mode, HPSF (Heating Season Performance
Factor) are what you want for heating.

But yes, they include the total cycle including any defrost. The *very
gory* details of calculating both are in 10CFR430, Subpart B, Appendix M.

http://a257.g.akamaitech.net/7/257/...s.gpo.gov/cfr_2004/janqtr/pdf/10cfr430.27.pdf

For air-souce heat pumps, there is a 47F 'high temperature test', (with 43F
wet-bulb temperature), a second 'high temperature test' with 62F outside
air, a 'frost accumulation test' with drybulb of 35F and wetbulb of 30F, and
a 'low temperature test' with drybulb of 17F and wetbulb of 15F.

daestrom

 

[daestrom]

Most now use an electronic temperature sensor and timer board. The sensor
closes around 30 degrees f and initiates the defrost cycle whether there
is actually any frost or not. This is done every 30 to 90 minutes
depending on jumper settings.
The defrost cycle is not included in any efficiency ratings.
Also be aware that efficiency ratings of any type are usually performed by
the manufacturer in a lab under controlled conditions.

I disagree. The HSPF rating calculations are specifically detailed in the
federal law. They include performance under frost conditions. And they
specify the testing details. So they work much like the mpg ratings on new
car stickers. Your exact performance might not meet the stated rating, but
all manufacturers are testing to the same conditions so the *relative*
rating between units/manufacturer's are a useful comparison.

daestrom

 

[daestrom]

You're right, to a point. According to ICP data, HSPF is based on
operation in frost, 80% occluded coil surface with temp of 30 degrees and
50% RH for 10 minutes. There is no mention of the defrost cycle though.

http://a257.g.akamaitech.net/7/257/...s.gpo.gov/cfr_2004/janqtr/pdf/10cfr430.27.pdf

Scrolling down to the 87th page in the pdf (reads 212 at the bottom of the
page though), 3.2.1.3 'Frost Accumulation test conditions'. This is the
section describing the testing required for air source heat pumps in order
to calculate their HSPF.

"....The Frost Accumulation Test requires that the unit undergo a defrost
prior to the actual test. The test then begins at defrost termination and
ends at the next defrost termination.. Defrost termination occurs when the
controls normally installed within the unit are actuated to cause it to
change defrost operation to normal heating operation. ..."

So the test explicitly *includes* the defrost cycle. Section 5.2 then goes
into detail of how the heating for a defrost cycle is converted to a heating
capacity. The calculation of Q(T) for 17 < T < 45 uses the heating capacity
from the 'Frost Accumulation Test'. So obviously, it depends on which
climate region you're calculating the HSPF for, but it does seem to have the
defrost part of the cycle in there.

There is some good information in this appendix as well. For example, later
in section 6.2, there is a chart of HLH (Heating Load Hours). It's kind of
interesting to note that although the Pacific Northwest is in zone VI and
has 2750 HLH, and the upper midwest as well as NY and parts of New England
have only 2250 HLH. *BUT* the temperature profile for the Northwest is
almost all above 30 F, while region IV and V have a significant portion of
temperatures much colder. So a heat pump that works well in Oregon with
2750 HLH would probably perform very poorly in parts of NY and PA.

daestrom

 

[Pete C]

Does anyone here know the inside details of heat pump defrost controllers?
Are they smart enough to tell when the coils actually have ice forming, or
do they just do it preventively whether there's frost accumulating or not?
Is the energy cost of the defrost cycle included in the SEER/COP ratings?

Hi,

Some electronic ones look at the rate of change of the evaporator
temperature.

I reckon it's possible to run a air-air heat pump by circulating water
indirectly from ground source (eg pond) over the evaporator.

cheers,
Pete.