cold weather operation

[hobbit]

I've got a couple of general questions about heat-pump operation
in cold ambients. One practical, one more theory.

Most units either have a crankcase heater or use the inverter to
bleed about 50W average through the motor windings to keep the
herm warm, and I totally understand the benefits of *not* trying
to run a cold compressor. I've read thread 1226341 that touches on this.
My concern is what happens after an extended power outage -- an
outdoor unit cools down to cold ambient, power is restored and
there's instant demand, what happens? Do any units try to protect
themselves against that? I haven't heard of compressor temp
sensors other than maybe just an overtemp cutout, so it seems
like it would happily try to start up with sludged oil and the
expected unhappy results. What, if anything, is done to prevent this?

The other Q has to do with ambient temp and defrost cycles. If the
outdoor air is below freezing already and the HP is bringing that
air even colder, is there an applicable "dew point" as such? How
does water vapor already under 32F condense on something, or does it?
I'd completely understand why a unit ices up in that typical cold
clammy just-above-freezing winter weather when the coil is running
like 10F, but I'm having trouble understanding how icing occurs
when it's already cold. Observing my fairly new unit so far has
shown a lot of ice in > 32F ambients, but almost none when it's
gotten down around 20F. The system still goes through defrost
cycles every so often regardless...

thanx

_H*

[ryan1088]

I'll take a stab at it....

There are no time delays I'm aware of other than thermostat delays that would prevent something like that from happening. Seems the only way to stop something like this from happening would be to turn the thermostat off and let the heater warm up for a while. Possibly running emergency heat in the meantime.

The second part is a little more interesting. Remember that the warmer the temperatures, the more moisture air can hold. When you start getting like freezer cold, you start to get frost instead of ice. The ice comes from doing a defrost, the frost melts then refreezes if all of it hasn't ran off. During a good cold spell you will see ice all around the bottom of the stand of a heat pump.

I hope this gets you what you're looking for.

[NY2GA01]

Not sure if this helps,

You have a reversing valve (aka 4 way valve) that reverses the flow of the refrigerant, and depending on the system that valve is energized in the heating mode, or cooling mode.

If you touch the line sets (AKA copper pipes), you will feel they are ice cold in the cooling mode, and will be extremely hot in the heating mode.

Also, unlike your normal AC system, your condenser is not a condenser in the heating mode, with a heat pump it's not even called a condenser, it's now called indoor coil, and outdoor coil.

In the coolinging mode it's a normal cooling system. In the heating mode, your outdoor coil is now your Evaporator, and your indoor in coil is your condenser.

There is a defrost control board on the outdoor coil on a heat pump, once ice builds up from the eaporation, the unit goes in to defrost.

Refrigeration & air conditioning Technology unit 43, heat pumps.

http://books.google.com/books?id=eS_...t%2043&f=false

[ryan1088]

I know this is a bit off topic Hobbit, but how are you liking your VRV system?

[hobbit]

I'm aware of how the indoor and outdoor coils have to swap functions,
and even know exactly where my reversing valve is... what I'm trying to
understand is what happens to subfreezing *air* when it goes through
an outdoor coil that's running "colder than cold" in heat-pump mode.
Like last night, ambient 20F or less and the 'stat in service-mode
was displaying an evaporation temp of -2F when running, but no ice
was forming on the coil. The outdoor RH was fairly low, which might
have had something to do with it.

This morning it's about 10F outside and the unit is *just* starting
to pick up a little light frosting while running. But nothing like
the heavy white blanket that clung to it fairly quickly when it was
40F and soggy outside a few weeks ago. [The whoosh of steam at the end
of *those* defrost cycles when the fan came back on was impressive.]
I guess it has more to do with outdoor RH than just temperature, right?
Although the point about colder air holding less moisture is also
well-taken, e.g. that must mean the colder air going through the coil
must have less water it can deliver.

Maybe I'm being confused by the concept of "dew point" when going
from subfreezing to subfreezing, but as there's no bump in the psych
chart around that region I guess it becomes the "ice crystal point"??

To ryan1088 ... the above probably answers your question; the Skyair
system is performing great so far, in fairly cold ambients and *without*
running the heat strips. It's heating entirely on the heat pump and
not even trying to bring the strips on -- the beeper I've got attached
to the "toaster" relay circuit would tell me if it tried to energize
the resistace heat. The outdoor unit sits close to my bedroom window
and ramps up so gently on a run that I can't even tell when it started,
my only clue is when the AHU fan starts to quietly push some air.

At some point over the Fall I had a different tech out who pulled the
old charge, ran through all the proper pressure and vacuum tests, and
weighed a new charge back in to also account for indoor coil and lineset
length like hadn't been done originally. He even watched some runs on
the Service Checker laptop he brought along to make sure all the target
numbers were being met -- the guy is one of the rare few that lives and
breathes both Daikin and Mitsu and we had a great conversation the whole
time. Everything was much happier afterward, including that annoying
picky homeowner.

A far cry from the 50-year-old oil monster it replaced, that's for sure!

_H*

[motoguy128]

Just because the temperature drops below freezing does not mean that all moisture in the air comes out of it's vapor state and freezes. If it did, you'd have an instant ice fog occur outside any time it was between 31-33F outside.

The best I can explain it is that air molecules have energy, meaning that they are bouncing around and smashing into each other... the warmer it is the harder they collide. Those collisions keep some water molecules dissolved and in a gaseus state despite it being below the freezing point outside. Once they come out of solution, they however become frost. Frost always forms on cold surfaces with a lot of thermal mass, like metal and wood because as frost forms, it releases energy and warms that surface. IF the object was light weight, it wouldn't be abel to absorb that energy. IF you think about it, air at even 100F is incredibly dry.... compared to air at 300 or 400F that nearly saturated. As a matter of fact, if you go the opposite way and remove all the air from water vapor, you now have steam. Steam is in fact tranparent... until it condenses.

One other point, the coil on a newer high efficiency condenser, due to it's large coil size, is usually only 5-10F colder than the ambinet air temp. Approach temps are usually only 1-3F and the airflow on 3 ton condenser might be roughly 3000CFM. So if it's putting out 20,000BTU's, that's a temp rise of just 6.2F for the air at that CFM. (CFM X 1.08 X Temp Rise = BTU's)

[Shophound]

Whether or not an outdoor heat pump coil will freeze or not depends on the dew point temperature of the air passing over the coil, and the surface temperature of the coil itself. If you're really curious about this, monitor both the dew point temperature of the air entering the heat pump, and measure the coil surface temperature. Ambient dew point temperatures can become quite low when a large, cold air mass (called a cold front) invades a region. In spite of what may appear to be high relative humidity readings during cold weather, when this high RH is translated to dew point, the actual moisture content of this same air can be quite low.

As for the crankcase heater on the compressor, if you experience a sustained power loss at your house in winter (and you live there...it's not a vacation home), turn off your heat pump, turn all of your appliances off and unplug all electronic devices in the house. Drip all indoor faucets.
There is no guarantee that when power is restored it will be clean, full power (i.e. "brown outs", etc.). Provided it is, place the thermostat for the heat pump into emergency heat mode. The electric heat strips will begin warming the house while the compressor crankcase warms up without running.

[beenthere]

At 20°F and 50%RH the DP is 6°F. At 15 and 50% RH the DP is 1.3°F

At 20°F and 50%RH, 11,268 CF of air has to pass through the coil for approximately 1 pint of water vapor to have passed through the coil.
At 15°F and 50%RH, 14,218 CF of air has to pass through the coil for approximately 1 pint of water vapor to have passed through the coil.

Keep in mind that not all of that moisture touches the coils or fins.

[ryan1088]

At 20°F and 50%RH the DP is 6°F. At 15 and 50% RH the DP is 1.3°F

At 20°F and 50%RH, 11,268 CF of air has to pass through the coil for approximately 1 pint of water vapor to have passed through the coil.
At 15°F and 50%RH, 14,218 CF of air has to pass through the coil for approximately 1 pint of water vapor to have passed through the coil.

Keep in mind that not all of that moisture touches the coils or fins.

How did you derive all of those numbers???! Great stuff.

[beenthere]

Carrier Psychrometric software to find the grains per pound and CF per pound. Then just did the math to find the volume of air needed to have 1 pound of moisture.

[ryan1088]

I guess I should have payed better attention in math class......

[beenthere]

At 15° and 50%RH, there is 5.898 grains of moisture per pound of air. 1 pound of that air is a volume of 11.98 CF. A pound of water is 7000 grains. 7000/5.898=1186.84 pounds of air.

1186.84X11.98=14218.34 CF

[NY2GA01]

At 15° and 50%RH, there is 5.898 grains of moisture per pound of air. 1 pound of that air is a volume of 11.98 CF. A pound of water is 7000 grains. 7000/5.898=1186.84 pounds of air.

1186.84X11.98=14218.34 CF

Do you: live, eat, sleep breath psychrometric.