27. Rod Speed
Dec 16, 3:47 pm show options
Newsgroups: alt.home.repair, sci.engr.heat-vent-ac,
Date: Sat, 17 Dec 2005 07:47:00 +1100
Subject: Re: prog. therm. and heat pump questions
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The whole point of set back is to lower the temperature for a
while.This would imply that the house cools down and the systems stops
running during this cool down period. Then it would run less per hour
due to the setback indoor temperature until such a time that it need to
start warming up the house so that it was back up to temperature
perhaps when the occupants awoke or perhaps returned home from work.
The problem is shortly after the temperature dropped to the setback
temperature, the heat pump would end up running steady just to get back
up to temperature.
Typically in the temperate states where air source heat pumps are used,
they are sized with the cooling load in mind and use the heat strips.
One sized for the full heat load will be grossly oversized for cooling
resulting in summer time humidity control problems.
No not wrong just unsucessful in educating you.
So you are saying size one for the cooling load, one for the heating
load and one for speedy recovery from set back then?
Three heat pumps now. Plus all the ductwork and backdraft dampers.
Let's try to keep this practical and not go to hypothetical extremes to
prove this is possible. Noah er I mean Nick is a bad influence on you.
Well like I said if you want to install three heat pumps, knock
You disagree with almost everything Nick. Have you EVER installed ANY
heating or cooling equipment? Have you EVER installed a heat pump? Do
you have test equipment and data loggers to measure the performance?
Do you have any PRACTICAL experience? Or do you just like to disagree?
You obviously have a brain, it is a shame you don't do something more
constructive with it.
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A two speed compressor gives two capacities. Typically low speed
compressor will give you 67% of the capacity that you get on high speed
compressor operation. A two speed fan just changes the SHR without
changing the capacity much. Youi may get a 5% variance in capacity by
changing just the blower speed. Note that when you change the
compressor speed you must change the blower speed as well. The
compressor will usually run at 1725 RPM on low speed and 3450 RPM on
high speed. Normally the indoor unit has a variable speed blower so
the air flow can change about 2 to 1. A standard multi tap motor has
trouble doing that. The ducts should be sized for high speed
Some systems use two swparate compressors (Trane). You have to be
careful of your refrigerant line sizing so you get good oil return to
Why do we need a 2-speed compressor? Abby suggests a high capacity heat pump
for winter setbacks will do poorly for summer AC because it will inefficiently
short cycle and won't dehumidify well. It seems to me that it can dehumidify
well with less indoor airflow and a freezestat that turns off the compressor
while the blower keeps running. What are the numbers for this "short cycling?"
When do we have to worry about premature wearout or inefficiency resulting
from too many starts? If a 3 ton heat pump runs 100% of the time at the winter
design temp, can it do 1 ton of AC in summertime with less indoor airflow?
Maybe the answer to this question depends on thermal mass. If the AC runs for
10 minutes out of every half-hour that might be fine. If it has to run for
1 minute every 3 minutes that might be no good. Is there some way to add
thermal mass to the indoor coil? Do we need to worry about short cycling
on mild winter days?
The next time you see an AC or heat pump, Use one of your data loggers
to measure the temperature drop across the indoor coil. The indoor
coil will take 10 to 15 minutes to reach max coldness. The colder the
ID coil gets, the farther below dew point it will be and the more
moisture it will take out. With a less cold coil, while it will cool
the house, it will not dehumidify well. That is why you want to size
it properly. The ID coil does not get cold instantly. A properly
sized unit will run longer and dehumidify better without losing
effeicieny. By the way, when they test efficiency, they run the
systems for an hour before they do the testing. Systems do not read
max efficiency till they have run an hour. Gas furnaces are the same
If you just lower the air flow, if you go below 350 CFM per ton, your
system will loose capacity, while using about the same electricity.
Again, use your data loggers to measure capacity and power consumption.
So the efficiency goes to hell. If you recall, the original idea of
setback was to save money on electricity. Now we are talking about
extreeme efforts to make an oversized system work at all, running the
electric bills out of sight!
Sounds like you are all going in the wrong dirrection.
Got numbers? "Max coldness" may not be important, if it dehumidfies at 50 F
but takes 10 minutes to drop from 44 to 43.8 F.
Why not? If we reduce the airflow, the coil becomes colder. In the limit,
we dehumidify with little cooling. With lots of airflow, we cool with no
dehumidification. And let's not forget hygroscopic house furnishings.
Got numbers? Is this HVAC folklore, or something to do with poor controls
in commercial systems? A 40 F coil might dehumidify equally well whether
the compressor runs 10% or 50% of the time.
Got numbers? An hour is a long time... to go from 99% of the max efficiency
to 99.5%? :-)
Got any numbers for us unbelievers, or simply your word as a God? :-)
In a humid environment, running the fan constantly is a bad idea. The
reason being, as soon as the compressor shuts off, the indoor coil
warms up. The mositure still on the coil will be re-evaporated
resulting in elevated indoor RH. 3 steps forward and 2 steps back.
It is even worse with a draw through air handler installed in a
horizontal position, the drain pans have a side connection. This means
that there is approximately an 1/8 of an inch of standing water tht
does not drain. So along with re-evaporating the moisture that did not
drain off of the coil, the constant fan makes the air handler into and
adibiatic saturator as it dries out the drain pan too.
Ballpark figure, constant fan will keep the indoor RH about 10% higher
than fan in "auto" mode. Hard concept to believe in the north but
perhaps search and see what Building Science or the Florida Solar
Center has to say on this matter.
Well you are the electrical engineer by training so I would assume that
you realize that motors are aged every time they are started.
In an environment where in the summer the outdoor dewpoint will not be
that high then you could size a heat pump (most likely a ground source
heat pump) to the heating load and be grossly oversized for cooling.
But with a relatively low ambient dewpoint you would not end up with
the high indoor RH.
Typically a heat pump is sized with cooling in mind to avoid the poor
humidity control in summer. Therefore in the winter during design cold
weather the compressor runs, with incremental auxiliary heat. Would
suggest a long run time, compressor on steady, with heat strips cycling
on and off. It could be to the point that it is so cold outside that
the compressor is off and the home could be on full electric heat even.
Under less than extreme weather the compressor will still have a long
When you want to analyze the system from a temperature and pressure
standpoint you should wait 15 minutes for the system to stabalize. Work
backwards from 15 minutes and figure out the time constant of the
Consider a furnace sized on the money for the heat load. Should be
running close to steady when the overnight lows reach and even exceed
the design levels. During weather that is cold but not at the design
level, they still run long enough to hit steady state and work at their
rated efficiency for a period of time, during mild weather they may not
run long enough to hit steady state before satisfying the thermostat.
Now consider a furnace that is oversized, runs 20 minutes on the hour
under a heating design ambient. Outdoor conditions warm up to a little
above the design level and soon not even 15 minute run time. Bulk of
the winter, at say half the design temperature differential, it never
even warms up to steady state before the thermostat is satisfied.
Wastes fuel, and heat exchanger is prematurely aged. HX ages everytime
it is heated up and cooled down.
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