prog. therm. and heat pump questions

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Couldnt bullshit its way out of a wet paper bag.
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Rod Speed wrote:

Pure dribblesnot! Dribblesnot I say!!!!!
Or is that pigdribble, I never can keep those straight...
JazzMan
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Wow, a troll has tried to set his hook in me. Same MO what a joke
http://groups.google.com/group/sci.physics/browse_frm/thread/640181ba052520ff/c32eba203aae58e4?lnk=st&q=&rnum=5#c32eba203aae58e4
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Abby Normal wrote:

http://groups.google.com/group/sci.physics/browse_frm/thread/640181ba052520ff/c32eba203aae58e4?lnk=st&q=&rnum=5#c32eba203aae58e4
All he does is troll and flush toilets.
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http://groups.google.com/group/sci.physics/browse_frm/thread/640181ba052520ff/c32eba203aae58e4?lnk=st&q=&rnum=5#c32eba203aae58e4 Couldnt bullshit its way out of a wet paper bag.
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27. Rod Speed Dec 16, 3:47 pm show options
Newsgroups: alt.home.repair, sci.engr.heat-vent-ac, misc.consumers.frugal-living
Date: Sat, 17 Dec 2005 07:47:00 +1100 Subject: Re: prog. therm. and heat pump questions Reply | Reply to Author | Forward | Print | Individual Message | Show original | Report Abuse

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 yourself out.
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I disagree.
Nick
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Go for it, Nick. :)
On 16 Dec 2005 16:17:40 -0500, snipped-for-privacy@ece.villanova.edu wrote:

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Figure out any Biot numbers yet Nick?
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I disagree.
Nick
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.
Stretch
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for day:= Monday to Sunday for post:= first to last if answering_mode then if person = respected then post_snipe else if rnd()>0.5 then ad_hominem_attack() else declare_previously_resolved_by_self endif endif else with theory_confusion post_basic_babble() end with endif next post next day
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I disagree.

Yes. No. Yes. Yes. Why? Need some help? :-) HVAC criminals are not Gods. Their assertions demand no more than counterassertions.
Now why do we need a 2-speed compressor vs a 2-speed fan?
Nick
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Nick,
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 operation.
Some systems use two swparate compressors (Trane). You have to be careful of your refrigerant line sizing so you get good oil return to the compressor(s).
Stretch
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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?
Nick
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Nick, 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 way.
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.
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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? :-)
Nick
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Go look at the link to your solar buddies in Florida
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"got numbers" = "I don't understand it"

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snipped-for-privacy@ece.villanova.edu wrote:

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 run time.
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 system.
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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If it rises above the dew point. Sounds like poor control.
Nick
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