proper system charging

As the owner of a very large and sucessful HVAC company.I just had to reply to this one,

If anyone doesn't thnk that you can cool an area faster with more 55 degree air than less 55 gdegree air you are just plain dumb and shouldn't be in this field at all. (See the section in VAV101)

If your answering questions about superheat, just remember you are taking that reading at the outside unit for conveience only, the only reading that matters is at the coil outlet.

Most of the air leaving the room is getting recycled across the coil and sent back into the room

It is the 55 degree air that is cooling the room nothing else. If you have more of it the cooler it will get. GEES O MAN

Cooling fast doesn't help if it doesn't dehumidify.

Bringing your Sh down to 10, isn't always agood thing either.

The original fast cooling that born was talking about was for eff, by way of decreasing his SH, and he wasn't taking any wet bulb reading when doing this.

Not a practice that many recommend.

Yes I agree with the not dehumidifying part, I was replying to the post that the guy was wondering if cool air could cool the space You can cool a space with 70 degree air if you move enough of it across it, if you want to ring it out you have to have dehum.

Born has a couple of post going on charging, and eff, and speed of cooling, he's getting hard to follow, and remmember what he posted where.

What sort of concerns me, is he teaches HVAC, and is asking the best way to check charge, so he can teach it to his students.

I commend him for wanting to learn, BUT, he should have learned how before he started teaching.

OK,

We are reading the same posts then,

In fact I read a post where the guy said that charging and troubleshooting were the same thing. Yeah thats what we should be teaching the new people to the field put your gages on first and charge it, you
ll find out whats wrong with it, !!!!!!!!! ????????????????????????

I don't know about him but I usually wait til I have made the repair before I charge the system.

Many years ago, a service manager told me I was an a/c's worst nightmare.
He said your going to put your guages on before you check anything else and come to the wrong conclusion, and add gas.

He was right once or twice, I'm sorry to say, I had to learn the hard way.

That was back in 80. And since then I found too many people do exactly that. We need better training in the schools, for the new techs.

And were not getting it.

Quote:

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Originally posted by fat eddy
In fact I read a post where the guy said that charging and troubleshooting were the same thing. Yeah thats what we should be teaching the new people to the field put your gages on first and charge it, you
ll find out whats wrong with it, !!!!!!!!! ????????????????????????

I don't know about him but I usually wait til I have made the repair before I charge the system.

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fat eddy, you are totally taking my remarks out of context and it is obvious that you have not read my articles on superheat, subcooling, troubleshooting or charging that are located in the "For Your Interest" area.

My point is simply that there is a relationship between properly charging a system and understanding what the subcooling and superheat should be on a normal system. The very same concepts that are required to know when a system is properly charged are those used to troubleshoot a system that is not operating properly.

You have taken me out of context and then tried to make it look like I stated something that I never said and totally disagree with.

Quote:

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Originally posted by fat eddy
As the owner of a very large and sucessful HVAC company.I just had to reply to this one,

If anyone doesn't thnk that you can cool an area faster with more 55 degree air than less 55 gdegree air you are just plain dumb and shouldn't be in this field at all. (See the section in VAV101)

If your answering questions about superheat, just remember you are taking that reading at the outside unit for conveience only, the only reading that matters is at the coil outlet.

Most of the air leaving the room is getting recycled across the coil and sent back into the room

It is the 55 degree air that is cooling the room nothing else. If you have more of it the cooler it will get. GEES O MAN

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You state that the only reason for taking superheat readings at the outside unit is for convenience only.

That is incorrect! A total low-side superheat measurement near the compressor is useful to determine if the refrigerant is flooding back to the compressor as well as determining if the compressor is getting sufficient suction cooling. Often high discharge temperatures are caused by high suction return temperatures which are often caused by high low-side superheat.

There is more but I will give you time to think that over before posting more.

Quote:

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Originally posted by bornriding
Shop, Doc, Thank ya'll very much. I've enjoyed reading and discussing your replies.

Here is my second try at explaining what I mean.

I've always thought that the reason for conditioning the air in an area ( say a home ) was to mixed the existing air in the home with cooler air from tha a/c system, at a proper level ( cfm's ), which determines how fast ( or slow ) the air is 'mixed' ( for lack of better word ) and then returned to the unit.
To me, there were two factors in determining how long it would take a 'home' to cool down from the time the thermostat turned the unit on until the thermostat was satisfied. And they were, the amount of cool air delivered to the home ( and recirculated within ) and #2 - the temperature of that air that was delivered.
I'm talking the same home, whatever the heat gain.
So, I've got a certain home, with a certain heat gain, and say my unit is sized correctly.
I kinda, at this moment, don't consider humidity or latent heat because by the time the air is delivered to the home from the system, only sensible heat is being sent.
I'm also not talking about the effects on a system from raising or lowering the volume of air or the temp. of the air.
But, to me, the amount of air and the temperature of that air are the two factors that actually govern the time that the a/c system will have to run to 'satisfy the thermostat'. And if I can get more air from a system, and maintain the same temperature of the air then the effect will cool the area ( home) quicker at less expense ( electric bill )
Or, if I cannot get more air, but if I lower the sensible temperature of the same air that is now being delivered to the home, then the effect will cool the home quicker at less expense. ( realizing that too much of either one is bad )

One of my points, I guess, is that all I here about how to set up a system deals with 'superheat' and yet, everyone gives a range of values, such as 10 to 15 degrees superheat ( + - 5 ).To me, the difference between 10 & 15 degrees superheat is a difference in the supply air temperature ( I have done tests for this ). If I set a system at 15 degrees superheat, the supply air will be (say) 59 degrees. But by setting the superheat at 10 degrees, the supply air reduces to (say) 55 degrees. So I prefer to set a system at 10 degrees s/h so that my supply air temp will be lower.... and I actually think that I am doing right.
All this, knowing of course that I must be careful to avoid liquid getting to the compressor.

What's ya'lls thoughts??

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I think your confusion is based on a misunderstanding about types of heat in the space.

Latent heat: heat that causes a change of state not temperature. (Humidity)
For example it takes 970BTU’s of heat per pound to change water to steam at 212° F.

Sensible heat: is measured by a thermometer (heat intensity).

O.K. I know that was basic so here goes the explanation: Air conditioning systems need to remove total heat (latent plus sensible) or a total amount of BTU’s, if you want to look at it that way, at a greater rate than the heat gain.

The temperature of the air delivered to the home helps lower the sensible heat which the thermostat is measuring. You are correct in assuming that if the air volume is in creased and the supply air (SA) temp is lowered (over sizing the system in a sense) that the home will cool off quicker, but it will only cool off quicker sensibly. Once the sensible temperature requirement is satisfied at the thermostat then the unit cycles off. Now the latent heat in the space begins to “emit” and causes a rise in sensible heat and the unit starts again. It quickly removes the necessary sensible heat and cycles off. This cycle takes a relatively short amount of time so the unity cycles several times per hour in this yo-yo effect of trying to remove total heat. Multiple starts are undesirable.
Each start of the compressor does several things:
1. Causes an amperage spike (pulls locked rotor amps) which uses about five times more energy than just running
2. Heats up the windings in the compressor which rely on cool suction gas to cool off. So the unit cools off the space fast and shuts off with hot windings because the unit did not run long enough to get cool gas back to the compressor. Then in 5 min when it starts again the windings heat up more without being cooled off then the cycle repeats itself until the compressor fries itself.
3. The heat is also not good for the compressor oil which begins to break down and circulates through the system as sludge.

A properly sized system needs to remove sensible and latent heat (total heat) to be effective. The operation of a properly sized system goes something like this:
1. There is a call for heat.
2. The unit cycles on and begins removing total heat
3. Due to the run time of sufficient length the compressor windings are adequately cooled by the cool suction gas and the space has been properly dehumidified (latent heat removed).
4. The stat is satisfied and the unit cycles off and the unit stays off for a lengthier amount of time than if sensible cooling alone is used to cool the space so less starts = less energy consumed and better savings.

Super heat is a safety factor and is a preference setting to some degree it depends on the risks you are willing to take. The lower you go the more risk of loosing a compressor but more efficient. I have seen several systems run through our hot summers with only 8° of superheat and they are over 20 years old with the original compressor this definitely is on the low end of the superheat spectrum and merely my observation not a recommendation.

Load calculations are tried and true and are based on some really intense engineering research that you can trust. If the load calculations are performed correctly and the unit is installed correctly and kept clean then you will have a very efficient system. I realize these are two big ifs but that is my spiel hope it helps some.





[Edited by hvacbear on 02-13-2005 at 07:26 PM]

Quote:

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Originally posted by fat eddy
In fact I read a post where the guy said that charging and troubleshooting were the same thing. Yeah thats what we should be teaching the new people to the field put your gages on first and charge it, you
ll find out whats wrong with it, !!!!!!!!! ????????????????????????

I don't know about him but I usually wait til I have made the repair before I charge the system.

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Born: Whoever said this was not me. Been a troubleshooter over 25 years. And I never tell my students to put the gages on first, always use your five senses for troubleshooting and your knowledge and you can usually locate the general type & location of the problem without any test equipment. Gages only go on when you are sure that it is a refrigerant problem.

And Hvacbear: no, i'm ot confused on two types of heat. I just realize that when the conditioned air reaches its area, it has already been de-humidified at the unit ( to whatever point that it can at that time and return air humidity level ). The air coming out of my supply register is producing only sensible effects of heat removal.

And all: Considering that a system will generally reduce the humidity in any installation the first few times that it may operate, after that less humidity removal is needed and more BTU's can be used for sensible removal. My consideration was for the, most of the time, ( by far ) that a system is operating under normal ( at least normal inside ) conditions.

[QUOTE] Oringinally posted by Bornriding
always use your five senses for troubleshooting

You'd be hard pressed to get me to use taste when working on someone elses system... Lol

Posted by bornriding

And all: Considering that a system will generally reduce the humidity in any installation the first few times that it may operate, after that less humidity removal is needed and more BTU's can be used for sensible removal. My consideration was for the, most of the time, ( by far ) that a system is operating under normal ( at least normal inside ) conditions.


As long as the evaporator temperature is at or below the dewpoint temperature of the air it will continue to dehumidify (remove moisture and create condensate) as long the system continues to run.

This is why undersizing a system can cause low indoor humidity because it continues to run and dehumidify while not satisfying the thermostat.

An oversized system will run a short time, satisfy the thermostat too soon and fail to properly dehumidify the air in the space. The result is a cold and clamy space.