# Superheat and Subcooling question



## hdung91 (Feb 13, 2013)

Dear all,
I have a question related to the calculation when I charge the A/C system.
I studied alot from the book but still not understand the situation.

I want to know the proper way to charge an AC system.

Some using the method of charging the system until it reach about 40 degree.

The method I am using is Superheat and Subcooling. The way I check is check the pressure and the temperatute of the line to determine it.
Superheat - 8 - 12, or 12 - 20, lower remove, higher charge
Subcooling - 10 - 20, lower charge, higher remve
Is that right?

But I also saw someone using wetbulb and drybulb temperature to calculate the subcooling.
Where is the wetbd temp located?
Where is the drybd temp located?

And why is it difference than my method?

Can you give me an advise?

Thanks.


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## Tom M (Jan 3, 2007)

I read it all a ways back but forgot. Looks like the your doing super heat correct. Not sure if the sub cooling means low charge. A dirty condessor would have low sub cooling.
The bulbs I think are a cross reference from the low side expansion valve and the bulb tempeture on the exiting line from the evaporator. The temperture drivers the valve to the right pressure going in.

I think


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## hdung91 (Feb 13, 2013)

Even my professor in school show me the way to charge is just:

Let the R-22 in and use the Thermometer to place on the evaporator supply air and wait until it reach 40 degree.

I just add R-22 to the exist system, It is reading 60 degree on the evaporator, superheat is corrected. And, I tried to add 1 lb, it dropped 1 degree, which is 59. But the superheat is too low, which is the refeigerant need to remove. 

Why is that happen???

Can anyone explain how they charge the ac.


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## beenthere (Mar 5, 2009)

Fixed metering device. use SH. measure wetbulb of return air at unit when possible. At return grille is ok if you don't have easy access to air handler.

Then use ((Wbulb temp times 3) minus 80 minus outdoor temp) divided by 2.

Using an outdoor temp of 90 for example.
67*3=201-80=121-90=31/2=15.5 target SH.

As the humidity comes down, so will the SH and coil temp.


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## hdung91 (Feb 13, 2013)

Thank You So much.


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## hdung91 (Feb 13, 2013)

Outdoor temp, is this the air blow out from the fan??? or the ambient temp?


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## beenthere (Mar 5, 2009)

hdung91 said:


> Outdoor temp, is this the air blow out from the fan??? or the ambient temp?


The ambient.


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## hdung91 (Feb 13, 2013)

beenthere said:


> The ambient.


Thanks,
So basically is
Return Air and Ambient

Thank You So Much


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## flashheatingand (May 3, 2008)

beenthere said:


> Fixed metering device. use SH. measure wetbulb of return air at unit when possible. At return grille is ok if you don't have easy access to air handler.
> 
> Then use ((Wbulb temp times 3) minus 80 minus outdoor temp) divided by 2.
> 
> ...


Where is the 90 coming from?


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## flashheatingand (May 3, 2008)

In general, keep the gauges off, unless you suspect there is a problem (opinion). When need to charge the system you want to make sure both guages are attached, or you risk seriously overcharging a system.

Usually, there is data on the panel on the condensing unit for what s/h you want to aim for. In general, 10 degrees puts you in the ball park. Know whether the system is a fixed orifice or txv.


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## beenthere (Mar 5, 2009)

flashheatingand said:


> Where is the 90 coming from?


Its a temp I made up for the outdoor air temp. I could have used 80 as an outdoor air temp. Then the target SH would have come out to 20°F.

67X3=201-80=121-80=41/2=20.5


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## flashheatingand (May 3, 2008)

gotcha


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## hdung91 (Feb 13, 2013)

What happen if the superheat or subcooling not right?


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## beenthere (Mar 5, 2009)

hdung91 said:


> What happen if the superheat or subcooling not right?


Then the system doesn't cool right, or dehumidify like it should.


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## hdung91 (Feb 13, 2013)

Is that superheat is following by manufacture detail? I can not find any on the system I am working. It just giving the low side and high side pressure. doesnt say anything related to superheat.


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## beenthere (Mar 5, 2009)

Brand and model number of unit your working on?

When a manufacturer list a SH for there equipment, they are assuming that the indoor RH is close to 50%.


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## flashheatingand (May 3, 2008)

If the data isn't available, you should be alright at about 10 degrees sh. But, make sure the high side isn't too high. If the high side gets above 300 (r-22), something is amiss. 

Also, keep in mind, you need to wait a spell for the sh or sc. Add charge at 1/2 lb increments, and wait about 5-10 min.


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## hdung91 (Feb 13, 2013)

What is the normally condition of suction line pressure and high side pressure if R-22 and R-410a


I remember that one I have from low side is 75 psi, high side about 250 psi.

I calculated superheat using my own method of measuring suction line pressure and its temp is 8 degree, which is almost over charged. 

However, degree come out from evaporator is 60, so I added 1 lb of R-22, its only dropped 1 degree.

I am not sure is that the coil not clean or what so ever. 

.....


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## hdung91 (Feb 13, 2013)

I know 1 lb I just added is overcharged. However, it did dropped 1 degree from evaporator. Why?


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## beenthere (Mar 5, 2009)

What was the SC. Post the suction and vapor pressures, and the Sh and SC.


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## hdung91 (Feb 13, 2013)

System I checked 2 day ago.

Low side: 75 psi
High side: 250 psi
Suction temp: 50

That all I checked.

Forget to check Discharge temp.

Because its R-22 so I measure Super Heat.


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## flashheatingand (May 3, 2008)

Sounds like money to me. Leave it alone


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## hdung91 (Feb 13, 2013)

???


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## flashheatingand (May 3, 2008)

hdung91 said:


> I am not sure is that the coil not clean or what so ever.
> 
> .....


You don't want to leave the indoor coil un-checked. It's something you shouldn't get caught overlooking.


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## flashheatingand (May 3, 2008)

hdung91 said:


> ???


Those pressures and line temp are money. If the performance isn't up to snuff, something else is amiss. I wouldn't mess around with the charge anymore.


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## hdung91 (Feb 13, 2013)

Is there anytools that can troubleshoot quickly for the AC system.

I saw 1 before. Its a digital tools that can guide you through the whole process


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## hdung91 (Feb 13, 2013)

I need a videos that explain the way to charge and check the AC properly.

It is always an arguement related to this topic


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## beenthere (Mar 5, 2009)

hdung91 said:


> System I checked 2 day ago.
> 
> Low side: 75 psi
> High side: 250 psi
> ...


5 SH is too low. Can end up flooding liquid back to compressor on low load days. 

What was the outdoor temp when you took those readings.


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## flashheatingand (May 3, 2008)

hdung91 said:


> I need a videos that explain the way to charge and check the AC properly.
> 
> It is always an arguement related to this topic


Speak with the local supply houses. They usually will have a seminar on charging systems.

Btw, what is the root of the arguments?


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## flashheatingand (May 3, 2008)

hdung91 said:


> Is there anytools that can troubleshoot quickly for the AC system.
> 
> I saw 1 before. Its a digital tools that can guide you through the whole process


I hope not. If so, I am going to need to apply for a job at the bowling alley


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## beenthere (Mar 5, 2009)

flashheatingand said:


> I hope not. If so, I am going to need to apply for a job at the bowling alley


Fieldpeice has a meter out to help with diagnostics.


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## hdung91 (Feb 13, 2013)

beenthere said:


> 5 SH is too low. Can end up flooding liquid back to compressor on low load days.
> 
> What was the outdoor temp when you took those readings.


About 80 (


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## beenthere (Mar 5, 2009)

Unless you had really low humidity in the house, 5°F SH is too low.

Probably either a restricted LLFD, or a restriction in a distributor.


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## hdung91 (Feb 13, 2013)

beenthere said:


> Unless you had really low humidity in the house, 5°F SH is too low.
> 
> Probably either a restricted LLFD, or a restriction in a distributor.


I see, thanks

How can I test if the LLFD or a restriction happened from visualize???


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## beenthere (Mar 5, 2009)

You sometimes can't. A LLFD won't always have a temp drop across it when its restricted. But if it does have a temp drop across it, it is restricted.

Often, just have to recover and pull the orifice and clean it and or its screen and replace the LLFD at the same time.


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