Charging - Superheat/Subcooling on fixed orifice systems.

[bundysnake]

Hello,

This may be a pretty basic question but im slightly confused.

I have been installing ducted a/c's and the manufacturer recommends to charge using the superheat method (it is a fixed orifice system & the system has a suction line accumulator). I thought a superheat test could only be done once the conditioned area reaches it design temp?

How can i accuratley charge this system if it won't get down to design temp?

I'm think it needs to be charged using the subcooling method then fine tuned by verify that the superheat is correct and no liquid is entering the compressor?

Another question is the superheat test done at the suction line compressor inlet (between compressor & suction line accumulator) or before the suction line accumulator? i have been doing it at the compressor suction line inlet.

Cheers,
bundysnake

[marc5180]

As a generaly rule of thumb fixed orifices are usually charged by superheat alone, TXV's and such use subcooling and superheat to charge.

Does the manufacturer not supply a standing charge for the units that you are fitting? If so then weigh this in and then charge bit by bit using the superheat method.

As you add refrigerant the temperature of the room will come down to design temperature, if you find you have slightly overcharged it then take some refrigerant out.

[bundysnake]

the units come precharged but i have found that the factory charge isn't very accurate and can sometimes be a kilo or 2 short with the standard pipe run length.

The manufacturer recommends charging to 8K super heat, this means 8K @ design temp right?

[Toosh]

the units come precharged but i have found that the factory charge isn't very accurate and can sometimes be a kilo or 2 short with the standard pipe run length.

The manufacturer recommends charging to 8K super heat, this means 8K @ design temp right?

Hi Bundy welcome to the form try searching there are numerous threads on here about superheat

Toosh

[Gary]

the units come precharged but i have found that the factory charge isn't very accurate and can sometimes be a kilo or 2 short with the standard pipe run length.

The manufacturer recommends charging to 8K super heat, this means 8K @ design temp right?

There is superheat at the evap outlet, superheat at the compressor inlet and everywhere in between.

There is superheat at the highest expected load, superheat at the lowest expected load and everywhere in between.

Manufacturers are generally more specific in their recommendations. If you want to know exactly what they mean by 8K superheat, you will need to ask them.

I would assume they want 8K superheat near the compressor inlet under lowest expected load... but I could be wrong.

[Gary]

Hello,

This may be a pretty basic question but im slightly confused.

I have been installing ducted a/c's and the manufacturer recommends to charge using the superheat method (it is a fixed orifice system & the system has a suction line accumulator). I thought a superheat test could only be done once the conditioned area reaches it design temp?

How can i accuratley charge this system if it won't get down to design temp?

A better question would be, "Why won't this system get down to design temp?"

[Gary]

the units come precharged but i have found that the factory charge isn't very accurate and can sometimes be a kilo or 2 short with the standard pipe run length.

This probably means you are setting the blower speed too high. Just as low airflow can cause low superheat/floodback, high airflow can cause high superheat, leading you to believe the system is undercharged. High airflow can also cause humidity problems.

Measure the temp at the thermostat and the temp of the air leaving the evap coil (measured close to the coil). The difference between these two temps should be about 11K for proper humidity control and the blower speed should be adjusted to attain the proper coil air off temp.

[bundysnake]

Hello,

Thanks for the replies. Gary i'm not trying to troubleshoot 1 specific unit here, as i said the units come pre charged but i have found that the pre charge is all over the place (for example one unit was meant to have 5Kg of R410, i reclaimed all gas and there was on 3kg and that was before i even opened up the valves to the blower.)

Your answers were very helpful and i will conatact the manufacturer and find out exactly how they want the superheat test done.

Blower speed cannot be controlled other than setting it to high, med or low speed.

Thankyou,
bundysnake