Checking Overcharge or Undercharge on a Domestic Split

Hi everyone,

I have a Mitsubishi 3.5kw split unit which is normally charged by weight. On these splits there is a service port only on the suction line so it is possible to measure superheat but not subcooling because liquid line pressure cant be measured.

Is it possible to use superheat to determine if charge is correct?

Or would it be a case of full recovery and recharge by weight again?

One final question do these domestic splits use a TXV or a fixed restrictor because if they use a TXV then it is my understanding one would check charge using subcooling which I cannot do.

Thanks for any help.

All manufacturers recommend only charging by weight. That is why these units come precharged. Units use capillary/restrictor expansion.

Hi Tec2K
is the unit not an heat pump? if so you could have put your discharge gauge on the service valve, is there no schrader valve inside of the unit?
yes I would be confident to test superheat to comfirm gas charge
best of luck

Hi Sedgy, thanks for the reply.

The machine is an MSZ-GC35VA 3.5kw inverter.

It does have a heat pump. I don't really want to open up the unit if I don't have to. I charged by weight according to the specification at 30g/m for every meter more than 7m piping.

I am assuming when they say the piping length they mean from outdoor unit to indoor unit and NOT liquid line plus suction line.

The machine IS cooling the air inside and I tested heat pump operation and it seemed to be working but suction pressure at the service valve is low with temperature close to 0 degrees C and I am sure it should be closer to 4 degrees C with pressure at about 115 psi. Would this indicate an undercharge?

Quote:

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Originally Posted by tek2k

The machine IS cooling the air inside and I tested heat pump operation and it seemed to be working but suction pressure at the service valve is low with temperature close to 0 degrees C and I am sure it should be closer to 4 degrees C with pressure at about 115 psi.

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Why do you think that it should be higher?

Well if the suction pressure at the service valve is 100psi that translates to 0 degrees C roughly which could cause the condensor coil to ice up? Or is that incorrect?

That is why I think the pressure should be a little higher.

The saturation temperature of the refrigerant inside the evaporator coil is not the same as the surface temperature on the outside of the evaporator coil.

I have seen saturation temps go much lower than this without freezing the coil.

This is not like pumping up the tires on your car. The pressure does not reach a particular setpoint and stay right there. It varies with conditions. There are several variables which effect pressures, the least of which is refrigerant charge. Trying to charge a system to achieve a particular pressure is a huge (and all too common) mistake.

M series not my speciality
but
dont worry about indoor coil frosting.
The unit has coil frost prevention function as standard.
In this case if the indoor coil temp sensor reading 3 to 6 deg C then outdoor unit slows compressor with the inverter.
If indoor coil is less than 3 for more than 5 mins then outdoor unit stops compressor & restats it again after 5 mins.

As the unit is inverter it also has LEV
'electronic expansion valve'
for the first 15 mins the LEV valve opens up in tandem with the compressor speed.
After 15 mins the LEV is controlled to make target discharge temperature.

The target discharge temp depends on the compressor speed from 47 at slowest comp speed up to about 67 at max comp speed.

So
If you think the system is short of gas suggest measure compressor discharge temp after about 30 mins max cooling, if its alot more than ,,,,lets say about 85 then it might be slightly SOG
If its over 95 then think about refrigerant recovery & start again
If its over 100 then it definately very SOG

If the compressor discharge temp gets up to 110 deg C then the outdoor unit starts to ramp down the compressor speed to protect the compressor.
When the discharge temp come back down to about 80 then the comp can start to ramp up again.
So if you watch the unit operating at max cooling & it ceeps slowing down & then speeding up every few mins that could indicate SOG.
But any way if the discharge temp get up to about 116 then the outdoor unit will protect the compressor by forced stop.

If the compressor discharge temp is excessive then the suction pressure will be very low & the compressor shell will be very hot.
This does not meen that the system is definately SOG but could also be LEV valve fault not allowing enough refrigerant through or some restriction in the pipe which is reducing refrigerant flow.

7 bar is not excessively low suction pressure but depending on the indoor & outdoor temps you would generally expect 7.5 to 8.5 bar suction.

So if the discharge temp looks about ok then best leave it.

And the discharge temp rises and falls in direct proportion to the compressor inlet superheat.

So if the compressor inlet superheat looks about ok then best leave it.

Ok thank you for the replies. I will investigate further and report back :)

Hi,

The obvious answer is by weight, unless you can access the bends on the condenser coil without comprimising performance, if so measure the temperature of the bends starting from the top, when you hit two or three bends where there is no temperature change that is your approximate saturated condensing temperature from which to determine subcooling, having said that splits are all critically charged so if you ever get the chance weigh it out.

Cheers