View Full Version : Over Charge

19-12-2007, 01:29 AM
Hope someone can help. I am not an air con engineer but work on the associated controls. I have been called to a job where a Mitsubishi P250YMFC 31.5kw outdoor unit is serving 13 Mitsubishi P25VMLA 3.2kw indoor units several times recently, usually when the night time outdoor temp has been below 0. The error code is 1500 overcharged. The problem has been suggested as a control problem despite testing correctly every time. Would the diversity be suspect in these conditions or has anyone any suggestions.

20-12-2007, 12:36 AM
The only way to be sure is have an engineer do a testrun with a pc!

Any takers on here?

The Viking
20-12-2007, 02:16 AM
Problem arises at cold nights........

Well, it wouldn't suddenly have charged itself....

To start with, check the indoor units' filters and coils.
(Should be nice and clean, if not you know where to start)

20-12-2007, 07:49 PM
The outdoor unit looks at the compressor discharge pressure sensor & the discharge temperature sensor & calculates the discharge supper heat.
If it detects low SH for 15 mins then it will stop & then restart the compressor.
If this happens 3 times in 60 mins then the unit considers that it judges that the suction accumulator is at liquid overflow & stops on this fault to protect the compresor from liquid flood back damage.
The first thing is to confirm the correct data from the temperature sensor & pressure sensor.
If outdoor circuit board is receving correct data from the sensors then the system may well be overcharged & so the system needs to be monitored carefully to look for any other overchage indicators.

There can be problems with the indoor unit LEV valves which cause liquid to suction bypass which can affect discharge SH.

If its a heat recovery system there could be liquid to suction bypass at the B/C box.

So it could be somthing as simple as a duff temperature sensor or a combination of a number of things.
Only carefull check & monitor of the complete system operation will locate the cuases of this fault.

21-12-2007, 12:10 AM
Thanks for an informative answer, my question is as Viking suggests. Could not the very cold nights we are experiancing, cause a problem.
I say this because on 1 of our Coastal Located sites, 1 of three chillers trips out on various low pressure / low discharge temp issues.
This is because it sticks out from the building furthest and the winds blowing through the condensor. Drop the residual heat in the condensor.
So that on start up there is so little "lift" developed that the system trips.
Therfore based on what you have explained, could lower than normal head pressure / temp be relevant
to Andy's problem?

21-12-2007, 04:47 PM

The older cooling only & heat pump models were prone to overcondensing at low ambient cooling.
So if this unit is cooling during night time low ambient with very small heat load & therefore very slow compressor speed then the discharge pressure & temperature will be low & the liquid level in the suction accumulator will be at highest level but still not at overflow unless overcharged.
At this low discharge pressure running condition the suction pressure / temp is also very low & the indoor units are stopping on coil frost prevention every 5 mins due to the low evaporating temp at the indoor coils.
But the system needs to see less than 10 deg C SH for 15 mins 3 times during 60mins including two 3 min compressor restarts.This is not very likely if the units are stopping all the time on coil frost prevention.
So low ambient cooling & overcondensing may be contibuting to the problem but I would suspect other factors such as faulty thermistor & or pressure sensor data most likely.

The system will be running on the hated R407c. Leak,,,, topup & charge rich in 134a ???? These systems constantly monitor refrigerant composition ( composition sensing circuit) & calculate a composition ratio so that can be checked with the system running provided you know how.

If the unit is installed in computer room then the system needs to be tricked out with some modifications to help it cope with that type of application for which it was never designed to operate.

19-07-2008, 09:21 AM
make sure your not having any tramps keeping warm and blocking the fan out let???

17-04-2009, 09:55 PM
Excellent tips .I really appreciate all these points, and I agree completely…