Mitsubushi City Multi

[hvacr1]

I have a puy-250-TM-C. We had a grounded compressor. I replaced and found that the sutction and discharge thermistors were out of calibration. We replaced them both. We started the unit and after warmup the compressor ramped up to 98 htz. The suction tem went down to-11oC units were going off on frost prevention. We replaced the main board. same thing whats next any Ideas.
Thanks for your help in advance.

[marc5180]

You replaced the suction and discharge pressure sensor or temperature sensor?

By suction temp do you mean Saturated suction temp or actual temp?

If its SST then first thing to check would be refrigerant charge?

[hvacr1]

we have replaced the temps sensors.
I do mean saturated suction temp.
We have recovered and weighed in the charge.

[marc5180]

I'm not familiar with the puy-250-TM-C and can't find any info on it. Is it a heat pump and not R2 heat recovery?

Couple of questions,
Is it only one outdoor unit?
How many indoors?
Are you running in heating or cooling when the fault occurs?
Are the filter clean?
Are the coils clean?
Do the fans run?

[hvacr1]

one outdoor connected to 4 indoor coils. Cooling. Coils are clean, filters are good. the condenser fan runs.

[marc5180]

You don't like giving out much info regarding the problem do you.

What refrigerant is it on?
What's the discharge pressure?
What's the discharge temp?
Are any of the indoors cooling?
Or is there one particular unit that is giving more cooling?
How old is it?

If it is relatively new then check for installation mistakes like crossed pipes or control cable.

If not then prime suspect would be indoor LEV locked wide open making liquid to suction bypass.

[install monkey]

have u measured the piperun and calculated the additional charge.did u change liq drier.did u drive open all lev's for pressure test and vaccing-did u check for a backdrop on the torr gauge.reset the levs ensure coils are clean and filters clean.

[hvacr1]

The system is running on R22
suction pressure is running 42 psig
discharge is running 250 psig
discharge temp 78oC
Another note when i turn on the dip switches is says my target evap teom is -15oC
the low suction pressure i thought ma rule out the lev being stuck open.
we weighed in the charge system has been running firn for 8 years.

[hvacr1]

all indoor coils are in cooling

[marc5180]

Have you checked each indoor LEV? Check indoor unit coil entering and leaving temp & SH by using SW1 data.

Also have you verified the HP/LP pressure sensors are correct?
Connect two sets of gauges to HP / LP service ports and HP / LP service valves and use SW1 to confirm correct data from pressure sensors & confirm correct refrigerant pressure through the outdoor unit.

[Thermatech]

TM-C ? not sure what this model is as we never had this type in the UK.

But R22 cooling only heat pump models we had in the UK in the early 1990's only had one outdoor coil & did tend to run at very low condensing temp / pressure when cooling in low outside ambient conditions.
With lower than ideal suction pressure the indoor units will tend to run in coil frost prevention conditions.
This could be a real problem if the indoor units were in a computer room with very low humidity in the room.

If this is the case then suggest put a cover over the outdoor unit fans to force up the discharge pressure & watch the indoor coil temp sensor data.
As the discharge pressure is increased You should see the suction pressure temperature improve.
By trial & error with outdoor air flow you may be able to find a discharge pressure & suction pressure which is high enough to allow the indoor units to run above 2 deg C TH2 coil sensor temp. Then the indoor units will not keep stopping on coil frost prevention.

Also after you have the system operating without the indoor units stopping on coil frost prevention all the time it will be much easier to monitor for the correct operation of the indoor unit LEV valves & assess the performance of the indoor units & outdoor unit.

If you find this works then you may want to deploy some additional head pressure control to the outdoor unit fans.
If its the old 4 fans outdoor unit we used to make up 4 pressure switches & 4 contactors to switch the fans on / off to force the outdoor unit to run at higher discharge pressures.

Later evolutions of the City Multi heat pump or cooling only outdoor units had two 50% outdoor coils & control stratagy to only use 50% outdoor coil when operating at low discharge pressure / low outside ambient cooling operation.
But even then the units could run at much too low condensing pressure if the indoor units are in low humidity computer room type application.

[hvacr1]

they have installed a low ambient kit to allow for low ambient operation. I am more concerned about the hz ramping up to 98 and staying there even then the sst is down to -10oC. We have replaced both the suction and discharge temp sensors.

[Thermatech]

The old R22 Y series outdoor unit contolled comp speed mainly on suction temp TH2
In this case the LP seems a bit too low
This could be due to low ambient cooling operation & resulting depressed discharge pressure which also results in low suction pressure & temperature.
However there could be incorrect refrigerant flow in the system which could cause the low suction pressure & coil frost prevention at the indoor units.

Suggest look carefully at the operation of the outdoor unit SV4 hot gas bypass valve & outdoor unit LEV1 subcooler control valve.
Monitor TH8 subcooler outlet to suction temp & TH2 suction temp.

Any of the indoor unit LEV valves could also have potentially failed & therefore fixed in a closed or only slightly open possition or fully open & dumping lots of liquid into the suction.

Switch indoor units to Fan mode one at a time but keep the outdoor unit running.
Monitor the indoor units coil temps on TH2 & TH3 to see if the refrigerant flow stops when the indoor unit is switched to Fan.
If liquid still flushes through the indoor unit back to suction then you will be able to see at the TH2 & TH3 coil temp sensors & that would indicate LEV valve failed in locked open possition.
If only one indoor unit has locked open LEV valve then liquid bypass to suction through that unit results in poor refrigerant flow through the other indoor units & coil frost prevention.

You will find it easier to see this type of problem if you force the system to run at higher discharge pressure because indoor units wont keep stopping on coil frost prevention & temperature differences will be greater & more noticable.

If you really want to go deeper then we can start to talk about the subcool coil bypass outlet super heat SHb & the subcooler coil outlet subcooling SCc & the Outdoor coil outlet subcooling SCo.
But more than likely you will be able to locate the problem without getting in that deep.

[hvacr1]

Thermatech
I did just as you have said and the lev's all close. We are still trying to maintain -15oC and compressore ramps up to 98 hz and pretty much stays there.
Does anyone know what controls the target evap temp. what is telling the system to maintain a -15oC target evap temp.

Thanks

[hvacr1]

by the way discharge preesure staying at approx 250psig