AC not performing

[R1976]

Hi all,

I had a look at a split in a server room recently, a Mitsubishi Electric MSZ A09YV/MUZ A09YV, this is one of the older and probably first generation R410A Mitsubishi splits.

Air on 30 degrees, air off 20 degrees?

Room is not getting cool enough and this td seems a bit low. I have recharged gas and no better.

Room only 3m x 3m.

Does anyone have any problems with these older splits?

[airmate]

what are the operating pressures and temperatures including the outside ambient. I have also experienced that additional equipment are added in the server room without any consultation with HVAC engineers. Addition of further equipment in server room adds more load on unit downsizing unit cooling capacity. Also check the return air sensor sometimes they start reading low but still inside the tolerance range which does not generate any sensor fault alarm. Warm the sensor by rubbing with your fingers will drive compressor frequency up and then measure your supply air temperature. also check your outdoor unit ambient operating conditions, because if the oudoor temp is too low and subcooling is low will cause the compressor to run at a lower frequency. Has the unit generated any alarms

[The Viking]

Also,
Check the humidity in the room, if it's below ~35% that's the problem

[Argus]

.


Over-condensing?

It's been damned cold recently.


.

[Thermatech]

Indoor coil frost prevention
When the indoor coil temp sensor is 6 deg c or lower the outdoor unit slows down the compressor to prevent frost fromation on the indoor coil.
This results in the higher off coil temp you have measured & therefore poor performance.

This is classic operating condition for basic inverter unit in computer room with low humidity.
The unit is constantly removing moisture from the room until the humidity is much lower than the unit is designed for & then the coil temp is always lower than the coil frost prevention function so the system copes with this extreem operating condition by reducing compressor speed to allow the evaporating temp to rise above coil frost conditions.

Any overcondensing at the outdoor unit will make even lower evaporating temperature.

This is why so many indoor units installed in computer rooms have iced up coil problems & why many ac contractors resort to installing a big drip tray underneath the wall mounted indoor unit to catch all the condensate overflow when the indoor unit defrosts.

Does this unit have any outdoor fan speed control ?
If it does then it will only be very basic & any overcondensing due to low ambient will tend to make low evaporating temp & activate indoor coil frost prevention function.

This is a comfort cooling split system which needs some latent heat load to work & although the unit is rated at about 2.5kw total cooling the sensible heat cooling will be alot less then 2kw due to the SHF.

The unit was never designed to operate in below zero ambient in cooling mode with indoor high sensible cooling load & you wont be able to find any data in the Mitsubishi tehnical manuals which can give you any indication of the cooling performance at the conditions you have.
The manufacturer does not publish the data for these conditions because they know it will not work well at these conditions.

So the only option is to make a guess at what size larger unit might cope with this extreem operating condition.
Why not consider some free cooling with fresh air as an alternative.

[R1976]

Thanks for your replies,

The unit also has a high air off in the summer but is worse now this cold snap is here which would explain the over condensing.

Suction pressure is always about 9 bar and it is the same for other models on that site.

I did not know of this coil frost protection although it makes sense to me now as years ago I was always coming up against basic R22 systems iceing up in server rooms.

The customer will probably have to have a bigger split then.

How could we raise the humidity?.... could we leave the door open slightly or should we fit a small ventilation unit (an extract fan with an intake grille in the door).

On the subject of heat reclaim units...VAM's and the like. I was in a server room and someone had fitted one in reverse... would this be to keep the heat outside of the building out of the room and is this a good idea as a way to introduce ventilation?

How could we introduce free cooling?This sounds a good environmently friendly idea!

Thanks again

[Temprite]

Hi all,

I had a look at a split in a server room recently, a Mitsubishi Electric MSZ A09YV/MUZ A09YV, this is one of the older and probably first generation R410A Mitsubishi splits.

Air on 30 degrees, air off 20 degrees?

Room is not getting cool enough and this td seems a bit low. I have recharged gas and no better.

Room only 3m x 3m.

Does anyone have any problems with these older splits?

I just started doing mitsu warranty work and have had a couple of systems with similar symptoms that have had a faulty LEV coil

[Yuri B.]

By Thermatech

Why not consider some free cooling with fresh air as an alternative.

Indeed, why this is so being avoided probably everywhere seemingly more as the most deadly sins? Should winter t. fall down to -70C for this to become at least worthy of some consideration?

[fridge doctor]

I am assuming a wall split. If cassette modify the instructions accordingly.

Stop the unit. Open up the vane(s) on the indoor, peer in with a torch. Specifically look at the blades on the indoor fan, which I am thinking you will find to have a coating on them many times thicker than the original blade. You will probably also find the exit airway to be filthy.

Remember all fan blades are designed to operate in one direction only. The blades are curved to ensure that the desired airflow is produced.

On an AC the indoor fan gradually loses its potential to pull/push air through the coil and out cleanly by virtue of this accumulation of dirt, and in extreme circumstances -which I have seen in computer rooms - so much crud has built up on the blades that the there is almost no visible space between them. At this point the indoor unit begins to work in opposition to itself. A percentage of the air which should be coming out of the outlet is in fact simply being circulated around the circumference of the blade and pushed upward and back through the evap, which apart from not cooling the room efficiently, will artificially reduce the temperature at the return air sensor (thermostat) causing the system to shut down the cooling cycle before 'room' temperature is reached.