mitsubishi vrf

[morgan c]

office building with 4 indenticle systems.4 units to each system using splitter/branch pipework.perfect pressures when heating but when cooling pressures 3b low and 10b high using r407. measured pipework and charge is correct.al systems operating the same way.outdoor freezing when cooling.:rolleyes:

[nike123]

Check systems separately.

[morgan c]

systems WHERE all checked seperately and running at the same low pressures

[acnerd]

I know Mitsubishi kit, like other VRF systems, will try to maintain a suction pressure that gives you an evaporating temperature of just above 0 degrees celsius. 10 bar is pretty much the minimum head pressure you would want in cooling mode.

I find it strange all 4 would be playing up .... or you're very unlucky! ;)

Start by comparing your pressure gauge readings with what the outdoor unit thinks the pressures are; via dip switch settings on SW1.

Silly questions too: is there enough heat load in the building? Are you running all 4 indoor units at the same time when reading those pressures? Are the evaporators clean and clear? Any fault codes?

[Thermatech]

The outdoor unit PUHY --- YMF-C ?
Outdoor unit suction accumulator a block of ice ?

The older Y series did tend to run at low discharge pressure anyway
but
3 bar suction
The unit has target evap temp of arround 0 so would expect suction around 4.5 to 5.5 bar.
It controls on the TH2 suction temp data.
So make sure the TH2 has correct temperature data at the outdoor unit display with SW1 setting.
For example if the TH2 sensor is reading 10 deg C high then the compressor will remain at high speed & overshoot on the suction pressure.
So when the suction temp is below 0 the compressor will start ramping down & thats why you will have poor discharge pressure.
You need to find out why the suction pressure is so poor.
As its a Y system the prime suspect is the LEV valves at the indoor units locked or blocked strainers.

If the system is operating in computer room application with lower than 15 deg C WB return air then you might get this type of poor performance as the system is operating outside its design limits.
In this case you would need to deploy some extra head pressure control to force the outdoor unit to run at higher discharge pressure provided no other problem with the system.

Please come back with mode details

[nike123]

Is that system just commissioned and you conducting start up?
Or that system is alreedy worked OK and now it plays up?

[morgan c]

found the problem 2 b that the outdoor / coil was oversized and was freezing up.the new models from mitsu have a split coil which have a solonoid valve that lets you use more of the coil when needed.we blocked the back coil wit cardboard and both pressures shot up to 3.8low and 17.4high.going to use a kontacter and pressure swith to delay the fan until pressure reachs about 19bar and that should balance the system. cheers

[nike123]

found the problem 2 b that the outdoor / coil was oversized and was freezing up.the new models from mitsu have a split coil which have a solonoid valve that lets you use more of the coil when needed.we blocked the back coil wit cardboard and both pressures shot up to 3.8low and 17.4high.going to use a kontacter and pressure swith to delay the fan until pressure reachs about 19bar and that should balance the system. cheers

Outdoor coil in cooling mode act as condenser and with your reported pressure of 10 bar (r 407C = 22°C/28°C) it cannot freeze even in theory.
Second, Mitsubishi have condensation control and could cool without problem (and control condensation pressure) even when outdoor temperature is below 0°C.
You should revise your writing here because it doesn't make any sense.

[Thermatech]

The contactor & pressure switch will work ok to stop overcondensing.

When setting up the pressure switch monitor the TH2 temp sensor of the indoor units at the outdoor circuit board using the SW1.
Increase the HP until you get the indoor units running at more than 1 deg C on the TH2.
Any lower than 1 deg C & the indoor units stop every 5 mins on coil frost prevention so if you can pull the suction temp up so the coil temp at the TH2 sensor is above coil frost prevention temp the indoor units will continue to cool untill room temp down to set temp.

[goshen]

HI THERE "The contactor & pressure switch will work ok to stop overcondensing"
this makes no sense the mitsu units work great till -10 c with no pressure switch aded!!
unless someone oversized the entire system and even then !!
i could not understand what u described here 145 psi is way above 0 deg c so how is there ice on the condenser ??
what is your outside temp?

[nike123]

HI THERE "The contactor & pressure switch will work ok to stop overcondensing"
this makes no sense the mitsu units work great till -10 c with no pressure switch aded!!
unless someone oversized the entire system and even then !!
i could not understand what u described here 145 psi is way above 0 deg c so how is there ice on the condenser ??
what is your outside temp?It looks that they are speaking some secret language. I think, same as you, that what is described in these few posts by Morgan doesn't make any sense if interpreted as written.:confused:
And I am not god in mind reading to know what is meant and not written.:rolleyes:

I could only guess ( as Thermatech did) that this system is used as cooling system in some server room with low humidity where it is not suppose to be, because it is not designed for that, and OP mean that INDOOR units are freezing (but nowhere in his text he said that)!

[morgan c]

sorry guys i just got mixed up the outdoor was freezing at the suction and out on to the accumalater bcause of the low bk pressure.the outdoor unit was oversized causing the over condensing.thanks 4 all the posts and advice(thermatech).