Mitsubishi Electric Intermittent fault

[marc5180]

Over the weekend one of the guys on call was sent to look at an aircon problem at a restaurant. He looked at the controller and found it flashing P8 02, he reset the fault and i was sent up there today to try and figure out the problem.

As i arrived on site i found 9 indoor units and 5 controllers, i worked out that there were 4 twins and one single split.

The indoors were all running as were the condensors. I found the P8 was an abnormal pipe temperature fault. I would normally associate this with a possibility of lack of refrigerant so i put the indoors on cooling and then heating and they worked fine.
I checked for cross wiring but everything seemed in order.

I then checked the resistance of TH1 TH2 and TH5 which were the return air sensor, Liquid line pipe temp and evap temperature of both units and they were all fine.
I rang mitsubishi up and they seemed to be going down the route that they could be undersized something which i can't beleive.

Has anyone come across a P8 fault on the new Mitsi Electrics?

The model number indoor PLA-RP125BA
outdoor is a PUHZRP250YHA2

[nike123]

Abnormal pipe temperature could also occur when you have insufficient air quantity at condenser or evaporator. I would check filter/heat exchanger cleanliness and proper functioning of fans. Also i would check for air short cycling and if there were present, in time of error occurrence, outdoor temperatures outside of unit operating range.

[Thermatech]

The indoor circuit board is looking in heating for a coil temp higher than return air temp as the indoor coil should have hot gas from the outdoor unit at about 45 - 50 deg C.

If the temp diff is only a few deg C then the indoor unit considers that there is a problem.

As you have checked the main things like crossed pipes & cables & SOG & correct sensor resistance then the next thing is to monitor the system operation.

With these systems you can monitor at the remote controller all the live temp sensor data from the indoor units & outdoor unit & also the data at the time of the fault which is recorded in the memory.
You need the service manual as the list of data which you can monitor or recorded data you can view runs up to about 150 items.

Look at the list & then call up the data which will help & write it all down.
Doing this you can build a picture of how the system is operating now & how it was operating at the time of the last fault without running arround taking temperature measurments.
Its also better because you are viewing the same data as the circuit boards.

Could be a problem like intermittent sticking 4w rv at outdoor unit.

[marc5180]

will the service manual show me how to interrogate the controller? Where do i get the manual from, the mitsi website?

[Thermatech]

There is a service manual for the outdoor units & another one for each of the indoor units.
The outdoor manual has all the instructions for remote controler engineer mode data.

Tip
Print off the outdoor manual as is covers all the outdoor units size 35 to 140.
Then select from all the indoor unit sevice manuals the few really important pages you will need for trouble shooting & print them off.

Then bind them all together to make your own custom manual.
Then you have just one manual for the complete Mr Slim range.
You can keep as many pdf manuals as you like on your laptop but I still think its easier to have a printed copy for on site trouble shooting.

Mitsubishi web site
register
they give you a password
you down load any technical documents you want
keep them on your pc or laptop & print off whatever you need

[sinewave]

We had this problem 6 months ago on a Mitsi Elec unit.

It ran for up to an hour and 15 mins before faulting on P8.

After checking all the options in the manual a decant revealed a shortage of gas due to a small leak on the shraeder valve.

All was well on a re-charge with fresh R22

[marc5180]

There is a service manual for the outdoor units & another one for each of the indoor units.
The outdoor manual has all the instructions for remote controler engineer mode data.

Tip
Print off the outdoor manual as is covers all the outdoor units size 35 to 140.
Then select from all the indoor unit sevice manuals the few really important pages you will need for trouble shooting & print them off.

Then bind them all together to make your own custom manual.
Then you have just one manual for the complete Mr Slim range.
You can keep as many pdf manuals as you like on your laptop but I still think its easier to have a printed copy for on site trouble shooting.

Mitsubishi web site
register
they give you a password
you down load any technical documents you want
keep them on your pc or laptop & print off whatever you need

Great stuff thanks. I've just registered it said it could take up to 7 days once my details have been verified, hopefully it will be sooner though.

[marc5180]

We had this problem 6 months ago on a Mitsi Elec unit.

It ran for up to an hour and 15 mins before faulting on P8.

After checking all the options in the manual a decant revealed a shortage of gas due to a small leak on the shraeder valve.

All was well on a re-charge with fresh R22

Were you having problems with the unit getting down/up to temp? I don't think it is short of gas because these get down/up to temperature quite easy, i could be wrong though.

[Daikin=Overated]

From my own experience of P8 faults with (earlier R22/R407C kit) I would first check to ensure that you can see light from your torch etc through both indoor/outdoor coils. Mitsi's are very sensitive when it comes to air flows and discharge temps. If theres any signs either coils blocked then blast with nitrogen not cleaning solution (outdoor easier to maintain than indoor obviously).

Failing this, check discharge side of compressor. I've had 2-P8 faults relating to this alone. Get your gauges on there and ensure compressors pumping on suction/discharge in accordance to R410 spec.

This is all based on R22/R407C series just for the record!

Regards,

J.

[sinewave]

Were you having problems with the unit getting down/up to temp? I don't think it is short of gas because these get down/up to temperature quite easy, i could be wrong though.

Yes Marc all was well with the two units getting to setpoint but after said timeframe of operation the units faulted to P8.


Trouble is a P8 code covers a ruck of possibilities and thus in it's self it's quite ambiguous.


The first A/C manufacture that can programme a system with diagnostic logic that can identify specificaly and accuratly, all possible faults will clean up in my book!

[marc5180]

I went back to the site today to look at the units and was met by a "U1" fault (high pressure fault) flashing on the controller.

I traced this to 2 of the louvers on one of the indoors, they wouldn't open because the last people to look at it had snapped the clip that hold on the louvers- meaning they didn't open and caused an airflow fault.

I ran it up and interrogated the controller but i could only go back to the last 3 fault codes and they were all "U1" faults not the "P8" that i wanted, so i ran it up and tested it in cooling and everything was fine.

All the sensors were reading as they should and temps and pressures were spot on.

I set it onto heating and left it for a few mins and one of the indoors got hot but the other one didn't.

I got above the ceiling and felt the flow into the cassette and it was hot but the suction coming out of the cassette there was no heat at all.

I recovered the refrigerant which wasn't short and disconnected the pipes from the indoor to check the strainers because it looked like a blockage- theres heat going to the unit but nothing coming out of it.

I blew through with nitrogen but there didn't seem to be a blockage. My thinking is that there must be some sort of obstruction in the refnet, wether it is oil clogged restricting the return to the compressor or the fact that the piperun from the refnet is too long.

So baically both indoors work in cooling very well, temps and pressures ok, unit doesn't fault.
As soon as it goes into heat mode, only one indoor works the other indoor has hot gas up to the indoor but nothing back out?

Has anyone ever come across this before?

[Thermatech]

Hot gas in
warm liquid out

The liquid line has good liquid flow in cooling but no flow in heating ?

Perhaps
There is contamination in the indoor unit which is enough to clog up the small liquid strainer on the liquid flare nut fitting with liquid flow out of the unit in heat mode
but
in cooling that contamination gets flushed through the coil to the gas flare nut fitting strainer & as this is a larger strainer it does not get completely clogged.

Does the hot gas get into the indoor coil.
Can you take out the fan motor & measure the coil temp. Then you could see if its getting through into the coil.
If the blockage is on the liquid line then when you first start up the system in heating after cooling operation the indoor coil should fill up with hot gas & then gradually cool down as the hot gas condenses to liquid.

[marc5180]

Perhaps
There is contamination in the indoor unit which is enough to clog up the small liquid strainer on the liquid flare nut fitting with liquid flow out of the unit in heat mode
but
in cooling that contamination gets flushed through the coil to the gas flare nut fitting strainer & as this is a larger strainer it does not get completely clogged.

That would make sense yes,

In heating the indoor has flow up to the unit but not coming out of the unit. I took out the tray and the fan earlier to get up to the coil thinking that there was an Lev inside but there wasn't... just a distributor.

As the flow goes through the flare nut and strainer about 2 inches past ( just inside the cassette) it loses heat, which is why we disconnected the pipes and tried to blow through with nitrogen..
I stayed with it for about 15 mins waiting to see if any heat was getting through the coil but if it was it wasn't noticable.

If there is a contamination in the indoor then it must be from factory because by the look of the strainers, nothing can get inside the coil past them(apart from liquid obviously )

The indoor does look the most likely but i can't see how something has got inside.

[marc5180]

The only suggestions i seem to be given is, change the indoor unit but these are not cheap.
I'm thinking of joining the 2 pipes together and seeing if i get flow back to the refnet.

[superswill]

The only suggestions i seem to be given is, change the indoor unit but these are not cheap.
I'm thinking of joining the 2 pipes together and seeing if i get flow back to the refnet.

oil logged indoor coil?

[Thermatech]

I have only ever seen oil logging at a Mr Slim system indoor unit once & thats in 20 years.

In that case the contractor had made his own special quick release couplings & flexible hose connection on R407c systems.
We considered that the unusual pressure drop through the couplings & flex hose was the cause of slugged up oil.

I would not expect to see oil logging problem at the indoor unit in heating mode.
Normally oil gets clogged at low temp such as at indoor unit cooling.

And why only one of the two units ?

Pipe run from the refnet too long ? might be part of the problem but if it was you would expect different cooling performance at the problem unit compared to the other indoor unit.

[marc5180]

It has to be a problem with the indoor coil then. There must be some kind of obstruction in the coil but i dont really want to start cutting the indoor coil apart

[Brian_UK]

I know that earlier you said that you took out the fan etc to check inside and afterwards you felt the heat fade away with the unit running.

Did you do this with the fan running or without it?

I was just wondering whether there could be a blockage elsewhere, refnet for eg., - can you run the unit without the indoor fan working at all. This would allow the 'hot' gas to perhaps get a bit further without being cooled by the fan.

Will the heat travel through the coil and out again ??

The refnet could be badly brazed causing an inbalance in the gas flow to the two units - if you follow my fuzzy-logic.

[marc5180]

I disconnected the fan from the board and ran the system up in heating. I was expecting it to fault because the fan was disconnected but it didn't.
If any heat was getting through the coil then it wasn't noticable..

[p_p]

Hi Marc

I had similar problem last year, unit would run ok sometimes then appear to have a restriction somewhere. however some butcher of an engineer had been attempting to fix it before it was passed to me and he had dessimated the outdoor unit.
I purged nitrogen through indoors then flushed with endoflush, purged with ofn again and replaced the outdoor unit, problem sorted.
Luckily I was able to get hold of a 2nd hand but good 407 unit.
As you have stated there is not much to restrict the indoor, I think your fault lies in the outdoor unit.

PP

[marc5180]

The system is a twin though. If there was a problem with the outdoor then the other indoor wouldn't work either, where as thats not the case.

I'm going back there tomorrow, where i will get at the refnet and cut it open and blow through both vapour and liquid lines to the cassette.
I'm also going to try and see if i can get anything out of the indoor. When im sure theres no restriction in the peipework then i am going to swap the indoor units around and see if the problem stays with the indoor unit that looked like it has a blockage, if it does then i will order a new indoor coil/unit.

If it doesn't then

[marc5180]

Well i went back this morning whilst the restaurant was quiet. I cut out the refnet and took it outside and blew through with nitrogen to be sure there was no blockage.

I then blew down the vapour line that had been cut with the ofn and took off the flare nut and found a big piece of armourflex about an inch long wedge in the vapour line as where the flare nut screws on.

It was a really big piece and totally blocked the suction flare, it had also melted and was more like plastic then armourflex. It took us a while to figure out what it was.

We vacced it out and recharged it and ran it up and its now working to spec.
My only concern is that the cowboys that put the system in have done this on the whole of the system ( not taped up the ends of the pipes when fitting the lagging) so there could be little bits floating about everywhere

[Brian_UK]

These things are sent to try us and unfortunately, sometimes they do.

Good result though Marc, well done.

[marc5180]

Just glad we found it but it's one for the memory bank.

[Brian_UK]

I suppose size is relative but many years ago, when commissioning a hospital boiler house we had trouble getting the main water flow rate up to duty.

Fault was due to a p*ssed off welder who was sacked.

He had stuck an empty oxygen bottle inside the main boiler 10" header.

It caused something of a restriction....

[marc5180]

that puts my blockage to shame!

[Slim R410a]

Good tip here, (plz dont shoot me down if you already do this!) the plastic end caps you get with copper coils, dont throw them away, collect them then when you are installing simply throw one on the end before armaflex, saves messing round with tape and the armaflex goes on easier!

[Thermatech]

Glad you found the cause.

Two years ago I was commissioning a large VRF project & the install engineers were just finishing off final pipe connections to the last few outdoor units.
One engineer showed us what he had just found when cutting a 3M lenght of 1 1/8 tube.
It was a piece of foam which was a perfect fit inside the tube.
It was impregnated with some sort of solvent.
It just happened to be inside the tube at the exact spot the enginner cut the tube open & there it was inside.

I think the copper tube manucaturers clean the inside of the tube & possibly blow a peice of foam through the tube. As the foam is impregnated with solvent it cleans the inside of the tube as it passes through.

In this case the foam got left in the tube.

I wonder how often that happens ?

[marc5180]

I'll keep that in mind next time i do an install. Always check the inside of the pipes in case there is anything trapped inside.
Thanks for your help