Daikin VRV Troubleshooting

[ral]

I have been trying to help a buddy of mine troubleshoot his Daikin VRV II air conditioner. The system has three indoor units on one outdoor unit.

He noticed that there might be a problem when the thermostat for one unit was set for 16C, but the room was only being cooled to 26C. The other thermostats were both set for 20C, but one was set to off. The rooms were at 19C and 24C. The system is not reporting any errors.

The system was serviced about 5 months ago and at that time was able to maintain 19C in all the rooms.

He was able to borrow a gauge set and check the pressures on the system. High side was only 115psi. Low side was 100psi.

I am planning on taking a trip to help fix his A/C. Right now I think that his system has lost most of its charge so I think that I will need to go and buy some refrigerant for the trip. However, I wanted to get a second opinion before I go an spend a bunch of money on refrigerant that he might not need.

What do you think? Is it likely that the system just needs to be recharged and I should buy the refrigerant, or is it possible that there is another problem that needs to be addressed instead?

Thanks for any help that you can give. I mostly just service A/C units. I'm still new to the more complicated troubleshooting.

[multisync]

Pressures are not to be relied on either way with these systems. However I would reclaim the refrigerant first and weigh it. Then at the very least one of the most common 'faults' is removed from the equation.

Multisync
London

[icecube51]

i think the best of help would be to read the instruction manual. its possible that you have to remove the refrig completely and recalculate the amount needed,and than do a recharge whit the exact amount. still , if there would be a leak,its best to do some tests first.

Ice

[Brian_UK]

Also remember that setting a 'stat to 16°C will not always get the room down to 16°C.

It depends on the unit sizing what the end temperature will be.

Perhaps the unit that is OFF is affecting the heat load calculation for the building.

[Thermatech]

If your buddy can take the compressor discharge/ suction temp & discharge / suction pressure together then you could make a better diagnosis for short of gas as you then can estimate the suction / discharge superheat.
If the comp discharge temp gets up arround 110 deg C then the outdoor unit protection control will be active & the inverter compressor will reduce speed to make lower discharge temperature / discharge super heat.
This will result in the low discharge pressure reported.

You could ask your buddy to switch off 2 indoor units & just operate 1 in cooling.
If the cooling performance at the 1 indoor unit gets a bit better compared to the poor performance with all indoor units cooling then that would indicate sog. There might be just enough refrigerant to have some cooling performance at just 1 indoor unit but not enough to provide cooling at all indoor units.
If your buddy can then take temp / pressure readings again & there is an improvement then that would tend to confirm sog.

If the system has long pipe run & or larger capacity indoor units than outdoor unit size ( over index ) then the system will be more sensitive to refrigerant sog.

[ral]

Hey, thanks everyone for your insight into this problem. Jimmy (my buddy) has gone on holiday for a bit so I will try to get him to get some more information and try a couple troubleshooting steps when he's back. I did ask him to try to take some temperature readings. He was able to measure 15C at high-side pipe where it comes into the house. He thinks that the couple shorted out since it stopped working, so we didn't get a temp on the vapor pipe. I don't know if that helps at all, though.

I was wondering if you could explain how the suction and discharge temp along with the pressure could get me an idea of how much refrigerant is in the system. Typically, I just know how much I put in originally and would need to pump down the entire system to find out how much was left.

I was also wondering how turning off the other units would affect the cooling at the one unit if the indoor units are not calling for cooling. I could understand this if the units were calling for cooling. Do the indoor units always cool a little and therefore take a little refrigerant at all times?

Thanks again for your help on this. I seem to be learning new stuff everyday.

[Thermatech]

If the discharge superheat is high & discharge temp is high this is a sure indication of sog.
Discharge pressure & temperature readings allow you to calculate discharge super heat.
When the disharge temp gets up into 110deg C then the outdoor unit protects the compressor by reducing speed & the cooling performance is reduced at the indoor units because the compressor is pumping less refrigerant & discharge pressure will be low.
The suction temperature & pressure will allow you to calculate the suction superheat. If its very high then thats another indication of sog.

If all the indoor units are calling for cooling then the electronic expansion valves will be at some % open. If the system is running sog then the indoor units are unable to achieve target superheat. In this case the indoor circuit board keeps opening the valve to a larger % opening. Then more liquid in the liquid line flashes off into vapor in the liquid line before arriving at the indoor units.This makes poor performance because expansion valve needs 100% pure liquid.
But if you shut down 2 indoor units & only run 1 in cooling there might just be enough liquid refrigerant in the liquid line to pass through the electronic expansion valve as liquid & then flash off in the indoor coil as it should & do some cooling.

I only suggested turn off 2 indoor units as this is a simple test the end user can carry out for you & can give a a further indication before your site visit.