HVAC

First we are not clear what you have installed?? Second why are you asking us?

It is under warranty and you should ask the machine supplier to answer you. Go through their Binder for trouble-shooting section and check step by step.

Ask the manufacturer's engineer to give you a briiefing of how it works rather than asing us. Everybody has in-house training for operation parameters for their products.

With your location and the comment about the fan, the head pressure must be going high and the hissing that your reporting indicates the head pressure safety pop off valve in the compressor is opening. This would increase the suction pressure.

You need to monitor the high pressure discharge side of the compressor at these times to comfirm this. It sounds as if the condenser fan motor has a distinct problem if it is slowing down, this would be where I would begin troubleshooting this unit.

Woody

From your postings it is assumed you have installed 10 TOR ( Tons of Refrigeration Unit) for comfort applications. IS it a duct able unit? How much inside conditions you maintain i.e. thermostat settings. If the inside conditions are maintained thermostat will cut off the compressor and the unit will be in off cycle mode. Day time due to high ambient conditions in Dubai there may high condensing pressure. The supplier must have supplied the Operation and maintenance manual. IF they have not please ask for it. Follow the instructions given in the manual.

Your condensers are inverter driven, the fans change speed in relation to head pressure, the compressor's speed can change on suction pressure, subcool or superheat depending on the metering device type. I don't have any experience with these specific units but have worked with other makes. As with any inverter driven compressor other than a screw, you are limited as to minimum speed because of oil return, the hissing could be hot gas by-pass to maintain vapor return velocity sufficient to maintain oil return. With 410a you also need to maintain suction pressure to reduce the separation of gases as this is a blend and not an azeoptrope. When you run at low pressures the separation reduces your cooling capacity in some cases but also tends to open up your glide to the point that you will frost your evap while not maintaining setpoint. Bottom line is on a 10 ton minisplit the inverter alone will have trouble maintaining capacity control so other measures can be required. The hissing sound would not bother me in your description because it follows a drop in manifold pressure. To verify my analysis you need only to look for a hot gas by-pass and if it's there put your finger on it's discharge line when you hear the hiss, should get hot.

If it is performing it's task, than relax.

There has been major changes in compressor design in the last several years.

when I first heard a "Copland digital scroll compressor" cycle, I thought that is was broken. I was wrong.

You need not, only to talk to the manufacturer of the equipment, but to those who manufactured the parts.