Inverter works on on/off cycles

[deian]

Hi, guys!
I have a RAC single split heat-pump with heating capacity 3.0 (0.5 - 5.0) kW. In heating mode, it turns the compressor on and off very often. Works like on on/off cycles (as a non-inverter), for examples 10 min on, 3 min off and this is repeated all the time. What can be the reason?
The minimum consumption I've seen is 330 W (I measure it with an energy-meter) and it can not go any lower, while in it's specifications it has a minimum of 90 W consumption. A cycle looks like this: starts the compressor, the power consumption goes up to 600 W smoothly, then starts going down to about 330-350 W and works a while and stops directly from 330-350 W to 23 W with the compressor and outer fan off, only the inner fan is working very gently. After 3 minutes again...
Is this normal for an inverter?

[nike123]

What about air filters?

[deian]

Perfect clean. I've cleaned them yesterday and I do this every 2 weeks.
I think that it cannot reach it's minimum capacity or even get close to it when it is necessary. It works on higher power than needed and it exceeds the set temperature and then turns off, instead of lowering its capacity and continue working.
What can cause that?

[paul_h]

Sounds like how they run when in fault. ie slow down when there's a problem, until a certain point when they cycle off.
Either it's chucking a wobbly due to discharge temp, indoor/outdoor coil temp or a similar reading they are unhappy with, or there's a control/sensor problem.

edit: oops, this isn't an LG is it?

[deian]

Nope, it's an MHI. It doesn't show any error codes. I've made self-diagnosis through the service mode of the model.
There are 3 sensors in the outdoor unit - changed them all, same thing.

[brunstar]

in regards to your unit cycling on and off it sounds like the unit has got something under it like a cupboard or the supply air is short cycling and turning the unit off, if you are not sure, a good way to check that is to run it in auto and if it goes into cooling you have something infornt of the indoor unit causing it to short cycle.

the other thing could be a problem with the return air sensor but i feel as though it sounds like it is installed with something under it.

[deian]

There is nothing under it and in front of it. The nearest thing is a wardrobe 4 meters away from the AC.
I think that maybe there is a problem with the indoor heat exchanger temperature sensor. Maybe it sends incorrect information to the software, like the heat exchanger is not warm enough and the software sends a command to the compressor not to lower it's rpms. But in fact the heat exchanger is warmer than needed for keeping the set temperature and this is causing the unit to exceed the set temperature and turn off, because the indoor temperature sensor (of the air) is giving right signals. It stops after it has exceeded the temperature with about 2 degrees C° and restarts again when the temperature drops below the set one.
What do you think, may I be right?

[deian]

Checked... the temperature sensors are OK. Any other ideas? What can make it cycle on and off? Refrigerant charge? It it with the factory load.

[nike123]

What is full model#?

[deian]

SRK25ZGX-S
Service manual:
ecocomfort.ee/public/files/Seinasplit%20SRK%20SRC%20ZGX%20technical%20manual%20english.pdf
PS: add www before it, I cannot post a URL.

[deian]

I found a similar thread:
refrigeration-engineer.com/forums/showthread.php?t=15706
The AC is different (but again MHI), the mode too, but mine cycles the same way when less power is needed. When I set 26° and the room temperature is 20°, it works for hours without stopping. But when it reaches it, starts cycling on and off.
I usually set it to 21-22° in heating and it cycles on/off all the time blowing warm air when working, instead of lowering it's consumption and blowing air with lower temperature thus keeping the set temperature constant without cycling the compressor on and off.

[nike123]

What are outdoor temperatures?
Check at Th2 temperature sensor place, actual temperature of indoor heat exchanger and note frequency change at compressor until you got OFF state of compressor.
Also, monitor relation of pressure and compressor frequency and then post here.
Please, take notes of these 3 values in relation to each other.
For example:
Pressure at service port is Z, indoor heat exchanger temperature is Y and inverter frequency is Z .

X,Y,Z at 12h 25 min.
X,Y,Z at 12h 45 min.
etc....

[back2space]

I am having the exact same probs with my LG system! It doesnt ramp down far enough and prefers to cycle off. Min it should use is 800watts on mine but it never gets below 1300w without shutting down.

Mine is multisplit though but if I have 2 units operating it will get to 1100watts sometimes never if only one unit operating!