Mitsubishi Heavy SRK20ZJX-S temperature control problem: +/- 3K too cold / too hot

[lexmark42]

Hi,

I have a setup consisting of a room of approx. 16 m^2 size and only negligible heat load (< 300 W) being serviced by a MHI SRK/SRC20ZJX-S single split unit.
While I've come to think that the MHI hyper inverter units perform great, I noticed that room temperatures are constantly off by quite a few degrees.

In cooling, a setpoint temperature of 21°C constantly results in a room temperature of 18°C or even less; conversely, in heating the resulting temperature is always approx. 3K too warm.

This behaviour should clearly rule out thermistor failure; these deviations apprently also occur largely independent of the outdoor temperature.

Anybody got a clue on that?

Cheers,
-Dave

[JoeAT50A]

Put some resistance let say 150 Kohm parallel with your room temp sensor. In serie put around 120 to 150 Ohm. (#not kilo-ohm)
That will off-set around 2 K to 3 K diff. You could fine tune ... somewhere around there.

[nike123]

What is distance from floor to bottom of indoor unit.

[lexmark42]

The indoor unit is mounted with the air outlet 210 cm above floor level; there is est. 12 cm above the air inlet to the ceiling. The install is fully within the MHI specs.

@Joe: I see that you aim at reducing the slope of the sensor response curve. Great idea, i will keep that in mind just in case.

However, I rather suspect the control software to be the culprit. Maybe they have incorporated a fixed minimum heat / cool load depending on typical operating conditions in their fuzzy logic. This would certainly reduce temperature oscillations and explain the strange behaviour.

[al]

Lex

I wonder is the unit oversized and is overshooting setpoints? i know comfort air con isn't close control but i would hope it would be better than 3k either way!

al

[stufus]

An SRK/SRC20 I believe is a 2kw high wall unit .
The control strategy on these as far as i know is +-1c. But you must remember these units use wet and dry bulb temp's to determine a mean temperature.
I would check to make sure the sensing thermistor has not come displaced
Cheers
Stu

[lexmark42]

Yes, it might indeed be oversized albeit being the smallest model of the hyper inverter series. On the other hand, what I experience is not really just overshooting but a large steady state error. However, when the control gain is too high, this rarely leads to steady state problems but rather strong oscillations. Those I don't have.

Of course I'm not sure what happens with fuzzy control. I know research showing that fuzzy control can perform similarly good if not better than PID control.

But who knows what special assumptions have been made.

Do you think it would be wise to try the wired control which includes another room thermistor?
Thus the control could avoid compensating for mounting positions and room size better, hopefully leading to more accurate control.

Any experiences / suggestions would be welcome.

Cheers,
-Dave

[al]

Apologies Dave (and Stu), i misread the model number, thought it was a 50. Does fan speed make any difference to the temperature swings?

al

[lexmark42]

I verified: The SRK20ZJX-S has a single room temperature thermistor, and it is fitted snugly in its place.
Only the larger SRKs (50/60) do have an additional humidity sensor.

I would be curious to know if any of you have got any documentation on possible DIP switches / jumpers possibly related to my issue.
Anyone who could lead me to a peek into the detailed service manual (that I have not found yet)?

Cheers,
-Dave

[lexmark42]

@Al: No, fan speed doesn't make any difference.

[Yuri B.]

Hello Lexmark
Do you measure the temperature at the point where the thermistor is placed ?

[lexmark42]

No, I measure the temperature in various places across the room about 1 meter above floor height.

The termistor is inside the air inlet panel; I have also*taken measurements there, but they are even more extreme (slightly when cooling; considarably warmer when heating).

But that is understandable since there is always a bit of thermal short circuit and convection (notably when heating).

I am now checking out how heating to higher setpoint temperatures checks out to find a more conclusive pattern in the behaviour of this otherwise quite nice unit.

Cheers,
-Dave

Cheers,
-Dave.

[Tayters]

Hi Dave,

I had to check a recent MHI install where staff complained they were too cold. Sorry. not sure of model numbers but it was a twin split cassette system installed summer this year. It was in a room where people gave blood. Units were oversized as in time the room will be enlarged. There were max/min thermometers all round the room.

Setpoint was 21*C cooling but with a thermometer on the inlet grill it cycled around 18*C also confirmed by other thermometers around room. I suggested to raise the setpoint a few degrees to compensate. Did also trying changing the dip switch settings to alter the detected capacity of the unit (as suggested by MHI) but this didn't make any difference. Before I went, another tech had tried setting the thermostat sensor to the one in the remote but to no avail.
To cut a long story short, as it was a new install, the MHI guy had a look and he could find nothing wrong and he put an offset on the thermistor through the controller.
Seems to have kept them all happy - and they didn't half moan when I was there!

Didn't try it on heating (plenty of heat in the room so cooling is all it would be used for) but it would suggest there were some bugs in the control of the system.

Cheers,
Andy.

[lexmark42]

Hi Andy,

this sounds a lot like my problem, albeit with a different unit. Since the room temperature sensor in the controller didn't help I doubt I will be spending the €200 for an RC-3 & kit just to try.

How did the MHI guy put the offset in? Would be great if there are any settings to be made that I do not know of yet.

Cheers,
-Dave

[Tayters]

Hi Dave,

I've emailed and PM'd you with some info.

Hope that helps,
Andy.

[lexmark42]

Cheers mate!

I will let you know what I find out.

[lexmark42]

Hi,

after a lot of reading I found only very little information on the control strategy of the hyper inverter units, except for the 4-way cassettes. With those it seems to be possible to set an offset for the room temperature thermistors although I'm not sure if this can be set separately for heating/colling.

About the wall mount models I found nothing at all, and they don't seem to have any dip switches or custom settings. Have you guys ever used an interface with an RC-3 or RC-4 with one of those units? I wonder if there is a way to set offsets using those as well.

At the moment the SRK is running comfortably on 21°C when I set it to 18°C setpoint temperature, which is the lowest supported setting.

Cheers,
-Dave.

[yinmorrison]

I also have this problem in 2 very small rooms with I/R set at 18 the room records at around 14 and this is charted on a Thermomax Datalogger.The max gradient is +/- 2Degc and it performs to this but we cannot tell someone to set at 22 so that it controls to 18 , what is the point in that? I reset the Thermomax to +2 to offset this and it now charts at around 16 but +2 is the max difference on the Thermomax so can go no further and would be very interested in what size resistor to install in series , any help appreciated.

[lexmark42]

Also a mitsi heavy?

[yinmorrison]

Yes they are SRK60ZIX-S Systems.MHI advise me that there is an inbuilt default in software that ramps up/down the unit manually every so often if no temperature movement is detected!! Far too complicated as far as I am concerned for a simple wall mount.I am thinking about taking out and replacing with Daikin as we cannot get paid because of the discrepancy.

[yinmorrison]

Andy, any chance you can send me the info you sent Dave please.

[lexmark42]

@yin: Very interesting! Basically same series and behaviour - do you have any details on what MHI told you - any way to turn that off?