panasonic ducted cycling off

[paul_h]

I've got a ducted unit here that's just switching off (normally with usual CF run on), showing no faults, just cutting out @26C when the setpoint is 18C.
So it cuts out after 5-10min. Does the same thing when indoor unit put into test mode too, and doesn't come back on.
When the unit has cycled itself off, I put the outdoor unit onto forced test mode and the unit started up again, so it doesn't seem like the outdoor unit is dead, it's just getting the shutdown command I think.

Indoor unit model is a CS-80E91ER, I have nothing in this unit, but the PCBs and layout is exactly the same as the cs24bd1p.
Going by the bd1p manual, the air thermistor should be 16K ohm resistance at 30c, and this unit had a 14.5k ohm reading after removing the covers, so that seems OK. (ceiling space temp was 36C).

So basically while checking the a/c, it starts ok, cuts out without error, doesn't restart (even though thermistor reading 14.5K ohm) Also cuts out on test mode (which thermistor shouldn't effect I thought)

Could be wall controller, indoor PCB controller, just be air thermistor, or even outdoor PCB controller, any ideas on how to narrow it down?

BTW, sp480 kpa, hp 1700kpa, suction temp 2C, discharge temp 66C, liquid temp 30C. I didn't get the air on off/s because it kept cutting out. I spent most of the time checking indoor airflow, ie filter, r/a duct, good fan flow, coil condition, thermisitor mounting position, dampers and outlet ducting. So everything seems OK except for the control side of things.


edit: after reading some more I found two possible causes:
1)
After running for 9 min, the outdoor unit will cut out and no errors displayed if the outdoor pipe temp is below 12C.
2)
After running for 9 min, the outdoor unit will cut out and no errors displayed if the indoor pipe temp is below 2C.

The outdoor pipe temp controls the condenser fan speed, and if that was reading below 12C, the cf would have been running at low/super low speed. To me it seemed to be running on high speed, I didn't check the oudoor thermistors though :(

Indoor pipe temp is something I didn't check either, coil wasn't icing up though.

[nike123]

Did you check air quantity thru the indoor coil. It could be that is clogged.

[paul_h]

First thing I thought of. I spent over an hour in a hot ceiling space because I was sure that was the problem myself, as airflow out of the ducts seemed low to me, but it's just a small system with too much ductwork so must be normal (I had all zones on to make sure no motors were faulty, and with the size of the unit it's too small to run all zones). The unit is seven years old, no no major design/install fault I think. Filter was clean, coil was clean, no ducts leaking or crushed.

I checked every duct and damper, and checked the coil and made sure the air thermistor wasn't in contact with the coil.
From what I've found out (and added in an edit), it's cutting out on one of the pipe sensors. So i'll have to go back and check them, but it's either a bad pipe thermistor or faulty PCB I think now.


edit: Lucky the thermistors are all the same for all pana conventional ducted units, so I'm just going to buy them all anyway as spares, then go back and have them to compare and test with. If they work it saves time, if not, then it needs a new PCB.

[Thermatech]

paul h

Does the indoor unit have coil frost protection mode if the coil sensor reads low temp ?

As the outdoor unit always runs for 9 mins it may be coil frost prevention mode.

If the coil temperature sensor is faulty & constantly reading very low temp that could then cause the unit to cycle on the coil frost prevention.

In the UK we often see this problem in computer room aplications because the humidity becomes very very low.
Standard split systems have sensible heat ratio of arround 0.75 & so need some latent heat load but in a computer room the heat load is all sensible. In this operating condition the standard split will cycle on coil frost prevention or the coil becomes a block of ice.

[paul_h]

Yeah, thats what it looks like, turning off on frost protection, Thats the fault #2) I found later on and added to my post.
Coil isn't that cold or iced up though.
Because of that I suspect faulty coil sensor (or could be PCB).
So I'll go back with a coil sensor and retest when it gets delivered.
Rather have a possile fix when I go back than just go back to retest ;)

[paul_h]

Went back today, it was a faulty indoor coil thermistor.
Funny how it never came up on fault though.
One to bear in mind for the future. Also at 20C, all the new and good thermistors were 25K ohm

[Brian_UK]

Thanks for the update Paul, also glad to hear that the fault is resolved.

[Thermatech]

Good one Paul

[nike123]

Everything you ever want to know about NTC thermistors:
http://www.thermometrics.com/assets/images/ntcnotes.pdf
http://www.specsensors.com/ntc-engineering.asp