Tumble dryer starts hot then turns cold

[Johnking]

My old AEG vented dryer head developed a new fault. At the start of a cycle, it heats up, but soon the heater goes off and it runs indefinitely blowing cold air. I have checked:

Door switch
Drum heat sensor
Heating element thermostats
Vent and hose

Seems like the behaviour I'd get if it was overheating?

 

[SpecialK]

Do you get power to the heating element? Sometimes the thermal cutouts on the heating elements fail?

 

[opps]

Not my area of expertise but my old Hotpoint tumble drier had two heating elements, high and low.

It had two overheat sensors, about the size of a 10p coin. It cost me about £3 to buy a new sensor.

The tumble drier was about 15 years at the time. One sensor and one new belt, we got 18(?) years out of a £150 machine, admittedly, the bearings were on the way out, it was very noisy.

 

[Johnking]

Thank you both. I took the heater to bits yesterday. Both cut outs were reading continuity and the two elements have the correct resistances for the rated power. The element does heat up sometimes, usually after the machine has not been touched for a day. But it always stops after a few minutes and the drum keeps turning, and will not stop. The blower is running all that time, and there are no restrictions to airflow. I've measured the resistance of the drum temperature sensor and it seems correct at 18deg (6k) going down when I press it to a glass of boiling water (to about 1.5k). I don't have a way to test that further but it seems ok.

When the heat cuts out, it then gets stuck in a no heat state. Suggests something on the mainboard is tripped and status that way until capacitive charge runs out?

But why would it keep running the drum if the heater trips?


I'm on the verge of giving up and going shopping!

 

[SpecialK]

Thank you both. I took the heater to bits yesterday. Both cut outs were reading continuity and the two elements have the correct resistances for the rated power. The element does heat up sometimes, usually after the machine has not been touched for a day. But it always stops after a few minutes and the drum keeps turning, and will not stop. The blower is running all that time, and there are no restrictions to airflow. I've measured the resistance of the drum temperature sensor and it seems correct at 18deg (6k) going down when I press it to a glass of boiling water (to about 1.5k). I don't have a way to test that further but it seems ok.

When the heat cuts out, it then gets stuck in a no heat state. Suggests something on the mainboard is tripped and status that way until capacitive charge runs out?

But why would it keep running the drum if the heater trips?


I'm on the verge of giving up and going shopping!

What drives the heater? A relay? Might be bad contacts...

 

[Johnking]

What drives the heater? A relay? Might be bad contacts...

From the attached diagram, it looks like live goes via the main switch and the door switch, then to the motor, and then to the heater. Neutal goes to P-2 on the main controller board, then the two elements in the heater are controlled from there by completing the circuit via P-1 and P-3. There are some pretty big components on the board and I expect power is being controlled by a relay (I hear clicks sometimes).

I assume relays are not something you can take apart and check/clean?

 

[mattylad]

that all depends upon the relay itself, most are sealed and cannot be taken apart, although they are usually inexpensive and easy to change with a soldering iron. Talking of, check all the connections/joints on the pcb as a dry or cracked joint can work then not work erratically.

 

[Johnking]

that all depends upon the relay itself, most are sealed and cannot be taken apart, although they are usually inexpensive and easy to change with a soldering iron. Talking of, check all the connections/joints on the pcb as a dry or cracked joint can work then not work erratically.

Thanks. I desoldered the main power relay and tested it, seems fine, nice loud click, switched poles show continuity when energised, <1 ohn resistance.

The board looks ok, I can't see any dry joints. There are other relays though, including one that looks like it switches on the secondary heater.

 

[SpecialK]

Next step is to hook up a multimeter to the element and see if its getting power when you think it should be...

 

[robinbanks]

But why would it keep running the drum if the heater trips?

Safety feature to cool everything down in an overheat scenario?

 

[Johnking]

So, thanks everyone, but I've decided to draw a line under this, it's a very inefficient machine and reluctantly I have to accept that buying a heat pump replacement is the sensible option.

 

[robinbanks]

Don't worry; at 50p/kWh your new dryer will pay for itself in a couple of years I'm sure!