Convector Heater Repair

[Samuel W]

Hello, I wonder if anyone could advise on a repair of a convector heater which I am attempted. I am a complete DIY/electrics novice so simple explanations particularly appreciated!

It is a typical convector heater which has two elements and two buttons (with a light on each) labelled 750W and 1250W, with both labelled 2000W if on together. But only one of the elements/lights (I think it is the 750W one if I remember rightly) ever comes on when the button is pressed -- and it only works occasionally, often when I turn it on (even from completely off and cool for days) it doesn't work at all, not even the one button (and its light doesn't go on either when the heater doesn't work). This is with the thermostat on max (and having heard the click at a lower thermostat setting, suggesting the thermostat connectors are working?)

I disassembled it and checked electrical continuity with a multimeter (first time I have used a multimeter) and there seems to be electrical continuity throughout -- the only time electrical continuity stops is if both switches are in the off position. Strangely, if either of the switches are on, there is continuity from the live prong of the plug to the end of both of the heating elements (one is fed from the switch by a red wire, the other by a white wire; one has a blue wire emerging which goes straight out to the plug, the other has a blue wire emerging which goes back up to the switches)-- whereas I would have expected each switch to relate to one heating element and the continuity to only be in one element if only one switch is on.

There is a thermal cut-off halfway along the elements (no thermal fuse) but it seems to click up and down appropriately. But I have just reassembled it and it still isn't turning on at all from cold (despite electrical continuity from the live to the neutral prongs of the plug).

Incidentally, the two elements look identical though the switches are labelled with different powers (750W and 1250W).

I don't understand how the switches can work as switches if there is continuity in both elements regardless of which one is on. Or why the heater doesn't work at all if there is continuity.

Any advice gratefully received!

 

[Marvo]

Can you post some pictures of the internals of the convection heater including the elements the selector buttons, the thermostat and the thermal cut-off?

The reason for your weird continuity reading might be that the selector buttons don't disconnect the elements, rather they may change the elements from series wired to parallel wired depending on the power setting.

This is obviously guesswork without seeing some photos.

 

[buzzlightyear]

Who's make is it.

 

[Samuel W]

Can you post some pictures of the internals of the convection heater including the elements the selector buttons, the thermostat and the thermal cut-off?

The reason for your weird continuity reading might be that the selector buttons don't disconnect the elements, rather they may change the elements from series wired to parallel wired depending on the power setting.

This is obviously guesswork without seeing some photos.

Thank you for the reply and sorry not to reply to yours sooner. I attach some photos, I don't know if you can make anything of the problem from them. I wondered if it could be something to do with a problem with the switches meaning the circuit shorts via the switches in some way and therefore the elements never heat up?

Who's make is it.

It says PROlectrix on the front

 

[Marvo]

I can't see anything untoward in the photos you've posted.

I'd suggest next step is, obviously with the heater unplugged, to very carefully remove the heater element and inspect the back of it. It looks like it's made of Mica so it will be crumbly and fall apart if you're not careful. There's almost certainly a thermal fuse or two somewhere on the element itself, it could be descrete components or it could be an exposed thermal wire or track somewhere that just melts open circuit.

Once the heater is removed and the wires are disconnected you can do a proper continuity test on both the heaters. Whilst the load is disconnected you can also do a proper continuity test across the contacts of the switches and thermostat.

 

[Samuel W]

I can't see anything untoward in the photos you've posted.

I'd suggest next step is, obviously with the heater unplugged, to very carefully remove the heater element and inspect the back of it. It looks like it's made of Mica so it will be crumbly and fall apart if you're not careful. There's almost certainly a thermal fuse or two somewhere on the element itself, it could be descrete components or it could be an exposed thermal wire or track somewhere that just melts open circuit.

Once the heater is removed and the wires are disconnected you can do a proper continuity test on both the heaters. Whilst the load is disconnected you can also do a proper continuity test across the contacts of the switches and thermostat.

Thank you for the reply. I can't test the continuity again at the moment, I'll have to reborrow a test meter from a friend to do that, but I did previously test it across the elements as far as I could - from the little bits (shown in the last attached photo) that stick up at either end of the element to those on the opposite side and to where the wires come in and it all seemed fine (I also tested across the switches -- though perhaps not the right bit of them as I don't understand the wiring -- and the thermostat). Though I don't actually understand at all how the element circuit is supposed to work -- presumably some of it is sandwiched within the mica?

Anyway I have removed the heater element and attach a few more photos, showing the underneath more clearly. I can't see anything resembling a thermal fuse, just the thermal cut-out halfway along. There are white, blue and red wires going in across one end and another blue wire a bit further along above the white one, going into the start of a metal strip along the edge of the mica halfway along which the thermal cut-out is found.

The fact that there seemed to be continuity in all the individual elements is what made me wonder whether it could somehow be shorting within the switch such that the electricity doesn't flow through the elements? Though I don't see if that can explain why the switch lights don't go on? I guess the thermal cut-out could be flicking as soon as electricity goes through it but would it really do that in a millisecond and wouldn't I hear a little click (which is audible when the cut-out is flicked with a fingernail)?

 

[Marvo]

I'm in rural Mozambique this week and my internet is running at 1990's dial-up speeds so even looking at the pictures takes about 15 minutes.... I'm also using a 20 or maybe 25 year old crt monitor so I'm probably not going to be much help.

I can see the thermal safety cut-out on the back of the element panel which makes sense and nothing is jumping out as a problem. If the two contacts are tight together under spring tension then that's unlikely to be the issue.

You're definately going to need a tester with a resistance mode to fault find this thing any further.

 

[Samuel W]

Thank you for taking the trouble to reply in such restricted circs -- I'll get hold of a tester with a resistance mode and perhaps I'll try to find someone around here who can show me how to use it!

 

[Marvo]

Maybe splash out and buy yourself a tester. It might be cheaper than the petrol if you're running around to borrow one, for under 20 of your UK pounds you can get one of these for example ;

Draper 41817 Digital Multimeter Electrical Electronics Tester Testing Tool AC/DC | eBay

Fitted with a backlit LCD screen, auto power off, continuity buzzer and data hold features. Supplied with test leads and 1 x 6F22 9V battery. Battery 1 x 6F22 9V battery (supplied). Resistance 200ohms/2kohms/20kohms/200kohms/2Mohms.

www.ebay.co.uk

 

[Samuel W]

Thanks! Though the harder bit will be knowing what to do with the tester and what its readings mean...