Home » How to test a Thermistor – Combi boiler faults – Multi Meter Fault finding
How to test boiler components. Using a Multi Meter. Testing a thermister. Boiler repair. Today I’m going to show you how to test a thermistor. If you need to test a thermistor in a combi boiler, if you’ve got problems with the boiler not working or not working correctly, I’m going to show you how to test the thermistor. Combi Boiler Fault Finding.

I’ve got a few different boilers here. We’ve got a Baxi 800. I’ve got an old Baxi Duo-tec. We’ve also got a Viessmann. We can have a look at some different thermistor in the boilers. We can test them cold and also we’ll be able to … On this boiler here because this one’s working, we’ll be able to test it hot as well. Just to show you the different readings in the boiler. Let’s have a look now.

Before we remove the case we’d make sure that we’d tested it. Done those electrical checks to make sure the boiler’s turned off. This boiler’s not connected so I know that it’s not connected. I’m just going to undo these two screws here.

When we look in the boiler here this is the thermistor here. Baxi call this a Domestic Hot Water NTC sensor. That’s what a thermistor is. What we need to do, we need to test this. We need to test … There’s two prongs in there and we need to test this. On this video what I’m going to do is I’m just going to undo this out. On this particular one it’s a 13 mm connection. What we do is we just un-clip that off. That’s got a little clip on there. Normally we’d just clip that down. Then we’d get a 13 mil spanner. We’d get the 13 mil spanner and then we’d just undo this out of here.

Now just to bear in mind, with this particular boiler, this is a wet pocket. If you disconnect that out of there then you’re going to get mains water pressure squirting out of there. You’d need to make sure that you’d turned your water off, and opened your hot tap and made sure no water were flowing through before you did it.

Just another way … Just before we undo that there you can also … You could do it on the board if you wanted to. You could remove this cover here. Then if you have a look in the manufacturing instructions you can see where these green wires go to. Then you could test it on board. On this particular one I think it’s X400 but we’ll just have a check of that. I’ve been on the Baxi training course and I’ve got their engineer service manual for this boiler. In the engineer’s service manual you can see there the electrical information. You’ve got you Domestic Hot Water NTC sensor. That’s your temperature sensor, your thermistor. If you follow them wires down they’ll go to the plug X400 there. If we have a look it’s terminals one and terminals three.

If we wanted to we could test it in there. We could put a multimeter into there and test it. But I’m going to take this thermistor out on this so we’ll have a look at it. But first of all what I’ll do is I’ll show you the multimeter and I’ll show you how we test it.

A little bit hard to see but I’ll just show you this now. I said it was number one and I said it were number three. It’s actually number one and it’s number four. If you’re going to check it at the board you would check it at number one and number four. I’ll just get the multimeter now. I’ll show you the multimeter.

This is the combi boiler thermistor, or boiler thermistor, or combi boiler sensor, or Baxi temperature sensor, or Baxi thermistor. This is what we’re going to test there. If you can have a look on there it’s a little bit hard to get in and get onto them wires. What I’m going to do is I’m just going to cut the lead off the boiler, just to make it easier for this test now. But first of all I’m going to show you the multimeter.

If we look on the multimeter I’ve got a Fluke 114 multimeter. What we want to do we change it to ohms on there. Ohms. Then what we’ll do is if we have a look on … If you can see on there. But that shows you M ohms. When we look at what M ohms means. That’s a megaohm. A megaohm is one million ohms.

I’ve got a little tester block here so I can show you the different readings on there. That one’s M ohms. Then if we go to the next one this one now, if you see there is K ohms. If we have a look in the book what K ohms is there, so it’s K ohms. That’s one thousand ohms. Then if we have a look again, if we go into this one, and we have a look on there. That one, that’s just ohms.

You just need to be careful when you’re doing your readings that you’re seeing the right reading. Let’s have a look at the thermistor now.

We’ve got this thermistor now. I’ve just cut the lead off out of the boiler. I’ve just bared the wires back. Obviously you wouldn’t normally do that but if we connect this … If we put it right way round and connect that onto there. Then what we can do now … This thermistor now is going to give us a reading or it should do if it’s working okay.

Allen Hart

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