HRM Wallstar - capacitor replacement

Another possible cause of my early morning lockouts is the motor run capacitor. This is used to keep the motor running, and if it's faulty it can result in the motor running too slowly, which consequently results in lower oil pressure, which can cause a lockout.  With a digital multimeter it's very easy to test.

So let's get inside the boiler and have a look.  First things first, turn off the power supply to the boilerat the switch.  We then need to go outside and remove the front cover by slackening off the four screws on the sides of the cover and lifting it up and off

Next step is to unscrew the red burner cover which has a single plastic screw holding it in place

With the cover off, you can see the capacitor to the side of the motor, with a plastic cap on it.

It's best to check if there is any stored energy in the capacitor, so set your multimeter to volts AC and put it across the terminals, you are expecting a reading close to zero. You can discharge it with a resistor, or by shorting it out with a screwdriver as long as there's not too much voltage in there.

You will then need to remove the connectors on the terminals, I believe you only need to do one, but removing both theoretically will give a more accurate reading.

With your multimeter set to capacitance, put the two probes on the capacitor terminals.

My capacitor is rated at 3uF +/-5% as indicated in the casing, and I measured it at 2.52uF, which is outside these parameters. It looks like the original from 1997 so it has had a good run.

Any sort of 3uF 400v motor run capacitor should be OK, most newer ones seem to be plastic cased rather than aluminium though. This shouldn't be an issue as the operating temperature ratings are the same, I was however worried about using a Chinese eBay special so opted for an AEG branded one which matches the motor. The ones from oil boiler parts sites seem to be stupidly expensive, upwards of £20, whereas mine cost £11. Plastic chinese ones can be had for around £3-4.  I decided to test it before fitting, and it measured 2.99uF which is within the parameters.






Removing the old one is far more difficult than it should be in my opinion, for what nowadays is considered a service item.  The motor has to be removed, and to remove the motor, the high pressure fuel line has to be removed.  I believe technically this means you should be OFTEC registered, as I'm working on my own system and feel fairly competent having spent many years maintaining diesel engines, I decided to have a go anyway.

The top union came undone with a 15mm spanner, and the smaller union on the side of the oil pump needed an 11mm spanner.  I suspect they are probably imperial size, but these were a close enough fit.  With this removed, keep this safe and make sure it definitely won't get any dirt in it, wrapping it in a lint free cloth would be a good idea.

You can now access the two bolts securing the motor to the burner casing, they are located at 2 o'clock and 8 o'clock, and require a 4mm allen socket/key.

Pull the motor away from the burner casing and lay it down, be careful of the flexi oil pipe.  The capacitor is secured to the motor lug with a 13mm bolt.  I hadn't realised this, and mullered my old one trying to get it off, so here's a picture of the new one fitted instead.







The motor can now be fitted back into place and the two allen bolts tightened back up.  The high pressure oil pipe can be refitted, taking care to nip it up tight but not too tight so you strip the threads.

I believe only the early style units used small width terminals on the capacitor, rather than the more common 6.3mm spade terminals (commonly known as 'red' spade crimps).  As I had an early unit, I had to swap the terminals, fortunately I had the kit in the garage to do this.










I cut off the old terminals, stripped back some of the insulation, and crimped the new terminals on.  As the cable is so thin, I had to fold it back over on itself so it fitted snugly in the crimp.  It's always best to give them a little tug test to make sure they have crimped down properly

With the proper terminals now in place, they just push on to the capacitor, followed by the plastic cover.  The cover didn't fit as well as before, owing to the longer terminals, but it does the job of protecting the terminals against fingers or other conductive things touching them and shorting it out.

That's it, all finished.  Stick the cover back on the burner, and the test mode switch can be operated to force the boiler to run.  Turn the power on and make sure it runs up.  Don't forget to push the switch back over again before putting the outer cover back on.









Fingers crossed this will cure the problem of the boiler locking out first thing in the morning, but only time will tell....

PS - you may have noticed the air box has been removed in some of the pictures, this is from when I was still trying to figure out how to remove the capacitor!  There is no need to take it off though if you follow the steps above.