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Boiler microprocessor noise

EMC Boiler microprocessor noise

Disclaimer: if you decide to copy any of the things I did here, you do so entirely at your own risk. If you are not completely confident you know what you are doing consult a qualified electrician.

After we had a new Vaillant 'ecoTEC plus' boiler fitted I discovered I had a high noise level on 15m. It was at about S4-5, which might not seem so high, but it's enough to wipe out most of the interesting DX. Using a cheap portable commercial radio that had shortwave band capability, I initially tracked this down to the wiring around the timer which is located in a cupboard upstairs, which also houses the unvented hot water cylinder. The boiler is in the utility room immediately below. I could turn off the power to the central heating and hear the noise on 15m disappear both on my amateur rig and on the portable radio brought up close to the wiring in the cupboard.

My initial thoughts were that the timer was to blame because the portable radio showed an increase in the noise when it was brought closer to the timer. However, I then removed the timer for inspection and turned the power back on to the central heating system. The noise returned! A bit more detective work showed it was coming from the boiler. Fortunately, there is a line switch to the boiler, so I was able to ascertain that the noise disappeared when the boiler alone was powered down.

The problem was easily solved using a common-mode choke formed out of the 5-core power/control cable and a ferrite toroid from Maplin (order code QT26D), as shown below:


Normally, the cable just passes through the two strain relief bushes and connects to the PCB terminal block. The addition of the connector block between the two bushes allowed me to make the common-mode choke at the workbench. You need to get as many turns on the toroid as possible, which is aided by removing the outer sleeving.