Trianco Aztec Boiler Issues

An unusual topic for my blog, but it offers something of a solution on an issue that has caused me to do work.

Our central heating at home is all electric. The boiler is a recently installed (May 2016) Trianco Aztec Classic. We had a major issue with it post installation, it was leaking badly and the heat exchanger needed replacing. We subsequently discovered that it was continuing to leak, though not as badly, and a seal was replaced on May 2017. (Given that it is also expensive to run my advice is to avoid them if possible.)

Last weekend it started to exhibit a new behaviour; it would stop heating. Rather like the magic porridge pot, even though the thermostat was not calling for heat the boiler was providing it. I’m rather hoping that this was the first time it’s done this otherwise my electricity bill will be astronomical!

The problem with this is that you can’t (shouldn’t) just turn the boiler off. Usually when the thermostat indicates that temperature has been achieved the boiler shuts off but the pump continues to run, ensuring that there isn’t a boiler full of hot water just sat there. My solution was to reduce the radiator temperature (front button controls on the boiler) to the minimum, let it run for a few minutes and then kill it.

So now the joy of getting it fixed.

I rang the service company and spoke to them. I suggested that any repair should be made under warranty, given the unit’s age and problem history. They said that it didn’t sound like a boiler problem, but rather a thermostat problem and asked if I’d changed the batteries. I knew that this was utter rubbish.

I’d already checked that the thermostat relay was “clicking” when I changed the temperature up and down to make it switch. Also it would be extraordinarily bad design to have it fail switched on if the batteries ran flat – it makes no sense. Not only that I’d had the cover off the boiler and checked the call/no-call operation with my meter.

I humoured them, as you have to, and rang back the next day to confirm that changing the batteries had made no difference. They said they’d have to call out, but that would be a £95 plus VAT charge.

So when I got home that evening I took the front off the boiler again and had a fresh look at it. I even referred to Trianco’s manual for troubleshooting guidance. Oddly it doesn’t cover this situation.

I double checked the wiring, mainly checking for cables that perhaps hadn’t been properly secured into plug blocks and had lost connection over time. Nothing.

Then I noticed a variable resistor on the front edge of the control board. It doesn’t seem to be referred to in the manual, but it is the only adjustable component on the board. It’s adjusted with a flat blade screwdriver. I noted the setting, adjusted off that position and then back, and then fired up the boiler. This fixed the problem.

I can only imagine that over time the wiper had formed a “dry joint” against the element, perhaps not helped by the early leaking which would have led to high humidity. I guess that it provides a balance that prevents the thermostat from triggering the call relay.

A final safety note. If you’re not confident with electrical work you should open up your boiler. If you do then be sure to properly isolate it at the switch and also at the consumer unit.

16 December 2018, UPDATE

On Friday the boiler failed again with the same problem. I attempted the fix detailed above but it had no effect. Today I found time to unplug/replug all of the connectors to the control board. This did fix the problem.

(I also took some time to examine the board. The microcontroller is a Microchip 16F1937 in the 44 pin TQFP package. The wiper of the variable resistor feeds the RB0/AN12/CPS0/CCP4/SRI/INT/SEG0 pin.)