Before attending the property it would be wise to ensure the customer has turned off the heating sometime before your arrival (especially on heat only and system boilers). Aluminium has considerable thermal mass and given the limited clearance and access above the heat exchanger the retained heat will be injurous to your hands.

Furthermore, the initial fan pressure check will prove invalid since the boiler must be run at maximum output - this can only be achieved if the system is cool. The heat exchanger on combi models can of course be cooled by running in DHW mode with the gas isolated (although they will require multiple resets due to lockout); this should provide sufficient cooling to allow the boiler to operate at maximum during the fan pressure test.

On arrival, hopefully with the boiler/system cool the service can begin.

Basics

Since the internal state of the heat exchanger is not visible Worcester have deemed a simple pressure test of the fan be sufficient to determine if full servicing is required. Each model will have listed in the MI a fan pressure reading. After removing the outer case a digital manometer is attached to the fan test point. The boiler is then put into service mode.

On the Junior this is done by running the hot tap on full, heating temperature on maximum, ensuring the radiator circuit is fully open (ie set TRV’s and Wheelheads to maximum) and pressing the service button for the required time. On other boiler models a switch on the pcb has to be set again ensuring the heating/hot water circuits are fully open.

The service mode operates the boiler at maximum output ie. it will run the fan at maximum speed. If the pressure reading is less than that published a full strip-down is required. The first item of note is the fan test point (see Image 1), removing the cap exposes the test nipple. The diameter doesn’t correspond to standard manometer hosing. Either sleeve your manometer hose or connect directly to the fan tube (see Image 2).

Experience so far shows the pressure readings obtained to be just over the published figures after only a year in service so it was felt prudent to carry out the full service.

On to the service

Isolate the power and gas to the boiler. As explained in the MI remove the siphon, gas valve connection, electrode and fan connections. Remove the retaining plate to allow rotation of the fan assembly. Lift out the fan/burner and remove the burner gasket and electrodes.

It was found on some electrode assemblies the retaining groove for the connectors has been machined too deep. This made it very difficult to unclip the wiring harness. Now remove the top baffle.

You will need to obtain the cleaning kit (Part No. 7 716 192 312). Within the kit you will find a baffle removal tool and cleaning brush (see Image 3). The baffle tool hooks the baffle from the base of the heat exchanger. The holes in the tool allow leverage with a large screwdriver. I can foresee this tool will provide insufficient leverage if heat exchangers are encountered that have not had regular servicing. A mechanical puller (perhaps an adapted slide hammer) may be more suitable and offer increased force.

I suggest you now move on an clean the siphon/trap. As can be seen (see Image 4 ) the unit contains significant aluminium oxide after just one year. It is doubtful the trap will not block with an extended service interval. Remove the cap and thoroughly flush out the debris,. Replace the cap, fill with water, dry the external surface, and leave to stand away from the boiler. This allows the cleaning cap seal to be tested whilst the heat exchanger is cleaned so preventing a possible call back.

Heat exchanger

Now to the heat exchanger. I suggest you cover the controls panel with plastic sheeting to prevent possible water ingress. A square carton shown in the image provides a suitable receptacle to catch the flushed contents of the heat exchanger. Cut down to suit, it fits under the sump pipe-work (see Image 5). Using the brush clean the accumulated debris from the internal surfaces (see Image 6) as described in the MI and kit instructions. Once thoroughly brushed the unit must be flushed out. I have fabricated a simple washer for this purpose (see Image 7).

As can be seen it comprises a water bottle connected to a flexible tube (sealed at the far end with a bolt). Near the sealed end the tube is perforated around its periphery to produce a shower effect. Starting at the top of the heat exchanger water is sprayed onto the internal surfaces washing out the debris. Whilst holding the bottle uppermost and rotating the tube a little, gradually move down the wall of the heat exchanger with each refill of the bottle. Empty the carton with each refill. I have found it requires 4 or 5 flushes before clean water is discharged via the sump.

Next remove the sump access cover and inspect for retained debris and flush the extremities if necessary. Replace the sump access cover using a new gasket (Part No. 871 010 319 10 but check for other models).

Check the siphon/trap for leakage on the access cap. Now empty the siphon/trap and re-fit to the boiler. Pour a quantity of water into the heat exchanger to refill the trap and test for leakage of the sump cover and trap seals. Clean the lower baffle of deposits and inspect baffles (see Image 8 and see Image 9). Refit as described in the MI.

Inspect the burner and electrodes for damage. The electrodes (see Image 10) show deposits and corrosion on the sense electrode. Using a new burner gasket (Part No. 716 110 534 0 but check for other models, see Image 11, shown with sump cover gasket) re-fit the fan assembly as described in the MI.

Finally

The final procedure is to carry out FGA. The MI specify a minimum working pressure on the gas valve inlet test point. It was found even with the casing seal removed (see Image 12) a 4mm screwdriver would only just fit - this was on the Prettl valve.

Whilst the boiler is reaching equilibrium check the heat exchanger vent tube connection is tight (it may have been disturbed during servicing) and run a gas sniffer around the gas valve connection. Also check the plug on the air side of the flue elbow is tight (see Image 13), it was found to be a very loose fit.

The Prettl valve can be adjusted with conventional flat bladed screwdrivers, the Sit and Dungs require 4mm or 2mm hex ball drivers. Gas rate the boiler at the meter. Useful formulae to note: 1 m3/Hr = 35.31 ft3/Hr, 1ft3/Hr = 0.0283 m3/Hr and 102 / (time in seconds for 1 test dial revolution) = m3/Hr

Take the boiler out of service mode, check ignition lockout, expansion vessel, system pressure etc where necessary. Allow at least an hour for the cleaning procedure.