During service life, it may become necessary to replace the control electronics to earlier Powrmatic NVx heaters containing a separate PCB in conjunction with a plug in Pactrol control box.
Upon receiving the modification kit from Powrmatic, read the instructions carefully. They are well written and document the procedure from start to finish in an easy to understand manner. Do not attempt to carry out the modification unless you are a qualified engineer, mistakes carry heavy consequences.
We began by isolating the gas supply to the Powrmatic heater. One of our Gas Safe engineers isolated the gas supply to the heater, plugged it off and made sure it could not be turned back on again. Next we isolated the electrical supply to the heater by turning off the fused isolator and removing the fuse. Using a test meter we proved that the electrical supply was off and therefore safe to work on. We will be removing the power cord that supplies the heater so a safe isolation is essential.
We checked that there were no electrical feeds being provided by the Powrmatic Powrtrol controller to the heater, supplied elsewhere. We confirmed that the Powrtrol mains supply was sourced from the heater after the isolation point and safe to proceed. We have found Powrtrol units fed from alternative supplies resulting in some control wires still being live inside the heater, even with the heater switched off. If this is the case, trace the supply to the Powrtrol and isolate it, lock it off and hang a warning sign from the isolator. If necessary, remove the wires from the supply point to prevent unauthorised switch on.
We then identified the Powrtrol wiring. The Live, Earth and Neutral feed to the controller was wired into the mains supply terminals inside the heater. There were also two additional wires which we identified and labelled as heat and summer. The heat signal starts the heater running when the controller calls for heat and the summer signal turns on the main fan when a switch is operated on the controller. Next we labelled up the supply cable correctly using coloured sleeving. The supply cable had been left with the colours brown, black and grey. We re-labelled them as green/yellow for earth, brown for live and blue for neutral in compliance with BS7671.
The whole chassis containing the PCB and Pactrol control box needs to be removed so we removed all the wires as indicated in the manual. The mains supply cabling and Powrtrol cabling were removed since we have identified the wires. There are flexible cables that provide power to the gas valve, main fan and combustion fan. Remove the connectors from the PCB and move the cables out of the way. The probe and ignition wires were removed from the old control box and leads carefully tied out of the way then the tube was removed from the pressure switch. The cabling to the overheat stat and neon was also removed from the stat and discarded as a new cable is supplied in the kit. The link between the neon indicator and the stat remains in place.
The chassis is held in place with four self drilling screws. They were removed using a nut spinner and the old chassis withdrawn and discarded.
There are specific details of how to mount the new chassis. We identified the position requirements for the NVx70 and marked the mounting holes using a pencil. We installed fixings back into some of the holes as required by the instructions. This plugs unused holes in the heater chassis. We used a small 3mm pilot drill and drilled the centre holes of the marked holes, this will assist in the self drilling screw locating. With the electrical chassis offered into place we installed the new self drilling fixings to hold it into place.
The supplied cable was installed between the controls and the overheat thermostat. The wire was installed to the terminal strip and then the connectors plugged onto the overheat thermostat. The terminals were easily identifiable on the rear of the stat and the wiring diagram shows the connections clearly. We needed to clean the back of the overheat thermostat to read the terminal markings aided by a small flashlight.
The flexible cables that supply the gas valve, combustion fan and main fan were connected back to the relevant connections on the terminal strip. The cable already installed to the main fan was of sufficient length and did not require replacing with the spare one supplied in the kit. We had to cut the Lucar Crimps off the wires and strip them back to install into the terminal strip. The cables can be made to run up the centre of the compartment and form a neat loom.
The ignition and probe cables are connected onto the Brahma control box directly. Ignition goes towards the bottom of the control box and ionisation probe towards the top. They use different sizes to identify the probes so check to avoid damage.
Next we re-wired the incoming mains supply cables into the mains terminal block and also connected in the power supply cables that feed the Powrtrol unit. The heat and fan circuits were wired back into the terminal strip.
All that remained was to push the flexible hose back onto the new pressure switch, connecting it to the – port, the + port remains unused. We followed up with some cable tie work and generally neatening up inside the panel.
It is important to note that the ignition and probe leads should not be cable tied or allowed to come into close proximity of other electrical cables inside the heater. The heater design is good and the leads remained towards the left hand side of the tray naturally. Avoid close proximity to the burner manifold.
Using a continuity tester on the ohms setting we next ensured that the earth connections from the gas valve, combustion fan, main fan, Powrtrol and the chassis were all reading low. This is an essential safety check and should not be avoided. Fully investigate high readings.
The most important step now is to check your work. I checked it from start to finish, hold each wire connected to every terminal, follow it to its destination and confirm that its connected to the right place. A colleague learning the replacement procedure then checked my work for me.
Once we were happy with the wiring, we put the fuse back into the isolator and turned back on the supply. The red neon on the chassis tray came on to signal the heater was locked out. We then confirmed that the supply polarity was correct and the supply earth was present.
With the gas supply still turned off, we set the Powrtrol to call for heat and held the reset button momentarily, the control box reset and the combustion fan started to safety purge the system with fresh air. After a short wait, the ignition electrode sparked to life and the gas valve opened.
After a couple of seconds the control box failed to prove the burner was lit and turned off the ignition and gas valve. The cycle continues several times and eventually the heater locks out and the red neon rocker switch lights back up.
Our Gas Safe engineer then reinstated the gas supply, carried out some safety checks and then informed me it was safe to test. The flue gas analyser was turned on and a manometer made ready to test and adjust gas pressures if necessary.
The lockout button was pressed again and the heater began its start up sequence. Almost as soon as the ignition electrode sparked to life, the six burners ignited on the manifold.
Our Gas Safe engineer then carried out various tests and measurements and printed out a flue gas analysis. He serviced and cleaned the heater and then gave the customer a Gas Safe service record.
Most warm air heater manufacturers are now using Brahma control boxes and they are working very reliably out in the field. We have only experienced two failures this year, both caused by energy management system engineers connecting the lockout reset wrongly. In all cases, the lockout reset should be switched to Neutral. Applying a live connection to this terminal will permanently damage the control box beyond repair.