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How does the BoilerMate III work?
A conventional central heating boiler heats the water in the BoilerMate III directly. The stored water is pumped to the boiler for heating and back again by the right-hand of the two pumps on the unit whenever the stored water temperature drops below a specific temperature set in the factory. This same pump also pumps water around the radiator circuits when central heating is required. There is an electric diverter valve on the outlet of the pump which switches the pumped hot water flow out of the thermal store to either the radiator circuit or the boiler, or shares it between both when the store is below design temperature AND central heating is required.
The boilerMate III (or 3) delivers hot water to the hot taps in a completely different way from previous versions. The coil of pipe inside the heat store was notoriously prone to water scaling and this problem is now (supposedly) addressed by using an external plate heat exchanger. the left-hand pump starts when a hot tap is turned on and pumps hot store water through the plate heat exchanger, thus heating the cold mains water very effectively on it's way to the hot taps or showers.
There are now two larger and more sophisticated circuit boards behind the cover panel. A domestic hot water pump speed control board (PSC board) controls the pump speed during hot water demand, and the Storage Appliance Control board (SAC board) controls the boiler and the electric diverter valve to maintain stored water temperature and operate the central heating.
The BoilerMate III abandons the user-adjustable thermostat for the heat store and the SAC board now performs this function, deciding for itself what temperature to keep the heat store at and when to fire the boiler.
Faults known to occur in the Boilermate III (or 3):
1) Depleted water in the thermal store.
BoilerMates have a filling cistern fitted to the top of them, which is there to fill the unit with water. This may or may not have a float valve connected to the mains water supply to fill it. When there is NO mains connection, there is usually provision for manual filling by the user by means of a tap on the wall nearby. When the user doesn't realise this, water lost from the thermal store through evaporation and/or leaks can prevent the unit from working. If the water level falls too low, the heat exchanger coil ceases to be immersed in stored hot water so when a hot tap is turned on, the unit will not deliver hot water. The answer is to check the water level in the top-up cistern and top it up to the waterline embossed into the wall of the cistern. This problem is usually accompanied by a noise of water trickling or flowing inside the unit. This is caused by the flow from the boiler discharging into the unit above the water level inside and making the 'pouring' noise. This can be very bad for the system as accelerated corrosion usually results from the constant aeration/oxygenation of the circulating water.
2) Circulating pump failure
Even BoilerMate IIIs are all quite old now, and many of those without corrosion inhibitor in the circulating water are suffering from advanced radiator and system corrosion. The corrosion deposits cause the circulating pumps to seize up and burn out. Fitting a new pump gets the system working again but doesn't address the cause of the original failure. A system cleanse is usually necessary (a 'powerflush').
3) Water scale-contamination of the plate heat exchanger.
The plate heat exchanger was hailed my manufacturers as the answer to water scaling, but this has proved not to be the case. Hard water in certain areas still seems capable of blocking a plate heat exchanger with calcium deposits causing restricted hot water flow from the taps and warm (instead of hot) temperatures. The answer now is to fit a replacement plate heat exchanger, which takes around an hour instead of several hours to descale the previous copper coil heat exchanger.
4) Blender valve failure.
The thermostatic blender valve limits the maximum temperature of the water from the hot taps for safety. It is prone to damage from accumulated water scale. This results in the water taps only ever being slightly warm. A new blender valve is necessary.
5) Heat sensor failure.
Hot water temperature from the taps and/or shower becomes unreliable and unpredictable. The hot water temperature sensor delivers a signal to the PSC board and this controls the pump speed. They seem to fail with age (after three or four years) and replacement restores reliable hot water performance. I believe they are thermistors but there is more to them than that as there are three conductors in the leads. Their true nature is shrouded in secrecy. No-one at Gledhill gives anything away when I ask questions... quite frustrating really but I'll get to the bottom of them eventually....!
6) Sticking diverter valve.
The heating and boiler pump actually performs two separate functions. It pumps store water around the radiator circuit during heating demand, and it pumps store water through the boiler for reheating when the store temperature has fallen below the minimum required. The pump is connected to one circuit or the other by a diverter valve. This diverter valve eventually fails by seizing up but before this happens it becomes unpredictable and unreliable. Sometimes the radiators fail to come on, other times they stay on when they shouldn't be! The fix is to either fit a new diverter valve body, or replace all the internal moving parts using components removed from a brand new diverter valve.
If your Boilermate III (or 3) has stopped working, call or text me to discuss a repair on 07866 766364.
Copyright Michael Bryant 2021
Site first created 21st January 2007
Site last updated Friday 13th May 2021
Gas Safe Register 197499. CIPHE reg no 56207