Causes of a blocked Plate Exchanger
- Waterways restricted with debris accumulation (rust/magnetite/black oxide)
- Scale formation (hard water areas depositing calcium carbonates)
- Filter blocked (sometimes located on the primary inlet to protect the plate exchanger from waterborne debris accumulation)
- Leakage across the plate exchanger channels
Plate Heat Exchangers are secondary heat exchangers in most Combi-Boilers. Combi boilers heat water in the heat exchanger and pump round the heating circuit, via radiators. When there is demand for hot water via the tap the diverter valve switches to “hot water mode” (see Diverter Valve in System Component Testing). The diverted heating water goes via the plate exchanger. The Plate Exchanger has 2 waterways:
- Central Heating (operating at heating circuit pressure typically between 1 and 1.5 bar)
- Tap Water (operating at mains water pressure between 2 and 6 bar)
Both water supplies do not mix but are separated by a series of plates. The plates heat via the hot water in one channel, with mains water going in cold and then heated via the plates. Water is pumped back to the main heat exchanger for re-heating. The plates provide the required surface area to heat the cold water. There are many plates and over time, due to the small space between the plates, debris and scale can form blocking the plate exchanger.
In areas where there is a lot of limescale you can use water treatments (where hardness is above 200ppm). The heating waterways in the plate exchanger is normally ok as the water is regularly being pumped around the system stopping any build up of debris. The domestic supply (tap water) can be an issue. Fresh water is supplied and heated via the plate exchanger. The flow rate may be inhibited via the build up of limescale and other debris, the build up can reduce the efficiency of the heating plates so the water may not heat efficiently.
Electrically isolate boiler / system controls
- Isolate the cold, flow and return valves beneath boiler. If water starts dripping through the glands of any of the isolation valves, they should be replaced, which then requires a full system drain down!
- Always check to see if the cold isolation valve shuts off the cold supply to the boiler by opening a hot tap and drain water contents via any built-in drain valves within the boiler. Alternatively soak up the draining water with absorbent mats /towels, taking great care not to damage any electrical components. (Do not use the pressure relief valve as debris accumulation may get caught on the valve seat and cause continuous leaking).
- If water continuously runs from the hot water taps, isolate the main incoming stopcock – the cold valve beneath the boiler is faulty and will also need replacing
Note: Due to the design of some boiler models, access to the plate exchanger may only be achieved by removing other internal components first. Should this be the case, always ensure you have prior arranged all necessary O rings or washers as replacing these parts with the existing dirty O rings or washers are likely to encourage further leaks. Mains water may be forced between the 2 chambers (mains water being higher pressure forced into the heating system side). The heating system pressure may rise dangerously over the 3 bar safety and the pressure relief valve will be opened, forcing heating water through the safety pipe. (See also Expansion Vessel Faults)
Debris may also be transferred to the heating system side which, if allowed to persist, will create problems.
Any necessary washers/O rings can be easily accessed via purposely designed computer software, found in any of the large plumbing/parts suppliers
Unless “power flushing” the existing plate exchanger with purposely designed adaptors and chemical flushing agents, then a new plate to plate heat exchanger is needed, not forgetting the new lip seals, which may need to be ordered separately depending on the manufacturer.
- Refit the plate to plate heat exchanger with the new lip seals, and other internal components (if any) and re-establish cold supply first
- Open slowly the return valve beneath the boiler, whilst at the same time re-pressurising the boiler via the filling assembly up to 1.0-1.5 bar (filling loop may be built-in the boiler or located remotely)
- Open the flow valve beneath the boiler and thoroughly inspect for any leaks (especially all the connections which had previously been stripped down)
- Re-establish power to boiler/system controls
- Run hot water and heating ensuring all air is fully purged from the primary circuit (via the pump head vent plug, unscrew plastic caps on air vents)
Note: Sometimes there can be a leak between the two waterways in the heat exchanger. This is not immediately obvious but can show as a leak from the safety pipe (may be continually dripping).
Other Issues to check
- Noisy Fan
- Noisy Pump
- Noisy Hot Water Flow Switches
- Noisy Solenoid
- Burner Not Igniting
- Boiler Fan Problems
- Boiler Shuts Down
- Explosive Ignition
- Blocked Plate Exchanger