icon 23t and hot water flow-rate 'firing' problem

6 Nov 2011
Reaction score
United Kingdom
icon 23t. Make etc (Hermann Icon 23t; Code: CWM 383U20; Number: 51415/39.99)

• Comments relate to domestic hot water – not central heating requirements.
• There has been no apparent change in water supply pressure.
• Inlet/Outlet pipework is ‘standard’ 15mm; and has not changed.

Characteristics before:
Prior to 31-10-2011 the boiler ‘fired’ at relatively low water flow rates via tap or shower.
The flame was maintained even at this relatively low flow rate. (This flow rate was not measured but might have been ± 4 to 6 L/min.)
The result of that was that taps were flowing a low volume of water, and a higher temperature of water was achievable.

Characterisics after:
After 31-10 things seemed to change, and the flow rate required to trigger ‘firing’ was considerably higher.
At that later stage the ‘trigger’ flow-rate to provoke firing is approx 8 to 9½ litres/minute.
Even if a high flow rate is used to ‘trigger’ firing (e.g. 9½ L/min), there is little or no scope to then reduce the flow (e.g. 7 ½ to 8 L/min) without the flame being cut.

Appliance Manual gives figures of:
• Flow rate at 35degree temperature rise = 9.6L/min
• Min water flow 2.5L/min
• Max. Supply pressure 6bar
• Min. Supply pressure 0.4bar
• Temperature range 35 to 65 C

I've read various posts, and there is often mention of 'diaphragm' and 'diverter valve' being associated with this type of problem. Additionally, that the 'diaphragm' can be replaced 'in situ'.
I am a patient DIYer, and time is not a problem.

• What would be the design or expected flow rate to trigger ‘firing’ of the boiler, and then maintain the flame?
• Does the above description indicate a fault?
• Is any adjustment mechanism available?
• What part is likely to be at fault (Water pressure/flow sensor/diaphragm/diverter valve/switch?)
• What is the availability and cost of the part?
• Would it be sensible to replace JUST the diaphragm? Any tips on technique?
• If replacing diverter valve, what steps needed and any tips?

If I’m on the wrong track, or more information needed please let me know – and thanks in anticipation of your helpful answers.
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