Worcester Bosch 24cdi RSF

[Donatella]

Hello,

Domestic hot water always comes out very hot regardless of the temperature setting on dial. Boiler fires up and then switches off (flames out) after about a minute and then comes back on again as the water begin to cool down. Believe this is called short cycling

- Hot water flow is fine
- no boiler error codes

Any idea what this could be? Reading various articles point to possible gas modulating issues.

Thanks
D

 

[DP]

Water always comes out hot initially as the water is purged from the heat exchanger heated by residual circulation from previous run. Once the boiler is running for hot water demand, the HW thermostat will play a part in controlling the temperature

When hot tap is run, boiler goes to full power and starts heating the tap water indirectly. If the boiler power is not transferred properly to water going to the tap, burner might get modulated or shut down completely until heat exchanger is at operational temperature again

You have a fault on the boiler. I assume boiler heats the radiators correctly.

 

[Donatella]

Thanks for the response. Yes the central heating radiators are working but what I also notice is that it burner is either in on or off state when CH is on the desired temperature, it runs for longer if temperature is set high. For example if I have CH on lowest temperature setting, it runs for about 5 minutes or so then shuts down, then start again once CH has cooled down.

Having read the operational flow diagram, what I think is happening in both CH and DHW mode the over temperature is shutting down the gas valve as modulation is not happening. In the case of the DHW shutdown is happening much faster due to shorter water cycle and PHE heating up much quicker.

Of course I could be 100% wrong : )

Thanks

 

[DP]

much faster due to shorter water cycle and PHE heating up much quicker.

The hot water heat exchanger will be blocked or the circulation through it will be the issue.

For heating, boilers often were not set to match the load- even the smallest combi was often rated to run at hot water demand rate therefore during heating cycle the burner went to max rate and then burner (if fully modulating) downsized accordingly to suit the load. As the flow temperature approached the CH stat setting, burner size was further tweaked by the electronics. Temperature overshoot resulted in burner shutdown and anti cycling timer starting up. When timer finished, burner fired up again

For hot water there is no anti cycling delay timer else you would be taking hot and cold showers.

If the burner setting is correct, hot water tap flow is as per the book (l per min) and hot water thermostat is at max, the temperature gauge on the front of the boiler will register 70 degrees. For a fouled HW HE, gauge will show 85/ 90 where overheat is occurring and boiler going to safety shutdown

 

[Donatella]

Just checked the water HE is brand new original changed about 2 months ago.

 

[DP]

Just checked the water HE is brand new original changed about 2 months ago.

What is the system temperature, flow rate of water from hot tap as mentioned in previous post. New heat exchanger means nothing.