need some help with my worcester bosche 30cdi bolier

[romster1]

got a couple of strange issues going on and wanted some advice.
firstly there are no errors codes and the boiler does seem to be operating in that I'm getting HW and CH but i have noticed that it seems to be in a cycle where it will begin heating then cooling then heating back up. so basically it fires up and heats to the temperature ive set for example 53 degrees then it seems to start cycling with the pump running and starts cooling till it gets to around 42 degrees then it fires up again till it gets to 53. is this expected behaviour because it seems pretty inefficient to heat then cool by 10 degrees only to heat back up instead of just maintaining a constant temperature? another issue which ive had which may or may not be related is that if the power has been turned off and on then the boiler seems to go into an overrun condition with the pump constantly running for hours even whilst there is no demand for heat and all heating channels are off. the last time this happened the only way i could get it to stop was to keep the reset button pressed but even then it would stop and then after about 15- 20 seconds the pump would start running again this happened a few times until i just powered it off on using the power button and then it stopped and settled down again. any help or ideas on this would be greatly appreciated
thanks in advance

 

[D_Hailsham]

firstly there are no errors codes and the boiler does seem to be operating in that I'm getting HW and CH but i have noticed that it seems to be in a cycle where it will begin heating then cooling then heating back up. so basically it fires up and heats to the temperature ive set for example 53 degrees then it seems to start cycling with the pump running and starts cooling till it gets to around 42 degrees then it fires up again till it gets to 53. is this expected behaviour because it seems pretty inefficient to heat then cool by 10 degrees only to heat back up instead of just maintaining a constant temperature?

The boiler is short-cycling because it is so warm.

A boiler is sized so it produced the correct amount of heat when it is very cold outside (-3C). As it gets warmer less heat is required so the boiler automatically reduces the output. This is called "modulation". There is a minimum output which the boiler can produce by modulation, so if it is very warm, the only way the output of the boiler can be controlled is by turning on and off.

A 30kW boiler is on the large size. Do you have a very large house with poor insulation?

Pump overrun is controlled by the boiler. You would need a qualified engineer to diagnose this.

 

[romster1]

The boiler is short-cycling because it is so warm.

A boiler is sized so it produced the correct amount of heat when it is very cold outside (-3C). As it gets warmer less heat is required so the boiler automatically reduces the output. This is called "modulation". There is a minimum output which the boiler can produce by modulation, so if it is very warm, the only way the output of the boiler can be controlled is by turning on and off.

A 30kW boiler is on the large size. Do you have a very large house with poor insulation?

Pump overrun is controlled by the boiler. You would need a qualified engineer to diagnose this.

thanks. yes itsa 4 bed semi rads in 2 unused rooms have been turned off but I'm getting the cycling issue even when there is a call for heat on the CH, HW and underfloor heating in a downstairs zone so I'd be surprised if that level of demand was not enough to prevent it from cooing sufficiently and to require modulation. the upstairs is fully insulated the open plan kitchen diner where the underfloor is has 2 rads too and that zone is probably not as well insulated as the upstairs. Is there a possibility that the heat exchanger on the boiler might be slightly blocked which is preventing the boiler from cooling down properly ?

 

[muggles]

I also live in a 4 bed semi (early 1970s) and my house runs quite happily in the middle of Winter on a 14kW boiler. I'd suggest that yours is too big and that's why it's short cycling - it can't modulate low enough to meet your actual demand. Could be a blockage though, but then I'd expect to see it overshooting your setpoint

 

[romster1]

I also live in a 4 bed semi (early 1970s) and my house runs quite happily in the middle of Winter on a 14kW boiler. I'd suggest that yours is too big and that's why it's short cycling - it can't modulate low enough to meet your actual demand. Could be a blockage though, but then I'd expect to see it overshooting your setpoint

thanks for your reply i haven't noticed it overshooting the temperature which has been set and there are no overhesting errors either.As a trouble shooting step i turned the temperature up but that resulted in the same cycle of heating cooling and heating but obviously heated up to the new temperature then cooled instead of doing it at 53 ( old set temp)

 

[D_Hailsham]

30kW for a 4 bed semi sound a bit over the top - unless the house is a large Victorian one with sold walls and 8 ft ceilings.

The room thermostats may be calling for heat but the boiler may not be able to modulate low enough.

For example:

Your boiler modulates between 7.5kW and 30kW. Let's assume the boiler is correctly sized, i.e you need 30kW when it is -3C outside to heat the house to 21C. That's 30/24 {21-(-3)} = 1.25kW per degree. The lower limit is 7.5 kW, which is equivalent to 7.5/1.25 degrees = 6C. This equates to an outside temperature of 21-6 =15C. So when the outside temperature is above 15C the boiler cannot modulate any lower and it will run in on-off mode to maintain the required heat output.

If your boiler is over-sized, the temperature at which it runs in on-off mode will be much lower. For example: if you really need only 18kW when it is -3C, the boiler will run in on/off mode when it is above 11C outside.

 

[romster1]

it is an Edwardian house with high ceilings and solid walls I'm pretty sure this cycling condition was going on even during the recent very cold spell but from your interesting explanation it might be that as you have said the boiler had been over specified so its struggling to cool down. Thank you for your time and explanation it has helped explain what might be going on.

 

[romster1]

update. ok, Ive narrowed this down a bit. whilst the boiler ( pump/ fan) was continuing to run despite it not being fired up i turned the water channel off after i heard the TRV from my megaflow tank close. it obviously closed because the internal stat had reached temperature. after turning that channel off about 1.5 minutes later the pump/fan turned off. obviously at this stage thier was no call for heat on the CW channel stat.
so the modulating id described is by design as per your previous reply but what I believed to be a possible overrun seems to be because the megaflow stat isnt shutting the boiler down after closing the TRV which according to BAXI it should.
Worcester have said that the pump is in a continually on state until its told to shut down which is by design and it will continueto run if it still detects voltage so looks like the megaflow stat wiring needs to be checked and that there could be a potential problem with the controller, which btw is making emitting a high pitch quiet but audible sound