Worcester 27CDi - Is this normal operation?

[stormyuk]

Hi,

Just got a new Worcester 27CDi fitted replacing an ageing thermal store and small ideal boiler.

The boiler seems very nice, DHW seems fine, boiler rund perfectly for that. I am slightly confused by the CH side of things.

The rads are all heating nicely, and my existing TRVs are kicking in as I would expect. Two rads have no TRVS so run constantly depending on what my wall thermostat is set to.

For purpose of testing I set it to 30c just to keep it running although the living room prolly got to about 23c-24c

I am not sure if my boiler is cycling properly or not though. Plumber will said he will come take a look if I am not happy but not sure I am wasting his time.

With the boiler on max the CH demand the boiler kicks into life and starts supplying the rads, the display on the boiler goes up to around 85-88 and then the burner switches off, boiler continues running and the temp of the return will gradually drop to 40 sth and then the cycle repeats. If I set the boiler to lower than max I get the same cycle but it will top out at 60-80 odd depending where the dial is.

Reading the manual for the CDi is a about as much use as a chocolate fire guard, with phrases such as "anti-cycling, burner modulation" etc.

From a supplier website I got this:

'Central heating mode

On a Central Heating demand, the boiler will initially fire at minimum output before modulating upwards to meet the actual system requirement. Electronics within the boiler continually monitor the heating flow temperature increasing or decreasing output on demand. An anti-cycling device makes the Greenstar CDi range particularly suitable for use on systems with TRV's (Thermostatic Radiator Valves).'

That to me suggests the burner should always be running and not ramp up the temp to 88 and then turn the burner off, it should keep the burner running and keep the temp from going over 88, ie to modulate and anti-cycle. Am I just understanding it wrong and should the boiler run and just switch the burner in and out?

Is my boiler working properly or is it doing something naughty?

Cheers,

 

[glasgowgas]

its doing something naughty

lets imagine all your rooms are 15 deg and your boiler is off.

switch on boiler and flow and return will probably be about 20 deg,boiler will fire up on a low rate, fan will speed up opening gas valve more,full fan speed full set gas pressure. as ntc thermistors pick up the rise in return temp boiler fan speed should ramp down redcing gas pressure (modulating) keeping temp at what you have set boiler thermostat to

if its not modulating down its probably not been set up properly

hope this helps

 

[stormyuk]

Yup, that's pretty much what I understood it to be, bugger. Looks like I will get the plumber back.

 

[bengasman]

That to me suggests the burner should always be running and not ramp up the temp to 88 and then turn the burner off, it should keep the burner running and keep the temp from going over 88, ie to modulate and anti-cycle.

The boiler will produce more heat if the difference between target temperature and actual temperature is high. (boiler temperatures, not air temperature). Compare this to putting your foot on the floor in your car.
Under normal circumstances, there is no need to set the boiler on max; provided the system is properly designed, 3 for spring/autumn and 4 in winter.

 

[stormyuk]

Thanks for the responses, yeh. I will turn the boiler down when its up and running properly, the problem above exhibits itself at all the temp range, it just switches the burner off sooner.

Going to ring Worcester today to just check the operation, then get the plumber back to look at it.

Edit: Worcester seem to suggest its a restriction in the CH flow to the rads, and that the boiler is too big for the system (although the 27CDi is the smallest DHW and CH Condensing boiler they do).

My pipework is crappy 10mm microbore although it has been powerflushed by the plumber when the system was commisioned.



I also noticed this morning while running hot water the boiler did the same thing for the the DHW although oddly, the water didn't get cold it just switched the burner off, and back on again after a few seconds without any actual heat loss at the tap.

Cheers,