Worcester Bosch Greenstar Combi Anti Cycling

[John.g]

Acquaintance has problem with a 5 radiator zoned system which takes ages to heat up. The boiler (don't know output) fires up normally and after ~ 20 minutes reaches its setpoint of 65C, over the next few minutes it will creep up to 70C and then cut out, again fairly normal if min boiler output is > heat demand. The problem he says is that it will not re fire when the temperature reaches 60c but may go as low as 45/50C before relighting. Now I don't know if this boiler has a anti cycling function but if it has then I assume the anti cycle time is set in a look up table and just looking at one for a Vaillant (spelling) it would seem that a fairly normal setting would be 20 mins so if one looked at the table for 20 mins and a boiler setpoint of 65 it might seem a boiler off time of ~ 5 or 6 minutes but he then said that after each cycle the boiler seems to stay off for longer and longer and he thinks it then doesn't even reach its 65C set point before again cutting out. Can anyone throw any light on this and if the cycle time is say 6 mins should this time be fixed for each subsequent cycle, also does the 6 min cycle time refer to 6 min off and then 6 min on or should it keep firing until it reaches its cut out temperature of 70C, in this case.

 

[Chuck]

Acquaintance has problem with a 5 radiator zoned system which takes ages to heat up. The boiler (don't know output) fires up normally and after ~ 20 minutes reaches its setpoint of 65C, over the next few minutes it will creep up to 70C and then cut out, again fairly normal if min boiler output is > heat demand. The problem he says is that it will not re fire when the temperature reaches 60c but may go as low as 45/50C before relighting.

Unless Bosch has changed their sofware recently the algorithm the boiler uses is to sleep (anti-cycling mode) for five minutes if two on-off cycles occur within the same five minute period. Typically, what happens is that the burner comes on and the temperature ramps up, there is insufficient circulation rate and the water temp rises to ca 75°C when the over-temp protection cuts the burners (first on-off event) the water termperature then falls to 60°C. At that point, the burner comes on again and the water temperature starts rising. When it reaches 75°C it cuts out for the second time in five minutes so the boiler goes to sleep for five minutes. During these five minutes the water temperature drops to perhaps 45°C. After 5 minutes the boiler wakes up and the whole sequence starts again.

Best performance without modifying plumbing or boiler will be achieved by setting all the TRVs to maximum and balancing the system correctly so that is is entirely controlled by a single thermostat. Worcester Bosch may be willing to give the installer some customised boiler settings, which may or may not help depending on what the laws of physics say about the system. Most people stop there as the next things to try are: install a ballast volume (to increase the heat capacity of the system water) or replace the boiler with one with a lower minimum output (larger turn-down ratio).

This behaviour is typically only real problem in autumn (i.e now) and spring because in the winter the emitters need to dump more than the minimum boiler output to keep the dwelling warm. Thanks to Building Regs requiring TRVs, installers not having time to precision balance radiators, improved insulation meaning the minimum output of many boilers is inappropriately high, etc. the behaviour your acquaintance has noticed is so common it's almost 'normal'.

 

[ShaunCorbs]

Down rate the heating side sounds like it’s way too big heating wise

 

[John.g]

Unless Bosch has changed their sofware recently the algorithm the boiler uses is to sleep (anti-cycling mode) for five minutes if two on-off cycles occur within the same five minute period. Typically, what happens is that the burner comes on and the temperature ramps up, there is insufficient circulation rate and the water temp rises to ca 75°C when the over-temp protection cuts the burners (first on-off event) the water termperature then falls to 60°C. At that point, the burner comes on again and the water temperature starts rising. When it reaches 75°C it cuts out for the second time in five minutes so the boiler goes to sleep for five minutes. During these five minutes the water temperature drops to perhaps 45°C. After 5 minutes the boiler wakes up and the whole sequence starts again.

Best performance without modifying plumbing or boiler will be achieved by setting all the TRVs to maximum and balancing the system correctly so that is is entirely controlled by a single thermostat. Worcester Bosch may be willing to give the installer some customised boiler settings, which may or may not help depending on what the laws of physics say about the system. Most people stop there as the next things to try are: install a ballast volume (to increase the heat capacity of the system water) or replace the boiler with one with a lower minimum output (larger turn-down ratio).

This behaviour is typically only real problem in autumn (i.e now) and spring because in the winter the emitters need to dump more than the minimum boiler output to keep the dwelling warm. Thanks to Building Regs requiring TRVs, installers not having time to precision balance radiators, improved insulation meaning the minimum output of many boilers is inappropriately high, etc. the behaviour your acquaintance has noticed is so common it's almost 'normal'.

Is it the case so that these boilers "prefer" to see the heating load higher than their maximum turn down so that they continually fire until shut down by the room stat or whatever as in the above case the boiler had modulated right down and was only outputting a kw or two higher than the heating demand and why should the over temp have reached 75C if the normal cut out is SP+5C, in this case, 60+5, 65C. I have seen my relations Vokera vision 20S ( and not range rated) when servicing only a 3 rad zone bring the boiler temp up to its setpoint of 65C in less than 5 minutes with only a 2C overshoot and then slowly reaching its cut out at 70C, it then cut in again at 60C. I did hear it cycle a few times after that but didn't notice any particular long shutdown periods and the rooms were up to temp in less than 2 hours. In the case above I was told that the room stat still wasn't satisfied after nearly 4 hours due to very short period of total firing time.

 

[RoryD]

As Shaun says, you can turn down the heat output (on a cdi you can do this in the settings)

 

[John.g]

Yes, I have suggested that on Shaun's advice., I think the total installed rad output is ~ 20 kw. I am finding it hard to figure out why one should de rate if say one needs a very fast heat up on a cold morning but later on one only needs say 5kw, Obviously, if the boiler max turndown is > 5kw the boiler will start cycling but apart from a bit more wear and tare, what is actually wrong with this? , Chuck gave a detailed post but if my acquaintance,s experiences are correct, the boiler firing time seems to get less and less despite the heating demand staying constant and the well insulated rooms take a age to heat up so why should he have to de rate by say 50% or more to get the rooms to heat up faster?, As I said above it would seem that unless the heat demand is > than the max turn down then it has a detrimental effect on heat up time.

 

[Chuck]

Chuck gave a detailed post but if my acquaintance,s experiences are correct, the boiler firing time seems to get less and less despite the heating demand staying constant and the well insulated rooms take a age to heat up

By 'demand staying constant' do you mean the blue light on the boiler is on permanently? What type of room thermostat does the system have? Where is it positioned?

 

[John.g]

He has 4 rooms on this zone with two rads in a combined dining/lounge, these two rads have no TRvs and the room stat is located in these combined rooms, the other 3 rads have TRvs but he has them opened fully and even removed the heads to ensure the pins weren't stuck., he has the old type room stat but even when he turns this up to maximum he said it still nearly takes 4 hrs to get the room(s) temperature up to 22C (from ~ 15C), don,t know about the blue light, if he has the other upstairs zone on then the boiler rarely cycles and he says that the downstairs will then heat up in ~ 2 hrs. I'm assuming "constant demand" as being the heat output from these 5 rads since the room stat isn't satisfied. He definitely is of the opinion (rightly or wrongly) that its the boiler cycling that is causing this slow heat up. I will ask him to repeat this with the upstairs zone valve opened only. I'm living over 200 miles from him but will probably be able to have a general look early next year and may well find that it is something far more obviously wrong.

 

[John.g]

Unless Bosch has changed their sofware recently the algorithm the boiler uses is to sleep (anti-cycling mode) for five minutes if two on-off cycles occur within the same five minute period. Typically, what happens is that the burner comes on and the temperature ramps up, there is insufficient circulation rate and the water temp rises to ca 75°C when the over-temp protection cuts the burners (first on-off event) the water termperature then falls to 60°C. At that point, the burner comes on again and the water temperature starts rising. When it reaches 75°C it cuts out for the second time in five minutes so the boiler goes to sleep for five minutes. During these five minutes the water temperature drops to perhaps 45°C. After 5 minutes the boiler wakes up and the whole sequence starts again.

Best performance without modifying plumbing or boiler will be achieved by setting all the TRVs to maximum and balancing the system correctly so that is is entirely controlled by a single thermostat. Worcester Bosch may be willing to give the installer some customised boiler settings, which may or may not help depending on what the laws of physics say about the system. Most people stop there as the next things to try are: install a ballast volume (to increase the heat capacity of the system water) or replace the boiler with one with a lower minimum output (larger turn-down ratio).

This behaviour is typically only real problem in autumn (i.e now) and spring because in the winter the emitters need to dump more than the minimum boiler output to keep the dwelling warm. Thanks to Building Regs requiring TRVs, installers not having time to precision balance radiators, improved insulation meaning the minimum output of many boilers is inappropriately high, etc. the behaviour your acquaintance has noticed is so common it's almost 'normal'.

Thanks for that insight, one other thing you may wish to comment on, this boiler also has a anti cycle time temp hysteresis range -0 to -30, default setting -6. which one would think would affect the anti cycle time but doesn'seem to have any effect in practice. Here is its definition from another WB site:
Anti-cycling time
(This function is also applicable to all other CH modes) When the boiler is switched OFF because the supply temperature reaches CH_Set-point + CH_Hysterese_Up, the controller will wait a period of time (Anti_Cycle_Period →180 sec. settable) before it is allowed to be switched ON again.
This function is to prevent short cycling ON and OFF of the boiler. However, when during the anti-cycle wait time the differential between setpoint and supply temperature gets greater than Anti_Cycle_T_Diff, anti-cycle will be aborted, and the boiler is allowed to start.