Mystery Worcester Bosch Combi Status Code 265.

[Peter155]

Can anyone please explain why my 12 month old WB Greenstar 28CDi Compact ErP Combi with fascia mounted Comfort ll room stat continually stalls with a 265 Status Code "Stand-by mode forced due to energy transfer limit", causing it to take twice as long to reach the set room temperature than it should. This has been a continual problem since installation in November 2021. HW has no problem and works fine.

If you can then you will be better than the installer, service engineer, and 3 WB engineers who say they have never seen this status before, and seem to be unable to fix it.

Thanks in advance.

 

[Chuck]

When the boiler is generating more heat than the water circulating in the system can remove and dump via the radiators the heat exchanger temperature rises and when it hits a preset limit shuts down the burners and waits for things to cool down before trying again.

Normally, this process results in WBs giving a message that says something like '20x Anti-cycling mode' (I forget exactly what they say but that's the gist). I've not seen your particular message myself but I speculate that it may be essentially the same root cause but instead of the trigger being flow '5°C above the set-point' it's 'HX temperature too high'.

This explanation is consistent with your observation that the HW system is working okay.

So if I were your engineer I'd want to check temperature sensor(s) for correct operation but once they're ruled out I'd expect the cause to be in this list: an ABV that is set incorrectly or faulty, inappropriate pump setting, undersized radiators, restricted circulation due to TRVs, lock shields set incorrectly, sludge, crushed section of pipe, incorrect pipework with a 'short circuit' between flow and return, etc.

If you post some more details about what the boiler flow temperature is doing before, during and after the boiler 'stalls' it may be possible to be more specific.

 

[Peter155]

When the boiler is generating more heat than the water circulating in the system can remove and dump via the radiators the heat exchanger temperature rises and when it hits a preset limit shuts down the burners and waits for things to cool down before trying again.

Normally, WBs give a message that says something like '20x Anti-cycling mode' (I forget exactly what they say but that's the gist). I've not seen your particular message myself but I speculate that it may be essentially the same thing but instead of the trigger being flow '5°C above the set-point' it's 'HX temperature too high'.

This explanation is consistent with your observation that the HW system is working okay.

So if I were your engineer I'd want to check temperature sensor(s) for correct operation but once they're ruled out I'd expect the cause to be in this list: an ABV that is set incorrectly or faulty, inappropriate pump setting, undersized radiators, restricted circulation due to TRVs, lock shields set incorrectly, sludge, crushed section of pipe, incorrect pipework with a 'short circuit' between flow and return, etc.

If you post some more details about what the boiler flow temperature is doing before, during and after the boiler 'stalls' it may be possible to be more specific.

Hi and thanks for that. Yes, it is certainly an issue caused by the return temperature being too high and triggering the safety/cycling shut down mechanism. There are no actual error messages being returned on the screen, but if I press the Info button I get the 265 Status Code which I can then see in the manual.

The piping is mostly well over 35 years old (I bought the house in 1987), but I have always treated it with Fernox in my time, and it was given a good flush before this boiler was installed, but I don't know how well it is designed and what issues there may be hidden in there. I previously ran a similar power non-condensing WB combi (which I appreciate would be far less fussy) on this pipework for over 20 years with no obvious problems. All the rads, 9 in total, and valves have been replaced with modern Stelrads, some doubles some singles, which my installer says are more than adequate for a medium size semi. TRVs are Drayton 4s.

By trial and error I have discovered today that a constant higher pressure pump setting, rather than the "variable" setting, improves things when it is heating up from a large room temperature difference, but it starts cutting out again when it nears the target room stat temperature, and when trying to maintain it afterwards. I am however concerned that this is probably not the most economical setting for running the pump.

I also noticed today that when set-up with the constant higher pump pressure it heats the flow by 2 or 3 degrees above the set temperature, and then the Status Code (no Error Message) changes from 200 "System being heated" to 204 "System waiting, primary flow temperature above setpoint" and the pump keeps going rather than stopping, as I would expect, rather than a full stop. I am guessing this is because the Energy Transfer Limit has not been exceeded as in the 265 Status. After it cools for a bit it returns to 200 Status and carries on heating.

Regarding the Flow and Return temperatures, it doesn't seem to matter where the flow temperature is set, it always seems to stall with the 265 code when the difference between the two reduces to about 4 degrees C. This is what appears to be meant by the Energy Transfer Limit.

The WB engineers replaced the PCB and one of the sensors on their first visit (I think it may have been the Maximum Safety Sensor), which didn't make any difference. I have also tried reducing the heat in the system by turning the 24kw power output right down to the minimum of 7kw and running the pump at the minimum 60% with a flow of 50 degrees C, with all the TRVs and lockshields fully open, and although this did allow it to run for longer in between stalls, as you might expect it took even longer to get up to the set room temperature, and still kept cutting out, so not very satisfactory.

What is an ABV please?

Peter

 

[John.g]

A ABV is a Automatic Bypass Valve, this is installed between the boiler flow & return pipework to ensure the required minimum flowrate when zones shut down or/and rad thermostatic valves start throttling down.
Some boilers also have a internal bypass valve.

Your problem is still happening with 9 rads on?, the combined output of these should be way above the minimum output of the boiler at 7kw.
How long does the boiler run for before cutting out?.
Suggest increasing the flow temp to 70C, even if only temporarily, a flow temp of 50C means that the rad output(s) are less than 50% of their rated output.

ABV

 

[Chuck]

Regarding the Flow and Return temperatures, it doesn't seem to matter where the flow temperature is set, it always seems to stall with the 265 code when the difference between the two reduces to about 4 degrees C. This is what appears to be meant by the Energy Transfer Limit.

Broadly speaking, the heat exchanger in a modern boiler is designed to operate with a difference of ca 20°C between flow and return so it's not surprising that the boiler starts to get upset when this falls to 4°C.

This small difference between flow and return suggests that water is returning to the boiler too rapidly and not having a chance to dump a decent fraction of its thermal energy as heat into your house. A combination of a powerful pump and "all the TRVs and lock-shields fully open" would be one recipe for this, an open manual-bypass would be another.

I would start by setting the pump to a medium or low constant pressure setting and rough-set the external ABV by tweaking it with all TRVs but one TRVs closed. Next, open all the TRV's fully. Then go round and set each lock-shield so it is "1 hour" (i.e. 30°) more open than the point where you can hear turbulence onset (i.e. a hissing noise starts). Having done this once, go round again as there will be some interaction between radiators. This should have the system close enough to balanced for it to be working in a fairly normal manner while you go round and check the flow and return temperatures at the boiler and each radiator. If all is well the radiators will all have roughly the same flow temperature, and a return temperature about 10°C below that. The flow temperature at the boiler should be a few degrees higher than the values at the radiators and the boiler-return a few degrees lower.

If all is well, you can then proceed to finesse the balance to get deltaT at each radiator to at least 10°C. If something is not right then one needs to diagnose the cause. For example, if the boiler flow and return are very similar and significantly above the temperatures at the radiators you might have a 'manual bypass valve' somewhere. These these were commonly installed until, IIRC, the 1980s and might be hidden behind or under more recently fitted cupboards, etc.

 

[Chuck]

Keep in mind that systems with modern boilers and TRVs everywhere do tend to cycle the boiler, particularly in the spring and autumn. TBH, it doesn't seem do the boiler much harm and I've not come across any quantitative evidence that it wastes a significant amount of gas.

If you have some patience and a some thermometers, you can achieve improvements by setting up the thermostat that controls the boiler and the TRV's very carefully. The aim is to end up with a system where the main thermostat shuts down the boiler before a majority of the TRVs close. This means you need to balance the radiator system using the lock-shields so that the rooms are receiving the right amount of heat with the TRVs fully open. The TRVs are then set so that they are normally open but reduce the heating in individual rooms to compensate for other sources of heat, e.g. solar gain.

Getting this sort of balance just right is time-consuming and also results in a system that raises the room temperatures relatively slowly, which may not be acceptable if the house is unoccupied during the day. If you want a CH system that can swing the rooms from freezing to toasty in 15 minutes flat then you need high flow and return temperatures, which reduces efficiency, and to accept boiler cycling in steady-state as a side-effect, which reduces comfort. You can reduce cycling by installing weather compensation and/or a buffer tank but these are relatively costly solutions and not every house has enough space for a buffer tank.

 

[Peter155]

A ABV is a Automatic Bypass Valve, this is installed between the boiler flow & return pipework to ensure the required minimum flowrate when zones shut down or/and rad thermostatic valves start throttling down.
Some boilers also have a internal bypass valve.

Your problem is still happening with 9 rads on?, the combined output of these should be way above the minimum output of the boiler at 7kw.
How long does the boiler run for before cutting out?.
Suggest increasing the flow temp to 70C, even if only temporarily, a flow temp of 50C means that the rad output(s) are less than 50% of their rated output.

ABV
View attachment 80312

Hi John. Thanks for that. As far as I can see I don't have an external ABV, only a shock preventer and a magnetic filter, but it looks from the parts list that the WB has an internal one, which I would imagine would not be adjustable.

Regarding the running times it depends very much on the starting temperature and the point in the cycle. If starting up on a cold morning from an overnight (system off on timer) drop to 17 degrees for example it can take anything up to 2 hours to get to 22 on the room stat, which to my mind seems too long.

It runs well to be begin with, presumable because it is able to shed more heat into the colder house, before getting nearer the target room temp and going into the 265 Code.

During this first stage it sometimes goes into Status 204 (system waiting, primary flow temperature exceeded) or 202/305 (anti rapid cycle mode) or very occasionally 358 (three way valve kick), but it recovers from those much quicker than it does from 265 (presumable because there is less excess heat to shed?).

After stalling at the 265 stage for the first time and sitting idle for 10 minutes or so, the Status Code of 265 then flips back and forth with Status 201 (system being heated), with a 10 minute wait every time, before eventually reaching the room target temp and Status 203 (system stand by).

I have tried setting the flow temp at 70 as you suggested, but it never gets beyond the low 50s before the burner goes out and one of the above codes cuts in.

I shall continue experimenting with various combinations of settings over the weekend, but what heating power do you think I should be using please? The full 24kW or something lower, down to the minimum setting available of 7kW?

Thanks. Peter

 

[Peter155]

Keep in mind that systems with modern boilers and TRVs everywhere do tend to cycle the boiler, particularly in the spring and autumn. TBH, it doesn't seem do the boiler much harm and I've not come across any quantitative evidence that it wastes a significant amount of gas.

If you have some patience and a some thermometers, you can achieve improvements by setting up the thermostat that controls the boiler and the TRV's very carefully. The aim is to end up with a system where the main thermostat shuts down the boiler before a majority of the TRVs close. This means you need to balance the radiator system using the lock-shields so that the rooms are receiving the right amount of heat with the TRVs fully open. The TRVs are then set so that they are normally open but reduce the heating in individual rooms to compensate for other sources of heat, e.g. solar gain.

Getting this sort of balance just right is time-consuming and also results in a system that raises the room temperatures relatively slowly, which may not be acceptable if the house is unoccupied during the day. If you want a CH system that can swing the rooms from freezing to toasty in 15 minutes flat then you need high flow and return temperatures, which reduces efficiency, and to accept boiler cycling in steady-state as a side-effect, which reduces comfort. You can reduce cycling by installing weather compensation and/or a buffer tank but these are relatively costly solutions and not every house has enough space for a buffer tank.

Thanks Chuck, it's good to know that this may not be damaging the hardware too much and hopefully not wasting too much gas. Thanks for the suggestion on how to roughly balance the rads, I have always wondered how that is done. As I said to John above, I don't think I have an external ABV, but it looks as if the WB has an internal one.

OK, I'll have a go over the weekend of balancing it all up as you suggest, but what heating power and flow temperature would you suggest I use please. I have a range from 24kW down to 7kW heating power, and max flow temp of 82.

Thanks. Peter.

 

[John.g]

Strange that the boiler only gets to the low 50s and brings up these codes etc with a 70C SP. What was the return temperature??
Wonder if the internal bypass was checked for correct operation.
Does the boiler run OK on DHW?
Some boilers also have another sensor somewhere near the combustion chamber that monitors the temperature rise but don,t know if yours has.


Yes, maybe just reduce the CH output to say 10kw for starters and see what happens, it doesn't limit the DHW output.

 

[John.g]

Also monitor the flow/return temps if you do reduce to 10kw or whatever and the flowrate can be calculated easily then.

 

[Peter155]

Strange that the boiler only gets to the low 50s and brings up these codes etc with a 70C SP. What was the return temperature??
Wonder if the internal bypass was checked for correct operation.
Does the boiler run OK on DHW?
Some boilers also have another sensor somewhere near the combustion chamber that monitors the temperature rise but don,t know if yours has.


Yes, maybe just reduce the CH output to say 10kw for starters and see what happens, it doesn't limit the DHW output.

Thanks John

It doesn't seem to matter what I set the flow temp at, it always seems to cut to the 265 Status once the return gets within 4 degrees of the flow temp. This point takes longer to reach when the starting room temp is lower or the room stat is set higher, but once the target room temperture has been achieved, and it is only maintaining it, it only burns for a few minutes before cutting out again with the 265, with a flow temp of only 30 to 40, irrespective of the SP.

The WB seems to have two heating sensors. From the manual, one is in the main heat exchanger and gives a reading in real time (as does the return sensor) and the other is called the Maximum Safety Sensor and is mounted on the front of the heat exchanger. These two seem to match each other as the flow temp goes up and down.

Yes the HW side is fine and hits the 60c max very quickly if I let it.

 

[Peter155]

Also monitor the flow/return temps if you do reduce to 10kw or whatever and the flowrate can be calculated easily then.

Also monitor the flow/return temps if you do reduce to 10kw or whatever and the flowrate can be calculated easily then.

I've already been doing that with various power settings between 7 and 24, and while it obviously takes longer for the rads to heat up at the lower setting it still hits the 265 cut out when there is the 4 degree difference between the two. Its very frustrating.

 

[John.g]

265 is flagged when the energy transfer limit is reached, so a 4C dT would require a huge flowrate, if the "normal" max flowrate is say 20LPM then that will only support 5.6kw at a dT of 4C!!.

Can you see what the pump setting is.

 

[John.g]

I've already been doing that with various power settings between 7 and 24, and while it obviously takes longer for the rads to heat up at the lower setting it still hits the 265 cut out when there is the 4 degree difference between the two. Its very frustrating.

Can you monitor the flow/return temps with a heat gun where the flow/return pipes enter/exit the boiler, or even just feel them,if all rads on then very difficult to see a rad dT of only 4C, its quite difficult normally to get rad dTs below 8/10C no matter how much water is forced through them.

What is the boiler dT while the rads are heating up, early in the cycle on reduced boiler output?.

 

[Chuck]

As I said to John above, I don't think I have an external ABV, but it looks as if the WB has an internal one.

The installation instructions I'm looking at do say:

"The boiler is equipped with an internal by-pass. The internal bypass is not intended to be a substitute for an external system bypass.
An external automatic bypass should be used if the system flow can be significantly adjusted or stopped by zone valves and thermostatic radiator valves (TRV)."

Puzzlingly, there is no external zone valve shown on the "Sealed System with Zone valves" (fig 4) but, based on simple physics, I suspect that it's the figure that is incorrect.

External ABVs need to be fitted so that they create a circulation loop large enough for the heat capacity of the water to prevent the HX overheating. This typically requires a round-trip of ca 10m of 22m pipework.

Given the problems you are experiencing, If you really don't have an ABV fitted, I'd want to double-check this is okay with WB Tech support 'cos it don't seem right to me.

 

[Peter155]

Can you monitor the flow/return temps with a heat gun where the flow/return pipes enter/exit the boiler, or even just feel them,if all rads on then very difficult to see a rad dT of only 4C, its quite difficult normally to get rad dTs below 8/10C no matter how much water is forced through them.

What is the boiler dT while the rads are heating up, early in the cycle on reduced boiler output?.

Yes, I've tried various combinations using those different pump settings. No, I don't have any measuring equipment other than a standard room thermometer. I am just using the flow and return temperatures from the boiler readout to get my 4 degrees.

 

[John.g]

but once the target room temperture has been achieved, and it is only maintaining it, it only burns for a few minutes before cutting out again with the 265, with a flow temp of only 30 to 40, irrespective of the SP.

Have you got TPI roomstat(s).

 

[Peter155]

The installation instructions I'm looking at do say:

"The boiler is equipped with an internal by-pass. The internal bypass is not intended to be a substitute for an external system bypass.
An external automatic bypass should be used if the system flow can be significantly adjusted or stopped by zone valves and thermostatic radiator valves (TRV)."

Puzzlingly, there is no external zone valve shown on the "Sealed System with Zone valves" (fig 4) but, based on simple physics, I suspect that it's the figure that is incorrect.

External ABVs need to be fitted so that they create a circulation loop large enough for the heat capacity of the water to prevent the HX overheating. This typically requires a round-trip of ca 10m of 22m pipework.

Given the problems you are experiencing, If you really don't have an ABV fitted, I'd want to double-check this is okay with WB Tech support 'cos it don't seem right to me.

OK, thanks. I'd best check with my installer to see if he did fit an external bypass and I just can't see it.

My plan is to go back to WB if I can't sort this whole problem out via the forum.

 

[Peter155]

Have you got TPI roomstat(s).

Not sure, it's the Worcester Comfort ll, allegedly a very clever piece of kit (and certainly not cheap)...

 

[John.g]

So no problems really until the roomstat SP reached?, its when the stat tries to maintain this SP by firing the boiler on/off that the problems begin?

 

[Peter155]

So no problems really until the roomstat SP reached?, its when the stat tries to maintain this SP by firing the boiler on/off that the problems begin?

Sorry, had to go shopping.

No that's not correct, sorry if I'm not being clear. If its still trying to raise the room temp first thing in the morning and the room stat is still calling and it cuts off with a 265 several times towards the end, then it significantly slows down the reaching of the SP on the room stat. I suspect it also costs more gas reheating the flow temperature every 10 minutes or so.

If it has already reached the room stat SP and is trying to maintain it, then more often than not the 265 has been replaced by 203 (system standby) so it fires up straight away, only to crash again with 265 after a few minutes before the room stat has returned to the SP.

 

[ShaunCorbs]

If you remove your trv heads as a test does it still do the fault ?

 

[Peter155]

If you remove your trv heads as a test does it still do the fault ?

Not tried that, but they were all new when the boiler was fitted 12 m ago. Drayton 4s. Pretty sure they all work OK.

 

[ShaunCorbs]

Worth a try incase there shutting down

 

[Peter155]

Worth a try incase there shutting down

OK, thanks Shaun. I'll give it a try 👍