Boiler Cycling

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Hi all,

I have an odd problem with my boiler that I am not sure how to resolve.

I have a Worcester Bosch boiler, a CDI 40 Conventional.

The boiler provides central heating (control by a thermostat in the house) and hot water. Both are also controlled by a timer system.

The house has two hot water cylinders. These have a solar heating element on a separate system that is set to a maximum temperature of 65 deg C. They also have heating elements from the boiler.

I noticed today that the boiler seems to be constantly cycling. With the central heating and hot water off (set to off with the control timer), the boiler shows a temperature of 65 degrees C and this slowly drops to 60 degrees C. When it hits 60 the boiler kicks in until the temperature is back up to 65 and water can be hear moving about the system. This cycle of heating and cooling continues every few mins.

Resetting the boiler does not change the behaviour, not does turning on the ECO mode (which I thought stopped the boiler keeping hot water ready to go).

I do not know much about boilers, but this behaviour seems to be rather odd, and I would expect the boiler to sit dormant until either the central is triggered (via the themostat / timer) or until the water is triggered (via the timer or turning on a hot tap).

Do I have an issue and is the system wasting gas????

Thanks

below are some snaps of the system. Note it says 58 degrees as the Mrs had a fiddle and turned the dial on the unit down a little..











 
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You are correct, the boiler should only operate when one of the thermostats is calling for heat. So, the boiler will be getting a live 'run' signal from somewhere. Because there are many different methods of wiring these systems up, it will require someone who understands how heating systems operate and can trace where the live is coming from to determine the cause.

If the system has always done this, that would suggest that the wiring may be incorrect.

If the fault has recently appeared, then a sticking motorised valve or microswitch somewhere may be the problem.

Do you have a frost thermostat somewhere? [Unit under the blue box photo 1 looks like it may be one, and the presence of a pipe thermostat in photo 2] if so, has it inadvertently been turned up? This would normally cause the radiators to come on, unless they all have TRV's set below the ambient temperature, or are manually turned off.

I came across one recently where the lady of the house whilst cleaning had unclipped a cylinder thermostat from the cylinder and left it lying on the floor, resulting in the hot water system not switching the boiler off. A faulty cylinder thermostat could also cause the same problem.

Has anything been changed lately? Any recent repairs / adjustments to other parts of the system?

Can you tell if the hot water leaving the boiler is actually being circulated, and if so where to. For example around the hot water cylinder(s)
 
Last edited:
Hi Stem,

Thanks for taking the time to reply.

In answer to some of your questions.



1) I am not sure if the system has always done this or not. I only moved-in in October and only recently noticed the behaviour as I was sleeping in a room next to the boiler room. It is possible that this behaviour has been going on since I moved in, but my gut feeling is that it has not been and is a recent development. The house used to be bed and breakfast, and I am not sure if they could have influenced how the boiler was set up.

2) I am not sure if we have a frost thermostat or not, I do not really know my way around heating systems (still learning). Below is a picture of the box that you thought could be a frost thermostat. Do you think it is one and does the setting look correct? As it goes from 3 to 10 I presume it is for frost..



3) With regards to has anything changed recently. The house has two water cylinders in the airing cupboard and a solar thermal installation in a loft (the boiler is on the ground floor). The solar thermal installation was recently serviced because the pressure release valve had activated. The engineer replaced the pressure release valve (from a 2 bar valve to a 6 bar valve), he also replaced the expansion vessel and recharged the system. The solar control panel was set to a maximum temperature of 50 deg C (presumably to stop guests burning themselves) and as it is now a private residence, the service engineer reset the maximum temperature to 65 deg C. My understand is however that the solar heating system is completely separate to the Worcester boiler and there only interface is in the water cylinder where there is a heating element from the solar system and a heating element from the main boiler.


4) With regards to the water from the boiler I am not sure where it is going. There may be the odd tricking noise from the cylinders,but I am not sure at this stage.

Thanks
 
You need someone with a multimeter rather than a wrench!
This is a signal problem and needs to be traced electronically to the "call" for "flame on" (Avengers Assemble reference) that is being asked of the boiler! This is not something that can be diagnosed at distance (ie: via this forum) but only hinted at! "Eyes on" and on site testing is the only way to resolve this issue expediently.
 
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[Below] is a picture of the box that you thought could be a frost thermostat. Do you think it is one and does the setting look correct? As it goes from 3 to 10 I presume it is for frost.
Yes, that's a frost stat (The model no FS1 is another clue.)

It is set to 5C, which means it will turn the boiler on if the air temperature drops below 5C - that's assuming it is wired correctly. You can check this by turning the stat down to 3C; the boiler should go off. If it doesn't go off, turn the stat up to 10. If the boiler then goes off, the stat has been wired incorrectly.

There should also be a pipe stat. This is normally set to 20C and will turn the boiler off when the water reaches this temperature.

What does the stat in pic 2 do (Its currently set to 60C.)?
 
[Below] is a picture of the box that you thought could be a frost thermostat. Do you think it is one and does the setting look correct? As it goes from 3 to 10 I presume it is for frost.
Yes, that's a frost stat (The model no FS1 is another clue.)

It is set to 5C, which means it will turn the boiler on if the air temperature drops below 5C - that's assuming it is wired correctly. You can check this by turning the stat down to 3C; the boiler should go off. If it doesn't go off, turn the stat up to 10. If the boiler then goes off, the stat has been wired incorrectly.

There should also be a pipe stat. This is normally set to 20C and will turn the boiler off when the water reaches this temperature.

What does the stat in pic 2 do (Its currently set to 60C.)?


Thanks for the info. I have no idea what yhe pipe stat set to 60 does. The boiler system is a bit of a mystery to me. Could this high setting be the problem??
 
You need someone with a multimeter rather than a wrench!
This is a signal problem and needs to be traced electronically to the "call" for "flame on" (Avengers Assemble reference) that is being asked of the boiler! This is not something that can be diagnosed at distance (ie: via this forum) but only hinted at! "Eyes on" and on site testing is the only way to resolve this issue expediently.

I do get the feeling that i will have to pay for an expert. .
 
Pipe stat should be set to 30c and fitted to boiler return.
 
I have no idea what yhe pipe stat set to 60 does. The boiler system is a bit of a mystery to me. Could this high setting be the problem??
Try turning it down to 20-30C and see what effect it has.

Is the boiler in a cold area? If not, I am surprised that the frost stat is turning the boiler on. I could understand if it was set to 10C, but 5C in the "summer"?

Have you done the test I suggested to check if the frost stat has been wired correctly?
 
Do you have a frost thermostat somewhere?

Just to clarify my earlier frost thermostat question.

The frost thermostat should have a low setting. Yours does, it is has a range of 3 to 10 degrees, its purpose is to override the rest of the system controls and turn the boiler / heating on in the event that the temperature where the boiler is located falls low enough to pose a risk of the pipes freezing. Sometimes thermostats with higher possible settings are installed and they can by accident, or by someone that doesn't understand what they are there for, turning them up overriding the other controls and causing the heating to come on. As yours is limited to 10 degrees maximum, this would not happen.

The room thermostat works in conjunction with the pipe thermostat. Because the purpose of the frost thermostat is solely to circulate warm water to prevent the pipes freezing, not to heat the property. Once the return flow to the boiler reaches 30 degrees the pipe thermostat will turn the boiler off.

the boiler will be getting a live 'run' signal from somewhere. Because there are many different methods of wiring these systems up, it will require someone who understands how heating systems operate and can trace where the live is coming from to determine the cause.

So unfortunately, it looks as further investigation will be required.
 
I have no idea what yhe pipe stat set to 60 does. The boiler system is a bit of a mystery to me. Could this high setting be the problem??
Try turning it down to 20-30C and see what effect it has.

Is the boiler in a cold area? If not, I am surprised that the frost stat is turning the boiler on. I could understand if it was set to 10C, but 5C in the "summer"?

Have you done the test I suggested to check if the frost stat has been wired correctly?


Hi again,

Looking again today, the pipe thermostat is actually set to 30 deg C not 60 deg C. The Yellow marker at the top confused me, but at the bottom there is a little arrow, which shows the setting.

The boiler is in a cold area, it is inside a separate outside boiler room, not in the main house. The boiler room is poorly insulated.

I have not done your test yet because last night I turned the boiler off completely as it was keeping me awake.

When I turned it back on today, with the central heating and the water set to "off" on the timer, the boiler showed a temperature of 23 deg C.

So after turning the boiler on I set the central heating and water to "on" on the timer.

Nothing happened.

So I ran a hot tap and the boiler remained off (clearly the solar thermal had put enough heat into the cylinders).

Next I set the room thermostat to 25 deg C (the ambient temp was 23 deg C). This made the boiler kick in and light and the displayed boiler temperature increased.

I reset the room thermostat to 20 deg C and the boiler kept running and moving water around until it showed a temperature of 38 deg C. The boiler then shut off. The temperature shown on the boiler has now dropped to 32 deg C, and I am waiting to see what now happens, i.e. if it starts cycling again but now at a new temperature...

If is does, your proposed test will be executed.

Thanks
 
Do you have a frost thermostat somewhere?

Just to clarify my earlier frost thermostat question.

The frost thermostat should have a low setting. Yours does, it is has a range of 3 to 10 degrees, its purpose is to override the rest of the system controls and turn the boiler / heating on in the event that the temperature where the boiler is located falls low enough to pose a risk of the pipes freezing. Sometimes thermostats with higher possible settings are installed and they can by accident, or by someone that doesn't understand what they are there for, turning them up overriding the other controls and causing the heating to come on. As yours is limited to 10 degrees maximum, this would not happen.

The room thermostat works in conjunction with the pipe thermostat. Because the purpose of the frost thermostat is solely to circulate warm water to prevent the pipes freezing, not to heat the property. Once the return flow to the boiler reaches 30 degrees the pipe thermostat will turn the boiler off.

the boiler will be getting a live 'run' signal from somewhere. Because there are many different methods of wiring these systems up, it will require someone who understands how heating systems operate and can trace where the live is coming from to determine the cause.

So unfortunately, it looks as further investigation will be required.


Thanks for the info. I am learning quite a lot about how the system works. Before you replied to this thread I had no idea at all!
 
So this morning the boiler is sat at 23 deg C and does no appear to be cycling. So perhaps shutting it off for 12 hours fixed the problem???
 
Continue monitoring and let us know (in this topic) what happens.

Well with the CH and Water set to off, the boiler is still sitting at 21 deg C (ambient temp) when the ECO light is set to on. So I think it may be ok!

Hopefully I can get through most of the summer with the CH and water off and all hot water provided by the Solar Thermal!
 

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