Worcester Boiler switching off(cycling) when ONLY heating HW

[theSouth]

I've noticed my boiler (Worcester 40CDi condensing natural gas) is cycling a lot of the time. It's the usual (usual because Google is full of these complaints) getting up to around 40-50 and then switching off with the thermal cut-out symbol.

Sometimes its OK, but most of the time it isn't.

The pump is recently changed so I'm confident it's not that. I suspect it's the flow from/to the boiler and the heated water therefore can't get away quick enough, thus boiler switches off.

Last night I tired turning on the CH value and - hey presto - the boiler liked that and all worked OK. Apart from a really hot house! The HWC heated up within 20 mins as usual.

So (you guess, I'm not a plumber) I'm inclined to think there is an issue with the HW circuit. It's an S-plan system. The pipework is 22mm plastic, apart from near the boiler. The HWC is about 6 or 7 years old, so my suspects thus far are:

- HW motorised valve no opening enough, restricting flow when the CH is off
- HWC clogged up with crap (i.e. the copper within it not allowing enough flow
- Plastic pipe to/from boiler clogged up with crap, again restricting flow

Am I on the right track?

 

[jeff the gasman]

By pass letting too much through.
Balancing valve not open enough.
Cylinder stat set higher than boiler stat
Or as you said.
Jeff

 

[theSouth]

thanks Jeff for the suggestions

By pass letting too much through:

sorry, I should have mentioned in the first post that I have been monitoring the temperature of the return at the boiler and it never got very hot before the boiler threw the error code and turned off.

the flow was, obviously, very hot (the boiler is set to max, though I've tried lower and no difference). so I think I can discount the by pass letting too much through.

if it was the by pass, wouldn't the boiler just turn off "normally" ?

i.e. the boiler believing the required heat of water was achieved and therefore could switch off. in my case I get the weird Worcester error that no-one can seem to explain in ASCII text

it's the square box with two corners chopped off - only the top left and bottom right; this symbol flashes alternatively with the current temperature. I think this means that the boiler has detected an overheat (or heating up too quickly) and this is due to inadequate flow.

Cylinder stat set higher than boiler stat:

again, if the cylinder stat was set higher than the boiler stat the boiler would again turn off normally, wouldn't it? the pump doesn't switch off, the boiler has the pump overrun feature and this is when the pump turns off (couple of minutes after the boiler switches off normally)

if the cylinder stat was higher than the boiler (which it isn't) then I guess the stat would cause the boiler to switch off normally and after the pump overrun the pump would switch off.

basically, the boiler is switching off abnormally - and I think it's because there is too little flow, not too much flow.

Balancing valve not open enough.

I have not looked at this....

 

[St0rmer66]

It's not an overheat as such but the temperature rising too fast or inadequate flow. Often a problem with the 40CDi (never had it on anything smaller, personally) because of being fitted on a system which either doesn't need 40kw or isn't piped correctly to make proper use of it; i.e. 22mm primary flow and return. With plastic pipes and inserts the internal diameter of the pipe is restricted yet further . A bigger pump can help (I've fitted a Grundfos 25-80 on one before which already had 28mm copper pipes) but a "Low Loss Header" is another option. If your primary flow and return are 22mm then a bigger pump is more of a "bodge" option and the speed of the water through the pipes will make it noisy.

 

[theSouth]

Thanks St0rmer66

You say temp rising too fast OR inadequate flow - isn't one caused by the other, or could the temp rise too fast for another reason?

I'm basing my investigations on a low flow rate at the minute; I tried again tonight with the motorised valve for the CH off and the HW on - same problem.

Then I manually opened the valve for the CH (so both open now) and within a couple of cycles the boiler sorted itself out - sort of. It wasn't perfect, it still did the cycling a few times. But the water was heated fairly quickly, just not in the "20 minutes" that is used to reheat the cylinder.

Is there any easy way for me to confirm 100% what the flow is around the HW circuit? This would tell me definitively that the problem is low flow, as opposed to, say, a boiler problem.

I've looked for flow meters but they seem quite expensive. the non-intrusive sort cost hundreds and hundreds.

I really like to find out what the problem is, then I can begin to fix it - whether that is a new HWC or pipework or whatever. But due to the intermittent nature of this problem, I haven't quite nailed the exact scenario in order to reproduce the fault accurately

 

[St0rmer66]

Yes, the two are very closely related. The temperature rise 'could' be the temperature sensor being faulty but if your works fine on CH then it's unlikely to be that and I'd assume a flow problem too.

Confirming actual flow is difficult but it can be done indirectly by measuring flow and return temperatures. If you've got 22mm pipes from a 40kw boiler though then you're starting from a bad point! 24kw is about the limit for 22mm as a rule of thumb. 28-30kw would work 'okay' and cycle a bit more than it idealy should. 40kw is ridiculous. Out of interest how many radiators have you got and how many bedrooms is the house? Just to give a rough idea of whether that size boiler was ever really needed.

 

[theSouth]

22 rads + 3 towel rails. 6 bed house.

It's a long story why I've ended up with a 40CDi.....

So, if 22mm is not enough, how come when the CH is on there is less of a problem? It's still the same 22 to the pump and 22 return from HWC and rad circuit. I'm not disagreeing with you, just trying to understand why having the CH seems to make a difference. I can only assume it reduces the drag/resistance by the flow being able to pass throw both HW and CH circuits ? That's been my theory up till now.

I'm able to swap out to 28mm if I need to, and all copper if required. Especially as some floor coverings have still to be put down. (We've extended recently)

Thanks for taking the time to help.

 

[Agile]

WE don't seem to have been told anything about the HW cylinder.

The heating coil could be well sludged up!

The boiler is about twice the maximum power the house could need if it was badly insulated.

Depending on the cylinder the boiler is probably four times over sized too. I would expect it to be cycling but with a normal differential on the boiler flow and return.

Tony

 

[theSouth]

Hi Agile

Ask and ye shall be told

The cylinder is a standard indirect cylinder, I think 900x450 (just over 100L I think) if I remember. It wasn't upgrade as it always provided enough water and still does, even though we have 3 baths (we don't all bathe at the same time )

So, let's say the rads plus 2k for the cylinder add up to around 40k, what is meant to happen when the CH is off and only the HW is on? Meaning that full 40k potential is focussed on the HW circuit only. I would expect this means, simplistically, that the water is heated up more quickly - we are used to that tank heating up in 20 mins, no more.

Obviously, it should be that the flow through the cylinder should be adequate to ensure the boiler doesn't heat up too quickly and thus cause the problems I am seeing (and BTW, plenty of others around the country are seeing judging from a simple Google search lots of people are asking this question).

The question I'd like to ask is, how is this 'supposed to be' achieved? Through a cylinder with larger coil?? or is that considered irrelevant if the flow and returns are adequate - which I concede they may not be any more in my set-up.

Or, since you are asking about the cylinder- are there different types of cylinder that need to be matched to a larger boiler? This I haven't heard before.

 

[Agile]

All I now know is the outside dimensions!

There are different types.

For a simple how they look there is bare copper, yellowey brown foam and the latest green or blue foam.

The size of the heating coil varies on those three basic types!

Do you know the actual flow and return temps when the boiler is firing?

Tony

 

[theSouth]

ermm...well you know the capacity, surely that's important?

it's blue. it was fitted in 2005/6 if I remember correctly.

it has one copper coil rather then two.

it has an immersion hole at the top with an immersion element fitted, but we don't use it.



I'm not sure what you are asking? so if I've not answered it above, and you want to help me, which I would appreciate, then just let me know what information I've missed.

surely all of these types of DIY discussions never provide ALL of the information at the beginning, so please bear with me if you do want to help.

cheers

 

[Agile]

The volume is irrelevant in this case.

What is relevant is that the heating coil is capable of absorbing about 8-10 kW when the cylinder is cold.

That means that the boiler still cannot modulate down to the power taken by the coil ( because the boiler is oversized ) and will be expected to cycle.

But as the cylinder is fairly new then it would not be expected to be scaled up and depending on the flow rate the temperature differential should be in the region of 6 C to 12 C but we have not been advised of that.

All the indications are that there is some kind of blockage or restriction. But real temperatures are needed to get some confirmation of this likelyhood.

Tony

 

[theSouth]

I was able to measure the boiler using an infra-red thermometer I used to balance the rads; I think it's pretty accurate and if not at least it's the same device providing all readings.

boiler cold, first cycle tonight:

boiler 38 (boiler cuts out when it gets to 38 )
flow 36
return 25

second cycle:

boiler 36
flow 26 (yes, 26)
return 25

it takes less then I would say 15-20 seconds for the boiler to initially cut out (i.e. when it reached 38 on the boiler display with the flow just below at 36 on the digital thermometer and the return at 25)

I then left it cycling for 5 mins and it started to work; I took measurements of the boiler digital temp, the flow temp and the return temp:

boiler display:flow:return
59:30:26
62:32:26
65:31:26
70:32:26
72:32:26

it has now settled down at the last measurements and seems to be working ok. I have both the CH and HW values open. if I don't do this, the cycling lasts for ages before it eventually works.

does that help?

my untrained opinion would be that there is some inertia to overcome before the system can function correctly.

 

[Agile]

The IR thermometers dont give very accurate results due to the emissivity of the pipes etc. But for comparative measure like rad balancing the errors are the same so its adequate.

For accurate measurements, a contact thermocouple sensor is needed.

I am not totally sure that I understand your measurements.

Am I right that the boiler flow temp is just half of what the boiler display is showing?

The differential seems about right.

Regardless the boiler should be able to operate on just hot water although it will cycle a lot because its over powered.

Tony

 

[theSouth]

yep, that's what it said.

on the first cycle the flow temp on the infra-red matched just about the digital display. but when the boiler shut down the pump overrun brought down the temp in the flow and return (I would assume it's the pump overrun as the temp went down quite quickly in the flow)

second cycle, the outside of the boiler heating matrix showed at around 35 with infra-red (same as boiler digital display) but the flow pipe was around 26. it didn't rise at all before the boiler switched off again (it did switch off within 30 seconds though). yes, I too would expect the flow to be around the same temp as the boiler and the second cycle, but it was not.

there is one bit I really just do not understand:

let's say I have a house that requires 40kw and I have a boiler sized to meet that requirement. what is supposed to happen in the summer months when the rads don't require any heat and only hot water is being heated and, as you said in an earlier response, the cylinder can only consume, say, 10kw when cold?

i.e. if you need a large boiler because you have lots of rads required for the winter, what is supposed to happen in the summer months when you just want to heat only hot water with the same boiler?