Greenstar 24ri - large temperature drop before re-fire

[Gwyndy]

Is this an old cylinder with the primary coil designed for gravity circulation from the boiler?
With 28mm primaries it would suggest so.

Yes, it is. The old Halstead boiler was gravity feed. This was commented on by the Bosch engineer who came out last Saturday but he didn't say that it was a 'no, no'. He did suggest that restricting the flow on the HW circuit might be effective.

 

[Gwyndy]

If this "Low Energy" pump is a A rated pump then at 95w minimum is going to circulate massive vols of water through a cylinder coil, just to repeat, try and get a feel for the dT, can this probe be attached to the boiler return?

Thanks for the useful comment.

My plan is: Pump speed set to the 'II' position. (This hasn't made any difference to the CH this evening.) Tomorrow I will monitor the flow temperature again and see if anything has changed. Wednesday I will monitor the return temperature and compare to the flow temp plot. This won't be precise but will give an idea.

One other thing for future examination may be the cylinder stat. These are not very precise devices - in fact the EPH stat that is fitted has something like a +/-5C switching differential. I'll worry about that another time.

 

[Gwyndy]

Something odd about today's data from the temperature logger - actual recorded figures do not look right (too low). Even so, the performance was still similar with pump speed at 'II' - large temperature drop and ~20 minutes before boiler re-fired. I will repeat tomrrow. However, HW heating shut off on the tank stat at just over one hour. May be getting somewhere.

 

[Johntheo5]

Thanks for the useful comment.

My plan is: Pump speed set to the 'II' position. (This hasn't made any difference to the CH this evening.) Tomorrow I will monitor the flow temperature again and see if anything has changed. Wednesday I will monitor the return temperature and compare to the flow temp plot. This won't be precise but will give an idea.

One other thing for future examination may be the cylinder stat. These are not very precise devices - in fact the EPH stat that is fitted has something like a +/-5C switching differential. I'll worry about that another time.

The switching differential of +/-5C is quite normal, I have installed a number of EPH stats on my familys cylinders and find them excellent, however, no harm in checking yours, note its present setting, turn it up or down until you hear a click, say it clicks when you turn it up, note the dial setting then turn it down slowly until you hear another click and again note the dial setting, there should be a max 6C/8C difference, certainly no more than 10C, return it to its original setting, normally, 60C, obviously if wired properly then it should call for boiler firing when you turn it up!!.

Looking forward to those return temps tomorrow.

 

[Gwyndy]

Looking forward to those return temps tomorrow.

May have to delay return temps until Thursday. Flow temps were definitely not right this morning - spot check showed return higher than flow! Temp probe may not have been located properly under the pipe insulation so I want to re-check tomorrow.

I have had an 'on-line' chat with WB this afternoon and have subsequently e-mailed their Tech Dept to see if they can shed any light on what is happening.

 

[Johntheo5]

Again, another basic check which should have been done is to identify the actual boiler flow and return, if the pump is on the flow pipe then it should be pumping away from the boiler, if pump on the boiler return then should be pumping into the boiler.

 

[Gwyndy]

Again, another basic check which should have been done is to identify the actual boiler flow and return, if the pump is on the flow pipe then it should be pumping away from the boiler, if pump on the boiler return then should be pumping into the boiler.

The pump is on the return and pumping in to the boiler. Flow is to the top of the HW coil and return at the bottom which is correct. The last WB engineer queried the pump location but in my (limited) experience it is quite normal. In any event, I doubt that it would have any bearing on my problem. I suspect that the issue relates to the rate of temperature fall that the boiler is seeing rather than an absolute drop (or maybe a combination). As it is a very short circuit with large(ish) diameter pipework I am guessing that the return temperature will rapidly equalise to the flow and the boiler will stop firing. How that then translates to boiler re-igniting remains a mystery!

The only other component in the boiler that could influence what is going on would be the over-temp sensor and, I think, if that kicked in it would show as a fault - i.e. flashing LED on the boiler.

 

[Johntheo5]

The boiler runs continuously for ~ 20 minutes while heating a cold cylinder and then cuts out, what generally happens is when the heat absorbed by the heating coil is less than the minimum boiler output, around 6kw for your boiler, then the flow temperature rises to the SP Temp+5C and the burner cuts out, dont know if this is happening in your case but anyway the boiler should refire in a much shorter time as you pointed out, ~ 10C cylinder coil flow/return dT is a good starting point, but some high recovery coils run at tremendous circ flow rates with 80C flowtemps so coil dTs of ~ 5/8C.

 

[Harry Bloomfield]

Flow temps were definitely not right this morning - spot check showed return higher than flow! Temp probe may not have been located properly under the pipe insulation so I want to re-check tomorrow.

Check the flow direction is correct. From cold, feel the two pipes, the flow should become warm much earlier than the return.

 

[Gasguru]

You shouldn't need to run the pump on speed 3 especially since the boiler is designed to run with a more favourable 15 to 20 degree drop.

Since the the heating side appears to function as expected (although it is only really tested when outside temperatures are down to zero) I wonder if the issue might be due to the single (or maybe 2) coils of 28mm pipe forming the cylinder coil (if it is indeed a gravity coil).

It wouldn't surprise me if the boiler software runs a PID algorithm and sees the rate of temperature drop so low that it gets confused.
There's no sense in firing up if the heat can't be dissipated without the burner rapidly ramping back down and cycling off again. We've seen similar odd behavior with other boiler makes/models when the system is not optimal.

Given the condensing nature, expected 20 degree deltas and a ton of eco regs. to contend with I can't see the engineers designing the control loops with old cylinders in mind (and many would not even be aware they exist).

 

[Gwyndy]

Check the flow direction is correct. From cold, feel the two pipes, the flow should become warm much earlier than the return.

Did that earlier today just to double check.

 

[Gwyndy]

The boiler runs continuously for ~ 20 minutes while heating a cold cylinder and then cuts out, what generally happens is when the heat absorbed by the heating coil is less than the minimum boiler output, around 6kw for your boiler, then the flow temperature rises to the SP Temp+5C and the burner cuts out, dont know if this is happening in your case but anyway the boiler should refire in a much shorter time as you pointed out, ~ 10C cylinder coil flow/return dT is a good starting point, but some high recovery coils run at tremendous circ flow rates with 80C flowtemps so coil dTs of ~ 5/8C.

When I do measure / log the return temp I am expecting to see it equalise with the flow after that initial heating period of ~30 minutes followed by a very slow fall. Yes, I agree, there should not be a reduction of 18C before re-ignition.

 

[Gwyndy]

It wouldn't surprise me if the boiler software runs a PID algorithm and sees the rate of temperature drop so low that it gets confused.

I think that you are probably right. Much of my working years were spent in the technical ceramics industry where precise temperature control of our kilns was imperative. I well remember the vagaries of PID control although after many years of retirement I've forgotten most of it!

I'm heading towards putting this down as a 'useful exercise' and just accepting that I do have hot water and that without changing the cylinder I'm perhaps never going to solve this conundrum. I'll wait and see if WB technical people come back with anything interesting.

By the way, the pump is now down to speed 'II' and will stay there.

Thank you all for your interest and suggestions. I'll keep you posted.

 

[DP]

Yes, it is. The old Halstead boiler was gravity feed. This was commented on by the Bosch engineer who came out last Saturday but he didn't say that it was a 'no, no'. He did suggest that restricting the flow on the HW circuit might be effective.

90 minutes to heat water is mental.
So is someone attending and replacing the pcb without foundation and bolting- sad

Is the cylinder insulated with a red jacket? That cylinder is not suitable for fully pumped primaries. Look at typical cylinder spec- reheat times are quite short. For gravity circulation, water circulates through the indirect coil with convection currents ( no pump used ). A cylinder needing pumped circulation will not function without a pump. Seems your issue is the opposite- heat transfer to water in cylinder too slow for the boiler- like a racehorse and snail doing 100 yards

Temperature differential across the flow and return should be 20 degrees. Short 28mm coil will really need to be choked mightily with balancing valve to achieve the required differential but reheat time will be hours then. What is the position of the cylinder thermostat on the cylinder. What is the temperature of water at the hot tap when boiler is doing 20 minute recycle

Microcontroller are go to components that control boiler operation these days. That boiler is doing its job. Maximum pump speed during HW heatup will result in minimal temperature drop across flow and return.

 

[Gwyndy]

90 minutes to heat water is mental.
So is someone attending and replacing the pcb without foundation and bolting- sad

Totally bonkers! And yes, the engineer who replaced the PCB was in a rush to get off so didn't wait to monitor a full HW cycle. Second engineer spent more time looking at the system and relaced the thermistor 'just in case'.

Is the cylinder insulated with a red jacket? That cylinder is not suitable for fully pumped primaries.

Cylinder is fully insulated (as supplied) so minimal heat loss. Rapidly coming to the conclusion from everyone's comments that the cylinder should have been changed 3 years ago when the new boiler was fitted. It did not occur to me that it was necessary and neither of the local (and reputable) companies who quoted mentioned it.

What is the position of the cylinder thermostat on the cylinder.

Total cyclinder height approx 42" - mid-point of thermostat 12" from bottom and 6" above return connection.

heat transfer to water in cylinder too slow for the boiler- like a racehorse and snail doing 100 yards

Great analogy and undoubtedly correct.

What is the temperature of water at the hot tap when boiler is doing 20 minute recycle

Don't know after 20 minutes but after shower and washing up this morning I was still getting HW at 60C+.

I'll continue to run some checks and I'll wait to see if WB Technical Dept come back with any comments and then I will challenge the installer. They were told about the issue within a few days of completing the installation but their solutuion was just to increase the boiler thermostat setting.

Thanks again everyone.