Greenstar 24ri - large temperature drop before re-fire

[Johntheo5]

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'.

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.

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

Great analogy and undoubtedly correct.

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.

42" high cylinder is ~ 150L??, I really can't see why that cylinder should be thrown out, why not simply install a gate valve on the cylinder coil and throttle it, this has to increase the coil dT and the boiler may cut back in "normally", heating say 120L? from 18C to 60C in 20minutes means a average coil/boiler output of 17.6kw.

 

[Gwyndy]

why not simply install a gate valve on the cylinder coil

That was a suggestion from the last WB engineer. Something definitely worth looking at. But @DP said in an earlier post ......

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.

....... so do you think that it will work? And flow or return? (Pump is on the return.)

 

[DP]

I believe or was lead to believe pumps used to be on the return to avoid pump getting subjected to ma I mum temperature.

so do you think that it will work? And flow or return? (Pump is on the return.)

Worth a try. You hot water heatup time will increase dramatically

 

[DP]

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.

Seems they did not take into account that cylinder not suitable for pumped primaries
Cylinder fully insulated in the factory would have caught out the unsuspecting installer but then the programmer should have raised a r3d flag as ch switch on would also activate hot water ( unless of course the programmer was replaced too when new boiler fitted)

 

[Gwyndy]

Seems they did not take into account that cylinder not suitable for pumped primaries
Cylinder fully insulated in the factory would have caught out the unsuspecting installer but then the programmer should have raised a r3d flag as ch switch on would also activate hot water ( unless of course the programmer was replaced too when new boiler fitted)

All re-wired and zone valves on HW and CH circuits so no chance of water heating unless demanded by time clock.

 

[Johntheo5]

Why not just run that return temperature test on the data logger?, that should help to determine which route to go.

 

[DP]

Why not just run that return temperature test on the data logger?, that should help to determine which route to go.

In agreement here.
Anticycling timer in the boiler is there to stop rapid firing of burner
I think what is happening in this case is, boiler fires, reaches terminal temperature, temperature at boiler output overshoots. Burner shuts down but pump keeps running.
Boiler controls are looking for flow temperature to drop during the anti cycling timeout but that is not happening because now the cylinder is acting as a heat source
Hence flow and return temperatures reading will assist further to pin down the cause. Cannot see why a data logger is required, simple temperature check will suffice.

Many years ago fitted a new cylinder to new boiler ( existing instal)
Fired the boiler to heat water in the cylinder.
The flow into the cylinder was near target temperature but return had hardly picked up any heat as indirect coil was massive 24kw output so heat transfer was good.
I ensured the system was balanced for 20 degrees drop with whole system calling.

 

[Gwyndy]

Attached is this morning's plot of return temperature. I haven't superimposed this on any of the recent flow temperature plots as the starting temeprature in the cylinder would be different. However, the shape of the curve is identical to the flow plots and, guessing, maybe initially within 5-10C of the flow. The return temp reaches boiler set point in around 35 minutes, very similar to the flow data that I have been seeing.

@DP I agree with your comment about the cylinder acting as a heat source once the boiler reaches set point, hence the slow cool down rate.

Conclusions -
Nothing wrong with the boiler.
Cylinder unsuitable for pumped system but working and providing adequate hot water,

Actions -
Do some spot checks of flow and return temps to confirm slow heat transfer.
Maybe look at restricting flow rate with gate valve in the HW circuit. (If the installer is sensible I think that this could be done using a pipe freezer to avoid a full drain down.)

Maybe I am being unnecessarily pedantic about all this. I do have hot water and, since changing the boiler, my gas consumption for HW heating has halved. (And significantly reduced for CH as well.) However, I am annoyed with the installer for not spotting this issue 3 years ago when they were told about it. I will see what they say now that I have some sensible conclusions.

I really do appreciate the interest and kind help from all of you. Many thanks again - I'll let you know what happens.

 

[Johntheo5]

You might ask WB if your boiler has a anti cycling time which refires the boiler when it has elapsed if the flow temperature is at its SP minus some number?, looking at the flow trend the burner seems to cut out at SP+5C so one might expect it to refire once the anti cycle time has elapsed as long as the flow temp is SP-5C. If not (ie no anti cycle time, it doesn't show any reference to it in the MIs that I've looked at) then is it looking at the RATE at which the flow temp reduces, a quick look at the return trend shows that the return temp reached 72C after 35mins and fell to 58C at 52mins when burner refired? at a return temp of 58C which probably means that the flow temp wasn't too far away from this, so a ~ RATE of 14C in 17 minuts whic = 0.82C/min.

Combined flow/return temps beloow, obviousaly taken on different days.
You said the flow/return through the cylinder coil are top/bottom but not sure if you have checked (but think you have) that the pump is actually pumping into the boiler return pipe, ie, that this pipe is actuall the boiler return.

 

[Johntheo5]

These boilers have the same anticycle&overrun time of 3 mins, it does state that the burner will cut out at SP+5C but does not state what the flowtemp must fall by to refire.

 

[Gwyndy]

You might ask WB if your boiler has a anti cycling time which refires the boiler when it has elapsed if the flow temperature is at its SP minus some number?, looking at the flow trend the burner seems to cut out at SP+5C so one might expect it to refire once the anti cycle time has elapsed as long as the flow temp is SP-5C.

The engineer who came a couple of weeks ago didn't know what temperature reduction to expect before re-ignition! He phoned back to base and was told '3 minutes (i.e. pump overrun time) plus 6C'. In my case the system never shuts off, of course, so I don't know if this has any relevance. However, looking at my temerature plots, the temp reduction from 3 minutes after the boiler shuts off to re-ignition is maybe 7 or 8C. I will ask WB - I have had a very confusing reply to the e-mail that I sent a couple of days ago as they are assuming that I have a general problem with both the HW and the CH. They have actually advised me to set the pump speed on III, which I have no intention of doing. I will contact again.

Edit: Comment about temp reduction after 3 mins to re-ignition removed. It's nearer 10C.

You said the flow/return through the cylinder coil are top/bottom but not sure if you have checked (but think you have) that the pump is actually pumping into the boiler return pipe, ie, that this pipe is actuall the boiler return.

Definitely flow to the top of the coil and return on the bottom - checked by hand.
Pump is just under the boiler and on the return lines from both the HW and CH circuits so pumping in to the boiler. Flow exits from the top of the boiler. Zone valves are on the return lines.

For clarity, tomorrow I will log the flow temp again and I will manually check the return temp at, say, 5 minute intervals.

Having read all the comments I would think that it is a running certainty that the issue relates to having the old gravity feed cylinder on a pumped circuit. Boiler firmware is probably looking for a faster rate of temperature reduction and / or a greater differential between flow and return.

 

[DP]

Attached is this morning's plot of return temperature. I haven't superimposed this on any of the recent flow temperature plots as the starting temeprature in the cylinder would be different. However, the shape of the curve is identical to the flow plots and, guessing, maybe initially within 5-10C of the flow. The return temp reaches boiler set point in around 35 minutes, very similar to the flow data that I have been seeing.

@DP I agree with your comment about the cylinder acting as a heat source once the boiler reaches set point, hence the slow cool down rate.

Conclusions -
Nothing wrong with the boiler.
Cylinder unsuitable for pumped system but working and providing adequate hot water,

Actions -
Do some spot checks of flow and return temps to confirm slow heat transfer.
Maybe look at restricting flow rate with gate valve in the HW circuit. (If the installer is sensible I think that this could be done using a pipe freezer to avoid a full drain down.)

Maybe I am being unnecessarily pedantic about all this. I do have hot water and, since changing the boiler, my gas consumption for HW heating has halved. (And significantly reduced for CH as well.) However, I am annoyed with the installer for not spotting this issue 3 years ago when they were told about it. I will see what they say now that I have some sensible conclusions.

I really do appreciate the interest and kind help from all of you. Many thanks again - I'll let you know what happens.

At 38 minutes the burner went out
Residual heat within the boiler is not going to be dissipated all that quickly if return is at high temperature. Return water from cylinder to boiler is at all times above 55 degrees, temperature to heat water would be set at 55 or similar. All that needs to happen is cylinder thermostat either slightly out, not fitted correctly, at wrong height etc, is not going to release the motorised valve. Remember, the stat is bimetallic strip- hysteresis is going to be quite broad unlike a digital stat
All considered, boiler is doing its job, behaving to designers parameters.

full drain down.

No need to do that.

Take it boiler functions correctly during CH demand.