Greenstar 24ri - large temperature drop before re-fire

[Gwyndy]

I have a Worcester Bosch Greenstar 24ri gas fired boiler for water and central heating. Hot water is via an indirect cylinder, tank stat set to 60C and boiler set to give flow temperature at around 73C (measured using contact thermocouple on pipe). Grundfoss pump set on speed setting 2 of 2. Time clock is set for a 1 hour cycle in the morning.

Problem: At the start of the cycle the boiler fires and runs for around 35 minutes. Boiler then stops firing but zone valve is still open and pump running because tank not up to temperature. Flow temperature drops to around 57C but boiler does not re-fire for 15 -20 minutes. System shuts down at the end of the one hour cycle. If I let it run beyond the one hour the boiler will re-fire eventually and the tank will get to temperature in about 90 minutes.

I first noticed this problem when the boiler was installed 3 years ago. The installer just increased the boiler thermostat setting but, because of the layering effect in the tank, the hot water was scalding. I persuaded Worcester Bosch to investigate in 2022 but the engineer said that the boiler was firing correctly (i.e. it went up to temperature) and did nothing. I have had another WB engineer to look at the issue this week and he observed a 20C drop in flow temperature before the boiler re-ignited. He changed the main circuit board (but did not change the temperature sensor) and said that had solved the problem but he didn't wait to check! (Was in a rush to catch a bus!!!) Nothing has changed and the system is still behaving as before.

WB has said that if the problem persists I will have to have the whole system flushed. The installer insists that the system was flushed during installation and that the problem must be with the boiler. I am stuck in the middle!!

Any ideas would be welcome please.

 

[Harry Bloomfield]

Any ideas would be welcome please.

Buy a couple of cheap, Chinese, digital temperature displays, with remote sensors. Clip them to the flow and return pipe close to the boiler, then you will be able to accurately see what the boiler is actually doing.

 

[Gwyndy]

Buy a couple of cheap, Chinese, digital temperature displays, with remote sensors. Clip them to the flow and return pipe close to the boiler, then you will be able to accurately see what the boiler is actually doing.

I've got one on the flow pipe which is how I'm monitoring the temperature. I am thinking about buying a simple temperature data logger so that I can prove to WB what is actually happening.

 

[Gwyndy]

I have had a temeprature data logger measuring the temperature on the flow pipe from the boiler. Temperature peaked at around 70C after 35 minutes and then dropped to 51C over the remainder of the one hour cycle. Boiler did not re-ignite. Bosch attending tomorrow to replace the temperature sensor which is what they should have done when I first reported this issue two years ago!

 

[Gwyndy]

Worcester Bosch engineer came yesterday to look at the boiler issue. Spent a lot of time looking at the system and eventually replaced the temperature sensor. It hasn't made any difference! When heating HW only, boiler goes to set point (~73C at the moment), boiler stops firing and doesn't re-ignite until temperature drops to around 55C approx 20 minutes later. All it does in the meanwhile is circulate cooling hot water around the system, presumably taking heat from the top of the HW tank.

The engineer did comment that the supply to the coil in the HW tank is in 28mm pipe and is a relatively short circuit to / from the tank. This was from the old installation and was retained when the boiler was changed. He suggested that maybe the flow rate is too fast and that the HW tank is not taking the heat out of the system rapidly enough. He demonstrated this by manually opening the zone valve for the heating circuit and the boiler fired instantly.

Possibilities -

1. Restrict the flow round the HW circuit with, say, a gate valve.

2. Install an automatic variable speed pump.

3. Reduce the set point on the HW tank thermostat by a few degrees. (It is probably reaching a temperature of 55-58C where the tank thermostat is sited around 1/3 of the way up from the bottom of the tank.)

4. Put up with it!

One thing that I have learned is that the WB boilers are actually very unsophisticated! I had assumed that there would be at least two sensors measuring flow and return temperatures to the boiler but there is actually only one.

Attached is a one hour plot from a couple of days ago showing what is happening if anyone is interested. (Set point was then around 70C - temperature dropped to appox 51C and boiler did not re-fire.)

This is driving me nuts! Suggestions or advice please.

 

[Johntheo5]

The boiler should have a settable anticycle time, usually ~ 5min or so. If the heat demand isnt satisfied, and if the boiler flow temperature rises to normally 5C above the setpoint temp then the burner will stop firing and will not refire after the anticycle time has elapsed and the flow temp is SP-5C, Some boilers also have a settable temp differential of up to 15/20C so check or ask WB what these are on your boiler or how to access them yourself. It should tell you what settings are available in your MIs.

 

[Gwyndy]

@Johntheo5 Thanks for your reply. Apparently nothing adjustable on this boiler - it either works or it doesn't! Certainly a temperature drop of 5C would be acceptable but mine is falling ~18C and around 20 minutes before re-ignition. My measurements don't show the boiler going over temperature.

I'll have another chat to WB technical people tomorrow. I know that they will not send anyone else to look at the problem as I have had two visits in the past couple of weeks and they have changed the PCB and the thermistor. Not much else left!

 

[Johntheo5]

What model Grundfos?, models like the UPS3 are very powerful with high heads, so maybe just reduce the setting to speed 1, which will give a higher dT through the cylinder coil, may help.

 

[Gwyndy]

Grundfos pump is marked as: Type: UPS0 15-60 A0KR. (From a web search it seems that this is now obsolete.) It has two speed settings marked as II and III. It is on the higher flow rate as the installer said that this was necessary for my CH system.

 

[Johntheo5]

Can't find any info on that model, strange that a pump with only 2 speed settings is marked II & III, I would try it certainly on speed II, if a speed I not available, maybe first try and monitor the boiler return temp as well even if only by changing the data logger clamp? from one pipe to the other regularly, if you only did this once or twice say near the end of the heating period, it will give a accurate idea of the boiler dT.
You said that switching to CH during this HW waiting period instantly refires the boiler, if you switch off the CH, does the boiler continue to fire on HW only? If not, does it continue to fire with both on, if so, look and see if the HW motorized valve continues to stay opened.

Is there a ABV, automatic by pass, installed? and if so what is its indexed setting?

 

[Harry Bloomfield]

It has two speed settings marked as II and III. It is on the higher flow rate as the installer said that this was necessary for my CH system.

There is bound to be a speed I, or it would have been marked I and II.

My boiler, by default, always shows the flow temperature, but long ago - I bought a pair of cheap, button cell powered temperature displays, with remote sensors. They are handy for keeping an eye on my boiler's operation, or for balancing the radiators. The sensor just clip onto the pipes, using a bit of pipe insulation.

 

[Gwyndy]

Is there a ABV, automatic by pass, installed? and if so what is its indexed setting?

Yes, there is an ABV but I'll have to do a bit of rummaging in the garage to read the actual setting. I assume that it is OK as I can hear it operating when zone valve(s) close and the pump runs on for 3 minutes.

I would try it certainly on speed II, if a speed I not available, maybe first try and monitor the boiler return temp as well even if only by changing the data logger clamp? from one pipe to the other regularly, if you only did this once or twice say near the end of the heating period, it will give a accurate idea of the boiler dT.

I will try reducing the pump speed to II and see what happens. I will also log the return temperature to the boiler tomorrow.

You said that switching to CH during this HW waiting period instantly refires the boiler, if you switch off the CH, does the boiler continue to fire on HW only?

Don't know - I'll give it a try.

-----------------------------------------

Today's log of the flow temperature showed the same pattern as before - system reached peak temp of 73.5C in 29 minutes and then boiler shut off (zone valve still open and pump running). Temperature dropped to ~56C over 20 minutes and then boiler re-ignited. Temperature back to 73C after 5 minutes and then off again.

The only other very remote possibility that has occurred to me is to wonder if the over-temperature sensor is faulty and misreading - i.e. sensing high temperature and shutting off the system until it has cooled sufficiently. A bit 'pie in the sky' maybe!

@Johntheo5 Thanks for your interest and guidance. I'll report back tomorrow.

 

[Gwyndy]

There is bound to be a speed I, or it would have been marked I and II.

If there is a speed I setting then it's very well hidden!

I bought a pair of cheap, button cell powered temperature displays, with remote sensors. They are handy for keeping an eye on my boiler's operation, or for balancing the radiators. The sensor just clip onto the pipes, using a bit of pipe insulation.

I have a digital thermometer with external probe and I have recently purchased a temperature data logger with probe which is how I am monitoring the flow temperature. Excellent device and not expensive.

 

[Gasguru]

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

 

[Johntheo5]

View attachment 357333
If there is a speed I setting then it's very well hidden!



I have a digital thermometer with external probe and I have recently purchased a temperature data logger with probe which is how I am monitoring the flow temperature. Excellent device and not expensive.

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?