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.