Worcester Greenstar CDi Compact - Return Temp (i14) much higher than pipe surface & rads

[MilindPuri]

Hi, I have moved into a 2 bed flat built in the 90s and trying to figure out why I heating problems even though the central heating and boiler "works" something is not quite right. There is a separate issue of a communal extractor system always pulling air out of the bathroom which I will ask the neighbours about separately, here I ask about a particular heating situation discovered after balancing the radiators.

System & Balancing:

• Worcester Greenstar 28 CDi Compact ErP in a 2-bed flat, installed in the kitchen.
• Pump: Mode 4 (Max Constant Pressure).
• Balancing: The lounge radiator is undersized for the large room, so I have opened the lockshield an extra full turn to maximize flow. All other radiators have TRVs/lockshields dialled back specifically to prevent them from stealing flow.

The Issue: The lounge and kitchen area (open plan) is taking an excessively long time to rise from an overnight 14°C to 19.5°C, with significant cold spots in the far end of the room from the single (undersized) radiator. Gas usage is high (55-70kWh/day including showers and hot water usage) because the boiler appears to be throttling or cycling prematurely. Reflective foil has been installed behind all the external wall radiators in the flat and the lounge radiator has 2 x 5 fan units running on the top to move the air through faster so as to get the heat to reach the cold spots.

Despite prioritizing the lounge, the radiator surface struggles to get past 50°C. However, the boiler's service menu i14 (Return Sensor) reports 60°C.

Temperature Facts:

• Boiler i14 (Return): 60°C.
• Manual pipe reading: 45°C on the Return pipe at the filter. (Measured using a digital kitchen thermometer held firmly against the copper pipe).
• Lounge Radiator: 50°C (measured with a radiator fan sensor and the same digital kitchen thermometer)
• Outside Temp: 2°C–5°C.

Question: How can the internal i14 sensor see 60°C when the physical return pipe entering the boiler is only 45°C and the radiators are at 50°C?

Thanks in advance for any advice!

 

[Johntheo5]

Possibly because the internal bypass is opening which it may do if the pump speed is very high and a possible restriction in the main heat exchanger, have a look at i16 which will give the pump speed (modulation) in %., with boiler running.
Is this the pump setting you are using?

Edit. Also what is the flow temperature?
Also, note the boiler modulation. i17,

 

[MilindPuri]

Possibly because the internal bypass is opening which it may do if the pump speed is very high and a possible restriction in the main heat exchanger, have a look at i16 which will give the pump speed (modulation) in %., with boiler running.
Is this the pump setting you are using?

Edit. Also what is the flow temperature?
Also, note the boiler modulation. i17,


View attachment 404366

Thank you for responding sir!

2.1E is set to 4. Boiler output flow is set to 66C though varies around that.
When boiler return flow temp i14 is 58C then i16 is 92% and i17 is 32%.
When return flow temp i14 reaches 60.5C then i16 is 96% and i17 is 35%

 

[Johntheo5]

Your boiler max output to CH (rads) is 24kW under your operating conditions, the output at 32% burner modulation is 7.68kW.
The circulation flow through the rads at 66C flowtemp and using your measured return temp of 48C is 6.11LPM, the circulation flow through the boiler at 66C+ 58C return, is 13.76LPM, so the internal by pass is recirculating 7.65LPM (at 66C), a huge recirc flow, you now have 6.11LPM at 48C mixing with 7.65LPM at 66C to give 13.78LPM at 58C, the pump is running almost flat out (no surprise) at 92/96% so IMO points to a restriction (fouling) either in the boiler HEX or in your CH circulating system due to possible sludge build up, it's also possible, but unlikely IMO that the internal bypass is faulty,.
So, (we) have a think about this overnight.

 

[MilindPuri]

Your boiler max output to CH (rads) is 24kW under your operating conditions, the output at 32% burner modulation is 7.68kW.
The circulation flow through the rads at 66C flowtemp and using your measured return temp of 48C is 6.11LPM, the circulation flow through the boiler at 66C+ 58C return, is 13.76LPM, so the internal by pass is recirculating 7.65LPM (at 66C), a huge recirc flow, you now have 6.11LPM at 48C mixing with 7.65LPM at 66C to give 13.78LPM at 58C, the pump is running almost flat out (no surprise) at 92/96% so IMO points to a restriction (fouling) either in the boiler HEX or in your CH circulating system due to possible sludge build up, it's also possible, but unlikely IMO that the internal bypass is faulty,.
So, (we) have a think about this overnight.

Good morning and thank you for that info with the Maths! It's much warmer outside today so the boiler won't work so hard, I dropped the output flow to 60C the actual readings 15 mins of running were output flow 63.5C return flow 55C Pump i16 is at 100% and i17 is 30%, radiator still managed to get upto 42C. 30 mins after starting the temp in the room has raised 0.5C rad temp is 45C. I think I am wasted gas as the boiler won't get into condensing mode and is forever cycling and running for longer as the radiator temp can't go up enough?

I have a landlord, it is not my place to fiddle with the boiler, the service was last done in July 2025, how do I persuade them that a gas boiler engineer needs to visit?

 

[Johntheo5]

Are you talking about the rad return when you say it managed to get up to 42C. If you cant get any more flow through the rads which apparently you can't with the pump running at that high speed then the only other way is to raise the boiler flowtemp, from the present 63.5C to say 70C which will give a 18.4% increase in rad output but the rad return will only rise from 45C to almost 48C to still give reasonable boiler efficiency, but of course this recirculation will then rise the boiler return to ~ 60C, reducing boile efficiency and using extra gas.
I would just point out this to them, you can give them those few calculations if you like, may not impress them, but if it helps to get them to agree get a engineer in, then good, you can't do any more really.

 

[MilindPuri]

To clarify, the i14 return sensor in the boiler menu was 55-56°C, but the actual radiator surface was only 42°C. I have never seen the i14 drop below the 55C. Since it's staying above 54°C, I assume I am getting zero condensing efficiency?

I'll wait for colder weather to try 70°C flow, today it is 10C outside and the flat is already warmed up. In the meantime, I’ll tell the landlord the boiler keeps cutting the flame off prematurely (cycling) even when the thermostat is calling for heat, making the flat take far too long to warm up.

Many thanks for your help!

 

[Johntheo5]

To clarify, the i14 return sensor in the boiler menu was 55-56°C, but the actual radiator surface was only 42°C. I have never seen the i14 drop below the 55C. Since it's staying above 54°C, I assume I am getting zero condensing efficiency?

I'll wait for colder weather to try 70°C flow, today it is 10C outside and the flat is already warmed up. In the meantime, I’ll tell the landlord the boiler keeps cutting the flame off prematurely (cycling) even when the thermostat is calling for heat, making the flat take far too long to warm up.
Many thanks for your help!

The radiator surface should read like my own below, very close to the boiler flow temperature where shown, along the TOP, my boiler and rad flow/return temperatures were 65C/58C, I don't have a condensing boiler so not too worried about the return temperature as + or - 10C (return) is only about 0.45% difference in efficiency. In a condensing boiler the condensing only begins at ~ 50C return and ~ 40C is required to get any decent level of condensing..effect.