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.