Info returned from google AI. Good Luck
I have a combi boiler, a Worcester Bosch 28SiII, more than 12 years old. Despite the boiler's age, my central heating is working fine. I am, however, having problems with hot water. Whether I get hot water or not depends on which outlet I use and whether the central heating is ON or OFF (see table below). Even when I do get hot water, it appears to have noticeably low pressure
Based on the specific behavior you are experiencing,
the primary culprit is a faulty or stuck diverter valve, likely compounded by a blocked plate heat exchanger. [
1,
2,
3]
Because your Worcester Bosch 28Si II is a combi boiler, it uses a diverter valve to switch the flow of heated water between your radiators (central heating) and your taps/showers (domestic hot water). [
1]
Here is a breakdown of why your boiler is acting this way and what needs to be fixed.
Breakdown of Your Symptoms
1. Why it works at the Bath but not the Basin/Kitchen (when heating is OFF)
- The Cause: Combi boilers rely on a specific flow rate to trigger the hot water. When you open a tap, cold water passes through a small internal turbine (flow sensor). If the flow rate is too low, the boiler never "realises" you want hot water and refuses to fire up. [1, 2, 3]
- The Symptom: Bath taps naturally demand a much higher flow rate than kitchen or basin taps. Your bath is drawing just enough water to force the boiler to ignite, whereas the lower-flow basin and kitchen taps are failing to trigger the sensor.
2. Why everything works when Central Heating is ON
- The Cause: Your diverter valve is physically stuck in the "heating" position.
- The Symptom: When your central heating is actively running, the boiler is already fired up and blazing hot to warm the radiators. When you open any tap, water trickles through the primary heat exchanger anyway. Because the diverter valve is stuck open to the heating side, you are essentially "stealing" residual heat from the central heating loop. [1]
3. Why you have noticeably low hot water pressure
- The Cause: A heavily blocked plate heat exchanger or a restriction in the cold water inlet.
- The Symptom: Over 12+ years, limescale and heating sludge build up inside the tiny channels of the plate heat exchanger. This physically chokes the water flow, creating a bottleneck that severely reduces your hot water pressure at the tap. This restriction also drops your flow rate, feeding back into the reason why your kitchen and basin taps can't trigger the boiler. [1, 2]
️ Recommended Next Steps
Because your boiler requires opening the gas-tight casing and draining down the internal hydraulics to access these components,
this repair must be carried out by a Gas Safe registered engineer. [
1,
2]
When you book an engineer, ask them to investigate the following:
- Inspect and replace/clean the Diverter Valve: They will need to check if the motor actuator or the valve mechanism itself has failed.
- Check the Plate Heat Exchanger: The engineer should check for blockages. Given the age of the boiler, it is usually more cost-effective to replace it rather than trying to chemically clean it.
- Test the Flow Turbine / Sensor: Ensure the boiler is accurately reading when water is moving through the system. [1, 2, 3, 4, 5]
The 12-Year Dilemma: Repair vs. Replace
A Worcester Bosch 28Si II is a highly reliable machine, but at over 12 years old, it has reached the end of its expected operational lifespan.
- Repair Cost: Replacing both a diverter valve and a plate heat exchanger can quickly run into hundreds of pounds in parts and labour.
- Advice: Ask the engineer for a fixed diagnostic quote. If the repair costs creep towards a significant portion of a new boiler installation, it may be wiser to invest that money into a modern, highly efficient replacement that comes with a fresh 10-year warranty.
To help you weigh up your options, could you tell me:
- Are there any fault codes flashing on the digital screen?
- Do you live in a hard water area (which makes limescale blockages highly likely)?
- What is the current system pressure reading on the boiler's round dial (it should be between 1 and 1.5 bar)?
