Worcester Bosch Highflow 400RSF & Tepid Shower

[KeepTriin]

Hi

I've recently encountered a problem with my Worcester Bosch Highflow 400RSF boiler in that I can only get a tepid shower. The basin hot taps are generally ok (I must check the bath taps separately) but it seems that the boiler isn't able to keep up with the demand for high-flow hot water like the shower.

The boiler has tripped out (overheat) a couple of times too lately. It was serviced and this was mentioned, but the engineer replaced only the expansion vessel.

Shower water temperature was okay before the expansion vessel was replaced. The expansion vessel had leaked and was full of water. This was replaced a couple of days ago and since then we've had tepid showers. The boiler has also cutout (overheat) once more since then too.

It may be coincidence, but I'm thinking one of:

1) Diverter valve failed

2) Sludge/restriction in the CH circuit causing diverter valve to stick

3) Water-water heat exchanger

4) Shower cartridge (Triton Sentiment thermostatic mixer)

Rather than replace everything, can anyone offer any advice as to how I might narrow down the problem? Or, if I'm completely wrong, has anyone else got any other ideas?

Thanks in advance

 

[Gas2Air]

Check flow rate to taps and temperature rise. If ok, do the same to shower. If not it's a mixer cartridge fault.

 

[ChrisR]

Exactly so - but to determine flow/temperature you need a thermometer and a measuring bucket and a watch. Wiggling fingers in the flow is not a way to tell!

 

[KeepTriin]

I checked the flow rate to the bath and it shows the same symptoms as the shower. Above a certain flow rate, there's a drop off in the temperature. In the basin taps it's fine. That rules out the shower cartridge I think.

That leaves me with either the diverter valve or heat exchanger. Any ideas how I might rule out either one?

Thanks

 

[ChrisR]

No it doesn't rule much, yet. What are the numbers? Your boiler can only provide so much hot water. It has a ""store" but even so, when the mains cold is cooler, you get less hot mixed water.

Whatever, you will always get a drop in temp if the flow rate goes high enough, ie if the taps allow the flow to go high enough.

You/we need the numbers. How many litres/minute at what temperature rise (hot flow alone, temp rise relative to cold)?

If you start with cold radiator pipes and run the HW, do the pipes warm up?

 

[KeepTriin]

I'll see if I can get the flow rates today, but in the meantime, I turned the CH off and ran the hot water. The rads got hot (they were cold beforehand)

Diverter valve?

I'll get back with those numbers as soon as I can. Before this problem started (last week) we've had to limit the shower temp at max flow as it was delivering more than enough hot water. Ambient temperature, if anything is warmer this week than it was last week. At low flow in the shower I can get those temps as they were before, but not at high flow rates.

Thanks for your help