Shower/Combi Boiler Cold Water Issue

Richie7

Hi everyone,

Got a bit of an odd problem.

Had a new Worcester 4000 boiler installed, and the shower is behaving very odd. It's a thermostatic mixer.

Sometimes it's fine and runs hot, other times it goes cold after a few minutes. The only way to get the hot back is to run the hot water tap and then it's fine again. As the boiler kicks in and heats the water.

The boiler is working okay otherwise, as are the radiators.

Do we think this is perhaps a shower problem? Or a valve/boiler problem? I can't find this specific issue anywhere.


Thanks,


Rich.

teaselMay

Is the pressure at the shower low? If it's too low it doesn't trigger the hot water at the boiler

Scot_39

Sounds your shower demand vs new boiler setpoints and operation borderline.

And that your shower power needs are too low for your boiler  min continuous power output. Or at least it's ability to derate to that power quickly. 

Two quick tests, if your tap flow needed to reignite burner minimal / low.

If boiler hw temp (not radiator temp) set high, say 60C, try dropping it to 50 or lower - say 45.

Some dermatologists don't recommend going over 40C at shower head.  45C or 50C should allow that.

(The tmvs in care scenarios supply a max of 43C at outlets - whilst maintaining tank system at 60C+ for legionella.  Modern homes at 48C max for bath taps too.
In both cases due to severe scald risk )

Will increase hot flow in mix at shower head - maybe enough to keep boiler at min derated power firing continuously



Try simply running higher flow or temperature from shower.

More power,  but if can save time showering maybe same energy / cost.



Combi Boilers can be much higher rated than electric showers - 25 kW-40kW total depending on no of rads / propery size - but the share of that for hot water max ??  - and derated  min contunous ??? -  compared to 9-10kW for simple electric  - where flow tiny for hot water in winter cf some hot fed.

And then how that power for hw changes if hw only or hw and rads at same time etc ?

Might explain more recent changes in behaviour as heating more likely to be on.


But back to electric shower rating analogy

With 60C from boiler and 10C cold for 35C shower - only 50% is hot water in mix - so to match electric - if boiler min power same 10kW for hw - would need double flow  to keep boiler running continuously.
Or to keep same flow - boiler would need to derate to 5kW for hw continuously.

No expert on combis or tmv showers though.

Richie7

Thanks for that, very useful information. I'll check back once I've had a tinker with the shower and the pressure. It could be just as simple as needing to increase the flow.

Farway

Richie7 said:

Thanks for that, very useful information. I'll check back once I've had a tinker with the shower and the pressure. It could be just as simple as needing to increase the flow.

I have a thermostatic mixer as well

I find the thermostat seems to stick at times, but just moving it a few times from max to min sorts it. No need for shower to be on at the time.

Furthermore, I put it down to my hard water, but maybe I'm wrong. However, it does sort it out

FreeBear

Farway said: Furthermore, I put it down to my hard water, but maybe I'm wrong. However, it does sort it out

If you are in a hard water area, turning the temperature down on the boiler will reduce the amount of scale building up.

Have hard water in my area too, and DHW temperature is set for 45°C or less.