Oil combi - Mixer shower

[Ziggur]

Please may I get some advice from the plumbing experts here.

Time for a new shower enclosure.

I want to replace my aging electric shower with a good thermostatic mixer supplied from a Worcester OIL Heatslave combi boiler.

I have looked at all the major brands and examined as many Installation Manuals as I can find. All of them state that the combi MUST be "fully modulating". Further research indicates that there are very few (if any) domestic oil-fired combi boilers that modulate - mine certainly does not.

My bath mixer tap works perfectly OK and I'm sure that If I stuck a high pressure bath shower mixer on the bath then that would also work. So why do a need a fully modulating boiler to supply a shower "tap"? All I want to do is to connect two water pipes to a mixer and have the water come out of a tap with lots of small holes in it!

Any help on a work-around would be very welcome. (A new electric shower is a last resort because I would need an electrician to install an MCB and he wouldn't like my wired fuses and I'll end up with a complete rewire.)

TIA.

[auctions]

I think the problem is that a thermostatic shower only works properly if the hot water temperature is pretty constant when the flow varies, as they work by restricting flow rates to vary the output temperature

- if the shower wants to decrease the output temperature it decreases the hot water flow rate. If that decrease in flow causes your boiler to increase the hot water temp, you've obviously got a problem.

I'd be inclined to fit a pressure balancing valve and a standard non thermostatic mixer, something like

something like much better than a thermostatic setup with any combi, and gets round your problem.

A

[John Rumm]

There is also a secondary problem with some non modulating combis. If your shower demands too little flow rate you can end up with the boiler cycling on an off (since the output water temperature rises above the limit set by its thermostat). The boiler may not be able to maintain a stable enough water temperature in this circumstance, and you get a fluctuating shower temperature.

[Ziggur]

Thanks John

If I kept the shower flow rate high enough and increased the boiler thermostat limit would the risk you outline be reduced?

Mike

[Ziggur]

Thanks a lot Andrew. Sound like a good plan. I even like the shower head in your link. :)

[John Rumm]

Quite possibly. Another solution that can work in this circumstance is leaving a basin hot tap running a little at the time.