Combi-boiler runs shower, intermittent hot/cold

natrapx

Hello all

We moved into a house with a Vokera Compact 25HE combi-boiler feeding all HWS in the house, including a centrally fed shower. This is in the attic above the bathroom.

Problem we have is when showering, it's not a consistent temperature. We've got the manual mixer set to a comfortable temperature, but it fluctuates between that and lukewarm at best, making a shower sometimes unpleasant! Additionally, filling a bath direct from the hot tap only takes quite a long time, and is just a tad colder than you'd want for a bath, this is with the temperature control on the boiler set to about 95%. It does reach above 50c at times, but then fluctuates enough that it brings the overall temp just too low.

I'm pretty sure this should be working better than this, any advice? I was looking at a unvented cylinder, but it's only two of us and I don't know if it would be worth it, especially as all washing up is done by the dishwasher!

Many thanks

Sedge123

We have the same with our shower and we've been told it's a problem with the pressures being unequal. Job for a plumber I think.

Le_Kirk

You seem to have two separate issues, one with the shower mixer and one with the boiler not delivering water at a high enough temperature. When you run an ordinary tap, does the water temperature (albeit too low) stay constant? If it does it is likely to be the shower mixer as indicated above. When the temperature varies, does the boiler demand cut in and out? You will need two people to see this, one running the shower and one observing the lights on the boiler control panel. When you run the shower does the boiler demand or flame light go out and then come on again? If so could be a water pressure problem.

natrapx

The hot tap at the bath fluctuates in the same way. I've got a thermometer so i'll do a temp profile over the course of a couple of minutes to see how it goes.

Pressure wise it reads about 1.5bar on the combi, is that mains pressure or would that be the pressure of the boiler system?

MX5huggy

The gauge in the boiler is system pressure (in the rads) not mains pressure reading.

Le_Kirk

If the hot tap fluctuates as well, next step is to observe the boiler to see if the demand light (if fitted) goes out along with the flame on light. If so, possibly the mains water pressure into the boiler for hot water is not high enough or the pressure switch is defective. As mentioned in post #5, the gauge is for the heating side not the domestic hot water (DHW)

booty40uk

As above, check that the demand light on the boiler stays on constantly while the tap/shower are running. 25kw wont give a great flow of hot water at the best of times and it is a vokera!!

macman

I agree. 25kW combi may be underpowered, but if the hot flow is cutting in and out at the taps as well then I'd suspect a sticking divertor valve. Not a major job to replace it.

southcoastrgi

I don't, I'd look at a problem with the plate hex unless the rads are heating while the hot water is running