Vokera Excell 80SP -Intermittent hot water

[che6mw]

Hi all. Got an annoying problem with the Vokera Excell 80SP I inherited with the house I bought! Hoping someone might be able to help me out so it can continue to be a decent workhorse rather than ripped out needlessly.

Central heating system works just fine and gets suitatably hot.

But when turning on the hot water a few things happen:

- boiler never fires up unless hot water tap opened to full flow (very fast)
- even with full flow half the time boiler does not fire up at all. Water comes through ice cold. If that happens tap needs to be turned off and left for a short period of time (almost like boiler needs to reset itself).

- if the boiler does fire up then 9 times out of 10 the water temperature then cycles from cool (not ice cold) to slightly warmed through. If you do nothing it seems to do this endlessly (it doesn't slowly get hotter)

- hotter water can only be obtained by turning tap partially closed, then partially open again repeatedly until you get lucky. When it does finally come through hot it usually stays that way until you dare to touch the tap again!

Any ideas for systemmatically identifying the problem please?

Would love to have it suitably repaired rather than throw it out (especially given CH seems fine).

I've owners and installers manuals to identify any problemmatic parts anyone suggests I check.

Thanks in advance. Can't offer much at all in return other than best wishes and a promise I offer my expertise on other forums that aren't to do with boilers!

Matt

 

[paulm34]

I had a simliar problem with mine it was the domestic diaphram which i have changed twice know since i have moved into the house. The big rubber diaphram tears slightly as it gets weak over time and lets the water past it instead of pushing the the arm against the switch to fire up the boiler to produce the hot water from the tap. They are a very common fault and i read somewhere these boilers seem to eat them Im not a boiler engineer im a car mechanic but found it easy to do hope this helps

 

[che6mw]

I had a simliar problem with mine it was the domestic diaphram which i have changed twice know since i have moved into the house. The big rubber diaphram tears slightly as it gets weak over time and lets the water past it instead of pushing the the arm against the switch to fire up the boiler to produce the hot water from the tap. They are a very common fault and i read somewhere these boilers seem to eat them Im not a boiler engineer im a car mechanic but found it easy to do hope this helps

Thanks Paul. Was this what you've been replacing please? Or something different?
http://www.buyaparcel.com/pageview.php?page=show_product&ecommerce_stockcode=T0019

 

[paulm34]

I had a simliar problem with mine it was the domestic diaphram which i have changed twice know since i have moved into the house. The big rubber diaphram tears slightly as it gets weak over time and lets the water past it instead of pushing the the arm against the switch to fire up the boiler to produce the hot water from the tap. They are a very common fault and i read somewhere these boilers seem to eat them Im not a boiler engineer im a car mechanic but found it easy to do hope this helps

Thanks Paul. Was this what you've been replacing please? Or something different?
http://www.buyaparcel.com/pageview.php?page=show_product&ecommerce_stockcode=T0019

Yes the T0019 is the service kit with more than just the big diaphram which i got the first time as i changed the sleeve which came in the kit. Or you can just buy the diaphram on its own which is part number 6882. If you are doing it yourself when you take the big white metal boiler cover off pull the dials switches down. And you can see where the diaphram is located in a round metal brass part with about 6 big phillips screws around it. You can test the boiler by pulling the arm against the switch and that should ignite the boiler with the tap running or just watch what happens when you turn the hot tap on to make sure this is the fault. But remeber to switch the boiler off when fixing it just in case

 

[MANDATE]

Beside switching the boiler off, you need to turn off the cold supply to the boiler and I don't mean the filling loop. This cold water passes through a heat exchanger and comes out hot. that is when the tap is opened.
With the hot tap shut this cold water which is at mains pressure is on both sides of the diaphragm so its kept in equilibrium.
When you open the hot tap it releases the pressure on one side so the higher pressure on the other side causes movement of the diverter valve and at the end of this movement it triggers a micro switch. Well that is the basic principle.
If there is a hole in the diaphragm, then the difference in pressure will not be as great as it should and may not move the diverter valve as far as it should and will not trigger the micro switch.
Sometimes by opening the hot tap fully and quickly might just allow it to function, but it all depends on how high a differential can be created.
Opening the hot tap slowly will allow water to pass through diaphragm and allow a build up of pressure