Worcester 350 RSF - DHW only warm if flow is low! UPDATED

Hello,

Here is a challenge i would like to ask you knowledgeable people for help with?

I have an old (1993) Worcester 350 combi boiler.
CH is working faultlessly.

i've used google, searched this site, the FAQ's and all the previous posts i have been able to find - without finding a solution, so far...

PROBLEM - DHW side has stopped supplying continuous hot water.

DESCRIPTION - A hot tap is turned on,
the green demand light illuminates, relays click, the fan runs, the burners ignite and fire up.
warm water comes from the tap for maybe 20 seconds,
the water from the HW tap starts to slowly reduce in temp until cold.
during this time, the boiler is still in operational mode ie, the burners are still alight and the DHW pump is still running.
once the HW tap (with cold water coming out!) is shut, the boiler ceases DHW mode, and after a delay of approx 2.5 mins the CH
resumes.

list of things checked so far -

i have read through the manual with the operational flow diagrams (the installation and servicing instructions) and have been step by step through the fault finding section:


the flow switch sends a signal/operates (tested by removing the unit from the DHW pipe - the DHW system stops running).
demand indicator (green) lights up.
fan comes on - high speed + drops to low speed
the HW pump runs.
gas valve operates, sparking starts, burners ignite, sparking stops.
warm water comes from tap for short time, then becomes cooler.

i have changed some parts on the boiler in the past,
driver board once,
sequence board twice,
relay on sequence board once (cheaper than a recon board!),
DHW sensor once,
pressure releif valve twice,
expansion vessel once,
system flushed every two years,
water to water heat exchanger flushed.

This current challenge has me stumped!
Have i missed something simple? (hopefully!)
Have you experienced/fixed this problem?
How do you think i should go forward with this?
what other parts could I check?


Thanks in anticipation for the sharing of wise words and experience!!



austin

Hi again,

Here is some more info in case it makes any diference, or helps with diagnosis -

our water mains pressure is quite good.


this problem happened literally overnight.

had a shower last night, no problem / hot water from the HW taps open full.

this morning - brrrr - freezing cold shower / cold water from HW taps open full.


after doing some more searching and reading, i tried varying the HW tap flow to see if there was any difference in the water temp.



if the HW tap is opened only a small amount (say 10-15% of full flow), then the water is warm/hot and the pipe to the water/water heat exchanger gets hot.

as the tap is opened to 30% the temperature of the water coming out of the tap drops and the pipe to the water/water heat exchanger gets very cold.

if the tap is closed back down to 10-15%, the temperature rises again and the pipe to the water/water heat exchanger gets hot again.



any ideas please?

thanks,

austin

Call an RGI who will fix the problem

Exactly.

But you could light gas hob and see what effect on flame size when hot tap is turned on.

Tony

hi,

thanks lightning, i appreciate you taking the time to reply.
calling an RGI will be my last option, due to a number of reasons.

my first option is to ask for help from any willing individual.
this obviously comes with the inherent challenges of mis-information, me not knowing the people who i'm talking to, not knowing peoples' qualifications, my description of the problem not being accurate etc ad nausium

in my (limited) experience with this boiler, all the problems that have arisen have been able to be fixed by -

RTFM! (reading the manual/service information).
taking the time to speak with other people, (including the RGI's i know through work, who by the way could not provide solutions to the last two problems i had),
checking components by swapping or testing,
researching on the net, in forums like this.


And thank you Tony (Agile), for suggesting a test that i can carry out.
when the HW tap is opened, the cooker ring flame reduces - although it is quite difficult to see it happening.
when the HW tap is closed, the cooker ring flame jumps/increases noticeably and almost immediately.

i've been able to get a warm shower (the shower is an old, 1993, Mira 415 mixer fed directly from the combi) by reducing the flow of cold and hot water via the ball valves on the feed pipes. Hurray!
it's more of a drizzle than a shower at the moment tho'

so, thanks to both of you, any other ideas please?

austin

There are some tests to be carried out which a diyer can not do, The problem you have got seems to be not enough gas to your boiler But an RGI will confirm this.

hi lightning,

brilliant!

i'll have a word with a pal who is RGI and ask him to check the gas pressure.

one more step along the road i go...

austin