DIY Doctor UK DIY Forums

Hi htg engineer, you seem to be the man to go to. I'll try not to bore you but i'll give you as much info as poss. Anyone, please feel free to answer, just joined and htg's name jumped out at me; I don't want to alienate anyone!

Boiler is 8yrs old but over last year or so has started to get noisy when CH is on. However, main prob started bout 3 mnths ago. Started being very tempermental and fitted a new PCB which did trick. At same time I noticed that there was a leak from the water flow switch. Only came to my attention when I heard hissing coming from it. What was happening was that water was escaping from the joint but the hissing was it being vaporised into steam!
When the hot water was on the domestic inlet pipe was keeping the pipe cool but as soon as the tap was turned off and the central heating kicked in the domestic inlet pipe got incredibly hot to the touch and 'steamed' the escaping water.
The following weekend I took off the flow switch and the flow switch restricter that sits within the brass fitting had melted! I replaced the flow switch restricter but the same has happened. I've had to make a makeshift restrictor from a coin with drilled holes in it but this isn't a long term solution!
I have flushed the system out about 5 times over the last weekend and added descaler & inhibiter. The water & CH work fine, the boiler is still noisy when the CH is on (pressure guage flickering frantically) but quiet when hot water demand. I was hoping that somehow a diverter valve wasn't shutting properly when the CH was on which was why the water was being forced back down the domestic inlet pipe but looking at previous posts it would seem that the 24i doesn't have one.
What do you think is causing the problem?
Something that you might need to know; when it was installed the Engineer used 15mm pipes on everything and not 22mm on the CH flow & return. Could the flow & return pipes somehow have become partially blocked over the years and therefore causing the water to be forced back down the domestic inlet pipe when the CH is on as the flow is restricted?
Any help & advice would be greatly appreciated. Thanks

It could be caused by some sort of build up like limescale etc etc as the heat struggles to get away from the boiler, and having 15mm flow and returns will increase the friction, if easily done I would advise upgrading to 22mm even if only until the first 15mm tee you will probably find the radiator will heat quicker and the heat will be more evenly distributed.

If in future you replace the boiler, make sure it is thoroughly cleansed/flushed, and inhibitors are used.

It could also be a problem with the gas pressures not being set correctly or it may not be modulating. Having a annual gas service carried out - is probably the best option as he will (should) check/set the gas pressures.

If the gas pressures are fine, you could try adding noise reducer to the system (read and follow instructions)


htg

Thanks for reply, I really appreciate it. It was sort of what I was expecting/hoping as I don't want to shell out for lots of parts or a new boiler. Just ordered a new flow switch as well asonly just noticed tonight that the CH remains on when the hot water is running.

Going away this weekend but next w/e I will get some 22mm pipe and do as you suggest.

The boiler is situated in the bathroom upstairs and all the pipes run up into the loft and across the joists. Should I run the 22mm as long as I can or would a 3metre run on both flow & return be sufficent.
Cheers Stewart

Put 4m of 22mm flow and 4m of return pipe, new flow switch, flow switch restricter and same problem. Hot water & CH working fine but the domestic inlet pipe still getting red hot when central heating is on and it has melted the new flow switch restricter (took about 30mins).Only cools down when I turn on hot water tap. Waiting for an engineer to come out to get to bottom of problem but still would like to know what could be causing it before then as I'd like to fix it myself.

As you've probably guessed, I'm not an engineer but quite like working on stuff. Could the heater exchanger be partially blocked or the pump on it's way out? Pump is set to highest setting but is it getting the water away quick enough?

Any new suggestions would be appreciated.

I know htg engineer mentioned the gas settings previously, but how would that cause the problems I'm now experiencing? It's the original gas valve and hasn't been touched.

Thanks

When the boiler first fires up it'll come on at low rate then step up to high until there's heat in the system, once the temperature in the boiler rises the gas rate will decrease. When you open the hot tap the gas rate will step up again as it has to heat the water from cold - if the gas pressures are incorrect or it's not modulating - it'll will stay at high rate the boiler will struggle to get the heat away from the boiler - next best route - DHW pipework.


htg

Hi, thanks again for looking at my post.

As soon as the CH is turned on, within 15 seconds the domestic inlet pipe gets red hot, hotter even than the CH pipe!

Chasing the engineer up again tomorrow. Should have been here over weekend!

Cheers

There are quite a few different Worcester 24's - don't assume it doesn't have a diverter because another posts says so, I think it's the worcester 24i that does't but there's the 24i junior - which does have a diverter, 24cdi which does etc etc etc.


You'll have to llok at the instructions - or lift off the front cover.

htg

Hi, its a Worcester 24i RSF-L Combi, s/n 7716130124 1921247238.

Has 4 pipes connected to the heater matrix, and can't see any sign of a diverter.

Does this help?

Thanks

Thanks for looking, Engineer came out and to be honest, didn't have the foggiest what was causing problem. He suggested a power flush and a new Heat Exchanger so I thanked him and said I'd be in touch.

Thought i'd put any money towards a new boiler.

Pump packed up on Friday, 2 days after ordering new boiler. Hot water works fine but obviously no central heating.

Makes me wonder if the pump was on the way out and was causing the problem.

Hay ho, guess I'll never know as I didn't want to shell out £80 for a pump and it still not work properly.

I know this post is very old now but anyone coming here hoping to find a possible result will be disappointed so having had this same problem and having resolved it I thought I would add my twopenny worth.
On the central heating flow pipe that runs horizontally across the back is a thermistor that is only held onto the pipe with a spring clip. This thermisitor needa to be seated properly on the pipe with heat sink compound, the pipe has been flattened where the thermistor seats. I had to bend the spring clip on mine to make it hold the thermistor in place correctly. This solved the problem for me and the domestic hot water pipe never got hot anymore.
I guess the feedback from this thermistor is used to determine the gas flow and modulation.
I am no expert on boilers, just trying to save myself buying a new one when this one works fine, mine is 14 years old now and still going strong.
I hope this is of some help to others.