WB 28CDi - CH fine... HW v. noisy

BishBashBosch · 18 Apr 2006

Problem is with a Worcester Bosch 28CDi installed 2 years ago.

System runs without problem for the central heating but (whether the heating is on or off) the boiler makes alarming noises when the hot taps are turned on... it's done this for the last eleven months becoming progressively worse.

When B Gas look they say the system needs flushing (£££!) But how can flushing the radiators (done last year at same time as heat exchanger replaced to fix the same problem) have any effect on the hot water?

They say that it's because the tank for the heating system in the loft is only a few feet above the boiler - which is situated in an upstairs cupboard (location of old hot water tank).

Their solution is to convert the system to a closed one.

Again, this seems to be making changes to address a problem with the central heating when the problem is with the hot water?

I understand that they use the same heat exchanger... but if the system doesn't bang when it's heating the radiators then why does it bang when it's heating the hot water?

Any ideas?

kevplumb · 18 Apr 2006

I understand that they use the same heat exchanger... but if the system doesn't bang when it's heating the radiators then why does it bang when it's heating the hot water?

the waterways in the h/w heat ex are much smaller bore therefore get restricted much easier

BishBashBosch · 18 Apr 2006

OK, so the symptom (loud banging) is a result of restricted flow through the (clogged up?) heat exchanger... that makes sense.

Short term solution = change heat exchanger...

However the damage to the exchanger which is resulting in a restricted flow must be being caused by something... presumably, whatever is the root cause of this must be what I need to address for a long term solution?

Is this possibly from the heating system?

Since the HW and CH must presumably take different routes through the heat exchanger (i.e. in different pipes) how can issues arrising from the configuration of the heating system cause problems in the hot water system?

This is what confuses me as to why the heating system needs flushing...

kevplumb · 18 Apr 2006

it's full of crud(technical term)

BishBashBosch · 18 Apr 2006

system was power flushed a year ago...

is it normal to clog up again so quickly?

plus how does crud in heating system effect hot water system?

kevplumb · 18 Apr 2006

it uses the same water just sends it in a different direction

so if one is dirty they both are

when the black valve on the tee changes over it sends the water to the heating system

BishBashBosch · 18 Apr 2006

This is really helpful.

Can you please let me know where the diagram comes from so I can see the key for all the numbered bits.

kevplumb · 18 Apr 2006

1 boiler casing
2 burner
3 condensate trap
4/5 temp sensors
6 auto air vent
7 pump
8 control panel
9 secondary heat ex
10 badge
11 c/h flow
12 hot water out to taps
13 flue outlet
14 expansion vessel charging point
15 expansion vessel
16 primary heat ex
17 flame falure sensor
18 burner fan
19 gas valve
20 gas inlet
21 diverter valve
22 system by pass
23 water flow restrictor
24 pressure relief valve
25 c/h return
26 cold water in

BishBashBosch · 18 Apr 2006

...much clearer now.

Combis seem a pretty useless design... seems guaranteed to fail...

can't say I'm clear how best to stop the problem re-occuring...

shelling out £600 - £800 per year to have the system flushed doesn't appeal... but what other choice is there...

will making the system closed help to extend the time between flushes?

ollski · 18 Apr 2006

Get Bg to powerflush the system and correct the reason it is sludging up. ie convert to sealed system. Then if you keep it on a contract they will flush it again for free.

solo · 18 Apr 2006

oooohhh!!!!!! Kevplumb, that's a fancy pic. How did you do that??

just-thinking · 18 Apr 2006

best pic of the year

kevplumb · 18 Apr 2006

found it on a website when i was looking for something else

Soggy_weetabix · 18 Apr 2006

kevplumb said:
it uses the same water just sends it in a different direction

so if one is dirty they both are

when the black valve on the tee changes over it sends the water to the heating system

Great piccy kev,

You should put that picture on FAQ!

Answers 90% of combi problems without the need for words.

Brilliant!

David

kevplumb · 18 Apr 2006

was thinking of doing that

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WB 28CDi - CH fine... HW v. noisy

just-thinking

Soggy_weetabix

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