Advise required r.e.Vaillant VCW-sine 18

AND1234 · 28 Feb 2008

Before you all say get a heating engineer out please hear me out.

Ive a problem with the above combi, hot water flow has never been great and in the last few weeks has become better pressure but little or no temp.
Initially opon opening a tap you may get some very hot water through but this soon dies off to luke warm and will only increase in temp if the tap is heavily gagged in. This is worse on taps further away obviously due to temp drops etc...
Ive tried searching the forums and seen similar problems but not the same.
My boiler knows I want hot water as it fires up when I open a tap - so not a diaphram?
My boiler seams to cope quite well with CH so not a gas supply issue?
My boiler does not heat the CH when i'm asking for HW so not a diverter problem?

My boiler is very old so my questions are -
1 - Does anyone have an idea what the problem may be.
2 - Is it worth spending alot of money on this old boiler? (Ive read mixed opinions already varing from old boilers are bullet proof to new boilers pay for themselves being more efficient.
3 - Obviously I could get a heating engineer in (And would have to for gas related work) but I want to know what the issue is before he replaces every flippin part under the sun...
4 - If its new boiler time. (which depends on answers in 1,2, and 3 above) how do I decide which to go for? Just because the guy who comes in usually fits say worchester, how would I know I could have done better for my money?

Any information or help off you guys would be much appreciated. Be nice its my first time.
Thanks in advance
Andy

ChrisR · 28 Feb 2008

Would still say diaphragm. Check pin stays right out while water is running.

LUNAT · 28 Feb 2008

HI

your boiler is very old and not worth repairing
i would recommend that you have your boiler replaced

DP · 28 Feb 2008

Onb the contrary, great boiler well worth looking after. In fact, at around 25 years of age, a lot of parts arre still available.

Might be worthwhile looking for an engineer who is not a boiler changer and knows how the boiler operates. If he can then show you parts that are broken, burnt etc and combined price is lumpy, then by all means consider a replacement.

Gasguru · 28 Feb 2008

Could be a sticking pressure differential valve preventing the gas valve modulator from increasing the burner pressure to max.

You could read the FAQs on measuring the gas rate and come back with the figure..measure in hot water and then heating mode.

Nixt · 28 Feb 2008

LUNAT said:
HI

your boiler is very old and not worth repairing
i would recommend that you have your boiler replaced

Agree with above. The VCW18 was a good reliable boiler but it's flow rate was pitiful (due to 60,000 output) and being old style BF with permanent pilot is just too outdated.

The next model, Combi-Compact range, with electronic DBI, fanned flue, good flow rate, plate heat exchanger conversion option and low NOx burners are still worth maintaining.

Agile · 29 Feb 2008

Gasguru said:
Could be a sticking pressure differential valve preventing the gas valve modulator from increasing the burner pressure to max.

I thought that was solely a function of the NTC reading as interpreted by the PCB?

Do you have circuit diagram of the pcb?

Tony

AND1234 · 29 Feb 2008

Gasguru said:
Could be a sticking pressure differential valve preventing the gas valve modulator from increasing the burner pressure to max.

You could read the FAQs on measuring the gas rate and come back with the figure..measure in hot water and then heating mode.

Right, thanks for this. These results look interesting to me. Not overly accurate but hope someone can make some sense of these...

HW only 42secs = 26kW
CH only 41secs = 26.6kW
CH + HW together 40secs = 27.3kW
CH + HW + hob (seven ring range burner with Wok on) 32secs = 34.1kW
All seven hobs only 84secs = 13kW

Hob just done for fun really.

AND1234 · 29 Feb 2008

If this helps anyone answer my questions the HW does seam to be hotter if I have the central heating on at the same time. Unsure why?

The CW pressure is very good so I gagged CW to combi feed in down quite heavily to try and help boiler out but made no difference.

HW flow is so poor, that if HW tap is open upstairs and another gets opened downstairs the upstairs will go off all together - but this maybe so with all old combis?

pls help

Agile · 1 Mar 2008

Please measure CW flow rate in litres per minute.

Next DHW flow rate.

Can you also say clearly in one sentence what problem you actually have with the boiler?

Tony

Gasguru · 1 Mar 2008

26Kw sounds about right from memory. If the boiler flow pipe does not get hot then the boiler is working fine..you are probably just opening the hot tap too much and expecting miracles from the boiler. (very easy to do on bath mixer taps). Get a bucket, with the incoming cold water in the middle of winter don't expect to get much more than 8 litres per minute flow rate raised to bath temperature.

AND1234 · 1 Mar 2008

God, you guys stay up late!

Right, not scientifically accurate but here it goes -

CW at kitchen tap (monoblock mixer set to cold) - 20L/min
HW at same tap (although very cold) - 10L/min
Cant get even luck warm at this tap (10m from boiler)

HW with acceptable temp at bathroom tap, (also a monoblock mixer set to hot) room next to boiler - 4L/min, if that. Thats warm not even hot.

Is this acceptable?

Gasguru · 1 Mar 2008

You say it's burning 26Kw of gas (correct amount) AND the heating pipes aren't getting hot. The heat energy must be going somewhere. You should easily be able to get 8 litres/min to bath temperature at this time of year.

Re-check the gas rate in hot water mode with the hot tap set fully open.
You're looking for about 42 seconds for 1 revolution of the dial (imperial meter) or a difference of 0.080 in the start/finish reading over 2 minutes (metric meter).

Have you got any single taps (ie not mixers). If so see what the hot's like. Some mixers can be faulty and still blend in cold water despite being on hot only.

AND1234 · 1 Mar 2008

Gasguru said:
You say it's burning 26Kw of gas (correct amount) AND the heating pipes aren't getting hot. The heat energy must be going somewhere. You should easily be able to get 8 litres/min to bath temperature at this time of year.

Re-check the gas rate in hot water mode with the hot tap set fully open.
You're looking for about 42 seconds for 1 revolution of the dial (imperial meter) or a difference of 0.080 in the start/finish reading over 2 minutes (metric meter).

Have you got any single taps (ie not mixers). If so see what the hot's like. Some mixers can be faulty and still blend in cold water despite being on hot only.

Hello, I have all monoblock mixers and themostatic mixer on bath and shower. So I isolated a cold water service valve on a kitchen monoblock and timed on HW only between 42 and 44 seconds... I understand heating energy must be going somewhere, presumably its just not efficient? It used to better than this though even on cold winters...

mrgas · 1 Mar 2008

You have a lot of mixer taps /showers.
Check the temp of the hot water pipe coming out of the boiler.
You could well have cold water mixing in with the hot somewhere along the line.

I think these boilers are generally worth repairing.
Most parts are still availlable and mainly cheap.
Of course it wont be as economical as a new condenser in gas bills alone however no new boiler will last the 25 years plus that these old beasts have.

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Advise required r.e.Vaillant VCW-sine 18

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