28CDi DHW - pressure fluctations, banging, and variable temp

[JDA2012]

Hello again all!

My boiler is on the blink again - it's a (now somewhat old) Worcester 28CDi RSF, and has previously had the DV diaphragm, fan and PCB replaced (by a competent tradesman where appropriate), as well as the manual timer replaced with a computerised one following various problems.

The current problem is that whilst CH is fine, DHW gives the correct lights and indeed (after a short delay) a supply of water at a good temperature, but pressure fluctates rapidly from 1.3-1.8 bar or so (set at 1.5 bar when cold) and after 20-30s or so there is a banging noise, lights and burner go out, water becomes cold, then shortly after that burner comes back on and the cycle starts again. Sometimes the banging occurs whilst heating continues as opposed to coinciding with it going off.

I have had the water-water heat exchanger off and checked for any blockages - seems to be clear. The pump is working, responds to speed changes (left on high for normal operation) and as noted CH is circulating OK. Hot water taps have good flow with boiler turned off.

The thread on the plastic manifold connecting water-water HE output to gas-water HE input (i.e. cold water flow) is completely shot and was leaking (very temporary fix with a thicker washer currently holding), so I've ordered a new manifold and will fit that, but it seems unlikely to be the problem.

Water has obviously previously escaped through the auto air vent on top as there is a brown patch on the ceiling above this. The cap has always been left on loosely to allow air to escape. Radiators do not release any air when bled, only brown water.

I think I have an overheating issue here, or maybe a thermistor fault - could I get a little help please? Happy with working on water parts, know not to touch gas ones, and believe this is the former at present.

 

[picasso]

Although the plate heat exchanger may look clear I reckon you have a blockage.

 

[JDA2012]

I see - thank you.

I will have to take the HE off tomorrow to fit the new manifold, and will be checking the expansion vessel pressure at the same time (whilst I have the system emptied).

Is there anything I can do to test the HE whilst I have it off, short of just squirting water through it? I have also removed the flow restrictor to see what effect this had (as the water was rather on the hot side but we could do with more of it even when working normally) - may be marginally better but could be in my head!

Would running some X200 or similar help with this sort of thing or is it a case of replacing the HE if blocked?

 

[JDA2012]

OK, all day today spent fiddling with the boiler, mainly due to this type's well-known propensity to spring leaks all over the place whenever you touch it!

Actions:
- New return manifold now fitted
- System drained (very brown) and expansion vessel charged to 13PSI/0.9 BAR (was essentially zero)
- Water-water heat exchanger rinsed out until gunk/bits stopped coming through
- System refilled and all radiators bled

Results
- I now have a leaky PRV (serves me right, will replace when the shops open)
- New return manifold has stopped the leak there
- Various HE leaks appeared (now dealt with)
- Central heating still OK (in fact better in some rooms after bleed)
- DHW still unreliable - can now run a luke-warm bath at least but behaviour is still about the same (hot>cold>hot>cold with banging from the boiler and along the pipes in between - I find this quite alarming to be honest)
- Had a large release of brown water through the auto-air valve at one point. On inspection, that is in a very messy state and I would think needs replacing
- One radiator is not heating up at all (it is on its own small "circuit" and has been bled) and one other is heating poorly but uniformly

Any advice on the auto-air valve, DHW problem or how best to clean the HE properly (still looking at X200 or similar) please?

 

[JDA2012]

New PRV fitted and leak stopped. Still a very slight leak from just below the pump, where the pipes look to have been coated in some sort of sealant, but much better

Also purchased some MC3 whilst at the spares counter - that will go in tomorrow night for a week to sort out the clogged radiators (assumed cause of poor heating and brown water).

Was going to replace the auto air valve, but this is a gas-side component so will not be going there. Not sure about the HE now as the DHW flow is good with the boiler off (and indeed on - just the temp that's the problem) and the CH flow is no problem (except the two dodgy rads) suggesting no blockage to me, but perhaps I have missed something?

Considered getting some MC2 (limescale/noise reducer) but this is again for the CH and I can't see that it would help with my DHW problem. Do not want to purchase a new HE at this stage...

Any advice still gratefully received!

 

[hanns21]

No offence but after all the time , money and effort i just dont understand why you dont just pay someone to fix it. My opinion would be to get a new boiler with a long warrenty instead of throwing money at this. Defo sounds like a blockage on flow to plate to plate but without being there its all a guess! Why you would want to just guess and put a new heat exchanger in it amazes me!

 

[JDA2012]

None taken, everyone has their own way of approaching things. At ~£70 that heat exchanger is not much more than getting someone out just to look at it (certainly a lot cheaper than getting them to fit one!), old enough that it would probably be of benefit to replace in any case and certainly represents a learning opportunity in itself - I value this knowledge and am here to further it.

The problem is not so urgent (dishwasher and washing machine heat their own water, shower is electric, DHW is enough to fill a basin) and the expense of calling someone out other than as a last resort / legal requirement is simply not something to which I would readily sign up whilst DIY is an option - feels too much like giving up and throwing money at the problem vs. solving it (which has appeal in itself).

As for fitting a whole new boiler, that is a relatively vast expense (£1000+) and one I wouldn't consider unless this one packed up completely and in an unrecoverable way (read melted, fell off the wall, drenched all the electrical components etc.) So far the manifold cost £15 and the PRV £11, hardly going to break the bank and still sub-£100 if I need to replace the HE. Jumping straight to the expensive options is just not logical to me and it would take a very good reason to justify paying that sort of money.

 

[Gasguru]

You really need to acid clean the plate...just flushing it through is not necessarily sufficient. I use Kamco FX2, fill the plate and rock it to expel any air. Simmering it in a bath of water on the hob gets the acid to work faster. Of course it's hazardous, you have been warned.

You might want to check the pipe from the lower end of the pump to the diverter...these sometimes block.

Untitled

• Gasguru
• 30 Sep 2010
• 7

Have you considered a Worcester fixed price repair?

 

[hanns21]

Your boiler was disconnected in 2007, its had new pcb , fan, blockages, Loads of leaks, prv, manifold.you started your thread with hello again my boiler is on the blink again..... If you want just keep changing parts every few months and spend weeks trying to work out how the boiler works. My thinking is you might want to invest in a new boiler which gives you piece of mind with a long warranty. Onces one component goes they all tend to go through age

 

[JDA2012]

I think you mean "discontinued"; it has certainly not been disconnected

The last problem with the boiler (and my last thread) was a year ago. Clearly we don't think in the same way, as I wouldn't replace any appliance because it required maintenance once a year. My car was discontinued in 2006 and I haven't replaced that; why would I? If something breaks, it is repaired - parts being readily available.

If parts become unavailable or a new model with compelling features appears, then I would consider buying a replacement - before then, it seems a waste - and certainly less interesting. My natural reaction when something stops working is to take it to bits (I would wager most of my appliances have had this treatment by now!), but I do see that just buying a new one might be the go-to action for a lot of people. The costs will, of course, add up eventually, but at £70(PCB)+£50(Fan)+£20(timer)+£15(Manifold)+£11(PRV) for all the components I've replaced so far and <£100 for labour there is a long way to go before it becomes more economical to replace the whole thing.

Gasguru - thank you, I will try that, but not this week as I'm away this weekend and whilst the other half is fine with limited DHW no heating (should things go wrong) would not sit well...

 

[hanns21]

Sorry. What im trying to say is im not a tight **** who will try to always fix things i know nothing about instead of paying people who are competent to do so. My car breaks down i pay someone to fix it, if it starts giving me hassle i would get a new one! If my boiler was the same i would pay £1000+ To Have piece of mind for alot of years that itz new and under warranty. But sure we think different.