Vaillant 242 Combi - DHW variation & sticky boiler drain

<Updated 27th Dec. 2006 to include knackered boiler drain tap>

Hi all...

Over the past few weeks the hot water from our 12 year old Vaillant 242 boiler has been fluctuating... as far as I can see the central heating temperature is fine.

In April 2005 the original "red" heat exchanger was leaking and was replaced with a smaller rectangular chromed unit, this came with a 1 year parts and labour warranty and 5 years anti-scale.

Searching the forums I have also found this //www.diynot.com/forums/viewtopic.php?t=12887 so wonder if the new heat exchanger is scaled up.

That said the water flow seems to be the same as it ever was and I have not noticed any sludge or rust deposits in the water.

The water temperature varys from hot (not scalding) to lukewarm so it is proving difficult to wash, bath or shower. For years the temp. adjusting dial has been scaled up so we can not change that. Back in 2005 I asked my plumber about this and he said... if it isn't causing a problem then leave it as they are prone to scaling up anyway - could this be the cause of the problem?

I can hear the boiler fire up heat some water and the switch off again, and then it starts again maybe ten seconds later - rather than staying constant, so I wondered if a sensor could be faulty.

If it is unlikely to be anything major then I'd fix it myself... problem is I don't know anything about boilers... We are also moving in a few weeks so I am loathed to spend 100's on it now

Any suggestions on what you think the problem may be please?

Many thanks

When the DHW heat exchangers are changed from the old black (not red!) plastic ones it is necessary to also change a disk inside the diverter valve to reduce the risk of scaling up the new heat exchanger. Since this doubles the time the job takes I doubt whether many people bother, even less understand why it's necessary.

Another more likely cause of DHW heat exchanger clogging is sludge from the CH side. If this is the case the whole system may need flushing and cleaning to prevent a recurrence.

You need to understand that there is "scale" meaning a Calcium compound blocking the hot water side of the heat exchanger. Thats rare in a properly functioning boiler except in some VERY hard water areas.

Then there is "sludge". Thats mostly metal oxides and largely comes from inside the radiators. That will block the boiler side of the plate heat exchanger.

The best solution is for the plate heat exchanger to be cleaned. It does not need to be replaced although some people like to replace it to jack up the bill. Its easy to clean if you know how!

Tony Glazier

Thanks guys....

So at the very least, draining the heating system and refilling it may help?

You are right the old exchanger was black (not red). The guy who fitted it is the same person that did the installation back in 1994... he's someone I think I can trust and he spent over 2 hours on the job (he was working constantly because I was with him most of the time).... I'm not sure if he changed a disc in the diverter valve.

So assuming I decided to remove the exchanger (first finding how to isolate the water supply) any suggestions on how I can best clean it?

I'm not sure if I have misunderstood but at the moment our heating is off... so there is no heat (I assume) passing into the DHW exchanger. For this reason the bolier is firing up as normal... but then fires down again - this happens at regular intervals of about 10 seconds....with correspondingly hot and lukewarm water.

Also the first occurrence of this problem seemed to happen over night... the problem has not got progressively worse really so that's what made me think it was a sensor/valve... something that might just stop working properly rather than get worse over a period of time.

Any other thoughts or do you still feel the exchanger is the likely cause?

dfarry said:

So at the very least, draining the heating system and refilling it may help?

Click to expand...

I don't think anyone suggested that. Flushing and cleaning is a much more complicated procedure than that. There's something on it in the FAQ topic above in the topic list, or in the Wiki.

No I realise that Chris.... it's also quite expensive I believe...

I'm just thinking about short term solutions really because I dont want to spend lots of money on a system I won't be using in about 4 weeks....

I could leave it and lumber my buyers with the problem but that's pretty unfair really....a least if I can get it working... it may continue like that for some months until it needs looking at again...

The symptom certainly sounds like a blocked plate heat exchanger.

However to confirm that would need a competent engineer to carry out some tests on the boiler.

Since you imply that you have a person you trust then why dont you call him to check it out?

As Vendor you have a legal responsibility to ensure all appliances are in good working order unless specified as not being in working order.

Tony

Chrishutt said

it is necessary to also change a disk inside the diverter valve to reduce the risk of scaling up the new heat exchanger

Click to expand...

I have always assumed that due to the change in pressure drop (on the primary side) across the new plate exchanger (rather than the old plastic exchanger) the control pressures across the diverter diaphragm will be different.

The new disc ensures the diverter valve will perform correctly with the different control pressure across the diverter valve.

Vaillant tech have in the past always told me there is no need to fit the disc unless TRV's are ffitted to all radiators (which is unlikely).

Anyone know the correct answer

If you look at the old diverter valve disk you'll see that it has by-pass grooves in the side that closes against the DHW circuit. These grooves allowed a small flow of central heating water to flow through the DHW HE when the boiler is in CH mode to maintain the DHW temperature.

However the new plate-to-plate HEs have a much smaller volume of water in them so such a by-pass flow would result in higher temperatures in the HE and thus scale formation (and possibly scalding). The replacement disk has no grooves and will close off the DHW HE circuit completely. TRVs on the heating circuit will tend to close and increase flow resistance on the CH circuit, thus increasing the by-pass flow.

It is also necessary to cut a link (W14?) on one of the PCBs if the boiler is a Hybrid model (plastic t/stat spindle). I don't know exactly what that does, perhaps someone can enlighten me?

Emptying and refilling the boiler wouldn't do any good at all. But sudden blockages can be flakes of grot, which can be relatively easy to clear out. The boiler would need draining and the heat exchanger taking out though. If the access to the left of the boiler is good it shouldn't be too big a job for a boiler man.
The sensor suggestion makes sense, but they don't often go that way. Something you could do more easily yourself, with the manual.

I must admit, I never got around to doing anything about this problem... partly because of the impending move (WHICH NEVER HAPPENED AFTERALL)....and I had loads of other stuff going on which made this a low priority....

But it is back on the Agenda again, plus I have another problem now with the boiler.

The draining tap on the right side of the boiler seems to be sticking open so consequently the boiler drains as soon as it is filled. I noticed the pressure on the boiler was low so had filled it a little.... but this morning even with the boiler cold the pressure seemed unusually high (I think the gauge may be knackered)... anyway I drained off some water and since then the water hasn't stopped draining!!

The space around the boiler is confined due to cupboards but I will take a closer look tomorrow to see if it can be removed and serviced in some way.... I may as well have a go at the heater matrix whilst I am at it unless anyone thinks I am better to leave this well alone.

I don't know if the drain tap can be replaced separately, it looks part of a cast metal housing that takes the other inlet/outlet pipes.

So at the moment we are without any heating and luke warm water.... I know, I know.... I only have myself to blame!

THe other option is get the guy in who fitted the system and in April 2005 the heat exchanger.... but with Xmas I could well do without the expense of that.

Well I managed to stem the flow of most of the water draining from the boiler...

So much so that it was just dripping....

I removed the plastic cap on the drain tap (twisting it anticlockwise 90 degrees opens the valve) and gave the brass spindle a few gentle taps with a hammer... it seemed to work anyway (well so I thought).... I noticed it would be possible to remove just this tape and fit a new one, so I will try to get some prices for a replacement part.

Strangely the water is still dripping from the drain pipe, but the boiler pressure is increasing - I don't understand how that can be - I would expect the reverse to be true. My only idea is that the filling loop tap is also worn and thats allowing a small amount of mains water to work into the boiler.

I assume you mean the pressure relief valve? If so, certainly you can get a replacement.

If the pressure keeps going up, remove the filling loop (as it's supposed to be!)
If the pressure still goes up then the secondary heat exchnger is leaking internally

Hi Chris...thanks...

I don't think its the pressure relief valve, I believe that is at the top right side of the boiler, it is slightly open (rather than screwed tightly down) and assume that it OK (but perhaps it isn't?)

The draining tap/valve is under the boiler, far right of the main inlet/outlet pipes - it has a grey plastic cover that then twisted anti clockwise opens the valve and releases water from the bolier... this then comes out of the drain pipe into the garden (this is a constant trickle at the moment)... the other day though the boiler was losing pressure immediately because the valve seemed to be sticking open.

I may be using the wrong term when referring to the filling loop... basically there is a silver braided/flexible pipe that runs from our 15mm cold water supply pipe up to the CW inlet pipe on the boil. This pipe has a inline tap and turning this at 90 degrees clockwise allows the boiler to be filled with fresh water. As you'd expect this rarely needs to be done but the tap still seems to move freely and I assume closes fully.

So assuming the system is fully sealed I am surprised that the drain pipe still leaks yet the pressure in the boiler remains constant.

Yesterday evening I released more water from the system (whilst cold) to take the pressure to about 1 bar. The heating was then on for about one hour until it timed off. The boiler was not used again until this morning, the pressure was at 2 bar then. The heating has been on for a further three hours and the pressure is around 2 bar currently - radiator heat and DHW all seem fine at the moment.....it's the pressure thing combined with the dripping over flow that is confusing me...

What you are calling a "draining valve" is the pressure relief valve and it should NOT be used for draining the boiler as thats likely to cause it to leak!

Tony