descaling

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ACOperson said:
(Detailed instructions for descaling)
I'm a newcomer to the forum, so please be gentle.

Like others here, I'm facing a situation where my Vokera Linea 28 is repeatedly cutting out when delivering h/w, apparently due to overheat on the internal side of the H/E.

This boiler provides one useful bit of data to help the diagnosis: I can see the boiler side H/E water temp, and it's very high - up close to 100C. The actual hot water out is in the 40C range. This seems pretty conclusive proof to me that, with a temperature differential like that, the exchanger is being very inefficient, and I assume that it's scaled up.

I can get along by reducing the output water temp, so this isn't an emergency yet. But it's bound to get a lot worse, especially living in an area where the mains water is actually a concentrated chalk solution. Questions, then:
  • Is replacing the exchanger a much better fix that descaling it? I assume they have some passivated surface that reduces scale, and that just descaling wouldn't necessarily restore the thing to pristine.
  • I can get a new exchanger for £70. Is there any way of knowing in advance all the little washers, gaskets, O-rings and other crud I'm going to need? Do spares places rotinely sell purpose-made kits with all the stuff in it?
  • Descaling the thing would probably also involve removing it from the system, so it will probably be just as sensible to replace it - and avoid all that fezzle with acids. Does that make sense?
  • Finally, although I'm strongly motivated to do the job myself, I've recently discovered the mixed joys of getting somebody in to do things for you. How likely am I to get a rip-off merchant who will try to flog me a new boiler etc.?
Any input from wiser heads most appreciated.

CD

MOD

please do not hijack posts
 
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This seems pretty conclusive proof to me that, with a temperature differential like that, the exchanger is being very inefficient,
Doesn't necessarily follow. It all depends on the flow rates. If the secondary water flow rate is too high the water cannot get hot enough, even if the heat exchanger is as good as new.
 
I would only diagnose faults like that with a contact thermometer as the boiler may not be measuring what you expect!

However it seems likely that it is blocked. Probably on the primary side unless the temperature sensor failed which you should have checked first.

We only clean them as this is totally effective when its done properly. Few boiler engineers seem to want to clean them or perhaps know how to do it.

We would charge £132 in our primary service area. Perhaps you would be better buying a new one and the four sealing rings if you are going to try to do it yourself.

If there is dirt in the system then this will be likely to happen again in the future.

Tony Glazier
 
Oops! Sorry, Mod - thought I was contributing to the other thread. Now I know.

Grateful thanks to chrishutt and Agile for their valuable input.

chrishutt said:
It all depends on the flow rates. If the secondary water flow rate is too high the water cannot get hot enough, even if the heat exchanger is as good as new.
I agree, but - when the system was first installed, five years back, it was perfectly capable of dealing with the flowrate requested by the shower. The flowrate's pretty much the same; the incoming water's the same; the only thing that seems to have changed is the differential temp on either side of the exchanger.

As this is a diabolically hard water region, in which a kettle will acquire about 1mm of CaCO3 per week, it looks like a good suspect to me.

Agile said:
I would only diagnose faults like that with a contact thermometer as the boiler may not be measuring what you expect!
Absolutely right, and thanks. I've not yet gone over the boiler circuit in detail, but as far as I know there are thermocouples all over the place, and the display unit will show their values under particular situations: when the h/w is flowing, it shows that temp. When you cut the flow, I think it shows the primary side temp, but I'm not sure - I'll check. When the system was cutting out, those temperatures were 50C and 95C respectively, and I know there's an overheat cutout at 100C on the primary - which suggests that the primary should never get that hot.

Reducing the output h/w temp lowered the temp of both sides, and the cutouts stopped occurring.

I'm interested by what you say about:
However it seems likely that it is blocked. Probably on the primary side unless the temperature sensor failed which you should have checked first.
You reckon it's the primary? I'm puzzled. Isn't that side effectively isolated, and not likely to have enough dissolved calcium to cause a problem? I would expect the secondary, although it's not as hot, to have a lot more available scale to build up on the h/e surfaces.

You say 'blocked', and maybe you're talking about a mechanical crud blockage rather than scale - which I admit I'm pretty fixated on as the actual cause. It screws up all the other appliances round here. I will check for primary side blockage, though.

We would charge £132 in our primary service area. Perhaps you would be better buying a new one and the four sealing rings if you are going to try to do it yourself.
I'm extremely grateful for both items of info in this bit: the guide price is very good to know and look out for. You also suggest that just the exchanger and the four rings are needed, which is excellent news.

Gents, you appear to have sorted me out, and I'm very thankful. Any further corrections to my ignorance would help, but I'm already a lot more educated.

Most appreciated.
CD
 
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Your boiler has NO thermocouples

It has just two NTC thermistors to monitor temperature and to diagnose your problem an external contact thermometer is ideally required. An experienced hand can do that too!

Owners often get fixated on what they decide are the problems. We just switch off our free advice when they do that! We cannot always diagnose faults every time by email but are likely to have better than 51% success rate.

Tony Glazier
 
If your secondary side is scaling you should expect reduced maximum water flow in the DHW circuit; this should give you a clue.
I think you have wisely decided to replace the H/E as, not only do you get an as new condition on both sides of the H/E, you also get something to play with later.
Agile is suggesting you clean the primary circuit in its entirety to prevent sludge and other dirt migrating back to the primary side of the new H/E and reducing its performance.
 

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