Vokera Linea 28 - No hot water

[niggle]

I appear to be suffering a sequence of failures with our Linea 28.

In April this year the boiler was repaired and fully serviced by a Vokera engineer. It was blowing off the PRV which turned out to be the common blocked expansion pipe problem.

Last week the PRV blowoff problem returned with a vengeance. I unblocked the expansion pipe again and all was well for a day or so. Then the expansion vessel failed. Replaced the EV yesterday and also the DHW heat exchanger. Radiators now heat up nicely and pressure stable and below 1.5 bar.

All was well until today. Virtually no hot water. Boiler ignites and runs in both HW and CH modes but in HW mode the CH flow pipe is getting hot rather than the HW flow pipe. No HW at taps, barely warm and temperature varies greatly.

Diverter valve motor seems OK so I removed the motor and pressed the diverter rod in while HW tap was running. Still no HW though, CH flow pipe still getting hot, HW flow pipe cool.

How do the diverter valves work? Does pushing the rod in allow the shuttle valve to move and redirect flow? Is it possible that the shuttle valve is stuck in CH flow even when the rod is pressed or do I have some other problem? Is there any possibility that the lack of HW flow is due to a stuck by-pass or non-return valve?

 

[Agile]

There is a non return valve in the boiler circuit at the plate HE. Sometimes they get jammed by dirt!

I think that its at the right of the plate HE.

Tony

 

[G60]

If the expansion vessel hose has blocked twice in a year you really need to get the system clean. If you are still getting heat to the ch flow with the the rod pressed fully in then the shuttle could be warped or lost its o-ring. If your system is a dirty as it sounds the dhw plate hex could also be blocked.

 

[niggle]

Thanks, gents.

I will service the 3-way and inspect and clean the by-pass and non-return valves.

Water lost from the sealed side when removing parts and captured from the PRV is clean. Obviously some oxide and magnetite solids moving about though and causing mischief. I'll run some X400 through for a week or two.

DHW hex replaced two days ago as a precaution. Old one was descaled a few years ago and now has a light coat of magnetite on the sealed side but appears to flow water from port to port.

 

[Agile]

X400 needs 4-6 weeks.

Its only a chellating agent and does not dissolve oxides!

Tony

 

[niggle]

I have no idea what a chelating agent is but does this mean that a "power flush" is my only option?

Are there any effective proprietary chemicals that I can purchase over the counter and leave in the system for a while to shift the oxides?

The pump seems to be healthy enough and we appear to have a good amount of hot water flowing to all radiators at the moment.

 

[Agile]

Citric acid will dissolve oxides but it has to be used very carefully as it can cause leaks in an old system.

Because of these risks I dont really recommend it for DIY use.

Its a constituent of DS40 and FX2 and some other sludge treatments. I prefer the basic citric acid.

Tony

 

[niggle]

So when my local expert turns up with his powerflush machine and a litre or two of X800, what is he actually achieving for £300+?

Is an aggressive acid-based cleaner normally necessary?