Biasi 24s problem diagnosed, but what now.

[David24S]

My boiler keeps cutting out on DHW.

Recently I replaced the diverter valve and the heat exchanger has been fully flushed.
I have now diagnosed the fault, but don’t know how to resolve.

The problem is the flow rate.
With the old diverter the DHW flow from the tap could be low and the boiler would stay lit.
Now on low, after about 10 seconds, the flame drops, then cuts out, it then fires back up after about 5 seconds.
If the tap is on fuller, then everything works ok.
CH not affected.

So how can I resolve the flow rate, so as the boiler stays lit, when the tap is on slow flow.

Thanks

 

[ianmcd]

your boiler should modulate like an accelerator on a car it should ease off the gas as the water gets hotter, sounds like yours is cutting out on the high limit thermostat rather than modulating, what did you use to flush the heat exchanger ?

 

[David24S]

Garden tap pressure !!!
Clean water
Flushed and reverse flushed through the 4 holes.
Never had the flow issue before the diverter was changed.

Thanks

 

[ianmcd]

You cant flush a Plate heat exchanger like that you need to use acid of some description, just noticed that you are running two posts on this at the same time, that is against site rules and you will not get the correct advice

 

[David24S]

I will delete the other post.
As for the material of the heat exchanger, then it would have to be a low acidic cleaner.
Would something like white vinegar solve it, or something stronger.
We are a soft water area and have never been plagued by limescale at all.
It’s only the heat exchanger I want to solve at the moment nothing else.

 

[ianmcd]

I will delete the other post.
As for the material of the heat exchanger, then it would have to be a low acidic cleaner.
Would something like white vinegar solve it, or something stronger.
We are a soft water area and have never been plagued by limescale at all.
It’s only the heat exchanger I want to solve at the moment nothing else.

There are lots of posts on youtube about how to do it, Brick acid seems to be a popular choice, I have never had to do it and if I did I would just buy a new plate heat exchanger rather than faff around, but understand that you are DIY and want to save a few bob, the plate heat exchangers are not too expensive if it is that, which is causing your fault, it can be other things though, here is a link how to actually clean one .

 

[David24S]

Ian,

thanks.

Ive seen Genuine new ones on ebay for less than £30, I will chance that.
Also, I detached the heat temp sensor plug from the sensor on the diverter valve and it continued with the same problem, should that happen ??
Surely the boiler should act differently with the sensor detached !!!
Finally, is it work checking the filter on the inline side ??

May as well check a few things whilst the water is off .

Thanks

 

[ianmcd]

yes it is the sensor that tells the boiler to reduce heat input, if that is faulty then the next step is, the high limit stat cuts in to protect the boiler, so removing it is exactly the same thing as it being faulty as the boiler doesnt know that you have removed it

 

[Agile]

If you want us to help you properly then you need to supply real information.

In your case the maximum and minimum hot water flow rate in litres per minute.

The diverter valve will only sense the water flow rate down to about 2.5 li/min and below this it will not give a demand for heat.

The gas settings will have an effect on the gas flame and when properly set will remain lit at the 2.5 li/min minimum design flow rate. But setting the gas parameters is out of the scope of a DIYer.

If the flow rate is reduced below 2.5 li/min and provided the demand is continued then the burner will need to modulate by turning on and off and this is quite normal and correct. Due to the thermal inertia of the boiler components the water temperature will not vary very much.

You seem very concerned by the boiler operation at a low flow rate. That is not the normal way to use a boiler. A wash basin tap will normally be used between 3-6 li/min.

Tony