Biasi 28s DHW is only luke warm.

[gno750]

I have a Biasi 28s, its around 10 years old & for the most part has been a very good piece of kit.
However its playing me up now & i seek some advice from whom ever can help.

When DHW is called for the boiler fires up & nice hot water is supplied for about 5-10 seconds, it then very quickly reduces temp to nothing more than luke warm even though the burner is still in operation, occasionally the burner will shut off but restart again but not in a regular pattern, it may go off and on once or twice at the first drawing of hot water ... for the most part the burner remains on.

Reading through similar problems on these forums i have changed both membranes & the DHW flow switch .. which have had no effect, when i purchased these items i also got a Temperture sensor/probe but havent fitted it as yet because the removal of the wiring clips to each of the sensors in turn that are already in situ (thus sending a nil temperture back to the pcb) doesnt make any differance either.

The C/H works as it should.
Any advice would be gratefully appreciated.

Best regards Glenn (southampton)

 

[ratonnastick]

I would investigate the plate to plate heat exchanger as it maybe blocked.
Do the heating pipes to the rads heat when running the hot water? If so the diverter may be duff.

 

[Agile]

You cannot make any judgement without measuring the DHW flow rate.

Set it to provide 10 li/min and see if its too hot to hold your hand in the water.

Then measure thegas rate at the meter, see FAQ for how to do that.

Tony

 

[gno750]

Thankyou both for replying.

Called for DHW, the water supplied reduced quickly to luke warm & neither of the 22mm pipes under the boiler heated up, so is the diverter valve in the clear?

I've measured the DHW flow & gas rate.
The water flow i did by filling a 5 litre container which took 30 secs, so 10 litres per minute, the water was luke warm.
The gas flow came out at 15.2kw, which seems low compared with the example in FAQ .. the gas taps are fully on & it appears to be giving its all, the temp reading went up to 80c while doing this test!

 

[Agile]

The boiler modulated back from the 35 kw inoput it should have been giving.

The high flow temperature indicates the plate heat exchanger is blocked or coated and is not transferring the heat into the DHW.

There could also be other causes like a weak pump but its most likely to be a problem with the plate HE.

Tony

 

[gno750]

Thankyou Tony.
That all makes sense to me, the DHW HE will then be the next item to check out.
It doesnt appear to be a problem to fit a new one on this boiler once the two circuits have been emptied.... however i would be concerned about another blockage occurring in the new one if there is that much rubbish floating about in the primary circuit.
Is flushing the system a DIY job?

 

[gno750]

I'll extend the question

Is flushing the system a DIY job, if there are instructions to follow?

 

[Agile]

It all depends on how dirty it is and how long it has taken to get to that state.

Just draining and refilling might be all it needs or fitting a filter on the return would be easier.

Tony

 

[gno750]

So i have replaced the DHW HE with the correct replacement.
However on refilling both the circuits & turning the power supply back on i have neither CH or DHW now.
The pump is working but no fan or burner.
The lock out light is not on.
The gas main supply is on but the burner doesnt not attempt to light (no spark flickers across the igniters.)


The main circuit is showing 1 bar on the gauge but would an air lock in this system cause my problem ?? (the auto air purge is open one turn & i've bled all the rads.)

 

[Agile]

Did you bleed the central pump screw?

The AAV cap may be loose but did any air come out of it?

You could gingerly push a needle down the middle but that can allow water to come out so have the cap ready in case.

If still not working see if the pin is coming out of the pump prooving switch to operate the microswitch.

Alternating demand for DHW and CH might help to get rid of the air.

Tony

 

[gno750]

Thanks Tony,

It was the pins not coming fully out to trigger the flow switches, i called for CH & got the missus to turn the hot tap on & off & fired the switches manually with a couple of electrical screwdrivers, the fan & burner came on.. after a few seconds the pins extended fully.... they went slack again when everything was turned off as they should but i guess any air lock in there moved on.
Replaced the flow switches & everything works as it should.

Regards Greatful DIYer