Biasi M90F 24S hot water problems.. (FIXED)

[Calmore]

Hi All,

I have a Biasi M90 24S which is causing hassle.

If I switch the wall thermostat on the boiler heats the radiators just fine. Pump spins, pressure is OK etc.

If I put on a hot tap I get cold or tepid water. The diverter valve pin comes out and pushes the micro-switch OK and the boiler fires up. Various pipes in the boiler get hot.

If I switch the tap off then the diverter valve pin pops back in and the boiler shuts down as I'd expect.

(We have has a few intermittent problems in the past but it always rectified itself and I put it down to a sticky diverter valve pin, which I'd investigate in the summer)

Any thoughts?

 

[picasso]

Diverter valve is stuck in the central heating position, run the hot water and see if the 22mm pipes under the boiler get hot.

 

[Calmore]

I'll do that once the pipes have cooled a bit (I have been running the CH)

Also I have:

1. replaced the three way diverter valve actuator (I have a spare)

2. Checked both temperature probes with a multimeter (They agree with the graph in the manual)

3. Checked that the LED lights on the flow switch when the basin tap is open (It does)

I have noticed that when calling for HW, the boiler will fire up for maybe 30 seconds, switch off for 30 seconds, fire up for 30 seconds etc. The DHW gets tepid, then cold, then tepid etc.. would this be consistent with a sticking three way diverter valve?

 

[picasso]

Always helps to get the info a bit at a time it sounds more like a blocked plate heat exchanger, any further info winging its way ?

 

[Calmore]

Have run the DHW for about 10 mins and I suppose the flow and return pipes from the boiler are moderately warm. I can't really tell whether this is due to heat conduction in the copper, though.

If I remove the diverter valve actuator again, will there be a pin in the middle that responds to being pushed? If it's stuck in the CH position, will it be stuck up or down?

 

[Calmore]

Would a blocked heat exchanger fail suddenly? It has been working OK over Xmas.

 

[picasso]

Turn the hot water temperature control down to about half way on the dial, if the boiler runs ok then its most probably a blocked plate, forget the diverter valve being stuck that was from the (lack of) info in your first post.

 

[Calmore]

OK, turning both heating controls down makes no difference; it still does the fire up, stop, fire up thing when the DHW is running.

When the DHW is running the heat exchanger is too hot to touch on the front but cold on the back. When not running the heat soaks through the whole heat exchanger and the whole lot is hot to the touch.

 

[picasso]

Then I would go for blocked plate but would check the resistance of the dhw thermistor before taking the boiler apart.

 

[Calmore]

Have checked both temperature sensors and both read correctly.

Have ordered a new heat exchanger to try. I will let you know the result!

If it turns out to be sludged up, is there a reliable way to clean it so that I can keep it as a spare/sell it on (It's stainless steel)

Thanks very much for your help.

 

[Agile]

This a is a Biasi M90 and not a 24S.

The DV motor pushes the pin down about 9 mm to get the CH position.

With the motor removed its in the DHW position.

With motor removed select CH and put the motor back and you should feel the resistance of the pin as you push it down.

In DHW with a clean plate the boiler temp gauge should read 60-65 C. If plate is blocked it will rise to above 70 C and can get to 90 C at which point the o/h thermostat will open.

Tony

 

[Calmore]

Hi,

It says "M90F 24S" on the label. What's the difference between an M90F 24S, M90 and a 24S? All very confusing!

Will check the motor operation tonight as you describe. It's a brand new motor but I suppose the wiring/PCB could be at fault. The pin moves up and down under thumb pressure sp I know that bit is working.

Heat exchanger is on order so in the absence of anything else proving to be faulty I'll swap it out and see what happens.

It has had inhibitor since being installed; are there any recommended brands when it comes to refilling the system or are they all pretty much of a muchness these days?

 

[Agile]

The 24S was one of the first Biasi boilers to be imported around 1996. They had several names like Riva, Prisma etc.

This was superseded by the M90 in 2002 and to indicate the boiler power the M90 was followed by 24S, 28S or 32S.

Sentinel X100 is about the best and the brand leader in inhibitors. But don't add any to your system until you are 100% confident the boiler is working properly.

On DHW with a clean plate the boiler temp gauge should read 60-65 C. If plate is blocked it will rise to above 70 C and can get to 90 C at which point the o/h thermostat will open.

Little point in changing the plate HE normally unless the gauge is showing an elevated temperature.

Tony

 

[Calmore]

I have ordered a new heat exchanger and service kit for the diverter valve as I noticed it's weeping very slightly around the pin (Although still working).

I'll report back once I have resolved things!

Thanks for the info about the boiler codes. Things are a little clearer!

 

[Calmore]

UPDATE

Parts haven't turned up yet but decided to take the heat exchanger off in preparation.

To be honest I'm not sure it's the culprit. The DHW side is surprisingly clear, so clear that it's still shiny in places inside, which I don't think is bad for a 10 year old boiler in a hot water area!

The 'boiler' side is coated with a little black magnetite, but I can pour water in one end and it comes out the other. If I connect up a rubber tube I can puff air through both sides with not a lot of resistance.

Result is I'm pretty darn stumped!

To sum up:

*DV valve pin moves in and out and switches micro-switch - boiler will fire up in response
*DV actuator works as expected; DV valve pin moves up and down.
*Both thermistors measure OK
*Pump is spinning OK.
*LED on flow switch lights OK
*CH mode works fine
*DHW mode doesn't work - boiler cycles between firing up and not - tap water goes tepid - cold - tepid

Any thoughts?