Biasi Riva M90 S24 no DHW

[Mikefromlondon]

Happy New Year all,

My DHW will only come when CH is turned on, but when it is turned off the DHW fails to heat up.
Checked the Primary Flow Microswitch which gets pushed by a pin when the CH runs the pump starts and the switch kicks in and fires up the boiler, but when CH is off the pin retracts and on opening a hot water tap the pin only just manages to reach the Microswitch leaver and does not fully push it. When I tried to operate the switch manually by pushing on the leaver, the boiler fires up and the hot water flows fine and with no overheating or any other problems, and hot water temperature is pretty much constant.

I changed the small diaphragm but to no avail. (the pin still will not push the Microswitch.

At this stage I am not suspecting any problems with plated heat exchanger as the flow of hot water from the tap is pretty strong and seems unrestricted by any blockage.


For the time being I have engaged the Microswitch permanently to buy some time as this happened during Christmas holidays and before I take it apart any tips and advise is welcome. especially from Tony as I had been reading other threads with similar problem on Biasi riva compact boilers.

mike

 

[Aquaheat]

@Agile The floor is yours

 

[Mikefromlondon]

I only mentioned Tony because he seems a genuine guy always willing to help with the right kind of advise and not refer me to a Gas safe Engineer, otherwise pointless having forums where such discussions are not welcomed, but on the other hand I am sure things don't just happen for no reason, I have never believed that even an atom could move for no apparent cause, the whole universe came into being through a cause, unlike many believe it came out of Nothing and spurted all by itself......Jesus! I could kill myself laughing out mad with that Scientific Statement! So for my boiler to behave as if Nothing had happened and the hot water is still being delivered at a constant temperature and at a good unrestricted flow rate, I am beginning to wonder if the Universe did come out of Nothing!

Ha ha, so what the heck is causing this fault I will find out with or without help, with your help it could save me time.

 

[Agile]

The first thing you need to do is see if the DHW Hall effect demand switch is being activated.

Most of them, several variants, have a red LED which is illuminated when it is activated.

Tony

 

[Agile]

I only mentioned Tony because he seems a genuine guy always willing to help with the right kind of advise

That sounds good to hear!

 

[Agile]

So the LED ?

 

[gazplumber]

I only mentioned Tony because he seems a genuine guy always willing to help with the right kind of advise and not refer me to a Gas safe Engineer,

So being referred to a Gas safe Engineer to be legal to work on your gas boiler is the wrong advice?

Ha ha, so what the heck is causing this fault I will find out with or without help, with your help it could save me time.

Don't you mean, with your help it could save me money?

 

[Agile]

Ah yes! I think you are right there Gaz!

 

[Mikefromlondon]

Thank you Tony, the Red led trips in the minute you run the hot water tap.

Would it help if I checked if the CH flow pipe gets hot when running hot water?

As for saving money gazplumber, indeed, that is the whole purpose of doing it yourself and joining Do It yourself why not in order to not being take for a ride.
as long as the part is not critical or gas related, most average people with mechanical skills can fix minor problems.

 

[Mikefromlondon]

Just to confirm, the red LED on flow switch trips in the instant the tap is opened.
Ran DHW with the Microswitch engaged manually and the boiler fires up absolutely fine, hot water starts to flows and within a minute it reaches 54C and maintains this temperature occasionally dipping just by a degree to 53c and boiler makes no noises and runs smooth with no overheat issues or tripping out. There are or have been no other issues with the boiler including any topping up, not sure if the installer filled any inhibitor, when he commissioned it 2 years ago in April 2013. ( was initially installed back in 2006 but the boiler was never used as the house was empty for much of the time and it was drained and left dry)
No kettling noises, or any trapped air, or other hints that something may be a problem, CH flow pipe remains cold as ice.
When I changed the diaphragm the old one did not seem to have any punctures or holes and was in a relatively good condition. Some water I drained was slightly coloured and I couldn't tell if inhibitor was used, and overall the old diaphragm that I took out had a few minor deposits on it.

Still that pin will not push out fully to trip the microswitch in, it pretty much looks like I may have to strip and check and clean the hydraulic block, possible for any partial blockages, I was hoping this would be unnecessary, the boiler was installed many years ago but was never brought into use until 2 years ago. so it is almost like a 2 year old boiler, it was installed but remained dry throughout that period when the house was undergoing refurb and remained empty for most of that time with no occupants.

 

[Calmore]

How far is the pin popping out?

A few years ago I had a similar issue and it was down to a duff micro-switch; yes, it worked when pressed like yours, but only responded at the end of its travel, which was too far for the pin to push it.

 

[Mikefromlondon]

Thanks for your reply Calmore, the pin practically reaches the microswitch but fails to exert enough pressure to completely trip the microswitch in, but when it is in CH mode the pin exerts full force and trips the microswitch in, so from this I cannot see any fault with the microswitch.

I was looking at a possible cause, studying the diagram on the Service and Installation Manual, and I can see that since CH flow is less restrictive provided the diverter valve is fully open, hence faster the flow through this body the higher the pressure created by this diaphragm differential switch, and when it is in DHW mode, the diverter valve operates diverting primary flow through the plated heat exchanger, which if it was restrictive due to partial blockage may cause a lower primary flow and thereby not generate enough pressure to knock the microswitch in, ( some guy called Daniel Bernoulli discovered that by accident when he was shagging his wife, she kept farting! (only joking) I guess this switch is intended kind of primary circuit flow switch to safeguard dry running of main heat exchanger or stop it from overheating when the flow rate drops, thus causing kettling, so now that means this bloody thing will have to be removed and looked at, cleaned and refitted or a new one.

 

[Calmore]

Worth checking that the electric DV actuator is fully operational and that the pin that controls the DV valve is free to move and not sticking.

(Another thought..a longshot..do you have an anti scale device fitted on the cold feed? Is this blocked at all?)

 

[Mikefromlondon]

No anti-scale device fitted, no filters, and seems like no inhibitors added either.

I know the one which is motorised, I haven't actually checked it yet, I can do, it is only a simple clip fastened and will try and see if it moves freely downwards and should come up with a spring force, however I am not actually suspecting it as when hot water tap is opened it provides good amount of hot water without any problems, today I allowed it to run for nearly 5 minutes and took digital temperature meter reading and the water was gushing out constantly at 53-54c and no heat detected on the CH flow pipe, it was cold as ice.

My guess is that there must be some restriction in the DHW circuit and is enough to cause this Microswitch unoperational though at this stage there is no obvious signs of any other problems, so either it has been designed to operate at a very tiny reduction in DHW flow, far too early may be, all in the name of prevention of major problems later on I guess. But i am glad that i understand that by operating this switch manually should not cause any safety issues for a few days unless the boiler was making kettling noises or other overheating issues, it has been going Ok since about 22nd December last year.when I permanently held this microswitch pressed on. I will probably have a look at it later this week to see what issues are causing the flow problems. I was even thinking is it the water pump? but if it was then the issue would also occur when it is in CH mode as well.