Intermittent hot water from Modena 80 combi!

[eastender3]

8 years old Modena 80E combi boiler.

Been getting intermittent hot water for the last couple of months, so called a boiler engineer.

He replaced 1 scaled thermister and cleaned the filter and tested the hot water taps, it worked but it was always intermittent, so wasn’t sure if it’s cured or not. But after 2 days later the problem started again, as the hot water fails to arrive on first attempt and it does not ignite. Only after several attempts, it starts. Also our mixer shower gets very hot water, when it works.

Not sure if it’s to do with low water pressure in our block of flats for the problem.

 

[adlplumbing]

posibly flow switch is scaled

 

[Agile]

As always in these cases "call him back" as he has not correctly identified the fault.

Unfortunately, I cannot remember if this model has a diverter or is a four pipe.

Tony

 

[adlplumbing]

it does not plate to plate heat ex agile thats why it might be scaled flow switch

 

[Agile]

Its pretty easy to test the flow switch.

Should actuate at about 2 li/min and the contact resistance should be less than 1 Ω and greater than 100K when not actuated.

Tony

 

[eastender3]

Thanks for your replies fellas.

He is comming back next wed for 'recall'. If it's flow switch is scaled, how easy is it to repair the switch?, does it just needs cleaning.

Thanks.

 

[kevindgas]

firstly never change a thermistor on these - change both as they work in series
the heat exchanger is twin pipe - no diverter valve, the water passes through a approx 10mm tube - this gets scaled up, descale it with a power flusher and chemicals it usually takes me around 20 minutes to do.
there is an overheat sensor on the top left hand side - these sometimes fail check the resistance / continuity
the flue has a venturi - again this needs to be washed out
these are the most common problems on the F24/30's and Modena 80/105's

 

[eastender3]

firstly never change a thermistor on these - change both as they work in series
the heat exchanger is twin pipe - no diverter valve, the water passes through a approx 10mm tube - this gets scaled up, descale it with a power flusher and chemicals it usually takes me around 20 minutes to do.
there is an overheat sensor on the top left hand side - these sometimes fail check the resistance / continuity
the flue has a venturi - again this needs to be washed out
these are the most common problems on the F24/30's and Modena 80/105's

The last time the engineer came round, he did mention about the heat exchanger but cautioned about descaling, saying it might damage it, better and cost effective, to replace the whole boiler, he told me! though he never checked anything else, just replaced 1 thermister and cleaned the filter.

What do you think the likely fault is with interminant hot water problem?

 

[kevplumb]

check your flow rate

 

[adlplumbing]

so what did this guy do

 

[glasgowjim81]

when you run the hw tap does the led under the tap light up? (on the boiler facia)this shows that your flow switch has been activated. i would check/replace the venturi as these are plastic and very brittle but you will need to get the gas installer back to check it out for you

 

[eastender3]

when you run the hw tap does the led under the tap light up? (on the boiler facia)this shows that your flow switch has been activated. i would check/replace the venturi as these are plastic and very brittle but you will need to get the gas installer back to check it out for you

Thanks for your reply.

Often it's ok, but sometimes it doesn't. I'll check and report back. When he comes back tomorrow, I'll ask him to check the venturi. Do you think he should replace the other thermister, as he only replaced 1. Someone mentioned that they should be replaced in pairs.

 

[eastender3]

The engineer came round this afternoon, he cleaned something with WD40, Mrs was at home, so she didn't know what he cleaned exaclty. But he said the flow switch, need replacing and may cost dear.

He thinks this intermitant flow of hot water is caused by faulty flow switch?

Is he right and how much is a flow switch?

Thanks for your help.

 

[adlplumbing]

possibly it would have been part i would have looked at

 

[Foobix]

http://www.boilersparesdirect.co.uk/parts-online/shopexd.asp?id=36856
If the hot water demand lamp doesn't light when you open the hw tap then 9/10 it going to be the flow meter or flow switch as others state.
A minimum of 2.5 ltr's a minute is required for the flow meter to operate.
Another thing to be wary of is the air pressure switch.If the switch has stuck in the closed position when there is demand for hot water the boiler will not fire.