Hot water fault with Saunier duval isomax

[Plumb Ugly]

hi new to the forum and looking for some opinions! went to a fault with one of the above boilers the other day heating working fine but the hot water supply is running out once the storage cylinder is depleted! the burner ignites to re heat the cylinder but goes straight down to minimum burner pressure and wont go any higher meaning the store is depleted after a few minutes! if left for ten mins or so the store will re heat but again only at min burner pressure. thermistor resistances checked ok and diverter valve moving correctly. seems to be a problem with the circulation through the cylinder as the flow to it is red hot but the return is never more than aired! checked the DHW pump and seems to be running ok! all i can think now i there is a blockage or restriction in the cylinder itself? anyone had this problem before with this model?

 

[lame plumber]

pump failing

 

[zeebeedee]

Air lock? How far is the divertor from the cylinder you sure its switching over and not stuck, could be heat transfer seen that few times

 

[Plumb Ugly]

The cylinder is built into the boiler so no more than a few inch's away from the diverter though I'm begining to think it is a blockage the cylinder has an anode that's never been changed I wonder if this could be the cause? Thanks for the input lads I'm going back today so I'll have a look at what you've said and keep you posted

 

[R1CKY]

Not seen one, but had a look at a PDF manual for it. It has a flow switch on the cold and a temperature sensor on the accumulator vessel.

My guess would be that when the sensor calls for heat and the hot tap isn't open, the boiler fires up on minimum to heat up the accumulator slowly and quietly. When a hot tap is open, the flow switch detects flow and fires up on full rate to keep up with demands.

I think you need to have a look at the flow switch and see if that is operating properly, sounds to me like it isn't. This is purely a guess though!

 

[Reg Man]

I have experienced Saunier duval boiler circulating pumps failing with the impeler shafts shearing off. Difficult to diagnose as the pump motor still runs, leaves you scratching your head wondering why the boiler keeps shutting down.

 

[Plumb Ugly]

Sorry forgot to mention iv replaced the flow sensor and checked the diverter is definatley shifting over ok. Checked out the resistances of the thermistors and vessel thermistor all seems well there.....seems the dhw thermistor is holding the burner at minimum because the pipe its connected to between the plate heat exchanger and storage vessel is red hot constantly. Unclipping it ftom the pipe then allows the burner to go up to maximum but the return pipe from the vessel stays cold regardless of whether the hot water is being drawn off or not? Could be the dhw pump you guys mentioned it appears to be running ok but who knows! next visit i'll take it out and have a look at the state of it and maybe try changing it, anyone know if theres a replacement head that will do the same job rather than getting the whole thing? If that doesnt work im stumped on this one!

 

[Plumb Ugly]

Finally fixed it! Everyone who suggested the dhw pump give yourslef 50 plumbing points!