Baxi Luna 310 Fi combi boiler
I generally don't get into extensive service on gas appliances.
Had a customer call with "no hot water at shower" but has very hot water at lav and kitchen sink. I suspected moen cartridge so I replaced it.
After nosing into it more, there is hot water when using the shower head, but not when using tub spout, unless moentrol valve is open to allow only a small amount of flow then it gets hot.
this tells me the boiler is not keeping up with the higher flowrate of the tubspout vs showerhead or sink faucets.
This boiler has been in for a few years I believe and this problem only started after the original installer performed a service on it and changed a diverter inside the boiler that controls the flow for heating/domestic water load.
There shouldn't be any control required for heat vs domestic flow as there is no call for heat, just to be sure I turned the boiler to only domestic hot water mode and the problem remains, making me suspect its not the diverter itself.
Any ideas what else could cause this?