Boiler is firing up but not heating the water

14 Jan 2010
Reaction score
United Kingdom
I have a Biasi Riva Compact M90E.24S combination boiler.

The central heating works fine but since last week only very cold water comes out of the taps. The strange thing is that when I turn on the DHW taps the bolier does fire up, but only cold water comes out of the taps and the hot water flows into the central heating side instead.

Any ideas?
Sponsored Links
If there is not even warm water then it could be the divertor valve.
Thanks for the 2 replies so far. If the diverter valve is faulty, wouldn't that mean that the boiler wouldn't fire up when the hot water tap is turned on? The boiler definitely lights in the normal way when I turn on the tap, which made me think that the diverter valve must be ok. Am I wrong in making that assumption?
Sponsored Links
The boiler will fire up but the hot water that would normally go around the boiler, goes around the central heating system.
A test... Ensure your central heating system is cold, then run your hot tap. As you say the boiler will fire up. feel the central heating flow pipe, if this is getting hot then the diverter valve is defective
Yes, that's exactly what is happening. With the boiler cold, if I turn on a tap the boiler fires and the CH flow pipe gets hot.

So the next question is which part of the diverter valve should be replaced? Is it likely to be the actuator motor or the diaphragm?
if you remove the actuator, you will be able to see if the pin inside is moving, in accordance with section 8.1 of your instructions. Chances are it may be the valve inside that is sticking.
Its most unlikely to be the pin sticking down.

Must more likely to be a faulty motor. Probably due to water ingress due to a failed gland. Can also be a failed motor drive relay on the PCB.

Simple test, unclip motor and the diverter should move to DHW position and give you hot water on demand.

Thanks for your replies guys; I really do appreciate your help and advice.
I've successfully replaced the diaphragm and microswitch on a Vailant in the past but I haven't even taken the cover off the Biasi yet so please forgive my ignorance.

I'll locate the diverter valve tomorrow and take off the motor. Presumably I don't need to depressurise the system or disturb any water connections to do this, do I? And if the valve moves to the DHW position when the motor is removed, does that mean that the motor is ok or faulty?
Hi Tony
I tried unclipping the motor and sure enough, the valve moved to the DHW position giving me hot water from the taps. Does that mean that the motor is siezed and needs replacing?
Many thanks.
Without testing the boiler I dont know!

Has water got into the area where the pin pushes?



PS Be careful of Ironnaz! He is a trainee boiler man! His profile says United Kingdom but he really lives in Cov !
Hi Tony.

Oh dear! I ordered a new motor earlier today!

I thought from your previous post that the test you suggested (removing the motor to see whether the actuator moved to the DHW position) would indicate whether or not the motor was faulty. Did I misunderstand?

It can be the motor or the drive relay on the PCB.

It needs testing with a voltmeter to see if the voltage changes from black to brown when the function changes.

If it does the motor needs to be watched to see if the pin moves accordingly.

I've now replaced the motor, a nice easy job, and the problem is fixed.
So much for the Biasi Technical Dept who diagnosed it as needing a diverter valve repair kit (which doesn't contain a new motor)!
Thanks again Tony.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links