Motorised valve seems way too hot

[Doh!]

As I feared, the new head seems to have failed this afternoon. I'm assuming the motor burnt out, like the one before.
The boiler still fires up to make hot water but won't respond to any signal for heating. This time the toggle lever can't be manually pushed across - it seems locked solid in mid position. The head's been running so hot it's completely melted the red plastic covering of the tank jacket.
Tonight, I took the head off the 3port valve and turned the spigot (easily) by hand ....and hey presto the heating came on straight away. I can move it across easily - but is it safe to use it temporarily like this ?
Would welcome any ideas. I appreciate trades guys rightly saying "get a expert in" but apart from the satisfaction of fixing it myself the short truth is that we're in our 70's, this place is rented, and landlord refuses to fix anything. Out of necessity I've got used to finding other ways.
Many thanks for the replies.

 

[moonlight]

Did you unlock the head when you fitted it?

 

[Doh!]

Yes, I did unlock the head when I fitted it (I think) - if that means was the lever moving freely between Man and Auto, then yes, it was. But of course since the second head failed I've now removed it completely - so I'm switching the spigot by hand between water and heating. That worked OK for the first day - the boiler came on pretty quickly in response to where the hot water was needed. But now, although I can still move the spigot on the divert valve the boiler does not fire up in response immediately ? It can take hours before the boiler finally gets a signal from somewhere to fire up.
Any idea what's going on ?

 

[moonlight]

Your boiler wont fire for heating unless the valve switches over. The default setting is hot water.
The only way you will get heating with the valve off, is to put the spigot in the mid position and turn the hot water on but that will only work while you need hot water.
View: https://www.youtube.com/watch?v=NzdSWUoTQhg

View: https://www.youtube.com/watch?v=M9DkNwVL4Dc

 

[Bobblehead]

Apologies for slow reply - been in hosp...God bless the NHS (and their heating system worked fine too).
Just want to say a big 'thanks' to everyone who replied - really helped me understand how the motorised valve works. It seems to me that the core problem is with the little ball valve inside the brass 3way port. Although it can be moved easily on the spigot from A to B i'm assuming there's something else wrong that's causing the motor to constantly spin and burn out. We're in an extremely hard water area so I quess that's got something to do with it. Unless you guys have better advice I think we'll wait til the warmer weather comes and have someone come and swap out the whole thing.
Can anyone recommend a reliable/reasonable CH plumber in the Stafford area ?
Thanks again for all your help.

Hi came across you post searching for possible solution to MV head overheating. Third new 3PV installed in Jan and actuator head looks like it’s failed already being so hot. Did you get to bottom of your issue? Seems I have similar problem to solve. Thanks

 

[SJB060685]

As Moonlight says above

 

[Doh!]

Apologies for slow reply - been in hosp...God bless the NHS (and their heating system worked fine too).
Just want to say a big 'thanks' to everyone who replied - really helped me understand how the motorised valve works. It seems to me that the core problem is with the little ball valve inside the brass 3way port. Although it can be moved easily on the spigot from A to B i'm assuming there's something else wrong that's causing the motor to constantly spin and burn out. We're in an extremely hard water area so I quess that's got something to do with it. Unless you guys have better advice I think we'll wait til the warmer weather comes and have someone come and swap out the whole thing.
Can anyone recommend a reliable/reasonable CH plumber in the Stafford area ?
Thanks again for all your help.