Oil Boiler - Won't Start Hot straight away!?

JohnnyTooSharp · Jun 22, 2025

Hi, this is about my own boiler. I'm a new-ish plumber but no boiler experience.

It's a...... Worcester Greenstar Heatslave External 18/25 Oil Boiler

It had a new burner and was serviced at the end of December 2024, and I've just re-pressured the boiler and it's expansion vessel to try resolve the issue.

BUT, for a few weeks now, when we run the shower, it comes on warm/hot for a few seconds then turns cold. Then we have to keep turning on and off, getting hot then cold water each time. It only stays hot after maybe half a dozen times, sometimes more. It's getting quite annoying now! No other plumbing in the house has changed.

Anyone know what the problem is and the solution?

ShaunCorbs · Jun 22, 2025

Could be diverter or store ntc

Does the boiler fire ?

JohnnyTooSharp · Jun 23, 2025

Does the boiler fire ?
[/QUOTE]
Well it does, but on and off for about 5-6 attempts until stays on!

JohnnyTooSharp · Jun 23, 2025

Is it worth changing the diverter and ntc then? Do I need my boiler guy or could I just do it? How much am I looking at roughly for the parts?

ShaunCorbs · Jun 24, 2025

JohnnyTooSharp said:
Does the boiler fire ?

Well it does, but on and off for about 5-6 attempts until stays on!
[/QUOTE]

What about heating does that fire eg not attempts for 5 etc

JohnnyTooSharp · Jun 24, 2025

ShaunCorbs said:
Well it does, but on and off for about 5-6 attempts until stays on!

What about heating does that fire eg not attempts for 5 etc
[/QUOTE]

Obviously it's been off quite a while, but just called for heat and got hot straight away, so don't think its a problem.

Btw, same happens at hot taps as well as shower, so not just the shower, that's prob obvious but thought I'd explain anyway.

ShaunCorbs · Jun 25, 2025

JohnnyTooSharp said:
What about heating does that fire eg not attempts for 5 etc

Obviously it's been off quite a while, but just called for heat and got hot straight away, so don't think its a problem.

Btw, same happens at hot taps as well as shower, so not just the shower, that's prob obvious but thought I'd explain anyway.
[/QUOTE]

Prob cold water ntc

JohnnyTooSharp · Jun 26, 2025

Ok I think I need the boiler man then. Can't see that in the manual, not even the location on the boiler!

You think it's that, not the diverter then?

ShaunCorbs · Jun 26, 2025

JohnnyTooSharp said:
Ok I think I need the boiler man then. Can't see that in the manual, not even the location on the boiler!

You think it's that, not the diverter then?

Can you see it moving when you open a tap ?

JohnnyTooSharp · Jul 1, 2025

ShaunCorbs said:
Can you see it moving when you open a tap ?

No, if I push it back it spring back as it should. Call for hot water and it stays where it is whether the water is running hot or cold

JohnnyTooSharp · Jul 1, 2025

ShaunCorbs said:
Can you see it moving when you open a tap ?

Am I assuming correct that my boiler hot water is always on and that the diverter is just for when heating is called for? i.e. the diverter lever doesn't move whether we get hot water or not, but it does move when heating is called for...(I just tested it again)

So if it is only for heating, does what I've described mean the diverter valve is in fact working correctly?

And if so, so that point back to the NTC?

ShaunCorbs · Jul 1, 2025

JohnnyTooSharp said:
Am I assuming correct that my boiler hot water is always on and that the diverter is just for when heating is called for? i.e. the diverter lever doesn't move whether we get hot water or not, but it does move when heating is called for...(I just tested it again)

So if it is only for heating, does what I've described mean the diverter valve is in fact working correctly?

And if so, so that point back to the NTC?

Picture of the diverter should be a Honeywell one

JohnnyTooSharp · Jul 1, 2025

ShaunCorbs said:
Picture of the diverter should be a Honeywell one

Search

Search

Oil Boiler - Won't Start Hot straight away!?

JohnnyTooSharp

ShaunCorbs

Super Moderator

JohnnyTooSharp

JohnnyTooSharp

ShaunCorbs

Super Moderator

JohnnyTooSharp

ShaunCorbs

Super Moderator

JohnnyTooSharp

ShaunCorbs

Super Moderator

JohnnyTooSharp

JohnnyTooSharp

ShaunCorbs

Super Moderator

JohnnyTooSharp

Attachments

Official Sponsors of Plumbers Talk

Similar plumbing topics

Featured Content

Sponsors

Featured Plumbing Threads

Thread statistics