Worcester greenstar heatslave 18/25 oil boiler - output

3 Jun 2019
Reaction score
United Kingdom
Hi, my first post here, but I sense not my last

I currently own this boiler, but up until now have been using an electric shower. It’s pretty pathetic tbh and I’d like to convert to a thermostatic shower. I’ve just run a bath to see how much hot water I’m getting. The answer is I get about 8cm before it goes colder, which is roughly 60 litres in our bath. The water temperature is good to use without adding any cold

(The water side is on all the time and the heating side is off)

This seems very little. Do you think there is a problem with the boiler or is this all I can expect?


Sponsored Links
The store is 69 Litres, you will get a good shower as long as your water flow rate is decent
Hi, thanks for the reply. Another person has told me since it’s a combi boiler I should get hot water for as long as I’m drawing it, not just that in the tank.
Is it a combi boiler, as my 18-25 is not, it has a cistern which is heated with thermo syphon and a programmer (timer) sets how long heated for with no thermostat on the cistern.

I am in process of working out how to add a thermostat, however seems RF type very expensive, so seems I need to run wires?
Sponsored Links
This is an excellent combi that should give you a great continuous shower....my own 12/14 can do that without any issues at all. In the summer it can even run two at once, much to my surprise......even after the Heatslave tank contents have been consumed.
John :)
@ericmark I’m not sure how you tell definitively. Some say they are combi, others not. I’ve been told they have an internal tank/store/heat bank while others have told me they do not. Someone has to be right unless there are lots of different versions of the Worcester Greenstar Heatslave (i) oil boiler.

My experiments so far seem to chime with @ianmcd that there is an internal tank of 69l, but the continuous hot water is not happening due to some fault or other

On another forum a petson with the same issue as me was told:

“You need to get the flow switch, plate heat exchanger, diverter valve and pump is working in DHW demand.
The flow switch opens the Diverter valve which starts the pump and circulates the water through the plate HX so you get continuous hot water”


The pressure gauge tells me that one is a combi.
Its capable of producing between 16 and 25kw depending on how the oil burner is jetted and set up.
The boiler has an internal Heatslave tank which holds water at a temperature decided by the thermostat dial on the front panel. Once the domestic hot water is called for, a flow switch clicks and fires the burner to keep the hot water coming. In central heating mode, the diverter valve moves to its other position and hot water is fed to the radiators.....the valve defaults to domestic on command but the internal pump is running in both cases.
Id venture a suggestion - if the hot water production is below par - that the diverter is passing some water to the radiators.....or the heat exchanger is becoming scaled. To be fair, running a bath is a stern task for any combi!
John :)
Thanks Jim, John

I’m not actually looking to run baths, was just doing that to test what volume of hot water I’d get before it ran cold. Approx 60 litres, or a heat bank’s worth. What I want to do is get rid of the electric shower and replace it with a thermostatic one

In the end I just pulled off the front and compared it with pictures from the downloaded manual. It’s definitely a combi :)

Anyway, time to get the heating engineer in

Thanks all
As I said on another forum, if you are measuring the output through a tap that demands more water than the domestic heat exchanger is designed to supply, then it will run cold. Check your manual for the DHW performance, then filling a measured container from the tap you are measuring will tell you whether you are exceeding the designed performance. If you run your bath tap slower, it will probably work as required.
@oilhead thanks for answering there and here :)

As I responded there, it made no difference what rate I ran the tap at. When run full tilt, I got 60-odd litres of hot water then it went cold. The next day when I ran the tap at half-rate I got 60-odd litres of hot water then it went cold. It was clear to me that I was getting a tank’s worth and then no more hot water once it had emptied. It was confirmed to me that the tank is actually 69 litres.

I’m grateful for all the responses that I received. I will post back when the problem has been rectified as a reference for other owners of this boiler
The slave tank is circulated across the DHW heat exchanger. When running the hot water, does the boiler fire? Does the diverter valve move to the DHW position? Follow the fault finding chart in the manual and you should be able to find the fault. It may well be advisable to call in an engineer. It should be 'meat and drink' to an experienced man.
Hi, I know this is an old thread but I have exactly this problem with the very same boiler, Did you manage to sort the problem?
The OP last logged in to this site two days after his first post so I wouldn't hold out much hope for a reply. Better to start your own thread

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