15 Mar 2021
Reaction score
United Kingdom
Hello. Our current external Grant Vortex Combi Oil Boiler has a problem with the initial hot water supply throughout the day. In summary:

- It takes about 5 minutes of running the hot tap until we get hot water.
- This is the same whether the heating is on or not and regardless of time of day.
- If someone has used the hot tap recently and managed to get hot water (i.e. for a shower/washing up etc... after following the first bullet above) then hot water will be readily available for a while despite the tap being turned off in between.

Is this likely to be a flow diverter issue? Or something to do with the small storage tank in the boiler itself? Or something else?

Had one engineer out and he wasn't sure, so wondered if anyone had initial thoughts before i call another! Many thanks for your help!
Sponsored Links
Could be there’s a prob lem with a thyrister playing up and not letting the hw get hot enough. You could try to contact grant technical. Bob
I would check the flow switch is working correctly,
Which indicator lights are illuminated .
Sponsored Links
Do you have sufficient system pressure? If no pressure, the pump cannot circulate through the slave tank. This isanother suggestion to look at along with the thermostat and flow switch.
It's a poor engineer who cannot check those obvious points.
Thanks for the replies. The pressure is fine and maintaining. Its an older model (c.2008) and no warning lights are on.

Its the same amount of water that seems to need to discharge before hot water kicks in. My assumption is that it is the capacity of the internal tank - but would need to measure.
When heating is off what happens when a water demand is made.
Does pump run or burner fire.
Q. When heating is off what happens when a water demand is made. A. Nothing. No burner fire. Once the water has run for c. 5 mins on moderate flow and filled large kitchen sink, it then kicks in and fires up the boiler. Hot water then works.

Q. Does pump run or burner fire. A.
First check is the flow switch . On off contact plugs into pcb.
If ok the next check is DHW pcb for voltages. Its the non adjustable one on a bracket.

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