Poorly Hot Water Temp

21 Mar 2017
Reaction score
United Kingdom

I'm hoping that someone can advise on this issue:

I've a vented, oil fired heating system with two separate circuits for HW and CH. At the moment there is simply a thermo syphon circuit for the HW and the temp is controlled by the boiler stat which results in extremely hot water when the CH is working.

I'd like to put a stat on the HW tank and control the flow through the circuit with a motorised valve once the required temp is achieved. But that raises the question of how best to plumb it in so that I don't dead head one or both of the circuits. The options I've been kicking about were:

1) Divert the flow from the HW circuit to the CH circuit, but the CH is pumped while the HW isn't.
2) Fit a pump to the HW circuit as well and divert the HW flow to the CH circuit, but presumably the flow is already filling the system, if I add the extra flow will I end up with both circuits performing badly as the CH circuit I guess was sized for the flow it already has.
3) Simply bypass the HW tank with a motorised valve and return the hot water to the boiler but I guess that would trip the boiler stat and shut it down.

Your advice would be welcome.

The rambler
Sponsored Links
The easiest solution will probably be to convert it to what is known as a 'C Plan' the motorised valve is simply inserted into the "thermo syphon"return pipe from the cylinder to the boiler as per the diagram below. It can be inserted in the flow pipe to the cylinder, but it must be done so that it doesn't block the vent pipe from the boiler when it's closed. This must be kept clear permanently.

c plan.jpg

At the moment, switching the hot water on will operate the boiler and when the heating is required, the pump will start. This means that you will need to modify the controls so that the boiler goes off when the hot water valve is closed and the heating isn't required, and also that the boiler stays on when only the heating is required. See the circuit diagram below that shows how it's done.


Probably the best solution would be to convert it to a fully pumped system, you would need to add a three port valve (Y Plan) or a second two port valve (S Plan), so the plumbing and wiring would be more involved and the cost higher. These don't offer any extra control functionality, but it would mean that the hot water cylinder would heat up more quickly.

Y Plan
Y Plan.jpg

S Plan
S plan.jpg
Last edited:
Hi Stem,

Thank you for your very comprehensive reply, I really do appreciate the trouble that you have taken. With these options in mind I'll review the system and hopefully select the one that best suits the installation.

Looks like I've no excuses now;)

Thanks once again.

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