DIY Doctor UK DIY Forums

Hi here.
I have a 25-30 year old CH system. My programmer is an electro-mechanical mini-programmer with controls only for;
1. Hot water, or HW and Heating.
2. Two on periods and two off periods.
3. Time periods: Off - Once - Twice - 24 hours.
The HW cylinder (tank) is vented, indirect.
I only have one thermostat which controls room temperature.
The boiler is a Potterton which has the thermostat turned right down. Even at this.... the water in the taps is too hot too handle. (OK the boiler stat probably needs replacing but the boiler is only 6-7 years old and its been like that since new. .....Yes the radiators get very hot.))
I now have a need to replace the hot water cylinder (tank) as it is leaking and corroded.
I have often considered putting a motorised valve into the indirect tank heating circuit and connecting it [u]directly to a tank thermostat [/u]strapped to the tank, and now that I have to change the tank, I am looking at the opportunity to do this while the system is drained.
In the past I have mentioned this to a boiler engineer when he came in to service the boiler, and he mentioned that I would have to put a bypass circuit in place that would come into play when the motorised valve shut down the indirect heating circuit to the tank.
Looking at the layout, it appears that there is one motorised valve in the circuit to the radiators, but there is a T junction ahead of this which I suspect is the feed to the indirect heating circuit in the cylinder.
I have now had two plumbers in to quote to replace the tank, and insert the motorised valve and tank stat.
One of them said that it would not need a bypass circuit and the other said it would.
I have been trying to read as much as I can on the need for a bypas circuit, but am still somewhat confused as to why one is/is not needed.
Can anyone please shed some light on this.

Hi, if you had a single 3 port divertor valve there is always some bypass, but with 2 single 2port valves as you intend, it means if both stats close their valves and the boiler needs a pump overrun there will be no where for it to go.
Old fashioned/cheap way was to put bypass gate valve leaving it part open, this means some hot water always being bled off so not very efficient! modern way is to fit a pressure operated automatic bypass valve just before tee to valves, they can be set to open only when valves shut.
regards SPARX

Hi Sparx, Many thanks for your post.
I now understand the need for a bypass loop, having read the CH boiler manual, and your note. However, the manual still calls for a bypass loop even with a 3 port diverter valve.
Anyway, I have decided to just make a straight replacement of the hot water cylinder without putting in the tank stat and valve.
Will plan to upgrade to a more controllable programmer and diverter valve at a later stage.
Your comments on the old fashioned/cheap way of providing a manual permanent bypass gate were very helpful as that was what one of the plumbers suggested, and I was not to keen on the idea. Prefer the pressure valve approach.
Again, many thanks for you post.

As is often the case when in doubt....RTFM
(Read The Flamin Manual)