In article .uk, Ed
Sirett writes

Thanks a lot. The latter arrangement I had worked out for myself, but I
can see your first design is less invasive.
You're welcome.

After thinking about this I'm not yet inclined to implement the mod.
At present the bathroom rad is also a simple restricted lock-shield
bypass, so it would become lethal at 80C during water heating.
It would also grossly over heat the 1.8m x 1.6m room.


I might eventually put in a smart bypass and put a TRV on the bathroom rad.

I often run the flow at 50-60 so just a little low to have a cyl stat
set to 60. The following mod would let you set the h/w demand flow to 65
or so to avoid the superheated bathroom rad. There's plenty of room on
the panel for an extra pot, perhaps with the spindle cut short and a
screwdriver slot cut to avoid accidental movement.

Just another one of many variations that could be applied but if your
system is working I understand it is hard to work up the enthusiasm to
tear it apart.

Variable h/w demand control board mod:

+5V
---
|
ctrl / 10k modified
panel \----/\/\/\----------------------- demand
pot / | level
(100k) \ +5V |
_|_ --- |
0V | |
1k \ 1N4001 |
h/w h/w /--||---|
demand ----- demand \ 240V~
| pot / ---
- | |
| | o o
RL1 | | RL1a / RL1b /
coil | | o o
- | |
| --- |
--- 0V c/h --------- boiler
240V RTN demand demand

Regards,
--
fred