Adding timer for immersion heater: sense check?

lb00

Hi all,

I have recently bought my own place and also started getting into DIY, so I'd really appreciate your help as I am a bit confused right now.

Looking for some advice / sense-check on the correct way to wire controls for a Megaflo immersion setup, as I want to get a timer for the main heater.
Setup:

• Cylinder: Megaflo DD210 with two 3 kW immersions (main + booster)
• Supply: Dedicated radial from consumer unit on a B32 RCBO (no other loads on this circuit).
• Location: immersions and controls all in the same airing cupboard.

The setup I inherited is currently a 20 A DP switch feeding the main and a 13 A FCU feeding the boost from the same point (daisy chained)→ I’m aware that in theory this 20 A could see ~26 A if both are on, so am currently keeping the boost off.

What I’d like to do now
I want to:

1. Replace the existing plate / backbox properly, it's a little bit burnt on the edges.
2. Install a Timeguard timer for the main (bottom) immersion only, so I can run it on a schedule. Was looking at the Timeguard NTT03
3. Keep the boost (top) immersion as a separate manually operated “boost” option.
4. Make sure everything is correctly rated if both immersions are on together (up to ~26 A).

Based on what I have been reading around, the proper way to go seems:

1. B32 RCBO → 45 A (or ≥32 A) DP isolator near cylinder (master isolator / joint point).
2. From isolator load:
   • Branch 1 → Timeguard NTT03 (16 A) or FST24/FST77 → bottom immersion only.
   • Branch 2 → 13 A switched FCU or boost timer → top immersion only.

So only the isolator/joint would carry the combined load; each control only handles its own 3 kW.

Is this correct?
Or is there an easier or better way to do this, without having to add the isolator?

Any advice is hugely appreciated, thank you so much!

WIAWSNB

What size cable runs from the CU to the immersion switches? 
Where do they split - 'daisy chain'?
The idea of two immersions is one for full-tank heating, or the upper one for fast, lower volume, heating - I don't think you would expect to have both running at the same time.
Q - are you sure they are both 3kW? A 13A FSU is really too small for that - it's bound to become warm.
Q - would you ever have/want both running at the same time?
Q - which one would be mainly used? 
Q - how often would the other one be used - rarely? If so, would manual control for it be ok? 
Q - Is your meter Smart - do you have access to economy power, say, overnight? If so, does it require a separate cable from a separate meter?
Pics of the current setup?
Anyhoo, for control, why not consider WiFi switches controlled by an App, or even Alexa? Around £20 a pop. Eg: https://www.amazon.co.uk/Immersion-Heater-Control-Assistant-White-1pcs/dp/B0CKTFNCB3/ref=asc_df_B0CKTFNCB3?
If your main heater is the lower one, then you could even leave the top one manual, for the occasional.time you might just want a quick heat up.
I'd first check the main supply cable size. I'd hope it's at least 4mm2, possibly 6mm, especially if a longish run. If so, it should be good to handle both immersions at the same time if needed. If it's just 2.5mm, then I'd fit a changeover switch near the tank so it can only be one or t'other. 
Then it depends on what you want. One Smart controlled and the other manual? Both Smart? 
I'd ditch the 13A FSU - that's too small, and you don't need a fuse there. You want two 20A isolators, each supplying its new timer, or going direct to the immersion if no timer. Everything from the main supply cable can be done in 2.5mm. so you can fit a JB to split the larger supply into two 2.5, one for each DP switch, or you can daisy chain as currently, but I don't see how a 4/6 cable and a 2.5 can squeeze into one set of terminals?
So, supply cable size? And how are they daisy chained?
Oh, I'm not a sparky...

QrizB

Normally this sort of setup is used with Economy 7 (or a similar time-of-use tariff).

The lower immersion is wired via a switched fused connector to a MCB in the the E7 consumer unit, supplied by the fifth terminal (or ALCS-switched external contactor) of the smart meter.

The upper immersion is wired via another switched fused connector to a MCB in the 24h consumer unit.