Timer and local thermostat control of 3 x heaters on 3 phases. Wiring logic help please!

[Mark42]

I'm fitting oil-filled panel heating into two rooms of a remote workshop, with (currently) a 20A x three phase supply.

Control, for occasional use only, is needed as follows:
1. A wall-mounted and easily-adjustable mechanical timer to set all the heating to come on an hour or two before people arrive.
2. Room thermostats to control temp in each room individually. Room 1 has one wall thermostat and one heater; room two has one wall thermostat and two heaters.

Due to supply limitations each heater must be on a different phase.

The block sketch below was knocked out by me over breakfast. I am not proud of the graphical standard

I'm unhappy with the second SP contactor. It feels clunky, over-complicated and ill-designed. I could of course fit a second thermostat in Room 2, simplifying the logic and deleting the second contactor, but that's even worse.

As always in life: Is there a better way of doing it?

ps. Can anyone recommend a model of oil-filled radiator which does not have any local room thermostat or complicated controls, designed specifically for central control. Difficult to find!
It has to be oil-filled with no exposed elements or blown/convected hot air, as explosives are handled in these rooms and it's the only type allowed.

 

[Mark42]

An alternative, depending on the cable runs involved, would be to put three DP contactors next to the DB, one per heater, each fed from an SP MCB and feeding the heater connection unit. Fit one stat in each room wired back in 1.0mm² to the contactor box. Feed the timer from a 6A control MCB either in the board or the contactor box, then take its output to both stats and the returns from the stat to the appropriate contactors. Advantages include not switching the heater load with the stats so they should last longer, and simplicity; three identical single-phase power circuits and one single-phase control circuit.

This I like, thank you for taking the time to think about it.

I agree yours is a more elegant system, but I wanted to avoid the complication and possible future confusion of feeding stat control lines back to the DB area. The runs are not easy.

I certainly agree about causing the stats to last longer - I like using relays for all kinds of things, and personally never use 13A stats direct wired on anything over about 6A, which this is, just!

I was using a 3P MCB only because there's a 4-pole contactor, and I like reducing component count. The 10A 3P MCB would simply be labelled 'Heating'.

Bloody hell this is a difficult call and I need to order parts and finish it by Thursday.

Here's a compromise I alluded to earlier: Keep the 3P MCB and one timer-circuit-controlled 4-pole contactor, which can be inside the DB so no external box is needed. Feed to three quality 13A stats, direct wired to the 6A heaters.

Do you think that under-run electronic stats with internal encapsulated relays are any less reliable than contactors? I agree that the old bi-metal type are trash, run hot and burn out, but those days are over. Aren't they?

 

[marconi]

INTELLIGENT CONTROL - Klima Underfloor Heating - https://www.klima.co.uk/intelligent-control

Nice simple control and wiring - a time-day programmable thermostat as used for underfloor heating? The one at link can switch 3.4kW. It could if you prefer control a relay/contactor for the heaters located next to the prog-stat in each room. One power cable run to each room.

 

[Mark42]

For completeness, here's what I did in the end.
TP breaker to 3-pole contactor in DB, controlled by local timer.
Switched L1 L2 L3 out to three separate wall-mounted thermostats, then onward to three separate heaters.
Not the most electrically elegant solution, but very simple, and consistent for all heaters so easy to understand in the future.

 

[ipf]

For completeness, here's what I did in the end.
TP breaker to 3-pole contactor in DB, controlled by local timer.
Switched L1 L2 L3 out to three separate wall-mounted thermostats, then onward to three separate heaters.
Not the most electrically elegant solution, but very simple, and consistent for all heaters so easy to understand in the future.
View attachment 56128

View attachment 56129

I'd have used SP breakers... so any future problems with a particular circuit wouldn't affect the other two.
External box for the controls, too.
...and (not your fault) that gunge in the cable entry point is horrible and pure daft IMO.

 

[freddo]

I like how the vents in the top and bottom rear of the enclosure are sealed up too.