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.

 

[westward10]

Don't most oil filled rads have stats on them.

 

[Mark42]

Don't most oil filled rads have stats on them.

Yes, but they are useless things which aren't accurate, are often complicated, and you need to visit each heater to see what it's set to.
A single wall stat per room is much more obvious to everyone how it works, is easy to set, and you can be sure the room is off simply by tuning it all the way down.

 

[westward10]

How big is Room 2.

 

[Mark42]

How big is Room 2.

Room 2 is 6m x 8m. Room 1 is 6m x 4m.
Both are 2.4m tall, in profile metal with some insulation (25mm Celotex to walls, new foam going in next week to roof structure, then drop grid ceiling.)

 

[westward10]

Room 2 I would switch both by way of the contactor. The only issue I see are the stats and a contactor in a possible explosive atmosphere. Do they not have a heating boiler.

 

[marconi]

Electrical Heating In Hazardous Areas - Zone 1 & Zone 2 ATEX Certified - https://www.heatingandprocess.com/electrical-heating-in-hazardous-areas/

Perhaps a call to these people for advice on suitable electric heaters for a hazardous area and how to install them including wiring system to meet the regulations..

 

[westward10]

Many of this type of area I have seen the heater is outside the room. Normally a space heater vented into it with dampers fitted.

 

[Mark42]

Room 2 I would switch both by way of the contactor. ...

You mean use a 2 or more pole contactor, for consistency? Can't be on the same single pole contactor as the two heaters must be on different phases.

... The only issue I see are the stats and a contactor in a possible explosive atmosphere. Do they not have a heating boiler. ... Many of this type of area I have seen the heater is outside the room. Normally a space heater vented into it with dampers fitted.

... Perhaps a call to these people for advice on suitable electric heaters for a hazardous area ...

A digression, but there is NO explosive atmosphere hazard!
Many times I have needed to explain this difference to regulators. The electrical regs are concerned with intrinsically safe fittings for use in potentially explosives atmospheres, which are entirely absent from most explosives processing/demonstration/teaching areas as in this installation.
The regs you refer to are aimed at flammable gases, volatile flammable liquids, or flammable powders which might become airborne.
When all that is present is bulk plastic explosives or enclosed ammunition, these cannot possibly give rise to an explosive atmosphere. All one needs to watch out for is hot surfaces and fire, so anything radiant, or convector heating with exposed elements, is out. A dodgy switch or relay can spark away in its box and nothing will happen

ps. These are remote sheds in a field. There is no oil heating. So it has to be electric.

 

[westward10]

All clear now. You can run two phases through a four-pole contactor, two lines and their neutrals. Using a four-pole gives termination points for the neutrals and supply it using a four core cable.

 

[Mark42]

Thanks, gentlemen, for the comments so far.
Unless someone comes up with a better way of wiring this, without the multiple contactors, I think I'll fit two wall thermostats in Room 2, one for each heater. It's the simplest, most logical way.
I suppose I can justify that by it providing the ability to leave only one heater on very low, limiting any background anti-frost load in Room 2 to only 1.5kW.

 

[Paulgwat]

Due to LOT20 requirements you may struggle to find any panel heaters that don’t have a thermostat of some sort integrated.

used to use Nobo heaters but even these now need the control module fitted even though we are controlling them separately.

if you find something let me know as I need half a dozen for an office project.

Paul

 

[Mark42]

Due to LOT20 requirements you may struggle to find any panel heaters that don’t have a thermostat of some sort integrated.
used to use Nobo heaters but even these now need the control module fitted even though we are controlling them separately. ...

Thanks Paul. Maybe they don't exist any more. I'm going to one these from EBay to experiment with:

At least they have only what looks like a very simple thermostat which can either be taken apart and shorted out, or left on max.
A lot of the models I found have complex electronic timers and stats, which I don't trust to default to max on after power interruptions.

 

[westward10]

I would fit some standard rads but manufacture some guards to prevent unauthorized regulation.

 

[Mark42]

I would fit some standard rads but manufacture some guards to prevent unauthorized regulation.

Yup, agreed. Or get the epoxy glue out!
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Due to LOT20 requirements you may struggle ...

I'd never heard of LOT20 so just looked it up. It explains everything.
What load of bol-ocks: now any wall-mounted heater must have complex timers, open-window sensors and so on, whether it's standalone (which is sensible), or part of a centrally-controlled system (a total nuisance, and daft).
Time for a bit of reverse engineering.