So the heaters take ~ 15V   and a touch under 4 A  each to generate 50W? Do they need to be in parallel, or is the standard of insulation such that each of your 4 banks or 8 could be wired as  8 in series  to make a 120 Volt AC driven section ? or you could run the heaters on the secondary of a transformer and switch the primary side with a normal thermostat, but I'm not sure how accurate you need to be.

How hot is it getting, and what level of control do you need - nearest ten degrees, nearest degree, nearest hundred milli-Kelvin?

There are standard thermistor units that are intended for underfloor heating, and there are controls for laboratory ovens, but none of that that may be really what you need. There are plenty of industrial PLCs that can be made to respond to a thermocouple and open or shut contacts at a preset level, but  that is probably at least as hard, if not harder to customize than sorting the Arduino.

What aspect is unreliable about the current set-up, and how much protection is needed to guard against or raise alarms for under or over temperature - what sort of thing  is at stake ?

It may be trivial or very hard depending on the answers.

Mike,

So the heaters take ~ 15V   and a touch under 4 A  each to generate 50W? Do they need to be in parallel, or is the standard of insulation such that each of your 4 banks or 8 could be wired as  8 in series  to make a 120 Volt AC driven section ? or you could run the heaters on the secondary of a transformer and switch the primary side with a normal thermostat, but I'm not sure how accurate you need to be.

How hot is it getting, and what level of control do you need - nearest ten degrees, nearest degree, nearest hundred milli-Kelvin?

There are standard thermistor units that are intended for underfloor heating, and there are controls for laboratory ovens, but none of that that may be really what you need. There are plenty of industrial PLCs that can be made to respond to a thermocouple and open or shut contacts at a preset level, but  that is probably at least as hard, if not harder to customize than sorting the Arduino.

What aspect is unreliable about the current set-up, and how much protection is needed to guard against or raise alarms for under or over temperature - what sort of thing  is at stake ?

It may be trivial or very hard depending on the answers.

Mike,

Hey Mike, 

I've got freedom to wire them however. I opted for screw terminal versions to avoid having permanent soldered connections. 

I'm aiming for a maximum temp of around 80 degrees. 

Control wise, anything within a few degrees would be better than what we have now. 

The existing system was set up by a predecessor, who developed a custom MOSFET PCB which I believe is rather overkill, as the MOSFET units are capable of around 60A per device, and there's 4 units being used for the 4 zones we have. 

They're being mounted to create thermal shrouds if it helps give a little more context, under vacuum. 

The existing MOSFET PCBs were designed to use a PWM feedback to check what's being sent from the arduino is actually being seen by the switching device, but this never worked, so was never implemented as things currently sit. 

The previous revisions of these MOSFET PCBs were just an off the shelf item from amazon, badged as capable of 30A, but the board wasn't designed to dissipate even a little amount of heat, so we had a few burn out fairly quickly.

I'd been looking into solid state relays and using an Arduino as a PID controller. 

I'm just struggling with how to lay this out to be honest, I've got capability of putting a max of 16 resistors per panel, so I was going to wire each panel separately to give zoned control, but it's a case of what supply voltage, do I go with a 230V a.c. solid state relay, or a 48, 24, 12V equivalent and use associated power supplies to try and manage the power distribution better. 

We're using Arcol HS50 4 Ohm resistors currently FYI, looking at the datasheet for them them shows I'm only going to be getting around 30w of power dissipation based on an ambient of 80 deg C.