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Wiring for ceiling mounted infrared heating

Postby forestcreature » Sun Apr 26, 2020 5:45 pm

I would like to install ceiling mounted far-infrared heaters in a number of rooms, controlling each room's panels with a programmable thermostat.

The entire array (all rooms) will be about 6kW. I am planning to supply it with a 4mm circuit on 32A MCB, direct-clipped to floor joists (no insulation in the floor space). I think that this circuit can be a radial. Direct clipped on 4mm is rated at 37A. 32 * 230 = 7.3kW. I am thinking to keep the radial all direct clipped, taking spurs off junction boxes to each wall-mounted thermostat (mostly behind plasterboard, with a couple to chase into masonry).

1) Is this plan appropriate? Is it better to use junction boxes, with a spur to each thermostat, or to loop the wire to each thermostat and back within conduit? Is the installation method (direct clipped) changed by looping a section up and back through conduit?

Some of the rooms will contain two panels controlled by one thermostat. After the thermostat, I am planning to connect each panel with appropriate flex to a FCU, mounted in the plasterboard ceiling.

2) How should I connect two panels (two FCUs?) to one thermostat? Can I do it in a radial arrangement to FCU 1 and terminate at FCU 2?

Thank you in advance! There are discussions elsewhere about whether infra-red panels are suitable as sole heat sources, but I am asking only about how to wire them.
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Postby ericmark » Mon Apr 27, 2020 2:31 pm

Job one is work out how to control, it is pointless running in any cables until you have worked that out.

The problem is any normal wall thermostat measures air temperature, and the normal control is mark/space i.e. on/off, but with the heaters your talking about they do not heat the air, so you control the output by selecting how many heaters are on, so for example if heating a church likely one would fit them in banks of three, each independent switched so it can be controlled by selecting 1, 2 or 3 bars.

The normal would be manual control, as the thermostat as said measures air temperature and they don't heat the air, but some times one can find a position where the thermostat will work, but how to wire is down to how many units.

The normal is to use a combination of heating methods, so the thermostat controls back ground heat, and the panels are not switched off/on with thermostat, often panels switched with a PIR so as soon as room is used they switch on.

But this is not a DIY job, it is rather complex, so you need to consult an expert, even as an electrician I would call in lighting and heating designers, I may install them, but I did not design them.
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Postby forestcreature » Mon Apr 27, 2020 5:04 pm

Thank you ericmark.

Far-infrared heaters as primary room heating are conventionally controlled by room thermostats. As far as I understand it, this works because the thermostat is mounted on part of the thermal mass---the wall---that the panel is heating. Here is a page from Herschel, a prominent UK supplier: d.co.uk/how-do-infrared-heaters-work/how-to-control-infrared-heating/. The smart controllers are fancy thermostats! Panel and thermostat placement in our small rooms isn't much of an issue, as all of the walls will be in effective range of the one or two panels on the ceiling.

I think that controlling output by switching discrete heating elements is done with near-infrared heaters.

Since posting this I had a word with a sparky that was working on our place. They said that for a given circuit, you take the worst-case installation reference method for the whole circuit. They advised against using unnecessary junction boxes as potential points of failure, so looping the 4mm up to each thermostat in conduit and going with that reference for the whole circuit (still works out fine).

In the case of panel installation, they said to follow the manufacturer's instructions! And that two FCUs in a radial circuit off a thermostat is fine if the panel manufacturer's instructions allow.
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