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Sometimes on a jobsite the lighting will be wired up but the lightswitches have not been installed yet ... so you drag a live extension cord up to the wall and stab the romex hot/neutral in and ... the room lights up.

Can you do that same thing with a warmfloor ?

If we have a warmfloor installed in the actual floor, I think we will have a single "cold lead" coming out of the floor and that cold lead will consist of two wires that connect to each other at the end of the cold lead, forming a loop ... right ?

So, in advance of the controller and thermostat being installed, can we directly apply mains power to the cold lead and expect the floor to warm up ?

Other than the obvious safety and OSHA problems with this scenario, is there anything wrong with it ? Can we damage the (extremely simple) system in any way ? Will it actually work ?

Are all of these warmfloor solutions basically one big short circuit under the floor ?

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    well, with the light it was easy, but the floor heating needs much higher power levels and will probably not work on regular wall outlet. So check what is the power requirement first
    – DIY75
    Commented Mar 15, 2023 at 7:07
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    Yes, but does a warmfloor heating element not generate heat in a linear relationship to power applied ? If I have a 300sf area that calculates out to needing 5400 watts and I only apply 120x15 = 1800 wouldn't I still get some warmth out of for testing purposes ? Or is there some threshold below which it wouldn't warm at all ?
    – user227963
    Commented Mar 15, 2023 at 16:46
  • A floor heating element is just a long resistance wire with an ordinary (non-heating) bit of wire at each end, surrounded by a grounded stainless steel braided sheath. I've never seen those leads "connected together" and can't imagine why they would be. You can certainly apply rated voltage to the 2 leads, and it will operate (or not) as it would in a finished installation. Proper resistance testing, per instructions, should be performed first. Powering a 240V element with 120V will just produce 1/4 of the heat output.
    – kreemoweet
    Commented Mar 16, 2023 at 4:51

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Electric floor heating will have two leads and a grounding jacket.
What you want to check is the continuity between the leads - it must be within what the manufacturer specs say - small deviations by couple ohms are fine, but if it reads close to zero or infinity, then you have a damage.
The second important thing to check is the resistance between the leads and the grounding jacket. If that is less than infinity, you have a cable damage.
Both this tests are best performed right after the floor has been poured or tiled.

If resistance between leads looks good, you "could" connect it to power, but that doesn't mean you should. Depending on floor type, it may be damaging to heat it up before it fully set.
Heating the floor up will take considerable time - hours, and without the thermostat cutoff you are risking overheating things. It is best not to leave it unattended and energized.

EDIT

There is one situation, where you might need to power the heating cable from mains to test it - when you identified a short between the heating wire and grounding jacket, and need to identify the area on the floor where the wire was damaged, to dig it out and attempt a repair. This should only be attempted when wire is still reachable (for example, laid in glue layer under tiles and tiles were pulled off before it set). This is last resort repair attempt and you should rather scream at your handyman and make them buy and install new heating mat if that happens.
A multimeter will tell you where between the end points of mat the damage lies, by comparing the proportions of resistance you can tell at which foot of heating cable the problem lies. You might not be able to see any obvious damage, so next step is powering the longer part of heating wire for several seconds by connecting mains to central lead and the grounding jacket. The grounding jacket serves as return wire. Then you seek by touch which part of the wire got hotter (after disconnecting power) - the spot where it goes from hot to cold is the location of short.
I had the above procedure performed by electrician on my heating mat that was damaged by screwdriver when laying tiles. The mat works, but I do not recommend doing that.

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    The meter tests described in this answer are in the documentation with precise required measurements. You have to do them. Some manufacturers require photos of the test to validate the warranty. Applying power to the cable is not an adequate test. Why not RTFM and learn to do something properly and to ask questions without scoffing at safety norms ? And if you want to use an extension cord to power the cable, just for fun, why not use the thermostat in that test? As long as the cable's power rating is within the cord and feed capacity. That's also in the manual btw.
    – jay613
    Commented Mar 15, 2023 at 12:09
  • @thomas Thank you. To your point about applying power without thermostat cutoff - when you speak of overheating things, you are referring to subfloor and maybe thinset layer and the other pieces of the warmfloor substrate, correct ? I don't think there is danger in overheating the wire, correct ?
    – user227963
    Commented Mar 15, 2023 at 16:50
  • @thomas also ... the two leads you speak of go into the same heating element and they meet at the end, making a short circuit, correct ? This heating wire just gets AC current from the thermostat device and has no polarity, correct ?
    – user227963
    Commented Mar 15, 2023 at 16:51
  • @user227963 There are two kinds of floor heating mats I know of - one will have two separate cables with central lead and copper grounding jacket, the other one is a cable with two leads in copper grounding jacket. You should not connect any power to leads unless you are sure they are the "working" wire with correct resistance - you risk creating a shortcircuit or shock hazard if you misidentify them. Ohmmeter is your friend, live AC is not. Powering the wire for extended time will heat up the floor that may be not fully set and disturb the binding process. It might overheat the wire too.
    – Thomas
    Commented Mar 16, 2023 at 14:36
  • In response to the "Edit" -- I'll just rabbit my earlier comment: there are ways to avoid that difficult situation, and if you read the documentation you'll know there is a $10 detector you can use during installation that will alarm you the second you put a screwdriver through the cable. IT's still a huge pain to fix but MUCH less painful than doing it after the floor is finished, and you don't have to "find" the damage because you hear the alarm while the screwdriver is still in your hand.
    – jay613
    Commented Mar 16, 2023 at 15:20
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I would just test for continuity and shorts with a multimeter.

Much safer than applying power to a faulty circuit - you could kill someone.

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