clamp meter to tell if someone's got a heater on?

This is what the Eco meter (discussed in another thread) would be perfect for, and better than your proposal. As long as you don't want to take accurate readings from it, it will give a clear indication of significant loads.

Heaters cycle on and off on thermostats, and you will only see the heater load when it's cycled on though.

It doesnt say 5% it said 0.5 thats a half a percent, about half a volt. What are you realy trying to do, is a tenant going out leaving stuff on, In the US we can buy something that goes on the meter with remote reader, you could get a data logger to the computer. If you want to find shorts get a Clamp On that goes to.01a. A Kill-a-Watt is great for apliance monitoring, if its a tenant and you pay the electric, thats a problem.

Can't you just ask ?

You do have to be able to clamp around a single conductor, not around the live/neutral pair, and not around screened conductors.

Not really......... I have a B&Q remote monitor and you can certanily tell if anh apliance is left on that takes significant power but small things are difficult to detect and account for. as an example, I have a fridget, freezers that switch on/off a thermostat in my aquarium all in all I use between 0.5 to 1 KWH . It's difficult to tell if it goes up from 0.5 to 0.7 is that my bedroom light switching on or my freezer motor, or perhaps the fish tank heater. (250W) I even noticed a change when I use dimmers on my lights but to be able to tell whether a small appliance has been left on is impossible. My two Macs computers use about 100W and 130W, I can just notice when the screen saver kicks in provided nothing else in the house has changed at that instant. Although I certainly notice when my kettle 3KW, electric cooker or Ecom 7 storage radiators come on. ;-0 So yes you could possible use it to 'monitor' certain appliances provided you know what else is eating your amps.

George (dicegeorge) expressed precisely :

It actually says +/- 2% on the current range, which is more than accurate enough to decide whether a light(s) have been left on. There are though better ways - look on ebay for a gadget which provides a wattage read out remotely. You clamp it around a cable, then a remote wireless display displays the wattage being used constantly.

What kind of screened conductor would not work with a clamp meter?

It looks as if you have 2 AC current ranges. The high one will be 0-999A and the low one (at a guess) 0-99.9A. Remember that the accuracy will be something like 2% +/-1 digit at best. So your indication with an accurate

Clip-on ammeters aren't usually all that accurate anyway. Keep the jaw surfaces scrupulously clean and the conductor in the centre of the loop, with other cables as far away as possible for best results.

Have you tested a clamp meter with the conductor not in the centre of the loop, or with other conductors nearby? I haven't, but I'd be surprised if the position of the conductors had a noticeable effect. Assuming of course that the jaw surfaces have closed properly.