I've used them in the past, and have a couple of kits I keep handy for emergencies.
In my experience, they don't work all that well. I think you're right that inhibitor lowers the freezing point.
The tricks I've found are these:
- Freeze a horizontal pipe if you can, as it will freeze faster
- Make sure you tie string round the foam collar near the ends, especially at the bottom if you're doing a vertical pipe.
- use electrical tape to seal round the slit where the thin plastic pipe goes,
- squirt *loads* in - more than seems right!
- leave it for a good long while before attempting to cut the pipe,
- Have some dry towels handy, to wrap round the pipe and collar if you can, to improve the insulation.
To be honest though, if you turn off both ends of all the other rads and isolate the boiler, you'll lose very little water when you drain down, and it's less stressful than wondering if the blowlamp is melting the ice! If the system is clean, trap the water in a bucket and use it to re-fill the new rad, so dilution of inhibitor is minimal.
If you're fitting new valves to an old rad, which I've had to do here a few times, a good trick is to hacksaw some slots through the conical thread of an old spigot (the bit that screws into the rad itself), parallel to the run of the pipe. This makes a handy thread cleaner - a poor man's 1/2" pipe-thread tap. It's much easier then to fit the new one, and far less likely to leak if the threads are clean.
