Okay by way of an update I now have a number of various PTC Thermistors for trial. The original ones I used in ID were 6cm x 2cm x 0.5cm and were rated for 230 C. I can't purchase these anymore so did some hunting around and found some alternatives:
1) Those shown in the picture next to the turrets 3cm x 2.5cm x 0.5cm rated at 220 C
2) Some older ones which are in a flat sleeve and are 6cm x 1cm x 0.3cm rated at 200 C
3) Some new ones which have just arrived 5cm x 2cm x 0.5cm rated at 230 C
4) The original ones 6cm x 2cm x 0.5cm rated at 230 C
They are all different with different characteristics and cable thicknesses. I've tried out 1) and whilst they certainly get hot it doesn't seem as hot as I would have thought. The trouble is there are various permutations to contend with - current consumption, surface area and rate of heating/time. So I've decided to take the plunge and have ordered a pyrometer to measure the actual temperatures so a series of experiments coming up:
Test the originals as I have one spare and I know these work perfectly. Check the current consumption every 20 seconds, final current consumption, temperature reading every 20 seconds and final temperature.
This should give me some parameters to play with when doing a comparison with the others so I can put this on a spreadsheet to see temperature rate and final temperature, time frame and current consumption.
I can then repeat the experiment with the thermistors strapped to the heat exchanger and measure the temperature of the heat exchanger, current consumption and time.
What I suspect is that whilst the smaller thermistors reach the required temperature the rate of conductivity to the heat exchanger is less as the surface area is less so the rate the heat exchanger heats up will be different. Depending on ambient air temperature its possible the heat exchanger will never get to the right temperature as the heat loss from the exposed surface will exceed the thermistors capacity to provide heat! However this would all be different once there is insulation so may just be a time issue.
I hope to be able to provide some guidelines for which thermistors are best suited to this process.
I'll report back later!
Cheers
Geoff