http://en.wikipedia.org/wiki/Thermocouple

http://en.wikipedia.org/wiki/Thermoelectric_effect

to quote from the text-

“The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference. At the atomic scale, an applied temperature gradient causes charged carriers in the material to diffuse from the hot side to the cold side, similar to a classical gas that expands when heated; hence inducing a thermal current.”

but there are other types that detect uv -

http://www.ehow.com/how-does_4924695_furnace-flame-sensor-work.html

a magnetic field can only influence material that is permeable.

http://en.wikipedia.org/wiki/Permeability_(electromagnetism)

hydrocarbons/oil based products are not great conductors, in fact they are good insulators and not permeable.

one thing about the fire being affected by magnetic fields though, its interesting that flame can be considered a plasma if it is hot enough, but thats in the thousands of deg c. plasmas can be altered by magnetic fields.

a good experiment is to put a lit match in a microwave and turn it on. watch the fireworks. note: you may damage the magnetron.

on the other hand, what about the effect on gas? when I place a magnet near to the flames in my gas boiler, the flames grow higher & burn hotter, the same applies when I wrap my magnets around the gas pipe.

The magnets were attached to the water boilers at work by a professional company, judging by the size of them compared to the domestic mains operated ones, I imagine there was a huge cost involved.
As far as the magnets on my fuel line were concerned, I was buyiong all my fuel on a company fuel card so each time I filled up I had to give my mileage and my records showed an increased mpg. I was travelling hundreds of miles a week, all around town and I didn’t have time to mess around driving economically so any placebo effect would have been quickly forgotten.
I felt it was worth a punt for a tenner & I have recomended magnets ever since.
I have had people try to tell me that it is impossible for magnets to have any effect on fuel – its composition simply doesn’t allow it.
Maybe it isn’t being affected in the way these websites are trying to explain (re-aligning molecules etc), maybe the fuel builds up a charge as it rushes through the pipes (in much the same way as a car or plane is charged as it moves through the air?) and it is this that the magnets act on?
Maybe the magnets don’t work on all cars due to the different composition of fuel lines used?

And the other thing is that we have laws about people making false claims in advertising – so why are magnets still advertised as having all these ‘fantasticla’ properties?

As you say, placebo effect. Everything you say is not evidence, it is only anecdote, when examples of what you claim are rested under controlled conditions (where outside influence and bias are removed and uncertainties understood) the effect disappears.