Just an idea not sure if any good as quite ugly and usually sited out the way, we use these to operate the gas valves , they sense the fans running via 2 hollow probes
http://www.otsuk.ltd.uk/differentia...MI6P2p7d621QIVR7ftCh0_Ow1rEAQYASABEgIuCPD_BwE
http://www.otsuk.ltd.uk/differentia...MI6P2p7d621QIVR7ftCh0_Ow1rEAQYASABEgIuCPD_BwE