Can you give more information on your heater? Is it inductive?
I have not worked with high power heaters (e.g 36kW), but an optoisolated triac driver (e.g. like moc3023 but I'm not sure if it can handle your heater) output would be a good solution to your problem. The optocoupler would protect your controller from high voltage side, while needing about 15mA to 30 mA current to trigger a triac.