Replaced heater core and thermostat had heat before replacing both now have no heat. Made a hissing sound after changing the heater core but had little heat changed the thermostat now have no heat at all. Checked the door for the heater core to make sure it wasn't stuck checked all the lines everything seems to be working properly but still no heat?

. Remove the thermostat and ensure it works properly, by suspending it in a pan of hot water. Using a thermometer, observe when it opens and closes.

. Purge any air from the cooling system.