Ive replaced the flow switch and the heater, but the spa still wont heat. It's a 220v and I'm reading 120 on both teminal studs of the heater. There is water coming out of the heater jet, but feels cold. The first time I filled it it heated at a rate of 4 deg. an hour. Now I left it on for an hour and nothing. This spa has no reset button for hi current switch. The heater is 00.00 for resistens on the studs to the case, and the same from stud to stud. Whats next?

In order to properly check a heater for voltage you need to check BETWEEN the two terminal studs going to it, on a 220V heater you will ALWAYS read 110V from each terminal to ground, but if you read 220V between the two leads then no matter what it is a bad heater. If you are however reading 0 between the two leads then if could be a flow problem or a circuit board gone bad...Ohms resistance between the two heater studs should read between 9-12 but in order to check this properly you have to disconnect BOTH wires going to the heater.