My new heater does not work on my hot tub.
Test across both terminals at the same time. If you are getting 230V, disconnect power and check the resistance across the terminals. For 4kW, you should get roughly 13 ohms. If not, the element is bad.
If you get 0 Volts across the terminals, the tub is not heating. ManyBalboa packs switch both leads to the same leg of power, so you can get 120V to ground from each terminal, but you get 0V from terminal to terminal.
The clincher here is that the "heater on" light is off. I would double-check the pressure switch. You should test it with the tub running in heat mode. Disconnect one of the flag terminal leads from the switch, THEN test for continuity across the terminals. If you get none, you have either low flow or a bad pressure switch. You can verify by jumping the two wires together and see if the "heat" light comes on. Again, check for flow. I would run any of these tests with the filter removed. The symptoms you describe are very commonly caused by a dirty filter causing low flow. Again, pressure switch test. A bad sensor would cause the "current temp" to read too high, and a hi-limit problem would cause the little "heater-on" light to flash. Unless that little light is just dead (I've never seen it go bad), everything you say points to the pressure switch (or a dirty filter).
on Dec 13, 2009