My spa is an unknown brand - on the controller it says "Spa Controls, Inc."
The heater only works when the pump is on high speed. What do you think the problem might be?

Your pressure switch is either out of adjustment, or needs to be replaced because it's on it's way out. It was previously working on low speed right?

Yes, the heater worked fine last summer.

I followed the instrctions on this site for checking the pressure switch (i.e.,I disconected the leads and checked for continuity.) There is continuity across the switch when the pump is on low speed. I performed this test without water in the system, does that matter?

Well that kind of changes everything.

The pressure switch should never show continuity without water in the system, unless your switch has three terminals and you're checking between the common terminal, and the NC (normally closed) terminal. Which of course would not do any good, since all of them work in the system using the NO (normally open) terminal.

Also, it's not safe to do this without water in the system because the heater element can burn up in seconds. They're not like dryer or stove elements, they MUST have water around them, and the water must be flowing. That said, even if it had water and it wasn't flowing, you'd boil out your heater cavity in less than 15 seconds on a 230 volt system, and burn out everything in about 30 seconds, including melting down your pvc attached to the heater after 60.

So, without the proper conditions being met, (water in the tub - pump running etc), you're going to get confusing results, and possibly damage your heater system. Be very careful with this and let us know if we can help you further.

Thanks! After repairing a leaky pipe, refilling the spa, and adjusting the pressure swirch, everything seems to be working fine.