The short answer is no, but is it harmful? Possibly.
Every time I hear about this when I was starting out, everyone had the same thing to say. “If you leave your hot tub empty, it will dry out the seals and gaskets.” That’s true. If you leave a hot tub empty for a long time, whatever is sealing the jets (silicone or gaskets) will eventually dry out and crack. Dried silicone or gaskets that crack cause leaks. You might know this already, but you don’t want leaks in your hot tub!
Hot tubs are built (you’re not gonna believe this!!) to hold water. Almost every step in the manufacturing process is this: not gonna leak here; not gonna leak here; not gonna leak here; not gonna leak here; not gonna leak here; not gonna leak here; not gonna leak here. Then there are some electronics and pumps and all that. But the resounding idea in every hot tub manufacturer in the world, is “let’s make sure this thing doesn’t leak”. Most of them are really good at it. My point is, they’re built with the idea that there will always be water in them. For that reason alone, it’s a good idea to never leave it empty.
All that being said, you would have to leave a tub empty for a crazy long time for seals to dry out. What I have found to be the biggest problem are the pumps, more specifically the motors. The longer you leave a hot tub empty, the higher the likelihood the motors are going to get locked up. Almost every hot tub is outside, and hot tub equipment is not shielded from the ambient temperature outside. The temperature is going to fluctuate. Here in the Pacific Northwest, you’re adding LOTS of moisture to that equation as well. In the springtime it can be 38° at in the morning, and 70° in the afternoon. Temperatures and moisture are going to cause corrosion to build up on the windings in your motor.
If your hot tub is only left empty for a short time, the corrosion won’t have enough time to build up. Most modern motors are strong enough at start-up to break any little pieces off. The longer the tub sits empty, the more build up there’s going to be. If you turn your hot tub on and the motor is locked up, can you smack it with a rubber mallet and break it free? Usually, yes. I have friends that are snow birds. I’ve wacked their motor for the last 3 years, and it still work great! That’s not going to work forever, and it’s probably not great for the motor.
There are other metal components in your hot tub too. Heater elements, heater tubes, ozonators, metal rings around the jets, etc. Yes, of course water is going to eventually make all these components corrode too, but that doesn’t change the fact that they were all designed and put together to live their lives underwater.
What I recommend is if you’re not going to use your hot tub for a while, just turn the heat down as low as possible. For most hot tubs that temperature is 80°. Whether it’s Summer, Fall, Spring or Winter, you’ll save money by not heating your hot tub to 104°. Additionally, and most importantly, this will allow your hot tub to run like it’s built and designed to. D
I hope this helps, and thanks for reading!
Hot Tub Mark