I have a hot springs hot tub that uses about 25% of my overall usage. This time of year we usually only use it on weekends and with all the rain and cold not much at that.
The hot springs controller has a “summer mode” that cycles the small circ pump and heater to maintain a much lower temperature but I found that it is too low if you actually want to use it.
So, I am considering installing a relay (connected to my home automation system) to cycle power to the heater during periods that I know there is no chance we will use it. That should reduce the overall usage as the lower the temperature of the water, the lower the losses. I understand that the heater will run longer when it has to catch up, but I am guessing that overall, the heater will run less. Freezing is not an issue for us, it gets cold but not cold enough to worry about it.
I think you will save money doing it like you said even though it will take a lot of energy to get it heated back up. As it is now, even keeping a lower temperature there is “heat loss”. No matter what you do the heat loss will always exist.
I know people with timers on their water heaters that completely turn them off at night or on vacation and they all have told me it has lowered their electricity use.
I think with a hot tub you will realize bigger savings than a water heater especially seeing that it’s 25% of your usage.
There are certainly savings to be had then, no question - whenever you can reduce the Delta-T you save.
If you have time of use pricing for your electricity, you can save even more by timing the cycles of the hot tub to favour your off peak electricity rates as well.
I use an HS110 to control our small 3 person plug-and-play hot tub. I programmed it so that it runs as little as possible during mid an on-peak electricity rates and focuses must of it’s run time during off peak electricity hours, and I’ve achieved some savings that way. As for lowering the temp, unfortunately, being a plug and play unit (120V) it can take a very…very long time to recover from any significant drop in temperature so I do tend to just leave it at 104 in the winter. In the summer however, I do plan to reduce it’s run time even more and let the temp drop off through the daytime and overnight hours as recovery will be much faster when it’s +30 out vs -20.
The only thing to keep in mind is to make sure that you leave things setup so that you do get enough basic filtration. Even if you don’t need heating, you will still need filtration - not getting enough will result in your sanitizer levels getting low (assuming you use a bobber style brominator? They require water flow to work) and scum build up and such, especially if the pump switches off not long after a bather load.