I came across an inflatable pool spa that would work great in an above ground pool. I works off your existing pool heater and filter system. That add has sure got me thinking.
Years ago I had a small hot tub sitting near my above ground pool. The hot tub finally gave way to old age and now I really miss it. The reason this add got me thinking is because I had used my above ground pool filter system to keep the spa clean. Every evening I would run the filtered pool water through my spa long enough to completely change the water, just a few minutes. I then shut the pool valves off and turned on the spa pump and heater. Being a small hot tub it only took an hour to heat. I thought it was a great system.
The spa shown below works off your existing pool heater and filter. Here in Arizona very few people heat their above ground pools so this product would not work so well. But in thinking back to my old hot tub and how quickly it heated I am sure that excellent results could be achieved with the addition of a three hundred dollar spa heater as opposed to a fifteen hundred dollar pool heater. If you just wanted to heat the spa for an hour or so day this would be the way to go.
Many of the smaller electric spa heaters work with either 110 or 220 volts. This means you could use your existing filter outlet in most cases. The plumbing for these units is simple enough. The return line from your filter back to the pool would have to run through the heater. It would also need to be either solid PVC pipe or my favorite, PVC flex pipe. Converting an above ground pool return to flexible PVC pipe is very simple. Most filters and returns use standard 1 1/2" thread so a few male adapters are usually all that's needed. For a Doughboy return I would suggest using a hard plumb adapter sold by Doughboy.
Since the water going into the pool spa is coming directly from the heater, the spa will heat up quickly and there is no need to heat the rest of the pool. The heater will run only as long as the spa is in use, then it can be turned off.