In summer, the big power hog in most homes is probably the air conditioner. But for homes with pools, heating pool water is often a significant factor too.
This raises the obvious question: When you’re using lots of power to cool the house at the same time as you’re spending money to heat the pool, shouldn’t there be a way to capture the heat you’re taking out of the home’s indoor air, and put that heat into the pool water?
Lindsay Scott, currently an energy auditor in Austin, Texas, had that same thought back when he was a builder in the Cayman Islands. Electricity is expensive in the Caymans, and Scott’s customers wanted high-performance homes with low energy bills. But they also wanted swimming pools. Scott’s answer was to heat the pools with waste heat from the houses, using a heat transfer system called the HotSpot FPH (for “free pool heater”), from HotSpot Energy (hotspotenergy.com).
The system is simple in concept: You just install a secondary condenser in the air-conditioner or heat-pump refrigerant line. When the air conditioner is running, if the pool needs heat, the secondary system cuts in and pulls heat from the air conditioning’s coolant loop.
John Williams, the CEO of HotSpot Energy, said his product sells nicely by word of mouth. “We’ve never advertised,” said Williams. “We’re engineering types, not marketing types.” But Williams was happy to explain the concept to JLC in a phone call. “It’s basically just a conventional commercial heat-recovery system, adapted for use with a swimming pool,” he said. “The only differences are some minor differences in controls, and of course we upgraded to a titanium heat exchanger to deal with salts or corrosive chemicals that could be in the pool water.”
“In a regular air conditioner,” said Williams, “you have a fan, and you blow outdoor air across a coil, and the heat gets thrown away into the backyard. When our system is active, the refrigerant uses the alternative water-cooled condenser, and instead of a fan, we use the existing pool pump to push water across the titanium coil, which puts the heat into the pool instead of throwing it away." Additional controls enable the system to choose which heat sink to employ, Williams explained: "When the pool needs heat, the system uses the water-cooled condenser. And when the pool is warm enough, the system switches back to use the air-cooled condenser, to prevent the pool from overheating.”
The system takes a week or 10 days to warm the pool up at the start of the cooling season (which, conveniently, is also the outdoor swimming season, Williams noted). But once the pool is warm, the HotSpot keeps it warm continuously. Said Williams, “You could never afford to keep a pool heated continuously like that with a paid-energy pool heater. Your bills would be $5,000 for the season.”
And, Williams explained, the HotSpot recovers the mechanical energy of the air-conditioner compressor as well as the waste heat from the house. "We are capturing 100% of the rejected heat from the home's indoor air," he said, "plus the heat of compression from the compressor. An air conditioner's rating is based on the heat rejection capability of the indoor coil, and there is always 12,000 Btu of heat per ton at the indoor coil. But at the outdoor coil, you typically have around 15,000 Btu per ton in radiant capacity, because the heat of compression—basically, the energy of the work done by the compressor—is added in. An average guy might think of a 3-ton unit as having 36,000 Btu of cooling. But they wouldn't realize that the same unit actually provides 45,000 Btu of heating capacity for the pool."
The simpler the home's cooling equipment, the easier it is to add the HotSpot, said Williams. "When you talk about high-SEER equipment," Williams said, "as long as it's a single speed compressor, our system is going to work fine with it. When you start to see variable-speed compressors, our system is fine with a variable-speed compressor too, it doesn't care; but variable-speed compressor units tend to have a lot of internal communications going on, and that can sometimes make it quite tricky for the installer to get everything working correctly. So for that reason, we don't recommend our system for variable speed compressors. You know, if you owned an air conditioning company, and told me you were putting this on your own house, I wouldn't have any hesitation. But I don't want to support highly complicated installations for customers who aren't expert themselves. We do a good enough business with this without having to look for trouble."
But by the same token, the HotSpot offers the biggest efficiency boost to the more basic cooling systems. So adding a HotSpot to a single-speed, relatively low-SEER cooling system makes that system's efficiency competitive with a much more costly and sophisticated high-SEER setup, Williams claimed—especially in warmer weather. As outdoor temperatures rise above 90°F, ordinary air conditioners work harder to push heat into the warmer outdoor air. At that point, it becomes easier for the HotSpot to push heat into 85°F pool water, especially because water is better at conducting, storing, and transporting heat than air is.
"And if you ever get one of those rare freak heat events, like a 120°F day," said Williams, "a regular air conditioner stops working altogether. Because at around 122°F, the outdoor air is above the saturation temperature of the R-410a refrigerant. So basically, an air cooled air conditioner can't work at all above around 122°F. Some may work a few degrees higher, but many won't. But if you have our system installed, you are going to be able to run your air conditioner, without regard to the outdoor air temperature."
"We don't sell this as an efficiency Improvement for the air conditioner," Williams said. "We sell this as a way to avoid $1,500 to $4,000 a year of pool heating energy cost. But the efficiency gain that you get is a nice thing too. This makes your air conditioner more efficient, and it heats the pool for free.”
Web Extra: see This Old House mechanicals guru Richard Trethewey install a HotSpot in this video (see: "How to Heat a Swimming Pool With an Air Conditioner").
