SanCo2 Issues

A homeowner on a building forum shared a question about their hot water setup, and it's a useful real-world look at a common heat-pump water heater quirk. The homeowner had switched from a propane boiler to heat pumps for home heating, then replaced their old water heater tank with a SanCo2 unit, an 83-gallon heat pump water heater that stores heated water separately from its compressor. They installed the compressor in a basement that stays around 50-60 degrees in winter and are happy with the efficiency overall.

The problem: after being away from home for four or five days, the water isn't as hot as some household members like. The homeowner suspects the unit waits until the tank cools below a certain temperature, maybe around 110 degrees, before the compressor kicks back on to reheat it, so returning home to a tank that's drifted down leaves less hot water than usual. They're considering reconnecting their old propane boiler as a backup heat source for the tank, but worry that mixing systems could disrupt how the SanCo2 tank naturally layers hot and cool water inside, a feature that helps it run efficiently.

If you have a heat pump water heater and travel regularly, this is worth knowing: these units are typically sized and tuned for steady daily use, and extended absences can mean a cooler tank greets you when you return. It's a tradeoff some owners accept for the efficiency gains, but it's a real quirk rather than a malfunction.