A London apartment resident with no option for traditional air conditioning turned to Reddit with an inventive question: Could a heat pump water heater cool the flat in summer while also providing cheaper hot water?
Commenters appreciated the creativity, but they quickly pointed to the physics problem that keeps the idea from working as a true AC replacement.
In a post on r/heatpumps, the resident described a 645-square-foot, east-facing, electric-only apartment with "a lot of solar gain" in a listed 1930s building — which means it's historically protected. Because external AC and a mini-split were effectively off the table, they asked whether installing a hybrid heat pump water heater indoors could serve two purposes at once.
The plan was to place the tank in a bathroom airing cupboard, draw air from the bathroom, and vent it into the hallway. In winter, the appliance would rely on its electric element for hot water. In summer, the resident hoped the heat-pump mode could warm the water while also helping cool the apartment.
Reddit users said the idea was not unreasonable — just limited. Several noted that once the tank water reaches a certain temperature, the cooling effect largely ceases unless the heat is dissipated elsewhere. One commenter estimated that many heat pump water heaters provide only about 4,000 BTU of cooling, while another said their unit uses a compressor rated at roughly 400 watts.
The original poster ultimately appeared to agree with that assessment. "I get that the cooling would be minimal and only likely to run for 4 hours a day based on how much water we use," they wrote.
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A heat pump water heater can still reduce costs, but mostly by lowering the electricity needed to heat water, not by cooling an entire home. The poster said their current electric resistance water heater operates for roughly four hours per day at 3 kilowatts, and later estimated that 6 to 7 kilowatt-hours of daily use may be tied to hot water for two people.
The main savings would most likely come from cheaper water heating in summer, not from replacing a conventional air conditioner. A few users noted that these systems can provide slight cooling and some dehumidification, which may make a stuffy room feel more comfortable even if the actual temperature drop is modest.
It may provide somewhat cheaper hot water and a little cooling, but commenters agreed that it should not be relied on as a primary summer strategy.
One commenter said that adding exterior white shades helped: "Blocking most sunlight before it hits the glass works well." In buildings with strict preservation rules, portable or fully internal systems may be the remaining options, if allowed.
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"I guess the only real advantage would be cheaper heating of the water in the summer," the poster wrote after reading the responses. Another commenter added, "Buy a heat pump water heater for what they're good at: cheap hot water."
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