Heat pump water heaters can lower home energy bills by cutting water-heating costs, and in the United Kingdom, one homeowner pursued similar savings with a secondhand cylinder that heats overnight on cheap electricity and then switches back to a gas combi boiler once the tank cools.

Here's what to know

According to a Reddit post, the home's Worcester Bosch Highflow 550CDi combi boiler stayed in place as the alternate hot-water source while the homeowner added a lightly used Gledhill Stainless Pro 300L Direct cylinder bought on eBay. The original poster said the cylinder, with a stated 76-gallon (287-liter) capacity, cost £300 (about $400), and the added sensing and automation came to under £100 (about $150).

The setup depends on low overnight power prices. Because the home already has solar panels and battery storage, they estimated the system could save about £350 (around $500) per year.

Home Assistant uses data from three temperature probes — one at the cylinder's upper section, one on the cold-water inlet, and one on the hot outlet — to determine when the stored hot water is running low. When the tank-top temperature falls to around 104 degrees Fahrenheit (40 degrees Celsius), it tells the Shelly 1 Gen4-controlled three-way valve to switch the home's hot-water supply back to the combi boiler.

The homeowner said, "We host Airbnb guests in the house, so 'the cheap hot water has run out' isn't a particularly attractive feature."

More background

The cylinder's two 3-kilowatt immersion elements can each be controlled and metered separately. On the dashboard, the homeowner can see all three temperatures, each immersion's live power use, that day's energy consumption, estimated electricity cost, and a rough gas-cost comparison. The homeowner also said they kept the existing safety equipment in place. 

One commenter wrote, "Sounds like an awesome project. I do something similar but we only have a cylinder! Three way valve is a cool addition."

Describing the sensor setup, the homeowner said the probes are attached to the pipes with foil tape, adding, "The temperature is extremely accurate already when it's flowing."

What can be done?

To make the system work, they kept the Worcester combi and installed a WRAS-approved 24V three-way L-port motorized ball valve to switch between the two hot-water sources. Home Assistant is set to run the bottom immersion overnight from 11:30 p.m. to 05:30 a.m., and the post also notes a one-hour boost for the top immersion, plus a two-stage boost option when needed.

For homeowners who want a lower-bill electric water-heating option without building a custom control system, Cala offers customizable smart heat pump water heaters that can help homeowners lower energy bills by heating water only when needed. That kind of timing control is a major part of what made this DIY setup appealing, as it was designed to take advantage of cheap overnight electricity and rely less on gas. 

Homeowners looking for a more packaged alternative to probes, relays, and dashboard automation may want to explore Cala.

Where can I learn more?

Here are a few more examples of how households and utilities are lowering water-heating and home-energy costs. 

• In Hawaiʻi, Shifted Energy turned water heaters into virtual power plants to help balance the grid.

• Across the U.S., homeowners are weighing heat pump water heater costs against long-term bill savings.

• Many homeowners are replacing furnaces with ductless mini-split heat pumps that cut heating and cooling bills.

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