Heating case study: Converting gas to heat pump hydronic

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This 1908 weatherboard Edwardian in Melbourne has been renovated, extended and insulated—and is making the switch to all-electric, powered by solar PV and 100% GreenPower. In a project completed in November 2017, the owners replaced the gas boiler on their existing hydronic heating system with an electric heat pump, while retaining the original 25-year-old radiators.

Owner Peter Hormann says they did a lot of work to check whether a heat pump system would enable the 25-year-old radiators to deliver sufficient heat for their winter heating requirements. An important consideration is the lower operating temperatures of a heat pump system which is most efficient up to 55 °C (though can run up to 60 °C or 65 °C) compared to 70 °C from a gas hydronic boiler.

While working out their upgrade options, they limited their old gas boiler to 55 °C through two Melbourne winters to test the existing radiator effectiveness at lower system operating temperatures. “We found that with the lower radiator temperatures the room heating was more gradual and took an extra 30 minutes to bring the house up from a 17 °C standby temperature to our 21 °C ‘comfort’ temperature,” says Peter. To compensate, their household thermostat was programmed for an earlier start in the morning and late afternoon heating periods.

Chris Siddons from Siddons Solar Hydronics (who installed the new system) says that there are some advantages to having a lower water temperature in a hydronic system: “The plumbing fittings have a longer life at lower temperatures, and the radiators are a safer temperature for toddlers and babies.”

Another approach used with heat pumps is to use larger radiators, but Peter was keen to use the existing radiators, which, despite their age, were in good condition.

Read the full case study, plus several more, in ReNew 144.