Air-source heat pumps are no greener than natural gas and propane heating systems, according to separate studies by Dutch and British researchers. The pumps, which some environmentalists have argued should be used to replace LPG heaters, release high levels of hydrofluorocarbon, a greenhouse gas with about 1,000 to 2,000 the effect of carbon dioxide. "This research shows that a heat pump is actually not more environmentally friendly than a gas-fired boiler," the Dutch researchers noted.

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