Retrofit hot water heating
For retrofits with radiators there are air to water heatpumps such as Gree Versati and Daikin Altherma which can do both space heating and DHW heat pump. However, sizing such systems (especially when reusing the existing radiators) might be an issue if the buildings are not well insulated and require a relatively large heating load which make the system relatively inefficient.
retrofit
idea for passive house