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Field Test Best Practices: A Resource for Practical Residential Building Science

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The residential boiler is a cast-iron, steel, aluminum, or copper pressure vessel heat exchanger designed to burn natural gas or oil and to transfer the released heat to water (in-water boilers) or water and steam (in-steam boilers). Boilers are most often used to provide heat for space conditioning but combined systems can be used to provide heat for water heating as well. A single appliance can be used to heat water for hot water demands and provide hot water for either a hydronic heating system or to a heat exchanger in an air handler. Providing space heating through the air handler heat exchanger means that space cooling can be provided using the same coils. Another option for a combined space heating and water heating boiler is to have the boiler and separate tank for the hot water supply and a closed loop to send hot water from the boiler to the adjacent tank. That arrangement is shown in the following figure.

A schematic showing a residential boiler and storage tank system used to provide space heating and water heating to the house.

Combined boiler (for space heating) and storage
tank (to provide domestic hot water)

  1. Hot water pipe (to taps)
  2. Heat trap
  3. Cold water inlet
  4. Hot tap water storage tank
  5. Heat exchanger (filled with boiler water)
  6. Stored hot tap water
  7. Pump
  8. Hydronic heating pipe (to house radiators)
  9. Boiler for house heat

Combined space heating and water heating boilers can save energy, installation costs and floor space, but must be sized properly if energy savings targets are to be met.