Appendix A

Space heating and hot water

* This situation is changing as manufacturers respond to the increased demand for products with the very low outputs needed in ultra-low-energy buildings.

A note on hot water storage

In the UK, it is common to use ‘combi’ gas boilers, which provide instant hot water and do not require any hot water storage (although better combi boilers do now sometimes include a small integrated water store to avoid the boiler firing up every time hot water, however little, is used). Combi boilers are popular because they avoid the expense of a separate, full-sized hot water store and also save valuable floor space (useful in the relatively cramped space of much UK housing).

Use of combi boilers or other instant hot water solutions makes it impossible to use solar thermal panels, as solar panels are not able to reliably deliver heat at a consistent and sufficiently fast rate to heat water as it is being used. While Passivhaus does not specify whether to use an instant hot water system or a system based on a hot water store, it does encourage the use of solar thermal panels, where the building’s orientation and site are suitable; this solution, then, would require a hot water store.

Thermal stores

A thermal store provides the same function as a conventional hot water tank or store. But, unlike in a hot water tank, where the hot water is drawn out and used directly in the taps, the hot water in a thermal store is never used directly.

Instead, cold mains water is fed through a coil in the thermal store, takes in heat from the surrounding water, and emerges hot. Thermal stores have two advantages over conventional hot water tanks. Firstly, they can work at a lower operating temperature. In a conventional hot water tank the temperature must reach 60°C at least once a week to protect against legionnaires’ disease. For this reason, in the UK they are generally set to maintain this temperature permanently. Current regulations require that hot water coming out of the tap must not exceed 48°C to avoid the risk of scalding, so hot water from the tank is mixed down with cold. A thermal store can be set to maintain a minimum temperature of, say, 50°C. This lower temperature reduces the small but unwanted heat gains in a Passivhaus and, if solar hot water panels are used, means the secondary source of heat (e.g. a boiler) doesn’t have to raise the water temperature so much. Secondly, because the water in them is not under high pressure, thermal stores can be connected to ‘uncontrolled’ heat sources, such as the back boiler of a wood burner, making them a more flexible option should logs be used to provide water heating, either now or in the future. The lower temperature also makes the use of heat pump technology easier, if this is your choice of heating method. (Heat pumps work more efficiently when the temperature difference between water in and water out is smaller, in the region of 35°C.)