The major energy cost in standard hotels is providing large quantities of hot water.

Rooms are heated, or cooled, to suit the comfort of occupants, but excess heat is lost to the atmosphere.

Whitbread tasked us with reducing the heating bill in its hotels by 30% using recycled energy.

Having determined there was no suitable ready-made solution on the market, our R&D team developed a bespoke system based on an air source heat pump (ASHP).

The system was installed and trialled at a hotel in Milton Keynes.

The project needed to meet specific objectives:

  • Reduce energy use as much as possible, with at least 30% of the saving coming from a renewable source.
  • Develop a robust, long-lasting and cost-effective solution requiring minimum maintenance while meeting the highest health and safety standards.

The object of an ASHP is to turn hot air into hot water. Crucially, the water has to be heated to a minimum of 60 degrees to eliminate the risk of legionnaires disease.

An ASHP is more efficient at capturing heat at lower temperatures, so our team proposed using the pump at lower temperatures then boosting the water temperature later by conventional means.

The way the system works is that when the air conditioning system goes into cooling mode, it extracts warm air from the rooms and, instead of venting this air to the atmosphere, uses it to preheat water via a coil.

This water is stored in a hot water tank before it is heated to the optimum temperature for use by guests. A 10kW boiler proved to be the best fit for the system. In the Milton Keynes hotel, the team trialled a Mitsubishi heating unit.

A tank holding 4,000 litres of water was heated to 45 degrees efficiently and regularly. A weekly booster cycle was introduced to ensure the system was periodically raised to the 60 degrees required for safety.

One factor that had to be overcome was coil resistance. The exhaust air needs to flow over the outlet coil at the optimum rate to allow heat transfer to the water in the coil without any back pressure.

When the team discovered the fan chosen for the prototype was inadequate, allowing exhaust air to flow back into the building, a larger pump was fitted to solve the problem.

By the end of the Milton Keynes trial, the team had succeeded in achieving the minimum 30% energy saving. In certain conditions it is possible to reduce the running of the heating system from 8 hours to 1 hour per day. As a result, the energy capture system is now used in all Antony Grice projects.