Most measures make the same system use less energy. A heat pump replaces the system and changes the fuel, which means its effect on a rating depends on how the assessment weighs electricity against gas, not just on how efficient the unit is. That is why two buildings can fit the same heat pump and see very different rating movements.
Large capital, and the rating effect depends on the model, not the unit
A heat pump delivers more heat than the electricity it consumes, which is a genuine efficiency gain. It also moves the building from one fuel to another, and the assessment applies different factors to those fuels. The rating outcome is the combination.
The practical consequence is that this is a measure to model for your specific building before committing, rather than to assume. It is also a measure whose rating benefit can change when the factors are updated, independently of anything you do to the building.
A heat pump produces heat at a lower temperature than a gas boiler. A building with poor fabric and emitters sized for high flow temperatures will be cold, or will run the heat pump hard and inefficiently to compensate.
So fabric and emitters come first in the design even where they come second in the programme. Retrofitting a heat pump into an unimproved building and hoping is the most common way this goes wrong and gets blamed on the technology.
Buildings with a steady, moderate heat demand and reasonable fabric. Buildings already running on electric heating, where the comparison is against direct electric rather than gas and the case is much stronger. Buildings with a simultaneous heating and cooling demand, where the plant can do both.
Less well: buildings with very high intermittent demand, poor fabric, or constrained electrical supply.
A heat pump is a significant electrical load, and so is EV charging, and so is any electrification of cooking or process. Several of those arriving together can push a building past its incoming supply capacity, and a supply upgrade is expensive and slow.
If more than one of them is on the horizon, establish the available capacity early. It changes the order you do things in, and occasionally it changes what is possible at all.
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