Sustainability
Heat pump and low carbon heat design
Heat pump design for new build and retrofit — flow temperature and emitter sizing, hot water and legionella control, electrical capacity and acoustics.
- Discipline
- Sustainability
WHAT IT IS
A heat pump is not a boiler swap. It delivers heat at a much lower flow temperature, which changes emitter sizing, pipework, hot water generation and control strategy, and it draws its energy electrically, which changes the incoming supply and the switchgear.
The design questions that decide whether the installation works are mostly settled before any plant is selected: what flow temperature the building can actually be heated at, whether the existing emitters can deliver the load at that temperature, how domestic hot water will be generated and kept safe, and whether the site has the electrical capacity.
In Scotland the New Build Heat Standard has made this the default rather than the alternative for new buildings, and the same questions now arrive on every project rather than on the ones with an ambitious client.
WHEN YOU NEED IT
On any new building in Scotland where the building warrant is applied for on or after 1 April 2024, and on any building where the heat source is being replaced.
On retrofit, before the plant is ordered. The most expensive heat pump installations are the ones specified as a like-for-like replacement and then discovered, after commissioning, to be unable to heat the building on a cold day at the flow temperature the emitters need.
OUR PROCESS
The building before the plant
Heat loss recalculated for the building as it is, not as the old boiler was sized for. Emitter capacity checked at the proposed flow temperature. Where the two do not meet, the options are stated — fabric, emitters, or a higher flow temperature with the efficiency penalty quantified.
Hot water separately
Domestic hot water is a separate problem from space heating and is where most low-temperature systems come unstuck. Storage volume, reheat, and the temperature regime needed for legionella control are designed together, with any supplementary heat identified rather than assumed.
Then the plant and the site
Selection at the design condition rather than at the rating condition, defrost behaviour accounted for, acoustic assessment for the location, and the electrical capacity confirmed against the incoming supply before anything is ordered.
DELIVERABLES
What you receive
- Recalculated heat loss for the building as existing
- Emitter assessment at the proposed flow temperature
- Hot water strategy including legionella control
- Plant selection at the design condition, with defrost and part load accounted for
- Acoustic assessment and location strategy
- Electrical capacity check and supply upgrade requirement
STANDARDS
Standards applied
The editions in force at the time of design. Where a standard is amended mid-project we confirm which edition the appointment is based on.
- New Build Heat StandardDirect emissions heating in new buildings, ScotlandScottish Government
- CIBSE Guide B1HeatingCIBSE
- CIBSE AM16Heat pump installations for multi-unit residential buildingsCIBSE
- BS EN 12831-1:2017Method for calculation of the design heat loadBSI
- HSG274 Part 2Legionnaires’ disease — hot and cold water systemsHSE
- BS 8558:2015+A1:2024Design of water supply systems within buildingsBSI
QUESTIONS
Common questions
Can a heat pump heat an existing building?
Usually, but not always at the flow temperature that makes it efficient. The determining question is whether the existing emitters can deliver the calculated heat loss at the proposed flow temperature. Where they cannot, the options are to improve the fabric, enlarge the emitters, or accept a higher flow temperature and the efficiency penalty that comes with it. That assessment costs very little and should happen before any plant is specified.
What flow temperature should a heat pump system be designed to?
As low as the emitters and the building fabric allow, because efficiency falls as flow temperature rises. The figure is an outcome of the heat loss calculation and the emitter assessment rather than a value chosen at the outset, and a system designed around an assumed flow temperature that the building cannot achieve will underperform in exactly the weather it is needed.
How is domestic hot water handled at low flow temperatures?
Separately from space heating, and deliberately. Hot water has to be stored and distributed at temperatures that control legionella risk, which is above what a heat pump delivers efficiently. The usual answers are a larger store with a longer reheat, a dedicated higher-temperature circuit, or supplementary heat for the final temperature lift — each with different capital, running cost and space consequences, which is why it is designed rather than defaulted.
Does a heat pump need an electrical supply upgrade?
Often, on retrofit. Replacing a gas boiler with a heat pump moves the entire heating load onto the electrical service, and the existing supply was sized without it. The capacity check is quick, and it belongs at feasibility, because a supply upgrade has a lead time measured in months and can change the viability of the whole scheme.