Installation case study
Two heat pumps. Three jobs.
Air source heat pump cascade — space heating, hot water & swimming pool
A high heat loss property, a full domestic hot water demand and a swimming pool to warm — solved with one intelligently split cascade rather than three separate systems.
- System
- 2× ASHP cascade
- Configuration
- Shared duty
- Serves
- Heating, DHW & pool
- Installer
- Elixa Renewables
The brief
A property that asked three things of one system
The design question was not how much heat do we need. It was how do we make the same kilowatts do different work at different times of year, without ever fighting each other.
Most heat pump projects have one job to do: keep the house warm. This one had three — and each of them pulled the design in a different direction.
The property carried a heat loss well above the 8–12 kW that covers the majority of UK homes — already at the upper end of what a single domestic monobloc delivers comfortably in mid-winter. Add a full stored hot water demand across multiple bathrooms, and then a swimming pool asking for a long, low-grade, high-volume heat input, and a single appliance solution stops being ambitious and starts being fragile.
One oversized heat pump? Size a single unit for peak heat loss plus pool and you end up with a machine that spends the other nine months of the year cycling badly against a fraction of its output. Short cycling is the fastest way to destroy both efficiency and compressor life.
Three separate systems? A heating unit, a hot water unit and a dedicated pool heater is clean on paper and expensive everywhere else — three sets of groundworks, three electrical supplies, three commissioning visits, three service contracts, and a plant area that no longer fits the space available.
The design envelope
- Property
- Large detached, approx. 250–350 m²
- Design heat loss
- 18–26 kW at −2 to −4 °C external
- Emitters
- 35–40 °C flow underfloor, 45–50 °C radiators
- Hot water
- 300–500 litre unvented cylinder, 3 kW immersion back-up
- Pool
- 45–55 m³; 4–8 kW steady demand with a cover fitted
- Electrical supply
- 25–30 A per unit — three phase common at this scale
Indicative parameters for a cascade specification of this type — typical ranges for this class of system, not measured values for this property.
The solution
A cascade, with one unit doing double duty
In mid-winter the property has the full output of two heat pumps behind the heating. In summer, one unit covers hot water while the other quietly heats the pool. Nothing is oversized for the job it is doing at the time.
Rather than one large appliance or three small ones, we installed two air source heat pumps in cascade, sharing a common primary circuit — but with the two units given deliberately different roles.
Unit 1 — dedicated space heating
The lead unit is committed to the heating circuit. It carries the base load on its own for the majority of the year, running long, low, weather-compensated cycles at the lowest flow temperature the emitters will allow. No competing demand, no interruptions for hot water, no compromise on efficiency.
Unit 2 — shared duty across heating, hot water and pool
The second unit is the flexible one. Its output is split at the header so it can support space heating alongside Unit 1 when outside temperatures fall, while also serving the hot water cylinder and the pool heat exchanger when those loads call. In practice it behaves like two half-machines pointing in different directions.
Hydraulic separation as the referee
Both units feed a hydraulically separated primary, keeping the heat pump circuit independent of the distribution circuits. Every circuit — heating zones, cylinder coil, pool plate heat exchanger — draws through its own pump and isolation, so flow rates stay correct whichever combination of loads is running.
Controls that set the priority order
The controls decide who gets the heat and when. Hot water takes priority for short, defined windows. Space heating takes precedence over the pool during cold weather. The pool picks up surplus capacity in the shoulder seasons, when there is plenty of it going spare.
The installation
Built once, built properly
Both units sit on level bases against the gable elevation, on anti-vibration mounts, with the primary flow and return runs kept short, insulated and neatly aligned between the two machines. Positioning was chosen for clear airflow across both fan banks and for acoustic performance at the nearest neighbouring window, assessed under MCS 020.
Internally the plant is arranged around the separated primary, with individually valved circuits so any zone, the cylinder or the pool exchanger can be isolated without draining the system.
Scope of works
2× air source heat pumps in cascade configuration · hydraulic separation, primary pump set and fully valved circuit manifolds · unvented hot water cylinder with immersion back-up · plate heat exchanger and secondary circuit serving the swimming pool · weather compensation and priority-based control strategy · dedicated electrical circuits, isolation and protection · system flushing, inhibitor dosing, commissioning and handover
Typical performance for a system of this type
3.0–3.5
Seasonal efficiency for a well-designed low flow temperature cascade
20–60%
Running cost saving vs oil — lower on a standard tariff, higher on a heat pump tariff
26–34 kW
Installed cascade capacity for two twin-fan units at A7/W35
Typical ranges for this class of system, confirmed per project at design stage. Cost comparison based on kerosene at approx. 92p/litre and electricity at 26.11p/kWh, or 13–15p on a heat pump tariff (August 2026).
Where a cascade earns its place: a heat loss above roughly 18 kW, a second heat demand such as a pool or annexe, or a property that cannot afford to be without heat if one appliance faults.
Have a property that asks more than one thing of its heating?
High heat loss, a pool, an annexe or awkward emitters — these are the projects where a standard single-unit specification quietly underperforms. We design cascade and multi-load systems from the heat loss up.