What is The Main Problem with Oversized Heat Pump?
- Mario Dodić
- 4 days ago
- 3 min read
Oversizing a heat pump can cause more problems than most designers expect, especially in modern, well-insulated buildings.
While a 20-30% oversizing margin is usually acceptable (especially in the UK, where defrost is highest on design outdoor temperature), problems often arise when systems are oversized by 100-200%, which is not uncommon in new homes with excellent insulation.
The result?
Reduced performance and SCOP
Frequent compressor ON/OFF cycles.
In poorly designed hydraulic systems, the COP can even drop to around 2, a level that is considered inefficient for a modern heat pump system.
Importantly, the issue is not at the outdoor design temperature. The real problem occurs during the 80-90% of the year when outdoor temperatures are significantly above the design temperature.
Here you can find historical meteorological data on the ASHRAE website.
Why Oversizing Happens
In practice, oversized heat pumps are more common than undersized ones. This is largely because many designers and installers are used to working with gas boilers, wood, or pellet systems - where oversizing does not create the same operational issues.
Heat pumps, however, behave differently.
What Happens When a Heat Pump Is Oversized?
The core component of a heat pump is the compressor, which has certain limitations, particularly in modulation.
What is modulation? Modulation is the ability of the heat pump compressor and circulation pump to vary their speed continuously between minimum and maximum capacity, matching output to the actual building load.
While a compressor can modulate over a wide range, this range sometimes isn't sufficient for the house and system to operate efficiently.
Let's explore this with an example.
Understanding Heat Loss
Heat loss depends on several variables, with the most important being the temperature difference (ΔT) between indoor and outdoor conditions.
When the outdoor temperature is lower, ΔT is higher, resulting in greater heat loss.
For example, consider a new 200 m² house with good insulation and high-performance windows:
At -10°C outdoor and 20°C indoor (ΔT = 30°C), the house requires 9 kW of heating power (45 W/m²)
At +7°C outdoor and 20°C indoor (ΔT = 13°C), the heat demand drops to 3.9 kW (around 20 W/m²)
This represents more than a 50% difference in heat loss between -10°C and +7°C.
Watch the full video on our YouTube channel to see how incorrect sizing and flow rate calculations affect heat pump performance.
The Impact on Heat Pump Performance
When sizing a heat pump, it’s important to understand that capacity varies with outdoor temperature.
As outdoor temperatures drop, the capacity of the heat pump also drops. As outdoor temperatures rise, capacity increases.
An oversized heat pump may not modulate low enough to match your home's lower heat loss at higher outdoor temperatures.
Consider a 9 kW heat pump below. At -7°C, it can modulate from 3 to 9 kW, providing a wide range of modulation.
However, at +7°C, it modulates from 4 to 9 kW.
In our example, the house requires around 4 kW at +7°C, placing it at the edge of the modulation range. This pump can cover the heat loss at both -10°C and +7°C due to its wide modulation range.
The Importance of Proper Sizing
If the water flow rate is insufficient or if the building requires a lower heat load, the heat pump may start to operate in frequent ON/OFF compressor cycles. Especially with thermostats.
Understanding the correlation between modulation, capacity, and your house's heat requirements is crucial.
In many cases, it is better to size the heat pump closer to the lower end of the required capacity range at the coldest expected outdoor temperature, using a backup heating element only when necessary.
For example, in our 200 m² house, a 12 kW heat pump may only be able to modulate down to 5 kW at +7°C, while the building requires just 4 kW. This mismatch can cause the compressor to cycle on and off frequently, reducing efficiency.
NOTE: Some larger units have a very good modulation ratio. If units of 5, 7 and 9 kW have the same minimum capacity (for example, 2 kW), then 9 kW might be a better choice in certain conditions (for example, faster domestic hot water production). However, always check with the manufacturer what the minimum capacity is and then decide.
Watch the full video on my YouTube channel to see how incorrect sizing and flow rate calculations affect heat pump performance.
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