Installing an air to water heat pump is one of the most energy-efficient ways to heat your home or building. But for it to work at peak performance and cost-effectiveness, correct sizing and system design are absolutely critical.

In this article, we’ll walk you through the key steps and considerations for sizing and designing an air to water heat pump system, ensuring long-term comfort, efficiency, and savings.

Why Proper Sizing Matters

Oversized or undersized heat pumps:

• Waste energy and money

• Lead to frequent cycling or inefficient operation

• Cause uneven heating or discomfort

• Reduce system lifespan

A properly sized system matches the building’s heating load and delivers the right amount of heat at the right time, improving both comfort and efficiency.

Step-by-Step: How to Size an Air to Water Heat Pump

1. Conduct a Heat Loss Calculation

The first step is to determine how much heat your building loses during the coldest conditions.

Factors include:

• Floor area and volume

• Insulation levels (walls, floors, roof)

• Window type and area

• Air tightness

• Local climate and outdoor design temperature

Use Manual J (for the US), SAP (UK), or ISO 13790 for accurate load calculation.

Example:
A 200 m² well-insulated home in Germany may have a design heating load of ~7–9 kW.

2. Factor in Domestic Hot Water (DHW) Demand

If your heat pump will also provide hot water:

• Add ~1.5–3 kW depending on household size

• Consider a buffer or DHW tank for storage

3. Select the Right Output Capacity

Choose a unit that matches or slightly exceeds your calculated heat load at your location’s design temperature.

For colder climates, ensure the unit can maintain output at -5°C to -15°C.
️ Check the low ambient temperature performance curves.

4. Check Flow Temperatures for Heat Distribution

Your heat emitter type impacts performance:

• Underfloor heating: Low temperature (35–40°C) – highly efficient

• Radiators: May require 45–55°C – ensure pump is rated for higher flow temps

Lower flow temps = higher seasonal efficiency (SCOP).

5. Design Hydraulic Layout

Key components:

• Buffer tank (optional but helpful for stability)

• DHW tank (with coil or external plate exchanger)

• Zone control valves or thermostats

• Circulation pumps and flow regulators

Make sure the flow rate and pressure drop match the heat pump's specifications.

⚙️ Tips for Optimal System Design

• Inverter compressors adapt to variable loads and improve efficiency

• Include weather compensation controls

• Use a thermally stratified DHW tank for better performance

• Size pipework to avoid flow restrictions

• Plan for defrost cycles in cold climates

️ Should You Use a Professional?

Yes. While you can calculate rough estimates, final sizing and system design should be done by a qualified heat pump installer or HVAC engineer using detailed software tools.

Benefits of Correct Sizing and Design

✅ Maximum seasonal efficiency (SCOP
✅ Long equipment lifespan
✅ Lower running costs
✅ Consistent indoor comfort
✅ Eligible for subsidies and rebates

Conclusion

Sizing and designing an air to water heat pump system isn’t a one-size-fits-all process. It requires attention to detail, climate conditions, home characteristics, and heating demands.

With a professionally sized and well-designed system, you'll enjoy the full benefits of this sustainable heating technology for years to come.