Air-to-Water Heat Pump Retrofit: 4 Scenarios Where AWHPs Shine

We all know the standard Air-to-Water Heat Pumps (AWHP) retrofit warning: You can’t just rip out a 180°F (82°C) gas boiler, drop in a 130°F (54°C) heat pump, and expect the building to stay warm.

But while a 1-to-1 boiler swap is a bad idea, AWHPs thrive when applied in the right context. If you want to reduce building carbon emissions efficiently, here are the four scenarios where Air-to-Water Heat Pumps are the optimal solution.

Key Takeaways:

• Engineers should consider AWHPs as the optimal retrofit choice in four well-defined scenarios, outlined below.
• Buildings with improved envelopes, oversized coils, or radiant floors are ideal for AWHPs.
• Bivalent (hybrid) systems can achieve 80–90% of carbon savings without replacing all terminal units.
• COP rises dramatically at lower water temperatures, making radiant systems the most efficient pairing.
• h2x allows engineers to model existing coil performance at reduced entering water temperatures before committing to a design.

An air-to-water heat pump extracts thermal energy from outdoor air and transfers it into the building’s hydronic loop — replacing or supplementing a traditional gas boiler without major pipework changes.

Why Air-to-Water Heat Pump Retrofits Matter for Decarbonization

An air-to-water heat pump (AWHP) is a heating and cooling system that extracts thermal energy from outdoor air and transfers it into a hydronic loop, the same network of pipes and terminal units found in traditional boiler-based buildings. Unlike a gas boiler, which generates heat by combustion, an AWHP moves heat using a refrigerant cycle, making it far more energy-efficient under the right conditions.

The key design constraint is supply water temperature. A conventional gas boiler can produce water at 160–180°F (71–82°C) without difficulty. Most AWHPs are enhanced to supply water at 100–130°F (38–54°C), though high-temperature models can reach 140–160°F (60–71°C). This difference should not be treated as a flaw of AWHPs, but as a design characteristic that engineers need to account for when evaluating a retrofit.

Buildings account for a significant share of global carbon emissions, and space heating is the largest single contributor within that category. The majority of commercial and multi-family buildings in the USA, UK, Europe, and Australia still rely on gas-fired hydronic systems. Replacing those systems with heat pumps is one of the quickest pathways to low-carbon heating (see the IEA’s analysis in The Future of Heat Pumps).

How an AWHP Hydronic System Works in Retrofit Projects

An AWHP integrates into an existing hydronic system in place of, or alongside, a conventional boiler. The refrigerant cycle extracts heat from outdoor air, compresses it to raise the temperature, and rejects that heat into the building loop via a heat exchanger. The same vapor-compression cycle can also reverse to provide cooling.

The six key steps in an AWHP retrofit assessment are:

1. Establish the building’s current peak heating load and design water temperature.
2. Identify the target supply water temperature the AWHP will deliver.
3. Check whether existing terminal units (coils, radiators, fan coils) can satisfy the load at the lower water temperature.
4. Calculate the pressure drop and flow rates through the revised system to confirm pump sizing.
5. Determine whether the system requires a bivalent arrangement to cover the coldest design conditions.
6. Size the AWHP to meet the base load, not necessarily the peak load.

Four Air-to-Water Heat Pump Retrofit Scenarios Where AWHPs Work Best

AWHPs are not universally applicable. But in these four scenarios, they are the most powerful tool available for reducing carbon emissions in hydronic heating systems.

1. The “Envelope-First” Deep Retrofit:

This is where building physics meets HVAC design. If the owner is replacing windows, adding roof insulation, and tightening the envelope, the building’s heating load drops drastically. Suddenly, those existing heating coils and radiators, which were originally sized to deliver 100 MBH (29.3 kW) using 180°F (82°C) water, only need to deliver 60 MBH (17.6 kW). Guess what? They can often hit that new, lower target using 130°F (54°C) water from an AWHP. Reduce the load, and you automatically reduce the required water temperature.

2. The “Accidentally” Oversized Coils:

Let’s be honest about historical engineering practices: we loved our safety factors. Many legacy systems were designed with a 20% safety factor on the load, plus a coil that was rounded up to the next available size. Before you assume an AWHP won’t work, run the existing coil performance at a lower Entering Water Temperature (EWT). You might be surprised to find that the “180°F (82°C) coil” can easily satisfy the actual space load at 140°F (60°C) simply because it was massively oversized on day one.

3. The Radiant / Underfloor Heating Dream:

If you are lucky enough to be retrofitting a building that already uses radiant / underfloor heating (or if you are adding it), AWHPs are a match made in thermodynamics heaven. Radiant systems thrive on low-grade heat, typically requiring water temperatures between 90°F (32°C) and 115°F (46°C). At these temperatures, the Coefficient of Performance (COP) of an AWHP skyrockets. It is the most efficient pairing you can design.

h2x models radiant / underfloor heating systems in both 2D and 3D, allowing engineers to calculate flow rates, pressure drop, and pipe sizing across every zone before committing to a design — essential when pairing a low-temperature AWHP with an existing or new radiant floor system.

4. The Bivalent Heat Pump Stepping Stone:

You don’t have to electrify 100% of the building on day one to make a massive environmental impact. Enter the bivalent system. You size the AWHP to handle the base load—covering roughly 80% to 90% of the annual heating hours. You then leave a small condensing gas boiler in the loop (or install a new one) to act as a peaking plant. The boiler only fires up during the coldest 10% of the year when the AWHP capacity drops and the loop needs a temperature boost. You get the vast majority of the carbon reduction without the massive capital cost of replacing every terminal unit in the building.

Common Air-to-Water Heat Pump Retrofit Mistakes

Even well-intentioned AWHP retrofits can fail when these mistakes are made:

• Treating the AWHP as a direct boiler replacement. An AWHP operating at 130°F (54°C) into a system designed for 180°F (82°C) will fail to meet the heating load. Always run a coil performance check at the target entering water temperature (EWT) before specifying.
• Sizing the AWHP to peak load. Peak load sizing leads to an oversized unit that short-cycles and operates inefficiently. Instead, size to the base load and use a bivalent arrangement to cover peak demand.
• Ignoring pressure drop at lower water temperatures. Flow rates often increase when supply temperatures drop, which affects pump sizing and system balance. Recalculate pressure drop across all branches when changing the design temperature.
• Overlooking frost protection. At low outdoor temperatures, AWHPs undergo defrost cycles that temporarily reduce output. System design must account for this, particularly in bivalent configurations.
• Underestimating controls complexity. Bivalent systems require sequencing logic that prioritises the AWHP and only brings on the boiler when needed. Poor controls strategy negates the efficiency gains of the heat pump.

Modern air-to-water heat pumps have a compact footprint and can be installed against an exterior wall with minimal disruption — making them a practical choice for residential and light commercial retrofit projects where outdoor space is limited.

Air-to-Water Heat Pump Retrofit Best Practices for Engineers

• Model existing coil performance at the target EWT before specifying any new equipment. Do not assume compatibility; calculate it.
• Size the AWHP to base load, not peak load. Use a bivalent arrangement to handle the coldest design conditions without oversizing the heat pump.
• Focus on buildings with envelope improvements or planned refurbishments. Lower loads reduce the required supply temperature and expand the range of viable terminal units.
• Check flow rates and pressure drop at the revised design temperature. Lower supply temperatures often require higher flow rates to deliver the same heat output.
• Design the controls sequence carefully. The AWHP should always run in preference to the boiler. The boiler should only operate when the heat pump cannot meet demand alone.
• Consider radiant / underfloor heating systems for new-build or major refurbishment projects. The lower operating temperatures increase COP and reduce running costs.
• Document the design rationale clearly. Bivalent systems are more complex than single-source systems; future operators need to understand why each component is in the loop.

How h2x Design Software Can Help

The most common reason an AWHP retrofit stalls is uncertainty: the engineer is not confident that existing terminal units will perform at a lower supply temperature, and recalculating that manually across every branch of the system is time-consuming and prone to error.

h2x removes that uncertainty. The software allows engineers to input the existing system layout and run coil performance checks at any target entering water temperature, so instead of assuming that a 180°F (82°C) coil will not work at 130°F (54°C), you can calculate it to verify. h2x also recalculates flow rates, pipe sizes, and pressure drop across the full system when the design temperature changes, flagging any branches that need attention before the design is issued.

For bivalent systems, h2x supports the modeling of dual-source plant configurations, helping engineers sequence the AWHP and boiler correctly and verify that the system will perform as intended across the full range of outdoor conditions.

Conclusion

An air-to-water heat pump retrofit is not a universal solution, but they are the most powerful tool available when applied in the right context. Envelope-first retrofits, oversized coil scenarios, radiant / underfloor heating systems, and bivalent hybrid designs all create the conditions where AWHPs operate at their best: efficiently, reliably, and with carbon savings substantial enough to justify the investment.

FAQs About Air-to-Water Heat Pump Retrofits

What is an air-to-water heat pump?

An air-to-water heat pump is a heating and cooling system that extracts thermal energy from outdoor air and transfers it into a hydronic loop. It uses a vapor-compression refrigerant cycle to move heat rather than generate it, making it significantly more energy-efficient than combustion-based systems when operating at suitable water temperatures.

Can I retrofit an air-to-water heat pump into an existing boiler system?

Yes, but not without an engineering assessment. An AWHP operates at lower supply water temperatures than a gas boiler — typically 100–140°F (38–60°C) versus 160–180°F (71–82°C). A successful retrofit requires checking whether existing terminal units can satisfy the building load at the lower temperature. In many cases, oversized coils, envelope improvements, or a bivalent configuration make the retrofit viable without replacing all terminal units.

What is a bivalent heat pump system?

A bivalent heat pump system pairs an AWHP with a secondary heat source (typically a gas boiler) to handle conditions where the heat pump alone cannot meet demand. The AWHP covers the base load for most of the year, while the boiler operates only during the coldest periods. This approach delivers most of the carbon reduction of full electrification at a significantly lower capital cost.

What COP can I expect from an air-to-water heat pump?

COP (Coefficient of Performance) measures how many units of heat output you get per unit of electrical input. At low supply water temperatures (such as 100–115°F / 38–46°C for radiant floors) AWHPs can achieve COPs of 3.5-4.5 under typical operating conditions. At higher supply temperatures approaching 140°F (60°C), the COP drops towards 2.5–3.0. Outdoor temperature also affects COP, as performance decreases as outdoor temperatures fall.

How does h2x help with air-to-water heat pump retrofit design?

h2x allows engineers to model existing hydronic systems and run coil performance calculations at different entering water temperatures. This makes it straightforward to check whether existing terminal units will satisfy the building load at AWHP-compatible supply temperatures, and to identify which, if any, need replacing. Flow rates, pressure drop, and pipe sizing can all be updated simultaneously as the design temperature changes.

Ready to model your next air-to-water heat pump retrofit in h2x? Watch a recorded demo to see how engineers calculate pipe sizes, flow rates, and coil performance in one connected workflow — or schedule a 1:1 discovery call to apply it directly to your project.