Replacing a Boiler with an Air Source Heat Pump Is More Than Replacing the Heat Source

Replacing a traditional gas, oil or electric boiler with an air-to-water heat pump is becoming increasingly common in residential, commercial and public-building renovation projects.

However, a successful retrofit is not simply:

Remove the boiler → install a heat pump → connect the existing pipes.

A heat pump and a boiler operate under very different hydraulic and temperature conditions.

A boiler can normally provide relatively high water temperatures with a comparatively large temperature difference between supply and return water. A heat pump, on the other hand, normally achieves its best efficiency when supplying lower-temperature water with carefully controlled water flow.

This means that before replacing a boiler, engineers should answer several important questions:

1. What is the actual heating and cooling load of the building?

2. What capacity should the heat pump provide at the design outdoor temperature?

3. Can the existing fan coils, radiators or underfloor heating system operate at the new water temperatures?

4. Can the existing pipes provide sufficient water flow?

5. Is the existing circulation pump suitable?

6. Is hydraulic separation, a buffer tank or a secondary circuit required?

Understanding these questions is the key to a reliable boiler-to-heat-pump retrofit.

1. Start with the Building Load, Not the Existing Boiler Capacity

One of the most common mistakes in heat pump retrofits is selecting a new heat pump according to the capacity printed on the existing boiler nameplate.

For example:

Existing boiler capacity = 100 kW
Therefore, install a 100 kW heat pump.

This approach can be seriously misleading.

Traditional boilers are frequently oversized, and their nominal capacity does not necessarily represent the actual peak heating demand of the building.

The correct starting point is therefore the design heating load.

In projects requiring cooling, the design cooling load must also be calculated.

A simplified preliminary estimate may use:

$$Q=A\times q$$

Where:

• Q = estimated building load, W

• A = conditioned floor area, m²

• q = estimated unit load, W/m²

For example, if a 10,000 m² building has an estimated heating load of 70 W/m²:

$$10,000\times 70=700,000W$$

Therefore:

$$Q_{heating}=700kW$$

However, W/m² values should only be used for preliminary estimation.

Final heat pump selection should preferably be based on a proper building heat-loss/heat-gain calculation considering insulation, glazing, ventilation/infiltration, occupancy, internal gains, outdoor design temperature and required indoor temperature.

2. Heat Pump Capacity Must Be Checked at the Design Outdoor Temperature

This is particularly important for air source heat pumps.

A heat pump advertised as a “20 kW heat pump” does not necessarily deliver 20 kW under all outdoor conditions.

As outdoor air temperature decreases, the available heating capacity and COP of an air source heat pump can change significantly.

Therefore, engineers should not select equipment based only on nominal conditions such as:

A7/W35

Instead, capacity should be checked against the actual project design condition, for example:

• A7/W35

• A2/W35

• A−7/W35

• A−15/W45

• A−20/W55

depending on climate and application.

The correct principle is:

$$Q_{HP,design}\ge Q_{building,design}$$

unless the system has deliberately been designed as a bivalent system with auxiliary heating.

For cold-climate projects, low-temperature heating capacity is therefore often more important than the nominal kW printed on a catalogue.

3. Heating and Cooling Loads Can Lead to Different Heat Pump Selections

For reversible air-to-water heat pump systems, another question appears:

Should we size the heat pump according to heating load or cooling load?

The answer depends on climate.

Cold Climate

In a cold climate, the heating load is usually dominant.

The heat pump is therefore normally selected according to the winter design heating load, after checking its low-ambient heating capacity.

Summer cooling capacity may then be sufficient automatically.

Hot-Summer / Cold-Winter Climate

The situation can be different in climates with both substantial heating and cooling demand.

Suppose a commercial building has:

$$Q_{heating}=700kW$$

but:

$$Q_{cooling}=1,500kW$$

Sizing the entire heat pump plant for 1,500 kW simply because of the cooling peak may produce an oversized heating system.

One possible engineering solution is a hybrid plant:

Air-to-water heat pumps → base heating + partial cooling

plus

Chiller → supplementary peak cooling

This can provide better equipment utilization and more flexible plant operation.

The exact configuration should be determined by load profiles, climate, operating hours, electricity tariffs, redundancy requirements and lifecycle cost—not peak capacity alone.

4. Can Existing Fan Coils Be Used with a Heat Pump?

In many boiler replacement projects, the building already contains hydronic fan coil units.

The good news is:

Existing fan coils do not automatically need to be replaced.

But their performance must be verified at the new operating temperatures.

Fan coil capacity depends strongly on the temperature difference between the water entering the coil and the room air. Therefore, lowering water temperature generally reduces heating output. Manufacturer performance tables should be used whenever available. (Caleffi S.p.a.)

For example, a fan coil originally designed around:

Boiler system

70°C supply / 60°C return

may deliver substantially less heat when operated at:

Heat pump system

45°C supply / 40°C return.

So the correct question is not:

“Can a fan coil work with 45°C water?”

It is:

“Can this fan coil deliver the required room heating capacity at 45°C water under the actual airflow and water-flow conditions?”

That distinction is extremely important.

5. Fan Coils Are Particularly Attractive for Heating + Cooling Heat Pump Systems

Fan coil systems have one major advantage in heat pump retrofit projects:

The same terminal can usually provide both heating and cooling.

During winter:

Air source heat pump → warm water → fan coil → space heating

During summer:

Air source heat pump → chilled water → fan coil → space cooling

This makes fan coils particularly attractive for hotels, offices, schools, apartments and commercial buildings requiring year-round HVAC.

However, when chilled water is used, the fan coil and piping system must also be designed for condensation control, including appropriate insulation and condensate drainage.

Modern hydronic design guidance also recognizes fan coils as useful heat emitters for air-to-water heat pumps, particularly when designed with sufficiently large heat exchanger surfaces for lower water temperatures. (Caleffi S.p.a.)

6. Do Existing Fan Coil Pipes Need to Be Replaced?

Not necessarily.

The fundamental relationship governing heat transfer in a hydronic circuit is:

$$Q=\dot{m}{c}_p\mathrm{\Delta }T$$

Where:

• Q = heat transfer rate

• ṁ = water mass flow rate

• Cp = specific heat capacity

• ΔT = supply-return water temperature difference

This equation explains one of the most important differences between boiler and heat pump hydronic systems.

For the same heat transfer rate:

$$\dot{m}\propto \frac{1}{\mathrm{\Delta }T}$$

Therefore, as the design ΔT becomes smaller, the required water flow becomes larger. (Caleffi S.p.a.)

7. Why Heat Pumps Often Require Higher Water Flow

Consider a simple example.

A system must transfer:

$$Q=100kW$$

For water, a convenient metric approximation is:

$$Q(kW)\approx 1.163\times Flow(m^3\mathrm{/}h)\times \mathrm{\Delta }T(K)$$

Therefore:

$$Flow=\frac{Q}{1.163\times \mathrm{\Delta }T}$$

At ΔT = 10°C

$$Flow=\frac{100}{1.163\times 10}$$ $$Flow\approx 8.6m^3\mathrm{/}h$$

At ΔT = 5°C

$$Flow=\frac{100}{1.163\times 5}$$ $$Flow\approx 17.2m^3\mathrm{/}h$$

So reducing ΔT from 10°C to 5°C approximately doubles the required water flow.

This is why replacing a boiler with a heat pump can create hydraulic problems even when the heating capacity has been calculated correctly.

The existing pipe may simply be too small for the required heat pump flow.

8. Why Simply Installing a Larger Pump Is Not Always the Solution

A common reaction is:

“If the flow is too low, just install a bigger circulation pump.”

Sometimes this works.

Sometimes it creates another problem.

Increasing pump head cannot indefinitely compensate for undersized pipework. Excessive velocity can result in:

• increased pressure drop;

• higher pump electricity consumption;

• flow noise;

• valve noise;

• erosion risk;

• poor hydraulic balancing;

• insufficient flow at distant branches.

The correct procedure is therefore:

Required thermal capacity → required water flow → pipe pressure drop → available pump head → circulation pump selection

rather than selecting the pump independently.

9. Existing Pipework Should Be Evaluated Before Replacement

Replacing all existing hydronic pipes can make a retrofit extremely expensive and disruptive.

Therefore, engineers should first investigate whether the existing network can still be used.

Check:

• main pipe diameter;

• branch pipe diameter;

• pipe length;

• fittings and valve pressure loss;

• existing pump curve;

• available differential pressure;

• required heat pump flow;

• allowable water velocity;

• hydraulic balance.

If the existing pipework can carry the required flow at acceptable pressure loss and velocity, there may be no reason to replace it.

This is one of the biggest opportunities for reducing retrofit cost.

10. Underfloor Heating Is Usually an Excellent Match for Heat Pumps

Existing hydronic underfloor heating can often be even more suitable for heat pumps than conventional radiators.

Why?

Because underfloor heating uses a very large heat-emitting surface.

A larger heat-transfer area allows the building to be heated using relatively low water temperatures.

That is exactly what a heat pump prefers.

Modern heat-pump design guidance identifies well-designed floor heating as particularly suitable for air-to-water heat pumps because it can deliver the required room heat output at relatively low supply water temperatures, improving heat pump efficiency. (Caleffi S.p.a.)

Typical low-temperature heating operation might be around:

30–35°C supply water

or

35–40°C supply water

depending on building load, floor construction, pipe spacing, floor covering and outdoor conditions.

The actual design temperature must always be calculated rather than assumed.

11. The Important Difference: Underfloor Heating ΔT

The existing floor heating circuit may have been designed for a larger supply-return temperature difference than the heat pump requires.

Suppose the old floor circuit operates at:

$$\mathrm{\Delta }T=10\mathrm{°}C$$

but the heat pump requires approximately:

$$\mathrm{\Delta }T=5\mathrm{°}C$$

For the same heating capacity, the heat pump side requires approximately twice the water flow.

This can create a mismatch between:

Heat pump required flow

and

existing UFH circuit flow capacity.

This does not automatically mean that all floor heating pipes need to be replaced.

Instead, the hydraulic system needs to be redesigned correctly.

12. Hydraulic Separation Can Solve Many Retrofit Problems

One effective solution is to separate the heat pump circuit hydraulically from the building distribution circuit.

A typical arrangement is:

Heat Pump → Primary Pump → Buffer Tank / Hydraulic Separator → Secondary Pump → Existing UFH or Fan Coil System

This allows the heat pump circuit to maintain the flow required by the heat pump while the existing building circuit operates at a different flow rate.

This is especially useful when:

• several heating zones open and close independently;

• the existing distribution flow differs from heat pump flow;

• multiple heat pumps operate in cascade;

• both fan coils and floor heating are used;

• minimum heat pump system volume must be maintained;

• defrost operation requires stored thermal energy.

Hydraulic separation allows primary and secondary circulators to operate at stable but potentially different flow rates. (Caleffi S.p.a.)

13. What About Existing Radiators?

Radiators require more careful evaluation.

Many older boiler systems were designed around relatively high water temperatures.

A heat pump operating at much lower water temperature may therefore produce insufficient radiator output.

Possible solutions include:

• increasing radiator size;

• adding additional radiators;

• installing fan-assisted radiators;

• improving building insulation;

• reducing building heat loss;

• using low-temperature fan coils;

• selecting a heat pump capable of higher leaving water temperatures where necessary.

The objective should still be:

Use the lowest practical supply water temperature that can satisfy the building heating load.

Lower required water temperature generally allows an air-to-water heat pump to operate at higher COP. (Caleffi S.p.a.)

14. Do Not Ignore Hydraulic Balancing

After a retrofit, total system flow may be correct while some rooms still receive insufficient heating.

Why?

Because water follows the path of least hydraulic resistance.

Short circuits can receive excessive flow while long circuits are starved.

Therefore, retrofit commissioning should include:

• branch balancing;

• manifold flow adjustment;

• differential pressure verification;

• pump speed adjustment;

• supply/return temperature measurement;

• terminal flow verification.

Parallel hydronic circuits are particularly useful because individual branches can be independently balanced and controlled. (Caleffi S.p.a.)

15. A Practical Boiler-to-Heat-Pump Retrofit Workflow

For engineering projects, the process can be summarized as:

Step 1 — Calculate the building load

Determine design heating and, where required, cooling loads.

↓

Step 2 — Determine design outdoor conditions

Use local winter and summer design temperatures.

↓

Step 3 — Select heat pump capacity

Check actual heating/cooling output at the required outdoor temperature and leaving water temperature.

↓

Step 4 — Determine required supply water temperature

Evaluate existing fan coils, radiators or floor heating.

↓

Step 5 — Calculate required water flow

Use:

$$Q=\dot{m}{c}_p\mathrm{\Delta }T$$

↓

Step 6 — Check existing pipework

Verify diameter, pressure loss and allowable velocity.

↓

Step 7 — Select circulation pumps

Use design flow and calculated system resistance—not pipe diameter alone.

↓

Step 8 — Decide whether hydraulic separation is required

Consider buffer tanks, hydraulic separators and primary-secondary pumping.

↓

Step 9 — Balance and commission

Measure actual flow, supply temperature, return temperature and ΔT under operating conditions.

16. Example: 500 kW Heating Retrofit

Consider a commercial building with a design heating load of:

$$500kW$$

If the heat pump system is designed around:

$$\mathrm{\Delta }T=5\mathrm{°}C$$

then:

$$Flow=\frac{500}{1.163\times 5}$$ $$Flow\approx 86m^3\mathrm{/}h$$

This means the main hydronic circuit must be capable of transporting approximately 86 m³/h under design conditions.

If the existing boiler circuit was designed around:

$$\mathrm{\Delta }T=10\mathrm{°}C$$

its previous design flow may have been only:

$$Flow=\frac{500}{1.163\times 10}$$ $$Flow\approx 43m^3\mathrm{/}h$$

The new heat pump may therefore require roughly twice the design water flow.

That single calculation immediately tells the engineer what needs to be investigated:

Can the existing pipe network and pumps handle the increased flow?

If yes, much of the original system may remain.

If no, hydraulic modification is required.

17. What Makes a Successful Boiler-to-Heat-Pump Retrofit?

A successful retrofit requires three systems to match each other:

Heat Source

The heat pump must provide sufficient capacity under real outdoor conditions.

Heat Distribution

Pipes and pumps must deliver sufficient water flow.

Heat Emitters

Fan coils, radiators or floor heating must provide sufficient room output at the selected water temperature.

If any one of these three is incorrectly designed, the system may experience:

insufficient heating → high water temperature → lower COP → higher electricity consumption → poor customer experience.

This is why heat pump retrofits should always be treated as a complete hydronic system engineering project, rather than simply an equipment replacement.

Frequently Asked Questions

Can I directly replace a gas boiler with an air source heat pump?

Sometimes, but the existing system should first be checked for heat load, required water temperature, terminal output, water flow, pipe diameter and circulation pump capacity. A direct one-for-one replacement without hydraulic analysis can result in poor performance.

Can existing fan coils work with an air-to-water heat pump?

Yes, in many cases. However, their heating capacity must be checked at the lower water temperatures normally associated with heat pump operation. Manufacturer performance data should be used whenever available.

Can existing underfloor heating work with a heat pump?

Usually yes. Hydronic floor heating is particularly suitable for heat pumps because its large emitting area allows relatively low supply water temperatures.

Why does a heat pump require high water flow?

Heat transfer depends on both water flow and supply-return temperature difference. When ΔT is reduced, water flow must increase to transfer the same amount of heat.

Do I need to replace existing heating pipes?

Not necessarily. The existing pipework should be hydraulically checked against the new required flow and pressure drop. If acceptable flow can be achieved without excessive velocity or pump head, the pipes may remain.

Should I install a buffer tank when replacing a boiler with a heat pump?

It depends on system design. A buffer tank or hydraulic separator can be useful when the heat pump and building circuits require different flow rates, when multiple zones frequently close, when additional system volume is needed, or when stable defrost operation is required.

Are fan coils or underfloor heating better for heat pumps?

Both can work very well. Underfloor heating offers excellent low-temperature heating efficiency, while fan coils can provide both heating and chilled-water cooling. The best choice depends on building type, heating/cooling demand and existing distribution infrastructure.

Conclusion

Replacing a boiler with an air source heat pump is not primarily a question of finding a heat pump with the same kW rating.

The real engineering question is:

Can the heat pump, hydronic distribution system and existing heat emitters work together at the required water temperature, flow rate and design load?

A professional retrofit therefore starts with building load calculation, followed by heat pump performance selection, terminal verification and hydraulic analysis.

Existing fan coils and underfloor heating systems can often be retained, which can significantly reduce renovation cost. But this is only possible when engineers carefully verify water temperature, ΔT, flow rate, pipe pressure drop, circulation pump performance and terminal capacity.

When these elements are correctly matched, converting a boiler-based hydronic system to an air-to-water heat pump can deliver efficient heating, cooling and long-term energy savings without unnecessarily rebuilding the entire distribution system.

Technical Note

The equations and examples above are intended for preliminary engineering understanding. Final equipment selection should always use project-specific heat-loss/heat-gain calculations, local design weather data, hydraulic calculations and the heat pump/fan-coil manufacturer’s certified performance data.

Wotech – Professional Air Source Heat Pump Solutions

For residential, commercial and OEM air-to-water heat pump solutions, including low-temperature heating, fan coil and underfloor heating applications, visit:

www.ecoheat-pump.com