Heat Pump DHW Tank Coil Sizing: Heat Exchanger Area, Flow Temperature & Recovery Time
Introduction
A DHW cylinder that stores enough hot water may still be a poor match for an air-to-water heat pump. The deciding factor is often the internal coil, not the storage volume.
This matters most in low-temperature systems. When the heat pump supplies water at 45–55°C, the temperature difference available across the coil is small, and coil performance can limit the whole DHW cycle.
This guide answers three practical questions:
1. Why does a larger coil area not automatically match a larger heat pump?
2. Why does a 45°C, 50°C or 55°C flow temperature change coil heat transfer so noticeably?
3. How can coil area, flow, pressure drop, heat transfer capacity and recovery time be evaluated together for a project?
1. Why DHW Coil Sizing Matters
In an indirect DHW cylinder, primary water from the heat pump flows through an internal coil. Heat passes through the coil wall into the stored domestic water, while the two circuits stay hydraulically separated.
How much heat the coil can pass depends on the temperature difference between the two sides, the effective coil area, the coil construction and the flow conditions.
If the coil transfers less heat than the heat pump delivers, the primary return temperature rises. The heat pump may then modulate down or reach its flow temperature limit, and the DHW cycle takes longer than planned.
Storage sizing and coil sizing are therefore two separate checks.
2. Heat Exchanger Area: What Does m² Actually Tell You?
Coil area sets the potential for heat transfer, but not the heat transfer itself. The governing relationship is:
**Q = U × A × ΔTlm**
- Q = heat transfer rate (W)
- U = overall heat transfer coefficient (W/m²·K)
- A = effective coil surface area (m²)
- ΔTlm = log mean temperature difference between primary water and stored water (K)
Area is only one of three factors. U depends on coil geometry, tube material, primary flow velocity and natural convection on the tank side. ΔTlm depends on the operating temperatures, which change throughout the heating cycle.
In a real cylinder, stored water temperature rises during heating and stratification develops. The equation explains the principle well, but it does not by itself predict a full heating cycle.
Nordtherm DHW cylinder coil configurations
The table below shows manufacturer design reference data for our most common cylinder sizes. For an example of the available tank arrangement, see Nordtherm's [multifunctional DHW tank with a stainless steel coil](https://nordthermglobal.com/products/multifunctional-dhw-tank-with-stainless-steel-coil).
| Cylinder volume (L) | Coil surface area (m²) | Coil tube (OD × wall, mm) | Coil material options |
| --- | --- | --- | --- |
| 200 | 2.0 | Φ22 × 1.0 | SUS304 / SUS316L / duplex stainless |
| 300 | 2.8 | Φ22 × 1.0 | SUS304 / SUS316L / duplex stainless |
| 500 | 4.0 | Φ22 × 1.0 | SUS304 / SUS316L / duplex stainless |
Single-coil and twin-coil versions are available. 800 L and 1,000 L cylinders can also be supplied, and other volumes are available on request. Standard insulation thickness is 50 mm.
Where water chloride content is higher, SUS316L or duplex stainless steel may be considered. Actual corrosion resistance depends on the specific grade, water chemistry, operating temperature and fabrication quality, so material choice should be confirmed against a local water analysis. A twin-coil layout can help where a second heat source, such as solar thermal or a boiler, is planned.
Coil area alone does not set a kW rating for any of these cylinders. Heat transfer capacity must always be stated together with its operating conditions, as explained below.
3. Why 45°C, 50°C and 55°C Flow Temperatures Change Coil Performance
The temperature difference between primary water and stored water drives heat transfer. With a heat pump, that difference is small, so a few kelvin of flow temperature can noticeably change what the same coil delivers.
For a wider system-level explanation of temperature selection and efficiency, see the guide to [45°C, 55°C and 65°C heat pump flow temperatures](https://nordthermglobal.com/blog/heat-pump-flow-temperature-45-55-65).
Illustrative comparison
Assume the same coil, a 5 K primary temperature drop, and stored water at either 35°C or 40°C.
| Primary flow / return (°C) | ΔTlm at 35°C stored water (K) | ΔTlm at 40°C stored water (K) |
| --- | --- | --- |
| 45 / 40 | approx. 7.2 | not achievable* |
| 50 / 45 | approx. 12.3 | approx. 7.2 |
| 55 / 50 | approx. 17.4 | approx. 12.3 |
*With a 40°C return and 40°C stored water, a coil of finite area cannot maintain a 5 K drop while still transferring heat.
The fixed 5 K drop is assumed only to illustrate the temperature driving force. In a real system, return temperature and heat transfer rate must be solved together for a given coil and operating condition.
Under these illustrative conditions, with stored water at 35°C, the calculated LMTD at 50°C and 55°C flow is approximately 1.7 and 2.4 times that at 45°C flow. These are ratios of temperature driving force, not predictions of actual coil heating capacity.
The final heating stage
The 40°C stored-water column highlights the more important effect. As stored water approaches the primary return temperature, ΔTlm collapses and heat transfer slows sharply.
A finite temperature difference is needed to keep a useful heating rate. How close stored water can get to the primary flow temperature depends on coil area, heating time, controls and the heat input required; there is no universal minimum approach for every coil. What does not change is the limit: a 45°C flow cannot heat the whole cylinder to 50°C, and a 55°C flow cannot hold the whole cylinder at 55°C.
The efficiency trade-off
Higher flow temperatures help the coil but may reduce heat pump efficiency. The best setpoint depends on the heat pump, the target DHW temperature, hygiene requirements and whether supplementary heating is available.
4. Coil Heat Transfer Capacity vs Heat Pump Output
A 12 kW heat pump does not automatically put 12 kW into any connected cylinder. Heat pump output and coil capacity are separate characteristics, and both must be evaluated at the same operating conditions. The same matching principle applies when selecting an [R290 air-to-water heat pump for heating, cooling and hot water](https://nordthermglobal.com/products/r290-air-to-water-split-heat-pump-for-heating-cooling-and-hot-water).
On the primary side, transferred heat can be estimated as:
**Q ≈ 1.163 × V × ΔT**
- Q = heat transfer rate (kW)
- V = primary water flow rate (m³/h)
- ΔT = measured primary flow-to-return temperature drop (K)
The coefficient assumes plain water; glycol mixtures need adjusted properties.
For example, at 1.5 m³/h with a measured 5 K drop: Q ≈ 1.163 × 1.5 × 5 ≈ 8.7 kW.
This is a heat balance, not a coil rating. It shows what the circuit transfers when that drop is measured. It does not show whether a given coil can sustain that drop at a given stored water temperature.
When comparing cylinders, ask for the coil's rated capacity together with:
- Primary inlet and outlet temperatures
- Primary flow rate
- Stored water temperature or test condition
- Whether the value is measured or calculated
5. Primary Flow Rate and Pressure Drop
A coil must also suit the hydraulics of the heat pump circuit. Higher primary flow may improve heat transfer up to a point, but it raises pressure loss through the coil.
As a reference, 20 kPa corresponds to approximately 2.0 m of water head. That figure covers the coil only, not the pipework, valves and fittings.
A pressure drop value is only meaningful together with its flow rate. Coil arrangement matters too: a single long coil and a coil split into parallel circuits can show very different pressure drops at the same flow.
Designers should check that:
- The circulating pump can deliver the required flow against the total circuit resistance
- The operating flow stays within the heat pump manufacturer's permitted range
- Velocity inside the coil tube stays at a level suitable for long-term operation
6. DHW Recovery Time: How to Estimate It
Recovery time is how long the system takes to raise stored water from a start temperature to a target. Tank volume should first be matched to demand; the related guide explains [how to choose the right heat pump water heater size](https://nordthermglobal.com/blog/how-to-choose-the-right-heat-pump-water-heater-size). A first recovery estimate starts with the energy needed:
**E ≈ 0.001163 × V × ΔT**
- E = heating energy (kWh)
- V = stored water volume (L)
- ΔT = temperature rise (K)
For a 300 L cylinder heated from 15°C to 50°C: E ≈ 0.001163 × 300 × 35 ≈ 12.2 kWh.
At an average net heat input of 6 kW, recovery takes about 12.2 ÷ 6 ≈ 2.0 hours.
The key word is average. Heat input is high at the start and falls as the cylinder warms, so the heat pump's rated output should not be used as the average. Reaching 55°C or more typically needs a primary flow temperature several kelvin higher, or supplementary heating.
Actual recovery also depends on stratification, sensor position, controls, standing losses and any draw-off during heating. Where recovery time is critical, use tested data with clearly stated start and end temperatures, heat source output and primary flow temperature.
7. Practical Selection Process
1. Define DHW demand: peak draw-off volume, delivery temperature and available recovery period.
2. Confirm the heat pump's available output at the intended DHW flow temperature and outdoor conditions.
3. Check that the coil can transfer that output, including as stored water nears its target.
4. Verify primary flow, coil pressure drop and pump selection.
5. Settle controls, sensor positions, hygiene strategy and any supplementary heating.
Worked example: 12 kW heat pump with a 300 L cylinder
Consider a 12 kW air-to-water heat pump, a 300 L cylinder with a 2.8 m² coil, a 55°C primary flow temperature and a target of 50°C from 15°C cold water.
| Check | What it tells you | Status in this example |
| --- | --- | --- |
| Heating energy | 0.001163 × 300 × 35 ≈ 12.2 kWh | Calculated |
| Heat pump output | 12 kW is the rated output; available output at the DHW flow temperature and outdoor condition may be lower | Check heat pump data |
| Coil capacity | 2.8 m² does not by itself show that the coil can absorb 12 kW, especially as stored water nears 50°C | Requires rated coil data at 55°C flow |
| Primary flow | Carrying 12 kW at a 5 K drop needs about 12 ÷ (1.163 × 5) ≈ 2.1 m³/h | Calculated |
| Pressure drop | Coil pressure drop at about 2.1 m³/h must be known to size the pump | Requires coil hydraulic data |
| Recovery time | Full 12 kW throughout would give about 1 h, a lower bound only; an average 6 kW would give about 2 h | Depends on the coil's real heating curve |
The useful question is often not "can the coil take 12 kW?" but "what average heat input does the project need?" In this simplified example, a 3-hour recovery window requires approximately 12.2 ÷ 3 ≈ 4.1 kW of average net heat input.
This does not mean a coil rated at 4.1 kW under one test condition will necessarily achieve that recovery time. Heat transfer falls in the final heating stage, and the heat pump's minimum modulating output, start/stop control and stratification all affect the result. The complete heating profile and control strategy must be checked.
Where only coil area and hydraulic data are available, coil performance should be treated as provisional. Request rated data, or apply a clearly justified design margin rather than a fixed kW-per-m² rule.
8. Common DHW Coil Sizing Mistakes
- **Selecting by storage volume alone.** Enough storage does not mean enough coil.
- **Comparing coil area without conditions.** Two coils of equal area can perform differently due to geometry, material and flow.
- **Treating a heat balance as a coil rating.** Flow × ΔT shows what was transferred, not what a coil can sustain.
- **Ignoring the final heating stage.** Heat transfer slows sharply as stored water approaches the primary temperature.
- **Ignoring hydraulic resistance.** A suitable coil can still underperform if the pump cannot deliver the design flow.
9. DHW Coil Sizing Checklist
- [ ] Usable DHW volume and delivery temperature
- [ ] Heat pump output at the intended DHW flow temperature
- [ ] Coil area, tube size, material and single/twin layout
- [ ] Coil capacity with full test or calculation conditions
- [ ] Primary design flow and corresponding pressure drop
- [ ] Required recovery time and heating window
- [ ] Insulation, sensor positions and control strategy
- [ ] Hygiene requirements and supplementary heating
10. Frequently Asked Questions
**Is a larger DHW coil always better for a heat pump?**
A larger coil generally offers more transfer potential, but actual performance also depends on U, operating temperatures and flow. Area alone does not guarantee a capacity.
**Can a 12 kW heat pump work with a 300 L DHW cylinder?**
Potentially, yes. The coil must be evaluated against the heat pump output, flow temperature, primary flow and required recovery time.
**Can a 45°C flow temperature heat DHW to 55°C?**
Not through a passive indirect coil alone. A higher primary temperature or supplementary heating is needed.
**Does a higher primary flow always improve heat transfer?**
Not necessarily. Gains level off, while pressure drop and pumping demand keep rising.
**What is the difference between coil capacity and recovery time?**
Capacity is a heat transfer rate at stated conditions. Recovery time is the duration of a full heating cycle, during which that rate keeps changing.
Conclusion
Matching a DHW cylinder to an air-to-water heat pump comes down to one question: can the coil transfer enough heat at the temperatures and flows the system will actually run at?
Coil area, flow temperature, primary flow, pressure drop and recovery time should be evaluated together. In low-temperature systems, the final heating stage deserves particular attention.
Nordtherm Global supplies DHW cylinders from 200 L to 1,000 L with single or twin stainless steel coils, and supports system selection for residential and commercial projects. [Contact Nordtherm] EMAIL：[email protected] with your storage volume, heat pump operating conditions, target DHW temperature and recovery requirement, and our team can help evaluate a suitable combination.
