Buffer Tank vs DHW Tank: What's the Difference in a Heat Pump System?
A buffer tank and a domestic hot water (DHW) tank can look almost identical: an insulated steel cylinder with pipe connections and a temperature sensor. In a heat pump system, however, they hold different water, solve different problems and are built to different requirements.
In short, a buffer tank belongs to the heating or cooling circuit and helps the system run smoothly. A DHW tank stores the hot water that comes out of taps and showers. This guide explains the difference in four steps: where the water goes, what each tank is for, why one cannot simply replace the other, and why some systems include both.
This article compares a typical closed-circuit buffer tank with a typical indirectly heated DHW cylinder. Combination tanks and other special designs are noted where they matter.
1. Where the water goes
The clearest way to tell the two tanks apart is to follow the water.
**Buffer tank: a closed loop.** The water in a buffer tank is part of the heating (or cooling) circuit. It travels from the heat pump to the buffer, out to the emitters — underfloor heating, radiators or fan coils — and back again. The same water circulates for years. It is normally retained and recirculated rather than replaced, may be treated with inhibitor (or glycol in some installations), and is never meant to be drunk or used at a tap.
In a reversible system, the buffer may also hold chilled water during cooling mode. That is why buffer tanks used for cooling typically need insulation designed to keep condensation off the tank surface.
**DHW tank: water that leaves the system.** The water in a DHW tank — often called a hot water cylinder — is drinking water. Cold water from the mains enters at the bottom, is heated and stored, and leaves through taps, showers and baths. Every litre drawn off is replaced with fresh, oxygen-rich mains water.
In a typical heat pump system, the heat pump does not heat the drinking water directly. It warms its own circuit water, which passes through a coil inside the cylinder or through an external heat exchanger. The two waters exchange heat but do not mix.
| | Buffer tank | DHW tank |
| --- | --- | --- |
| Water it holds | Heating/cooling circuit water | Potable (drinking) water |
| How the water moves | Closed, recirculating | Fresh water enters as hot water is drawn off |
| Water replaced? | Normally retained and recirculated | Continuously, as hot water is used |
| Ends up at | Underfloor heating, radiators, fan coils | Taps, showers, baths |
2. What problem each tank solves
Because they hold different water, the two tanks also do different jobs.
**A buffer tank addresses system water volume and hydraulics.** A heat pump generally runs best with long, steady cycles. When only a few zones are calling for heat, the emitter circuit may not take enough heat to keep the compressor running steadily, and the unit may start and stop more often than it should. Extra water volume can help absorb that mismatch. A buffer can also help supply heat for defrost cycles and, in some layouts, hydraulically separate the heat pump circuit from the emitter circuit when their flow rates differ.
None of this has anything to do with hot water at the tap. A buffer tank is a system-side component.
**A DHW tank addresses hot water storage and supply.** A shower or bath can draw heat much faster than a residential heat pump can deliver it continuously. The cylinder bridges that gap: the heat pump reheats the stored water over a period of time, and the stored water covers short, high-demand draw-offs. Storage temperature also matters for hygiene, and many countries set rules or recommendations for periodic higher-temperature operation to manage Legionella risk.
**Sizing is a separate question for each.** Buffer volume depends on factors such as the heat pump's minimum modulated output, the volume already in the emitter circuit, zoning and the manufacturer's minimum water volume — not on a single litres-per-kW rule. This is covered in [Do You Need a Buffer Tank with an Air-to-Water Heat Pump?](https://nordthermglobal.com/blog/do-you-need-a-buffer-tank-with-an-air-to-water-heat-pump)
Hot water capacity depends on the number of users, draw-off habits, simultaneous use, storage temperature and recovery time. For the general principles of hot water demand and recovery, see [How to Choose the Right Heat Pump Water Heater Size](https://nordthermglobal.com/blog/how-to-choose-the-right-heat-pump-water-heater-size). With a separate heat pump and cylinder, recovery also depends on the heat pump's output and the cylinder's heat exchanger.
3. Why one cannot simply replace the other
Two tanks with the same litre rating and a similar outline are not interchangeable. The differences sit inside the tank and in the requirements it is built to meet.
**Purpose.** A buffer is designed to stabilise a circuit; a DHW tank is designed to store and deliver drinking water. Each design follows from that job.
**Materials and hygiene.** A DHW tank is in contact with drinking water, so its inner surface and fittings are generally expected to be suitable for potable water under the rules of the country of installation. Enamel-lined steel tanks usually rely on an anode for corrosion protection. Whether a stainless steel tank needs an anode depends on its design and the manufacturer's requirements. Many buffer tanks are plain carbon steel. That can work in a closed loop, where the water quickly becomes low in oxygen. Exposed to a constant supply of fresh, oxygenated mains water, the same steel may corrode, and it would not normally be approved for drinking-water use.
**Heat exchange structure.** A buffer tank usually has no internal coil: circuit water flows straight through it. A DHW tank needs a suitable heat exchanger to transfer heat into the stored water without mixing — an internal coil, or an external plate heat exchanger with its own circulation. Because a heat pump works with a smaller temperature difference than a boiler, heat pump cylinders usually have a larger coil surface. Coil area alone is not the test, though: the match should be checked against heat transfer capacity and pressure drop at the actual operating temperatures. A cylinder designed around a boiler may limit how well a heat pump can recharge it.
**Product requirements.** Unvented (mains-pressure) cylinders are built and tested for their rated pressure and are fitted with safety devices — typically pressure and temperature relief and an expansion vessel — according to that rating and local regulations. Vented cylinders operate at low pressure from a feed tank. Buffer tanks are rated for the pressure of a sealed heating circuit. Connection positions, sensor pockets, insulation (including condensation control for cooling) and labelling requirements can also differ.
Some products combine both functions, such as tank-in-tank designs or buffers with a fresh-water module. These are purpose-built combinations, not a buffer and a cylinder swapped for each other. Before choosing any tank, check what it is designed and approved for — not only its volume and shape.
4. Why a system may have both tanks
Many air-to-water systems provide both space heating and hot water from one heat pump. In that case, the two tanks sit on different branches of the system rather than in a row.
In a common layout, a three-way diverting valve after the heat pump sends heat to one branch at a time:
- **DHW mode:** heated circuit water flows through the coil in the DHW cylinder, warms the stored drinking water, and returns to the heat pump.
- **Heating (or cooling) mode:** circuit water flows to the emitters, through a buffer tank if the design includes one.
The cylinder is there because the household needs stored hot water. The buffer, if present, is there because the heating circuit needs extra volume or hydraulic separation. One does not replace the other, and having a DHW cylinder does not by itself mean the system needs a buffer.
Whether a buffer is needed still depends on design: the heat pump's minimum water volume, the emitter circuit, zoning and defrost strategy all play a part. The full decision process is covered in [Do You Need a Buffer Tank with an Air-to-Water Heat Pump?](https://nordthermglobal.com/blog/do-you-need-a-buffer-tank-with-an-air-to-water-heat-pump)
Buffer tank vs DHW tank at a glance
| | Buffer tank | DHW tank |
| --- | --- | --- |
| Main job | Adds system water volume; helps hydraulics and steady operation | Stores hot water for taps, showers and baths |
| Water type | Closed-circuit heating/cooling water | Potable water |
| Heat exchange | Usually no coil; circuit water flows through | Internal coil or external heat exchanger, matched to the heat source |
| Materials | Often carbon steel, suited to low-oxygen closed loops | Potable-suitable materials; enamel-lined steel usually with an anode, stainless steel per maker's design |
| Pressure and safety | Rated for sealed heating circuit pressure | Unvented: rated for mains pressure, with safety devices per local rules; vented: low pressure |
| Cooling use | Possible, with condensation-proof insulation | Not used for chilled water |
| Always needed? | No — depends on system design | Only if the system provides stored hot water |
FAQ
**Can I use a hot water cylinder as a buffer tank?** Generally not as a straight swap. A cylinder's coil, connections and pressure rating are designed around drinking-water storage, not circuit volume or hydraulic separation. If a buffer is needed, choose a tank designed for that role.
**Can a buffer tank supply hot water to taps?** Not a standard buffer tank. Its water is circuit water, and its materials are typically not suitable for drinking water. Purpose-built combination tanks or fresh-water modules can produce hot water from a buffer, but they are designed and approved for that function.
**Is the water in a buffer tank safe to drink?** No. It is closed-circuit heating water that may contain inhibitor or glycol and is not treated as drinking water.
**If I have a DHW cylinder, do I still need a buffer tank?** Not necessarily. The cylinder serves hot water demand; a buffer serves the heating circuit. Whether a buffer is needed depends on the heat pump's minimum water volume, emitter circuit volume, zoning and defrost strategy.
**Can one tank do both jobs?** Some products do, such as tank-in-tank designs or buffers with a fresh-water module. They are engineered for both functions; an ordinary buffer or cylinder is not.
Continue Exploring
- [Do You Need a Buffer Tank with an Air-to-Water Heat Pump?](https://nordthermglobal.com/blog/do-you-need-a-buffer-tank-with-an-air-to-water-heat-pump)
- [How to Choose the Right Heat Pump Water Heater Size](https://nordthermglobal.com/blog/how-to-choose-the-right-heat-pump-water-heater-size)
- [Heat Pump Weather Compensation: How to Set the Heating Curve](https://nordthermglobal.com/blog/heat-pump-weather-compensation-how-to-set-the-heating-curve)
- [R290 Heat Pump vs Gas Boiler: Can You Keep Your Radiators?](https://nordthermglobal.com/blog/r290-heat-pump-vs-gas-boiler-can-you-keep-your-radiators)
- [Stainless Steel Buffer Tank for Heat Pumps (20–1000 L)](https://nordthermglobal.com/products/stainless-steel-buffer-tank-for-heat-pump-20l-1000l-a-water-storage)
- [Multifunctional DHW Tank with Stainless Steel Coil](https://nordthermglobal.com/products/multifunctional-dhw-tank-with-stainless-steel-coil)
Planning a heat pump system with hot water?
Share your heating load, emitter type, number of zones and expected hot water demand. Our engineering team can review whether your layout may need a buffer, how the DHW cylinder should be paired with the heat pump, and what to check before installation. \[Contact our team →\] *(routes to Hazel)* Email: [email protected] Tel: +86-13727963335
