Pressurized vs. non-pressurized water heater: what is the difference and what to choose?
Pressure vs. non-pressure water heater: what is the difference and what to choose?
When a customer first looks at the range of electric flow-through water heaters, they will sooner or later come across two terms that may confuse them: pressure operation and non-pressure operation. At first glance, this seems to be a technical detail, but in reality, it is a fundamental structural difference that will affect not only how you install the device, but also the comfort you experience on a daily basis. Choosing the wrong type of water heater can mean problems with low water pressure, the need to replace the tap, or even the impossibility of connecting the device to existing plumbing.
In this article, we will examine both systems in depth – how they physically work, what their strengths and weaknesses are, what the difference in installation is, and most importantly: how to decide which one is suitable for your specific case. If you are also looking for a comparison of flow-through and storage water heaters, see the article How to choose an electric water heater: storage vs. flow-through in our Knowledge Centre.
Basic principle: what lies behind the terms "pressure" and "non-pressure"
To understand the difference, we first need to clarify what these terms mean from a physical and structural point of view.
A pressure water heater (also known technically as "closed-pressure") is a device whose internal tank – whether a storage tank or a flow-through body – is hermetically sealed and firmly integrated into the water supply piping. Pressurized water from the public water supply or from the home plumbing passes through the heater under full network pressure (typically 0.2 – 0.6 MPa, i.e. 2 – 6 bar). One heater can supply multiple outlets at the same time – for example, a sink and a shower – and each of them has a standard tap with hot and cold water.
A non-pressure water heater (technically "open-pressure" or "pressure-open") works completely differently. The inlet pipe for cold water passes through a shut-off valve, but the interior of the heater is open to the atmosphere – that is, it operates at atmospheric pressure (0 bar overpressure). The heated water flows out by gravitational force and by the slight pressure of the cold water pushing it out. There must be no shut-off tap at the outlet of the heater – otherwise, the overpressure would damage the device. Therefore, special "shower" or "non-pressure" taps with an open discharge arm are mounted on non-pressure heaters.
Constructional details: why the closed/open nature of the system is so important
Many customers think that pressure/non-pressure is just a matter of setting a pressure valve. It is not. It is a fundamental structural difference – pressure water heaters have a more solid, hermetically sealed tank or flow-through body, which is designed to withstand continuous water pressure. Non-pressure devices, on the other hand, have thinner tank walls and an outlet opening freely communicating with the atmosphere.
This is even more pronounced in storage water heaters – a pressure storage tank must withstand not only the static water pressure, but also the pressure generated by the expansion of water when heated (water expands when heated, which increases pressure in a closed system). Therefore, a pressure and temperature relief valve (also known as a PTBV or combined safety valve) is always installed in pressure storage tanks, which opens and releases part of the water when the set pressure or temperature is exceeded. If it were missing, there is a risk of tank explosion in extreme cases.
In flow-through water heaters, the situation is a bit simpler – in a pressure flow-through heater, water passes through the heating coil and immediately drains away; it does not accumulate, so the risk of expansion is lower, but the entire construction must still withstand the pressure of the network.
Storage water heater in pressure operation
A storage water heater in pressure operation is the most common variant for households in terms of daily comfort. It works simply: cold water fills the sealed tank (5 to 150+ liters, depending on the model), an electric resistance element heats it to the set temperature (usually 55–75 °C), and the thermostat maintains the heat. When you open the hot water tap, water from the tank flows out under water pressure and cold water flows in from the bottom.
An example of a compact pressure storage water heater is the electric storage water heater BD 2.0 kW 30 l top. This 30-liter tank with a 2 kW power is a typical solution for a smaller bathroom or kitchen, where you need a supply of hot water for the sink and shower without having to change the taps. A pressure tank can be easily connected to the existing hot and cold water supply – no special taps are required.
Advantages of a pressure water storage heater:
- Supplies multiple outlets simultaneously (basin, shower, kitchen sink)
- Standard taps without the need for special valves
- Full outlet pressure equal to the water supply pressure
- Comfortable temperature setting using a thermostat
- You have hot water available at any time; heating occurs during off-peak hours at a lower cost
Disadvantages of a pressure water storage heater:
- Limited capacity – with 30 l of hot water and a shower, the tank can be depleted in 5–8 minutes
- Larger physical size, requiring space for installation
- The safety valve must be connected and functional
- Energy loss due to maintaining the temperature in the tank (so-called stand-by losses)
Pressure water flow heater
A pressure flow heater is a modern solution for those who do not want a tank but still require full comfort – including the ability to connect multiple outlets. Water passes through the heating chamber directly under water supply pressure, is heated, and immediately flows out. A pressure flow heater is therefore connected similarly to a tank: you feed cold water into it and lead hot water from the outlet to the tap, where it is mixed with cold water.
In our range you will find, for example, an electric flow heater MK1 3.5 kW with electronic switching and pressure operation or a more powerful MK1 4.5 kW. Both models have electronic switching, which means reliable regulation and the possibility of more precise temperature setting. Learn more about choosing the correct power in the article What power flow water heater do I need: 3.5 kW, 4.5 kW or 5.5 kW? in our Knowledge Centre.
Non-pressure water flow heater
A non-pressure flow heater is a solution that at first glance appears simple and inexpensive – and in many cases is indeed a suitable choice. It works by cold water entering through a control valve (usually a single-lever or screw valve on the inlet), passing through the heating chamber, and exiting through an open outlet arm at atmospheric pressure.
Key technical feature: the outlet side of the heater must always be open. This means that a non-pressure heater cannot be connected to a standard water supply tap with valves, because closing the outlet valve would cause excessive pressure inside the device, which could damage the seals or even deform the body of the appliance. Manufacturers therefore supply special non-pressure taps (so-called shower or basin taps) with a freely hanging outlet arm – the opening is always free, and water only flows out when it is opened by supplying from the cold side.
In our range you will find, for example, an electric flow heater PM 3.5 kW with hydraulic switching and non-pressure operation or a more powerful PM 4.5 kW. Hydraulic switching of these models means that the heater automatically turns on by water movement – not by electronic sensing. Learn more about the differences in switching in the article Hydraulic vs. electronic switching of flow heaters: what it means and which is better?
Advantages of a non-pressure flow heater:
- Simpler construction – lower purchase price
- More pressure-safe – no risk of damage from back pressure from the water supply
- Suitable for places without a constant supply of cold water under full pressure (e.g. garden houses, cottages with low pressure levels, wells with pumps)
- Easy installation – does not require a safety valve on the outlet
- Low maintenance
Disadvantages of a non-pressure flow heater:
- Supplies only one point of use at a time
- Need for a special non-pressure valve – you cannot use an existing standard valve
- Lower outlet pressure and flow – subjectively weaker water flow
- Not suitable for shower panels, hand showers with long hoses or thermostats
Practical scenarios from practice: when to choose what?
Over the years of working with customers, we have seen many situations where the choice between a pressure or non-pressure heater was crucial. Here are the most common scenarios:
Scenario 1: Bathroom renovation in an apartment with existing plumbing
This is a typical situation where a boiler is replaced with a flow heater. The existing plumbing is dimensioned for pressure operation – at the sink, you have a classic lever valve with cold and hot water. In this case, the pressure flow heater is the only reasonable choice. Installing a non-pressure heater would require replacing the valve with a non-pressure valve, which can be problematic with built-in or wall-mounted valves. Moreover, if you have a shower and a sink in the bathroom, a pressure heater can supply both points from one device.
Scenario 2: Summer cottage without pressure water supply
At the cottage, you have your own well with a pump and pressure tank, but the pressure is unstable and sometimes drops below 1 bar. You supply only one sink in the kitchen. Here, the non-pressure heater is ideal – it does not require a minimum water pressure, a simple non-pressure valve is sufficient, and since you supply only one point, the limitation to one point of use will not cause any problems.
Scenario 3: Summer kitchen or garden pavilion
You want to have hot water near the garden grill or in the garden kitchen. The water supply is from a hose connected to the water supply. The pressure is sufficient, but you do not need the comfort of a shower – just washing hands or dishes. A non-pressure heater is suitable and cost-effective. A smaller 3.5 kW model will provide comfortably warm water for washing without a complicated installation.
Scenario 4: Office with one sink
In the office, you need hot water only at the sink in the WC. The existing valve is a standard two-handle valve. Two solutions are possible: a small pressure storage heater (e.g. 5–15 l, preheats a water reserve) or a pressure flow heater with low consumption. A non-pressure heater would only be possible with a valve replacement.
Scenario 5: Historic building with low water pressure
In older apartment buildings, the water pressure on higher floors can be very low – sometimes only 0.5–1 bar. Pressure flow heaters usually require a minimum pressure of 0.3–0.5 bar and a maximum of 0.6–1.0 MPa. If the pressure is below the minimum, electronic switching may not function properly. In such a case, a non-pressure heater may be more reliable, or a pressure storage heater can be chosen, where low pressure is not a problem for the device's operation.
Comparison of pressure and non-pressure water heaters: a clear table
| Parameter | Pressure water heater | Non-pressure water heater |
|---|---|---|
| Number of draw-off points | Several simultaneously | Only one |
| Type of valve | Standard (two-way) | Special non-pressure |
| Outlet pressure | Same as tap water (2–6 bar) | Atmospheric (low) |
| Pressure relief valve | Required (tank) | Not required |
| Purchase price | Slightly higher | Lower |
| Installation complexity | Medium (pipe connection + pressure relief valve) | Simple |
| Suitability for cottages, gardens | Yes, if pressure is sufficient | Yes, including low-pressure sources |
| Compatibility with shower panels | Yes | No |
| Use in a regular apartment | Standard solution | Limited (battery replacement required) |
Installation and electrical requirements – what you need to know before purchase
Regardless of whether you choose a pressure or non-pressure water heater, you should consider several installation aspects that differ partially for both types.
Electrical connection: Flow water heaters with 3.5 kW and higher require an outlet or a fixed supply dimensioned for the corresponding current. At 3.5 kW (230 V), this is a current of ~15.2 A, which requires a B16A circuit breaker and a conductor of at least 2.5 mm². At 4.5 kW (230 V), it is ~19.6 A – here, a 2.5 mm² supply with a B20A or B25A circuit breaker is also required, or a fixed connection. Always consult an electrician, as a standard 16A socket is not sufficient for power above 3.6 kW. More details can be found in the article Installation of an electric flow water heater: procedure, connection and requirements for electrical installation.
Water connection – pressure water heater: A shut-off valve and check valve (or combined valve) are installed before the inlet of cold water. A pressure and temperature relief valve (PTBV) with a drain to the sewer is installed on the outlet of hot water from the tank. Never connect a pressure tank water heater without a pressure relief valve – in the event of a thermostat failure, dangerous overpressure can occur.
Water connection – non-pressure water heater: The inlet of cold water is the same – a shut-off valve. The outlet of hot water goes directly to a non-pressure valve with an open arm. No pressure relief valve is required at the outlet, but no shut-off valve must be installed after the outlet.
Location: Flow water heaters are installed as close as possible to the draw-off point – every meter of hot water pipe means heat loss and waiting time for hot water. Tank water heaters can be located further from the draw-off point, but with the same philosophy: the shorter the pipe, the fewer heat losses in the pipes.
Impact on energy consumption and operating costs
From the perspective of energy efficiency, pressure and non-pressure flow water heaters differ minimally – both heat the same amount of water to the same temperature with the same power. The decisive factors are the power of the device and the volume of heated water.
A non-pressure water heater has a lower flow of hot water at the same power (e.g., 3.5 kW), because the lower pressure at the outlet means a slower flow. The water therefore stays longer in the heating chamber and reaches a higher temperature with a smaller amount – which is physically understandable. A pressure water heater at the same power produces warmer water at a higher flow. Therefore, a higher power is sometimes required for the same thermal comfort with a pressure flow water heater.
Tank water heaters have so-called stand-by losses – energy consumed to maintain the water temperature in the tank, even when you are not drawing water. Modern tanks have good thermal insulation and stand-by losses are in the range of 0.5–1.5 kWh per 24 hours, but during longer absences (holiday), it is advantageous to turn off the tank.
Most common mistakes in selection and installation
From practice, we know that the most common mistake is installing a non-pressure water heater with a standard valve. The customer buys a cheaper non-pressure water heater, installs it on existing pipes, does not change the valve – and the heater does not work properly or is damaged. Solution: always check the compatibility of the valve before purchase.
The second mistake is underestimating the power. The customer buys a 3.5 kW pressure flow water heater with the intention of showering – but at full water pressure, the flow is too high for the 3.5 kW to heat the water to a pleasant temperature. Solution: when showering with a pressure water heater, consider at least 4.5 kW, ideally 5.5 kW. More about power dimensioning can be found in the article What power of flow water heater do I need for my apartment.
The third mistake is installing a pressure tank without a pressure relief valve, or with a pressure relief valve without a drain pipe. The pressure relief valve must have a free outlet – if you connect it without a drain hose, dripping hot water can cause damage to furniture or the floor.
Most frequently asked questions (FAQ)
Can I connect a non-pressure water heater to an existing standard valve?
No, this is not possible. A standard two-way valve closes the water outlet, which causes excessive pressure inside the device with a non-pressure water heater. This can damage the seals and the body of the heater. A non-pressure water heater requires a special non-pressure valve with an open outlet arm, which never fully closes the hot water outlet.
Is a non-pressure water heater suitable for a shower?
It depends on the specific conditions. A non-pressure water heater has a lower outlet pressure and flow, which is noticeable in the shower as a weaker water stream. For basic showering with a simple handle, it may be sufficient, but for shower panels, massage nozzles or thermostatic heads, it is not suitable. If you want comfortable showering, choose a pressure water heater with at least 4.5 kW power.
Does a pressure storage water heater need a safety valve? Where to buy it?
Yes, a safety valve is mandatory for a pressure storage water heater and is absolutely essential from a safety perspective. It prevents dangerous overpressure in the event of a thermostat failure. The safety (pressure-temperature) valve is commonly sold separately as part of the accessories for water heaters and is set to the maximum pressure and temperature (typically 0.7 MPa and 90 °C). It must always have a free discharge into the drainage system.
What is the difference between a pressureless flow water heater with hydraulic switching and a pressure type with electronic switching?
These are two independent features. Hydraulic vs. electronic switching refers to the way the heater detects water flow and activates heating – hydraulic is mechanical (via a membrane movement), while electronic is more precise and sensitive. Pressure vs. pressureless, on the other hand, describes how the heater is connected to the water supply system. Our pressureless heaters (PM 3.5 kW and PM 4.5 kW) have hydraulic switching, while the pressure models MK1 have electronic switching. A more detailed comparison of switching types can be found in the article Hydraulic vs. Electronic Switching in Flow Water Heaters: What It Means and Which Is Better?
Can a pressure flow water heater supply multiple rooms?
Theoretically yes, but in practice it is limited by the power. At 4.5 kW and under pressure operation, the heater can supply one outlet with acceptable comfort. If you open a sink and a shower at the same time, the power will be divided and the temperature at both outlets will drop. For multiple simultaneous outlets, a storage heater or a more powerful flow heater (6+ kW, which requires a three-phase connection) is more suitable.
How can I tell if my existing heater is a pressure or pressureless type?
The easiest way is to look at the faucet on the sink: if it has two pipes (hot + cold) and a classic lever or screw-type faucet, it is likely a pressure system. If the faucet has one pipe and an open outlet arm (water flows out without the possibility of fully closing the hot side), it is a pressureless system. Another indicator: a pressure storage heater usually has a visible safety valve with a discharge pipe. Detailed instructions can also be found in the article Common Faults in Electric Water Heaters and How to Fix Them.
Conclusion: How to decide?
Choosing between a pressure and pressureless water heater is not always straightforward – it depends on the specific conditions of your home, cottage, or business. To summarize in simple rules:
- A standard apartment with existing plumbing and standard faucets: always a pressure heater – storage (e.g., BD 2.0 kW 30 l) or flow type (e.g., MK1 4
Do you have a question about this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.