Home bathroom and sanitaryPressure lower with hot water than with cold water - solve the problem

Pressure lower with hot water than with cold water - solve the problem

  • 1. Check the perlator / shower head
  • 2. Check fitting
  • Hot water systems
    • Local hot water appliances
    • Central heating systems
  • The KFR valve
  • Call a specialist

More than just annoying - because you want to enjoy a hot shower or a warm bath but experienced a disappointment: The pressure of the hot water pipe is weak. Instead of a blazing jet, only a tired trickle comes out of the pipe. Such a damage picture is more than annoying. It is an indication of a problem that can grow into a very expensive refurbishment measure. That's why systematic action is now indicated.

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Now put the bucket under the tap and let it run fully turned on exactly 15 seconds. Since one liter of water corresponds to exactly one kilogram of weight, the filling volume of the bucket can be precisely calculated with the aid of the balance. A measuring cup fulfills the same purpose. The determined number of liters takes you 4 times and you already have the flow rate in l / min, which is present in the apartment. To reinterpret this in the pressure unit "bar", many more calculations are necessary. However, the drinking water supplier can use the value of the flow rate to determine whether everything is in order with the pressure in the apartment.

A tip: It is generally advisable to determine the actual state of the flow rate and take note of an apartment when referring to a dwelling. This is best done on every single faucet. So you have a comparison value when changes are detected and can later problems narrow down more quickly.

1. Check the perlator / shower head

Perlator and shower head are the entry points of the warm water. These can lime and disturb the flow. Both elements can be unscrewed safely and easily. As a rule, not even tools are necessary. Both are placed after unscrewing for 2 hours of vinegar essence, equipped with a new seal and screwed back - done. If the water pressure in the hot water is then still too low, you must proceed with detective instincts. With sealing tape, the threads of the perlator and shower head become watertight.

2. Check fitting

The biggest enemy of a water pipe is the lime. This mineral is unavoidable in drinking water and is given by the "water hardness" of the water supplier. The harder the water is, the more lime is contained.

Lime sets first at narrow flow rates. The fittings are therefore particularly at risk for calcification. The narrow channels of a fitting are simply faster than the several centimeters wide pipes of the drinking water pipe.
To overtake a fitting you need:

  • 1 pipe wrench with rubber jaws (from approx. 5 Euro)
  • possibly small wrenches and screwdrivers (about 5 euros each)
  • Seals (in the set approx. 5 Euro)
  • Faucet grease (2-5 Euro per 30g tube)
  • Essig Essence (about 1 Euro per 0.5 liter bottle)
  • Seal kit consisting of hemp, sealing paste and, if necessary, sealing tape. (about 5 euros)

There are also special Kalklöser for fittings for about 5 € per liter. However, this is usually not necessary, vinegar essence is very effective here.

First, the supply line is shut off. In general, each fitting has a small, separate valve under the basin or the tub. Just let the water run and turn on the inlet valve. If no more water is running, the fitting can be unscrewed with the pipe wrench. The rubber-coated jaws prevent scratches on the chrome-plated or enamelled surface of the screw connection. Fittings can be completely disassembled. Put in vinegar essence overnight and equipped with new gaskets, a calcified fitting is made feasible again. Do not forget to grease the taps too, otherwise the taps and corner joints may jam.

Of course, you can also replace the entire valve at the same time. These are available as a new part from about 30 euros.

When installing the valve, the hollow threads, to which it is connected to the water pipes, are attached. For this purpose, the thread is wrapped tightly overlapping with installation hemp from top to bottom and then rubbed with sealing paste.

Hot water systems

If the overhaul or replacement of a fitting did not restore the water pressure, then you need to take a closer look at the hot water system.

Two fundamentally different technologies are available for the production of warm tap water: local or centralized systems.

Local hot water appliances

Local hot water appliances are connected to the cold water pipe directly at the sampling point. One differentiates between heating boilers and water heaters. Both devices work with electricity. A basic word about these devices: Hot water terminals are very sensitive to strong pressure fluctuations. The connection of a high-pressure cleaner to a hot water pipe, which is fed by a hot water terminal, can quickly destroy the unit due to the setbacks in the pipe!


The boiler is sold as a undercounter or hanger. It costs about 50-150 euros and is basically just a permanently installed kettle in which a defined amount of water can be heated. These devices work without pressure, which means that they are decoupled from the water pressure of the cold water pipe. Pressure losses on a boiler indicate an advanced calcification in the device itself. Worse, however, are breaks in pipes where some of the water runs into the housing. These leaks can be a major threat due to the soaking of the electronics in the devices. Therefore, in case of pressure loss in boilers: Immediately disconnect the device from the power supply and inspect the device for leaking water.


Instantaneous water heaters the water from the cold water pipe while it passes through an internal pipe system. They also work electrically. Instantaneous water heaters are pressure equipment that maintains the pressure of the cold water pipe. They are much more expensive than the water heaters. Brand appliances are available from about 250 euros. However, to operate a shower, devices of the 500 Euro price range are necessary. In these devices, pressure loss can also mean internal damage or advanced calcification. Damage to the water supply is also considered. For this reason, the pressure loss on instantaneous water heaters is handled in the following order:

1. Disconnect power, switch off fuse if necessary
2. Turn on cold water and check for pressure
3. Open the housing of the water heater and check for water loss
4. Check the flow heater for calcification or other damage.

If the pressure loss in the water heater itself is present, then the unit should be replaced or left to a specialist for repair. Wherever water and electric power come together, no attempts should be made to repair but trust a specialist.

Central heating systems

In central heating systems, the water is heated at a central point and fed via separate hot water pipes to the sampling point. Central heating systems are technically much more complex than the local devices. However, they work more economically and comfortably. They can be installed in the basement, as well as in the garage or in the apartment itself, usually in the bathroom or a separate chamber.


To rule out that the pressure loss in the hot water line is not a problem with the supply line, it is always necessary to first compare the flow rate between the hot and cold water lines. A small pressure difference between the two networks is normal as the warm water has to go a much more complicated route. Each component such as valves, bends or longer lines have a pressure-reducing effect.

A pressure loss in the hot water pipe can have three causes with central heating systems:

  • Line is added
  • Line is damaged and pressurized water leaks out
  • Hot water treatment is damaged

Line is added

There are three ways to seize a pipe: foreign matter, rust and lime. Foreign objects such as sand and gravel are held back by a fixed water filter located close to the feed station of the waterworks. The filter is easy to change. The procedures are type-dependent. In general, the water filter sits between two shut-off valves. These are turned off first and then the filter element replaced.

Rust and lime usually occur only on galvanized steel pipelines. These were used as standard until the 1980s, but are now being replaced by brass and plastic piping due to their greater resistance to clogging. Galvanized steel pipes shine silver. On the sleeves and arches can be seen emerging rust marks and limescale also from the outside. There are procedures that can clean rusty or calcified pipes and even recoating from the inside with waterproof epoxy resin. It has to be checked on a case-by-case basis whether this intervention is worthwhile or whether the water pipes should be replaced.

Damaged cables

A loss of pressure in the hot water pipe, where a leak is the cause should be found quickly. If the damaged area is so large that it causes a noticeable pressure difference, then the location of the damage can not be overlooked. The leaking water has to flow somewhere and draws attention to itself through wet walls or puddles of water. Here only helps a replacement of the damaged pipe. Caulking and plastering work can be done by the DIY enthusiast himself. The replacement of the defective pressure water pipe should be left to a specialist.

Damaged hot water treatment

A central heating system can be fired by various types. However, the functionality is always the same: Fuels, such as oil, gas or pellets, heat water via a heat exchanger, which is temporarily stored in a tank and passed on to the sampling points. Also additional systems such as heat pumps or solar thermal collectors are only supplementary and are irrelevant for the pressure in the hot water pipe.


The warm water is led to the fitting via a complex system. A variety of valves, regulators and other components to ensure the same pressure. Each of these components can be damaged by overaging, lime or rust. Often the components can be made workable by replacing seals, thoroughly decalcifying and lubricating. Nevertheless, they usually have to be upgraded for this.

However, at one point the do-it-yourselfer can try to restore the water pressure himself: the KFR valve, if it is not moving for a long time, can jam and thus provide for the pressure differences in the domestic water supply.

The KFR valve

The combined free-flow valve with non-return valve is located directly behind the water meter. It can be recognized by its sloping design. It has two functions: The KFR valve opens and closes the water supply of the domestic water supply from the drinking water supplier. The built-in backflow preventer ensures that the water can only flow in one direction. If this backflow preventer is jammed or damaged in any other way, this leads to pressure fluctuations in the system. It can also be installed several KFR valves in a drinking water network of the house.

By simply turning the KFR valve on and off again, the backflow preventer can loosen again. The problem is solved.

Call a specialist

However, if the water pressure is still unsatisfactory despite the replacement or decalcified fitting, aerator or shower head, and if the release of the non-return valve in the KFR valve has also failed, then a specialist should take care of the matter. Even the most committed handyman must know his limits. Amateur repair attempts on a pressurized water system can only end in an aggravation of the situation without the necessary experience, training and work equipment.

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