Identifying and fixing a hot water system

Fixing a problem boiler or heater must start with identifying the kind of system you’ve got.

Most houses today have two separate hot water systems, and although both systems share components, they do not mix. The central heating system uses water that is heated in the boiler, then circulated around radiators. The domestic water system is the water that comes out of the taps, and that is the water system discussed below. In order to fix any problems you have with the heating of your domestic water system, first you must figure out which system you have.

Level of difficulty

Level 3: Advanced

Hot Water Tank – Direct boiler system

Hot water tanks are usually one of three kinds: direct boiler systems, indirect boiler systems and immersion heaters. Direct boiler systems draw cold water into the bottom of the tank, into the boiler, which returns it to the top of the tank. Direct boiler systems can be identified by a vent pipe running between the hot water tank and the cold water tank. This allows excess steam to be redirected into the cold tank to prevent damage to the hot water tank.

In all systems which use hot water tanks, water is drawn from a tap at the bottom of the tank, and as it heats, it rises in much the same way that hot air rises above cold air. The cold water pushes the hot water up through the pipes when the hot water tap at the sink or tub is turned.

If it goes wrong – kettling

One common problem with any hot water tank system is something called ‘kettling’. This refers to a build-up of limescale, like you would find inside your tea kettle. It is exactly the same process. The hot water leaves deposits of minerals, which turn into limescale. This limescale can be corrosive to the tank, causing leaks, or it can restrict the flow of water, causing it to overheat and boil. This boiling then causes a knocking sound in the boiler.

Getting rid of a boiler’s limescale is as easy as adding noise reducer, a liquid that is added to the system and slowly gets rid of the limescale, without the need to flush out the system. If that doesn’t work, more aggressive descaling liquids can be added, but they do need to be flushed out of the system once used.

Hot Water Tank – Indirect boiler system

An indirect boiler system has a length of coiled pipe in the hot water tank which is connected to the boiler. The boiler heats the coil, and the coil then heats the water. This coil is part of the central heating circuit, and the circuit’s primary purpose is to provide heat to the radiators. This is called the primary circuit, and heating the water is just a by-product of the primary circuit. Once the water is heated, the hot water tank works the same as the tank in an direct boiler system does.

You can distinguish an indirect boiler system from a direct one quite simply. An indirect system uses two hot water tanks. The smaller one heats the water for the primary circuit and has the vent pipe over the top.

If it goes wrong – low water pressure

Both indirect and direct boiler systems are called vented systems because of the vent pipes. These pipes leave the tanks open in atmospheric pressure changes, which can affect the water pressure. If it becomes a problem, pumps can be added to improve the water flow.

Hot Water Tank – Immersion heaters

In most domestic water systems in the UK, immersion heaters are used as a backup method of heating the potable water. These heaters warm the water with an electrical element that screws into the water tank. Generally these emersion elements work with timers to heat the water when energy prices are lower, between midnight and 7am. Then the well-insulated tank will largely retain the water’s heat until it needs to be used.

If it goes wrong

The most common problem with immersion heaters is that they simply don’t heat the water. Normally either the element has stopped working or the thermostat has, and there are very simple tests to determine what has gone wrong.

  1. Check the heating element terminals using an electronic multimeter or multi-tester
  2. Turn off the electricity supply to the heater, and wait for the water to cool down. Test if the water is cool by turning on the hot water at a tap and waiting for it to run from hot to cool or lukewarm water
  3. At the heating element terminal block, remove the cover panel and fold the insulation back, away from the heating element
  4. Loosen the screws holding the wires to the terminal screws and remove the wires
  5. Set your tester to ‘ohms’ and connect the leads to the terminals, connecting the red lead to one terminal and the black to the other
  6. If the ohms read 0 on a digital tester, or if the needle doesn’t move on an analogue tester, there is no flow of electricity, and the heating element needs to be replaced
  7. If the ohms are around 20 (though the ohms can vary), then the thermostat is likely what needs to be replaced

Unvented system

Unvented systems were approved for use in 1989, and they differ from vented systems because they are connected directly to the mains water. Because of this, the water pressure is much more consistent: it is affected by the mains water pressure, without any influence from the air pressure. Luckily, these systems are designed with many safety valves and pressure releases to ensure the steam doesn’t build up dangerously. Otherwise, the principles of heating the water are pretty much the same as those in the indirect heating systems, complete with the coiled piping and immersion heaters.

If it goes wrong

Unvented systems can have some problems, but it is usually easy to diagnose and fix any issues if the hot water runs out.

  1. Check the breaker. Some unvented systems have two breakers, one for the main immersion heater and one for the boost heater. Make sure both are in the on position. If they won’t stay on, you may need to replace the immersion heater
  2. In addition to circuit breakers, some systems have residual-current devices (RCDs) that disconnect the supply of electricity if the current is unbalanced. Try to turn your RCD back into the on position. If it still shuts off the power, leave it off and turn off the breakers protected by the device. Turn the RCD back on, then turn on the breakers. If you continue to have problems, there is either a fault in the heater, or the circuit that links up to the boiler may be overloaded by other appliances also using that circuit.  Try turning off the RCD and the breakers, then unplugging all other appliances that use the circuit. Turn the RCD and breakers back on, then plug in the appliances, one at a time, to see which one is overloading the system and tripping the RCD
  3. Check whether the controller is switched on and set correctly
  4. Check the fuse. The 13amp fuse in the fuse spur may need replacing

Thermal storage system

One of the newest forms of heating domestic water, a thermal storage system is connected to both the heating and the mains. Water from the mains supply is heated in the insulated cylinder via a boiler, an immersion heater or even solar power. It is heated to a very high temperature, and that very hot water is cycled through the radiators. For the taps’ supply, cold water from the mains is mixed with the heated water before continuing throughout the house. This ensures the water coming out of the taps is hot, yet not scalding.

The thermal storage system is unique in that it uses a gravity feed, or head of water, to circulate hot water through the radiators. Because it relies on the effects of gravity, it must be installed higher in the house than the highest radiator.

If it goes wrong

The system needs to mix hot and cold water before sending it to the taps, and it does this with a mixing valve. This valve causes many of the issues unique to the thermal storage system. When the valve fails, hot water is unable to get through the system, so the water from the taps remains cold. To check if the valve is indeed the source of your problem, do the following:

  1. Check the temperature in the cylinder. It should be around 75 to 80°C, and there should be a brass boss near where the heating element is to indicate the temperature in the cylinder
  2. Turn the valve to its lowest setting, and turn on a hot tap
  3. Check the temperatures of the hot and cold inlet pipes that are running to the valve, as well as the temperature of the output pipe. The hot and cold inlet pipes should have very different temperatures, and the outlet pipe should be similar to the cold pipe in temperature
  4. Turn the valve to its maximum setting. If the valve is working, the temperature of the outlet pipe will rise to around 55°C. If the valve is faulty, the pipe’s temperature will remain largely unchanged
  5. Replace the valve if necessary

Single point water heater

Sometimes people just want one particular water source heated. In this case, they often prefer to use a single point water heater. These heaters can use gas or electricity. Single point gas heaters have a reservoir of cold water. When the tap is turned on, it senses the arrival of new cold water, which triggers the ignition of the burners used to heat the water. The hot water at the top of the reservoir comes through the tap, whilst the reservoir is replenished with cold water from the mains.

Single point electric heaters are most famously used in electric showers. They heat water similarly to coffee makers, but with much higher water volumes and pressure. Heaters up to 3kw can be plugged directly into a regular 13 amp socket, though care has to be taken to ensure the socket is not within access of the water supply. Heaters using more than 3kw must be wired into the fuse box with a double pole isolating switch in the circuit.  In a kitchen, the switch can be wall mounted above the sink, but in bathrooms, the switch should be either be a cord-operated ceiling switch or one installed on the outside of the bathroom.

If it goes wrong

The most common problems with single point water heaters are the same problems associated with any other appliance or device that relies on electricity or gas. In the case of electrical heaters, that is the breaker being tripped, and with gas, it is the pilot light going out. Correct these as you would with any other device, and the heater should begin to work again.

Multi-point water heater

The most common multi-point water heaters are combination heaters. All multi-point water heaters heat water exactly the same way as the single point ones, but they can be connected to several points. One multi-point heater, for example, could provide water to taps, a washing machine and a dish washer. Ordinary multi-point water heaters need to be used in conjunction with central heating boilers, as they heat only domestic water supplies. Combination heaters, however, can be used on their own, as they heat both domestic water and also heat the water for central heating systems. They do give heating priority to domestic water, however.

If it goes wrong

One of the most common problems associated with combination heaters in particular is a leak. Checking for a leak is a fairly straightforward matter, and it should always start with your shutting off the electricity to the heater, then draining the heater of water. Whether the leak is in a valve, a pipe or a seal, you have to do both those things before you can replace the part or repair the leak.


Please note that all our DIY guides and ‘Expert answers’ advice have been written strictly for reference only. Rated People do not accept any liability for any damage caused to an individual, property or anything else as a result of following our DIY guides and using our ‘Expert answers’ advice.

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