Electric water heater installation: how it works and what to know beforehand

What electric water heater installation involves and when it is needed

Installing an electric water heater is a whole set of steps: fixing the unit to a load-bearing surface, building the cold and hot water connections, fitting the safety valve and shut-off valves, making the electrical connection, and finally filling and checking the system. Each of these affects whether the heater runs for ten years or causes problems in its first one.

The need usually arises in one of three situations. The first is a new or renovated bathroom with no hot water solution yet. The second is hot water independent of district heating — a house, a summer place or a flat where central hot water is absent or unreliable. The third is replacing an old unit whose tank has failed and where repair no longer makes sense.

These situations are worth separating, because the scope of work differs. A completely new water heater means the pipework and power supply still have to be built. Replacing an old unit usually means the connections already exist, but they need to be inspected and are normally partly renewed.

• a new bathroom or kitchen area with no hot water solution
• a house or summer place without central hot water
• a flat where hot water is unreliable or needed independently
• an old heater that is worn out and due for replacement
• a heater being moved to another spot during renovation

What the choice of heater and capacity depends on

Capacity is the first decision that cannot be changed cheaply later. A tank that is too small means hot water runs out mid-shower; one that is too large takes up space, loads the wall more and keeps heating water nobody uses. In practice the starting points are the number of residents, their habits and whether there is a bathtub.

For one person, 50–80 litres is often enough. For a household of two or three, 80–100 litres usually fits; for a larger family, 120–150 litres or more. A bathtub changes the calculation noticeably, because filling it uses a large share of the tank at once. The same applies when hot water is drawn in several places at the same time.

Besides capacity, the unit's dimensions and shape matter. In a narrow recess or a low-ceilinged room, a horizontal or slim cylindrical heater may be the only option instead of a vertical model. It also matters which side the connections are on, since that determines the pipe routing and the service space left afterwards.

The heating element type is a separate topic. A direct-contact element heats faster but sits in the water and collects scale more readily in hard water. A dry element sits in a protective sleeve and is easier to replace later. Which makes more sense depends on water quality and how regularly servicing is planned.

• number of residents and hot water habits
• whether there is a bathtub or only a shower
• whether hot water is used in several places at once
• free wall area, ceiling height and recess dimensions
• connection positions and the shape of the unit
• heating element type and later serviceability

Choosing a suitable installation location

The location decides three things at once: whether the unit stays firmly in place, whether it can be serviced later, and how long the pipe runs become. The most convenient spot has the water connections nearby, power available, and enough room in front of the unit to replace the heating element or anode later.

The wall has to carry a full tank. A hundred litres of water alone weighs close to a hundred kilograms, plus the unit's own mass. On lightweight partitions, tiled stud walls and old soft walls an ordinary plug is not enough — fixing into the load-bearing part, a support frame, or a floor-supported solution is needed. This is always assessed on site, because walls that look alike can have very different load capacity.

The second key condition is the safety valve's discharge. The valve must be able to release water somewhere it causes no damage: a floor drain, a drain connection, or at minimum a container that is checked. If a heater goes into a room where water cannot be directed anywhere, the consequences of a leak are incomparably worse.

The distance to the taps affects daily comfort. A long pipe from heater to tap means waiting longer for hot water, and each time part of the already heated water runs off unused. That is why a spot near the bathroom or kitchen is preferred over the far end of the house.

• a load-bearing wall or floor support for a full tank
• room in front of and below the unit for servicing
• a nearby discharge option for the safety valve
• a sensible distance from bathroom and kitchen
• a dry room with no freezing risk
• free access to shut-off valves and the electrical panel

What to check before installation

Most surprises during installation are foreseeable if the right things are checked half an hour beforehand. The most important is knowing where the water is shut off and whether the shut-off valve actually works. In older flats the valve often turns but water still keeps coming — in that case the valve has to be fixed or replaced first.

The second checkpoint is the condition of the existing pipework. Greenish or white deposits at the joints, rust marks or softened plastic fittings mean there is no point putting a new unit onto an old connection. The new heater will last years, but the old connection leaks first, and then the whole job has to be opened up again.

Third, it is worth looking at the power supply in advance: whether the heater has its own protected circuit, what condition the breakers are in, and whether the panel has residual-current protection. An electrician assesses this, but photographing the panel and noting the breaker markings speeds up the decision considerably.

In an apartment building the housing association's procedures matter too. If the work requires shutting a riser, that usually goes through the building manager and has to be timed with the neighbours. This affects the actual installation schedule more than the work itself does.

• where the shut-off valve is and whether it closes fully
• condition of existing joints and flexible hoses
• whether the heater has its own protected circuit
• whether the room allows a safety valve discharge
• whether the unit fits the planned spot with service room
• in an apartment building — whether a riser shut-off and approval are needed

Water connections and the components needed

A heater's water connection has two parts: cold water enters the lower inlet and hot water leaves the outlet. They are marked on the unit, and swapping them is one of the most common mistakes, turning the operating principle upside down. The joints are made so that they can be taken apart later without cutting into the pipework.

The cold side gets a shut-off valve and a safety valve. The shut-off valve lets the heater be isolated on its own without closing the water to the whole flat. The safety valve limits the tank's internal pressure and releases excess water. The hot side normally also gets a shut-off option, which makes later servicing and draining simpler.

It is sensible to leave a separate way of draining the tank — a tap or a hose connection point. Without it, every later service becomes awkward and means letting water out somewhere nobody wants it. Details like this distinguish a considered installation from a rushed one.

Material choice depends on the existing pipework. In a mixed system where copper meets plastic, proper transitions are needed, because a direct joint between dissimilar metals accelerates corrosion. Flexible hoses are an acceptable solution, but they are a wear part and should not be hidden where nobody will ever inspect them.

• a cold water shut-off valve before the heater
• a safety valve on the cold water inlet
• non-return function according to the solution
• a shut-off option on the hot water side
• a drain tap or hose connection point
• correct transitions between copper and plastic

Electrical connection and safety

The electrical connection is the most sensitive part of installing an electric water heater. It is a relatively high-power appliance working in a damp environment, so the connection must be made by a competent person and follow the unit's documentation. If there is any doubt whether an existing connection meets requirements, the only sensible step is to have an electrician inspect it.

This guide deliberately keeps that part general. The purpose here is not to teach electrical work but to explain what gets checked during installation. Replacing a plug connection with a fixed one, breaker selection, residual-current protection and earthing are decisions that depend on the specific panel and appliance, and general advice cannot substitute for them.

In practice the work is often split. The plumber makes the water connections, mounts the unit and prepares everything for commissioning; the electrical connection is made by a suitably qualified electrician. That division is normal and sensible, because each side has its own responsibility. We agree on the need for it before the work starts, so the installation does not stall halfway.

One rule always applies: the heater must not be switched on electrically before the tank is completely filled with water. An empty or partly filled tank means the element runs in air and fails very quickly. This is the most common way to ruin a new unit in its first minute.

• the electrical connection is left to a qualified electrician
• the unit's documentation sets the requirements, not common practice
• the panel and breakers are reviewed before the work
• the heater is not switched on before the tank is filled
• damp-room solutions are decided case by case

Safety valve, pressure and expansion

Water expands as it heats, and in a closed tank that means rising pressure. The safety valve exists precisely for this: when pressure exceeds the permitted limit, the valve releases a little water and the pressure drops. A few drops or a small amount of water from the valve discharge is therefore normal, not automatically a fault.

The problem starts when the valve is missing, has the wrong rating, or is fitted the wrong way round. Then the pressure is carried by the tank and the joints, and the consequence appears at the weakest point. Equally risky is a discharge that someone has closed off, or one left blocked with sediment.

If water runs from the valve constantly and in a noticeable amount, it is worth looking further. Common causes are excessive incoming water pressure, a worn valve, or a system with nowhere else to absorb expansion. In some solutions a small expansion vessel is added on the hot water side to take up the volume change and reduce constant valve operation.

Whether an expansion vessel is needed in a given case depends on the system: incoming pressure, the pipework layout and the manufacturer's instructions. Fitting one to every heater automatically makes no more sense than avoiding it on principle — the decision follows measured pressure and the actual situation.

• the valve rating must match the appliance requirement
• the discharge is always left open and checkable
• a small release of water during heating is normal
• a constant strong flow points to pressure or the valve
• with high incoming pressure a reducer is considered
• the need for an expansion vessel follows the system

How the installation goes step by step

The order of work is mostly the same whether the unit is new or a replacement. First the water is shut off and confirmed to be off. Then the location is prepared: fixing points are measured, the wall is checked, and mounts able to carry a full tank with margin are fitted.

Once the unit is firmly in place, the water connections are built. The cold side gets the shut-off valve and the safety valve with its discharge; the hot side gets the connection towards the taps. The joints are made so they stay visible and removable — a hidden joint is the most expensive option when a leak eventually appears.

Next comes filling the tank. Water is let in slowly and the nearest hot tap is held open so air can leave the tank. Filling is complete when the tap gives a steady stream with no air bubbles or surging. Only then can the electrical connection be energised and the unit switched on.

Finally the checks: all joints are inspected, the first heating cycle is observed, and the safety valve and its free discharge are confirmed. The customer is shown where the shut-off valve is, how to switch the unit off, and what to do if something seems wrong. That last part takes a couple of minutes and saves the most trouble later.

• shutting off the water and confirming it is off
• measuring the location and fitting the mounts
• lifting and levelling the unit
• water connections, shut-off valve and safety valve with discharge
• filling the tank slowly and releasing the air
• the electrical connection only with the tank full
• the first heating cycle and a leak check
• showing the customer the operation and shut-off valve

Common installation mistakes

Most later problems start with a handful of repeated mistakes. The most common is swapping the connections: cold water onto the hot side and vice versa. The unit seems to work, but hot water runs out fast and the owner starts suspecting the heating element when the real cause is the connection.

The second common mistake concerns the safety valve. Sometimes it is not fitted at all, sometimes its discharge points somewhere nobody notices water, and sometimes the discharge has been closed off because 'water was coming out of it'. All three make the unit significantly riskier, because the last safeguard against pressure has been removed.

A third group of mistakes involves the mounting. An ordinary plug in a lightweight wall holds an empty unit nicely, but with a full tank the fixing starts to give way. Such mistakes rarely show up immediately — they surface months later, when cracks appear on the wall surface or the unit is visibly tilted.

The fourth mistake is leaving no service room. The heater is pushed exactly into its recess with no thought for replacing the element or anode later. The result is that the first service requires taking the unit down, or even cutting the pipework open. The same goes for hidden joints and unreachable shut-off valves.

• cold and hot water connections swapped
• safety valve missing, reversed or closed off
• mounting that ignores the weight of a full tank
• the heater switched on before filling
• no service room and hidden joints
• old worn fittings left in front of a new unit
• air not released from the tank

When connecting the unit is enough and when pipework has to be redone

Not every installation means reworking the pipes. If the new unit goes where the old one was, the dimensions are close and the existing connections are in good shape, the job often comes down to swapping the unit, renewing the fittings and fitting the safety valve. That is the fastest scenario.

Changing the pipework becomes necessary when the unit moves to a new spot, when the new heater's connections are on the other side or at a different height, or when the existing pipework is so worn that putting a new unit on it makes no sense. The same applies when the earlier installation was done without shut-off valves or a proper valve discharge.

A separate case is the first heater in a house or flat with no hot water line at all. That is already pipework: a hot water line is built out to the taps, taking run lengths and insulation into account. Such a scope cannot be assessed accurately over the phone — the room and the existing pipework have to be seen.

Drawing that line matters, because the duration and preparation depend on it. That is why we ask beforehand whether the heater goes where the old one was or into a new spot, and what condition the previous connections are in. If the picture is unclear, we look at the situation on site and talk it through before anything is opened up.

What is different about replacing an old heater

Replacing an old unit starts with draining, and that takes time. A hundred litres of water does not go anywhere quickly, and if there is no drain tap, some ingenuity is required. While draining, a hot tap has to stay open so air can enter the tank and the water can move out.

Underneath and behind the old unit, conditions nobody has seen usually come to light: rusted fixings, damp marks on the wall, limescaled joints. That is why it is sensible to expect the fittings to be renewed along with the unit, not just the unit itself. Old flexible hoses and a worn shut-off valve are the most likely spots for the next leak.

Dimensions need attention. A new heater of the same capacity may have a different height or diameter and no longer fit the recess. Fixing points often differ too, which means drilling the wall again and, where needed, closing the earlier holes.

Taking the old unit away is worth agreeing in advance. Even drained, a heater is a bulky, heavy object that is awkward to carry down a stairwell. If the plan is clear before the work, the unit does not end up standing in the hallway.

• draining time and where to direct the water
• renewing fittings together with the unit
• the new unit's dimensions and fixing points
• the condition of the wall behind the old unit
• agreeing on removal of the old unit

Checks after installation

The work does not end when the unit is switched on. The first check is inspecting the joints with the tank full and under working pressure — a dry piece of paper or tissue under a joint reveals even slight damp that the eye misses.

The second check is the first heating cycle. The water should reach the set temperature and the unit should then switch off. During this the safety valve is watched: a small release of water is normal, a constant strong flow is not. It is also worth listening for unusual rumbling from the tank, which would point to air in the system.

The third check happens a day or two later, usually by the owner. By then the unit has run several cycles, and if something is off, that is normally when it shows. Worth looking at: the base of the heater, the wall behind it, and the area around the valve discharge.

It is useful to note down the model, capacity and installation date. That information is needed at every later service and makes finding spare parts considerably easier.

• a dry check of joints under working pressure
• the first full heating cycle and switch-off
• watching the safety valve and its discharge
• checking the base and wall after a couple of days
• noting the unit's details and installation date

Servicing and everyday use

A heater's working life depends largely on the operating temperature and servicing. Constantly holding a very high temperature speeds up limescale and loads the element. A moderate setting covers normal household needs and keeps sediment building more slowly.

The most important part of servicing is checking the anode and removing sediment from the bottom of the tank. The anode protects the tank against corrosion and is consumed in use. If it goes unchecked for years, corrosion continues on the tank itself — and then there is nothing left to repair.

In hard water, sediment is a bigger problem and the service interval is shorter. The sign is usually simple: hot water runs out sooner than before and heating takes longer, even though the unit is otherwise sound. Our article on heating problems covers that in more detail.

The safety valve is worth checking now and then, so the discharge is neither blocked nor closed. If a long winter absence in an unheated room is planned, freezing risk has to be considered and the unit drained along with the pipework.

• a moderate temperature setting to limit limescale
• anode inspection and replacement as needed
• removing sediment from the bottom of the tank
• inspecting the safety valve and its discharge
• allowing for freezing risk during long absences

How long installation takes and how to prepare

Three things set the time needed: whether the unit goes where the old one was or into a new spot, what condition the existing connections are in, and whether the wall fixing needs a separate solution. A replacement with matching dimensions and sound connections is the quickest. The longest is the first heater in a room with no hot water line yet.

Preparation on the customer's side shortens the work more than it seems. If the space in front of and below the unit is clear, furniture is moved aside and the route to the heater is free, work can start straight away. If the new unit has already been delivered, it is worth unboxing it and keeping the documentation to hand — that is where the fixing dimensions and requirements come from.

In an apartment building, allow for a possible riser shut-off. If one is needed, separate time goes into approval with the building manager and notifying neighbours. Better to settle that before the installation day rather than on the morning itself.

During the work the water is off for a shorter or longer period. A practical tip is filling a couple of containers beforehand, so washing and kitchen tasks do not stop entirely. In a household with small children or pets it makes sense to keep the work area clear, since there are open pipes, tools and water there.

• clear space in front of, below and on the way to the unit
• the new unit's documentation to hand
• in an apartment building, riser shut-off approved in advance
• water set aside, since the line is temporarily closed
• information about the previous connection and the shut-off valve location

When to call a professional plumber

Part of the preparation an owner can do alone: measure the room, choose the capacity, find the shut-off valve and gather information about the existing pipework. Beyond that, the work moves into territory where mistakes are expensive — water and electricity in one room leave little room for error.

A plumber is definitely worth calling when the heater goes to a new location, when the existing connections are worn, when the shut-off valve does not work, or when there is doubt about the wall's load capacity. For the electrical connection the answer is clearer still: that work is done by a suitably qualified electrician, however simple the connection looks.

Quick intervention is needed if water appears under the heater, if a noticeable amount runs constantly from the valve, or if the unit trips the breaker. The last one means stopping: leave the unit switched off and have the cause inspected rather than resetting the breaker repeatedly.

We carry out installations in Tallinn and Harju county. If you are planning a new electric water heater installation or replacing an existing unit, the simplest step is to call and describe the situation briefly — from the capacity, location and previous connection we can already say on the phone what the work involves and what is worth preparing.

• the heater moves or the pipework needs changing
• existing connections or the shut-off valve are worn
• there is doubt about the wall's load capacity
• the electrical connection needs making or inspecting
• water appears under the heater or the breaker trips