If there's significantly less hot water in your water heater than before — the shower turns cold mid-wash, there's no hot water left after washing dishes, or the water is warm, not hot — it's not a coincidence, but something has changed within the water heater or its controls.
Many residents in Tallinn and Harjumaa, upon noticing the problem, immediately assume the water heater is old and needs replacing. However, in most cases, it's a specific, identifiable, and often repairable fault.
In this guide, we'll go through all the common reasons why hot water in a water heater runs out quickly, explain how to identify them yourself at home, and provide instructions on when it's wise to call a specialist.
Limescale on the Heating Element — The Most Common Cause
Estonian tap water in many regions is quite hard, meaning it contains dissolved lime salts that, when heated, precipitate onto the heating element's surface as a white, hard layer.
Limescale acts like an insulating layer — the heating element has to transfer the same amount of heat through a thicker and less conductive layer, which causes water heating to take longer and the element to work continuously under greater load.
If limescale has accumulated thickly, the water heater cannot heat the entire tank to the intended temperature within the allotted time, meaning a significantly smaller portion of the volume actually reaches the desired temperature — resulting in the feeling that hot water runs out quickly, even though the tank itself is the same size.
- Limescale accumulation is a slow process, so the problem usually develops gradually, not overnight.
- In areas with harder water and for older water heaters (over 3-4 years without maintenance), limescale is almost always a contributing factor.
- As the limescale layer thickens, the heating time for the entire tank also increases, meaning hot water regenerates slower than before.
How to suspect limescale buildup yourself?
If your water heater makes more noise than before during heating — rattling, bubbling, or sizzling — it's often a sign that water is boiling under the limescale layer and in areas where heat transfer is uneven. Also, your water heater's water level indicator or electricity bill might show that the appliance is running longer than usual to achieve the same result.
Faulty or Inaccurate Thermostat
The thermostat is the water heater's 'brain,' measuring water temperature and commanding the heating element to switch on or off. If the thermostat is faulty or out of calibration, the water heater may not heat the water to the intended temperature.
A common fault is the thermostat switching off the heating too early — for example, at 40-45 degrees Celsius instead of 60 degrees — making the tank seem full, but the actual water temperature is lower, and the hot water cools down faster when used at the tap.
Another variant is a thermostat that doesn't react correctly at all, switching the heating on and off erratically, leaving part of the tank unheated.
- Check if the water heater's set temperature (usually between 55-65 degrees Celsius) actually corresponds to the water temperature coming from the tap.
- If the water heater clicks on and off more frequently than usual, or doesn't click at all, it could indicate a thermostat malfunction.
- Precise thermostat checks and replacement, if necessary, require a specialist's help due to electrical safety.
Leaking or Broken Mixer Tap/Valve Mixing Cold Water In
If the internal mechanism of a bathroom or kitchen mixer tap (ceramic cartridge or old-style washer tap) is worn, it can allow cold water to flow through even when you have the tap fully on hot water.
This mixing isn't visible, as the flow remains a single stream, but it lowers the actual outgoing water temperature and gives the impression that the water heater's hot water has run out sooner.
A similar effect can be caused by a tempering valve (thermostatic mixing valve) installed near the water heater, which is designed to dilute excessively hot water but is incorrectly set or faulty, letting too much cold water in.
How to check for a mixer tap issue?
Connect the tap directly to the water measured from the water heater's outlet, or let water flow from another tap supplied by the same water heater — if the water is hotter in the other location, the problem is likely with the specific tap, not the water heater itself. You can also dismantle the tap's interior and check the cartridge or seals.
Incorrect Temperature Setting or Deliberately Lowered Temperature
In some cases, the reason is simpler than it first appears — the water heater's temperature regulator has been accidentally or deliberately turned down, for example, after cleaning, maintenance, or a child accidentally adjusting a knob.
A water heater with a lower setting heats water to a reduced temperature, meaning the same amount of hot water, when mixed with cold, will provide a comfortable shower for a shorter period, as the difference between the hot and desired temperature is smaller.
For energy-saving purposes, the water heater temperature is sometimes deliberately set lower, but this leads to precisely this side effect — hot water seems to be less, even though the tank is full.
- Check the position of your water heater's temperature regulator and compare it with the manufacturer's instructions.
- The usual recommended setting for home use is 55-65 degrees Celsius — high enough to prevent Legionella bacteria, but not too high to pose a scalding risk.
- If the temperature is set below 50 degrees, it's worth checking and increasing it if necessary.
Water Heater Capacity No Longer Meets Actual Household Consumption
If the family has grown, washing habits have changed (e.g., longer showers, more frequent baths), or more hot water is used simultaneously (washing machine, dishwasher, and shower at the same time), a previously sufficient water heater capacity may become too small.
This is not a technical fault, but a natural discrepancy between tank volume and actual consumption — an 80-100-litre water heater suitable for two people may no longer be enough for a family of four using it at the same rhythm.
If the problem has developed gradually with household growth or changes in habits, rather than suddenly, it's worth considering this cause first before looking for a technical fault.
- Measure how long hot water lasts under typical use and compare it to how many people and which hot water appliances are being used simultaneously.
- If several hot water-demanding activities (shower, dishwasher, washing machine) occur at the same time, the tank will naturally empty faster, regardless of the water heater's condition.
Damaged Internal Dip Tube or Cold Water Diffuser
Inside the water heater, there's a cold water inlet pipe, usually with a diffuser at its end, which directs incoming cold water to the bottom of the tank so it doesn't immediately mix with the hot water at the top.
If this pipe or diffuser is damaged, broken, or dislodged — which can happen due to age, corrosion, or previous maintenance — incoming cold water immediately mixes with the hot water at the top of the tank, instead of staying at the bottom.
The result is that even before a large amount of hot water has actually been used, the water coming from the tap starts to cool down, as cold water reaches the hot water outlet too quickly.
Can this be checked yourself?
The condition of the internal pipe can only be checked by opening the water heater, which requires a specialist's help. If other simpler causes (limescale, thermostat, mixer tap) have been ruled out, but the problem persists, a damaged diffuser is the next likely suspect, especially in older water heaters.
Partial Heating Element Failure (Two Elements, Shared Tank)
Some larger capacity water heaters (usually over 150 litres) are equipped with two heating elements that work together or alternately to speed up heating and distribute the load.
If one of the two elements has burned out or is faulty, the water heater will still heat water, but at half the power — meaning the tank will only partially heat up within the intended time, and the actual available hot water quantity will be smaller.
Such a fault is difficult to distinguish at home from slow heating caused by limescale, as the symptom (slower and incomplete heating) is very similar — an accurate diagnosis requires electrical measurement.
How Does This Issue Differ from Water Heater Cleaning and General Maintenance?
Our guide on how to clean an electric water heater specifically addresses the step-by-step cleaning process, and our water heater installation and maintenance guide focuses on installing a new appliance and regular maintenance schedules.
This article is for diagnosis — it helps determine WHY hot water runs out quickly before you decide whether a simple clean, part replacement, or a complete appliance upgrade is needed. If you've already identified limescale as the cause, this guide will naturally direct you to the cleaning article; if the problem seems more serious, it helps you decide when to call a specialist.
How to Diagnose the Problem Step-by-Step at Home?
Before calling a specialist, it's worth going through a simple checklist to help narrow down the cause and provide the specialist with more accurate information later.
- Check the position of your water heater's temperature regulator and ensure it hasn't been accidentally turned down.
- Measure how long hot water lasts under typical use and compare it to previous experience — was the change gradual or sudden?
- Test different taps in the house — if the problem occurs only in one place, it's likely a tap issue; if everywhere, then it's the water heater itself.
- Listen for any unusual noises from the water heater during heating (bubbling, rattling, sizzling) — this indicates limescale.
- Observe if household water usage has increased recently (new residents, more frequent showering/bathing, simultaneous use of multiple appliances).
- Check if the water heater switches on and off more or less frequently than usual during heating.
- Investigate if there is a long pipe section between the water heater and main taps that might amplify cooling, especially in colder rooms, such as an uninsulated basement.
When will the problem resolve itself and when won't it?
If the cause is simply changed usage habits or an incorrect temperature setting, the problem will resolve itself by adjusting behaviour or pressing a simple button. However, all technical reasons — limescale, thermostat, diffuser, heating element, mixer tap — require physical intervention and usually a specialist's help, as they involve either electrical parts or opening the water heater's interior.
When Is It Time to Call a Specialist?
Some causes can be quite reliably identified at home, but the repair work itself usually requires the help of an experienced specialist, as a water heater is an electrical appliance connected to the water system.
- If you have ruled out temperature settings and mixer tap problems, but hot water still runs out quickly.
- If the water heater makes unusual noises during heating — this likely indicates heavy limescale accumulation.
- If you suspect a faulty thermostat or heating element — these require electrical testing and safe replacement.
- If the water heater is over 4-5 years old and has never been cleaned or maintained.
- If the problem occurred suddenly, not gradually — this points more to a specific breakdown than natural wear and tear.
How Can ToruFix Help?
We check the water heater's heating element and thermostat, assess the extent of limescale buildup, clean the tank if necessary, and also inspect mixer taps and the internal dip tube. If the appliance has reached the end of its lifespan, we can help you choose and install a new water heater that meets your actual hot water needs.
How to Prevent Hot Water Running Out Quickly in the Future?
Regular maintenance and a few simple habits can help keep your water heater working longer and avoid unpleasant surprises.
- Have the water heater's heating element and anode checked and cleaned every 1-2 years, depending on water hardness.
- Avoid setting the water heater temperature too low — keep it within the recommended range, considering both comfort and energy saving.
- Periodically check the technical condition of mixer taps, especially if they are over 8-10 years old.
- If household size or water usage changes significantly, assess whether the water heater's capacity is still sufficient.
- In hard water areas, install a water filter or softening system to slow down limescale formation.
Can Hot Water Running Out Quickly Indicate an Electrical Hazard?
Most causes of hot water running out quickly are technically straightforward and don't pose an immediate danger, but some signs warrant quicker attention, as they may indicate an electrical safety issue, not just a matter of convenience.
If the water heater casing feels warm or even hot externally, if the circuit breaker repeatedly trips during heating, or if you notice moisture around the water heater combined with either of the aforementioned symptoms, you should immediately switch off the water heater's power supply and contact a specialist, rather than waiting for the hot water to run out completely.
In such cases, it is no longer merely a convenience problem, but a possible insulation fault or leak between the heating element and the casing, requiring immediate electrical inspection.
- Unusual external heating of the water heater casing indicates a possible insulation problem.
- A repeatedly tripping circuit breaker during heating is not normal and requires immediate inspection.
- Moisture or water around the water heater combined with electrical signs is a reason to cut off the appliance's power immediately.
Do the Causes Differ Between Electric and Gas Water Heaters?
Most of the causes described in this guide — limescale, temperature settings, mixer tap problems, pipework heat loss — apply to both electric and gas water heaters, as they relate to water, the tank, and pipework, not the heating source itself.
However, with gas water heaters, there is an additional possibility that the problem lies with the burner settings, ignition automation, or gas pressure, affecting how quickly and how high the water heater can heat water. If the gas water heater's flame is uneven or the burner doesn't stay lit, it's worth checking this first, rather than looking for the fault in the pipework.
For electric water heaters, heating element and thermostat faults are typical, as these are the only parts responsible for generating heat. The diagnostic logic, however, is similar — first, simpler and cheaper checks, then involving a specialist for more complex parts.
Does Pipework and Its Insulation Affect How Long Hot Water Lasts?
The water heater itself is not the only factor affecting how hot water actually comes out of the tap — pipework length, diameter, and insulation also play a role, especially if the water heater and taps are far apart or the pipework runs through cold areas.
If there's a long, uninsulated pipe between the water heater and the tap, the water in it will cool down before reaching the tap, meaning the first few seconds or even minutes will deliver lukewarm water, even if there's plenty of hot water in the water heater itself. This is not a water heater fault, but heat loss in the pipework, and it can be alleviated by insulating the pipe.
In older apartment buildings in Tallinn and Harjumaa, where risers and branch pipes are long and often uninsulated, this effect is particularly noticeable during colder months.