If your water heater has significantly less hot water 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; something in the water heater or its controls has changed.
Many residents in Tallinn and Harjumaa, upon noticing this issue, immediately think the water heater is simply old and needs replacing. However, in most cases, it's a specific, identifiable, and often repairable fault.
In this guide, we'll cover all the common reasons why a water heater runs out of hot water quickly, explain how to identify them at home, and provide advice on when it's sensible to call a professional.
Limescale on the heating element — the most common cause
Estonian tap water in many areas is quite hard, meaning it contains dissolved lime salts that, when heated, precipitate onto the surface of the heating element as a white, hard layer.
Limescale acts like an insulating layer — the heating element must deliver the same amount of heat through a thicker, less conductive layer, causing the water to take longer to heat up and the element to work continuously under greater strain.
If limescale has accumulated thickly, the water heater cannot heat the entire tank to the intended temperature within the expected time. As a result, a significantly smaller portion of the volume actually reaches the desired temperature — giving the impression 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 recovers more slowly than before.
How to suspect limescale build-up yourself?
If your water heater makes more noise than before during heating — clanking, bubbling, or hissing — it's often a sign that water is boiling beneath the limescale layer and in areas where heat transfer is uneven. Similarly, your water heater's meter or electricity bill might indicate 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 when the thermostat switches off the heating too early — for example, at 40-45 degrees instead of 60 degrees — making the tank appear full, but the actual water temperature is lower, and the hot water cools down faster when used at the tap.
Another scenario is a thermostat that doesn't react correctly at all, switching heating on and off inconsistently, leaving part of the tank unheated.
- Check if the water heater's set temperature (typically between 55-65 degrees) actually matches 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 faulty thermostat.
- Precise checking and, if necessary, replacement of the thermostat requires a specialist's help due to electrical safety.
Leaking or broken tap/valve mixes in cold water
If the internal mechanism of a bathroom or kitchen tap (ceramic cartridge or old-style washer tap) is worn, it can allow cold water to flow through even when the tap is fully on the hot setting.
This mixing is not visible, as the flow remains a single stream, but it lowers the actual outlet water temperature and creates the impression that the water heater has already run out of hot water.
A similar effect can be caused by a mixing valve (thermostatic mixer) installed near the water heater itself, which is designed to dilute excessively hot water but is incorrectly set or faulty, allowing too much cold water to be added.
How to check for a tap-related cause?
Connect the tap directly to the water heater outlet with measurable water or run water from another tap fed by the same water heater — if the water is hotter elsewhere, 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 intentionally lowered temperature
In some cases, the reason is simpler than it initially seems — the water heater's temperature regulator has been accidentally or intentionally turned down, for example, after cleaning, maintenance, or a child accidentally adjusting the knob.
A water heater with a lower setting heats water to a lower temperature, meaning that the same amount of hot water, when mixed with cold water, provides a pleasant shower for a shorter period, as the difference between hot and desired temperatures is smaller.
For energy-saving purposes, the water heater temperature is sometimes intentionally set lower, but this leads to precisely this side effect — there seems to be less hot water, even though the tank is full.
- Check the position of the water heater's temperature regulator and compare it with the manufacturer's instructions.
- The typical recommended setting for domestic use is 55-65 degrees — 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 water consumption
If the family has grown, washing habits have changed (e.g., longer showers, more frequent baths), or more hot water is used at once (washing machine, dishwasher, and shower simultaneously), the previously sufficient water heater capacity may become inadequate.
This is not a technical fault but a natural discrepancy between tank capacity and actual consumption — an 80-100 litre water heater that was suitable for two people may no longer suffice for a four-person family using it at the same pace.
If the problem has developed gradually with household growth or changes in habits, rather than suddenly, this cause should be considered first before looking for a technical fault.
- Measure how long hot water lasts during typical use and compare it with how many people and which hot water-consuming appliances are running simultaneously.
- If multiple 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 is a cold water inlet tube, usually with a diffuser at its end, which directs the incoming cold water to the bottom of the tank so it doesn't immediately mix with the hot water at the top.
If this tube or diffuser is damaged, broken, or dislodged — which can happen due to age, corrosion, or previous maintenance — the incoming cold water mixes immediately with the hot water in the upper part 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, as cold water reaches the hot water outlet too quickly.
Can this be checked by yourself?
The condition of the internal tube can only be checked by opening the water heater, which requires a specialist's help. If other simpler causes (limescale, thermostat, 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 heating elements, shared tank)
Some larger capacity water heaters (typically over 150 litres) have two heating elements installed, which work together or alternately to speed up heating and distribute the load.
If one of the two elements has burnt 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 allotted time, and the actual available hot water quantity will be less.
Such a fault is difficult to distinguish from slow heating caused by limescale at home, as the symptom (slower and incomplete heating) is very similar — a precise diagnosis requires electrical measurement.
How does this topic 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 unit and its regular maintenance schedule.
This article is intended for diagnosis — it helps determine WHY hot water runs out quickly before you decide whether a simple clean, part replacement, or a complete unit 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 precise information later.
- Check the water heater's temperature regulator position and ensure it hasn't been accidentally turned down.
- Measure how long hot water lasts during typical use and compare it with previous experience — was the change gradual or sudden?
- Test different taps in the house — if the problem only occurs in one place, it's likely a tap issue; if everywhere, then the water heater itself.
- Listen for any unusual noises the water heater makes during heating (bubbling, clanking, hissing) — this indicates limescale.
- Observe if household water usage has increased recently (new residents, more frequent shower or bath use, 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's 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 not?
If the cause is simply changed usage habits or an incorrect temperature setting, the problem can be resolved by adjusting behaviour or a simple knob adjustment. However, all technical causes — limescale, thermostat, diffuser, heating element, tap — require physical intervention and usually specialist assistance, as they involve either electrical parts or opening the water heater's interior.
When is it time to call a professional?
Some causes can be 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 also connected to the water system.
- If you've ruled out temperature settings and tap problems, but hot water still runs out quickly.
- If the water heater makes unusual noises during heating — this likely indicates heavy limescale build-up.
- 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 inspect the water heater's heating element and thermostat, assess the extent of limescale build-up, clean the tank if necessary, and also check taps and the internal dip tube. If the unit is at the end of its lifespan, we help select and install a new water heater that meets your actual water needs.
How to prevent hot water from running out quickly in the future?
Regular maintenance and a few simple habits can help keep your water heater in good working order longer and avoid unexpected surprises.
- Have the water heater's heating element and anode checked and cleaned every 1-2 years if necessary, 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 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 reasons for hot water running out quickly are technically straightforward and don't immediately endanger anyone, but some signs deserve quicker attention as they may indicate an electrical safety issue, not just a convenience problem.
If the water heater casing feels warm or even hot from the outside, if the fuse (circuit breaker) repeatedly trips during heating, or if you notice moisture around the water heater along with either of the aforementioned symptoms, you should immediately switch off the water heater's power and contact a specialist, rather than waiting until the hot water completely runs out.
With such signs, it's no longer merely a convenience problem but a potential insulation failure or leak between the heating element and casing, requiring immediate electrical inspection.
- Unusual warming of the water heater casing from the outside 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 disconnect power to the appliance immediately.
Do the causes differ between electric and gas water heaters?
Most of the causes described in this guide — limescale, temperature setting, tap problems, pipe heat loss — apply to both electric and gas water heaters, as they relate to the water, tank, and plumbing, not the heat source itself.
However, for gas water heaters, there is also the possibility that the problem lies with the burner setting, ignition automation, or gas pressure, which affects how quickly and how hot the water heater can heat water. If a gas water heater's flame is uneven or the burner doesn't stay lit, check this first rather than looking for the fault in the plumbing.
For electric water heaters, heating element and thermostat faults are typical, as these are the only parts responsible for generating heat. However, the diagnostic logic is similar — first the simpler and cheaper checks, then involving a specialist for more complex parts.
Does plumbing and its insulation affect hot water longevity?
The water heater itself is not the only link affecting how hot water actually comes out of the tap — the length, diameter, and insulation of the plumbing also play a role, especially if the water heater and taps are far apart or the plumbing runs through cold rooms.
If there is 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 though there is plenty of hot water in the water heater itself. This is not a water heater fault, but heat loss in the plumbing, 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, such an effect is particularly noticeable during colder months.