If water from the cold tap but none at all from the hot tap then you could have a faulty valve or other supply issue with the line that feeds the water heater. If you are getting water and it's not hot, then you have a water heater problem.
It is usually one of two problems. One is that one of the cold water valves are close or a blockage in the cold line. Two would be a defective anti-scald or mixture valve in the faucet.
You Have A Faulty Water Heater
One possibility is that the heating element in your water heater has burned out. This can happen over time due to normal wear and tear or result from sediment buildup in the tank. Another potential issue could be a faulty thermostat, which regulates the water temperature inside the tank.
If it's winter and your hot water pipes are exposed to the cold, they might be frozen. In this case, you will need to safely thaw them out. If you suspect a blockage in your pipes, it might be best to call a professional to inspect and clear them.
Check the circuit breaker.
If you have an electric water heater, the first thing to check when experiencing hot water issues is your circuit breaker. A tripped circuit means the current was disrupted. As a safety measure, the circuit breaker flips.
As water travels through the pipes in your home, the temperature surrounding the pipes is generally cold in winter. Unless your pipes are insulated, the water coming into and exiting the water heater will be affected by the external cold temperatures, causing the heater to work harder.
There are several reasons why you might not have hot water, ranging from common problems, such as a problem with your water or power supply, boiler settings, thermostat, timer, recent power cut to more complex issues, such as boiler pressure, a potential airlock or frozen pipes.
If your cold water has frozen but water comes out of your hot water pipe, let the hot water run for 20 minutes. Sometimes this can help to thaw a frozen cold water pipe. If that doesn't work or your hot water pipe is frozen, use a hair dryer set on high heat.
Airlocks happen when air is trapped in the system. This air forms a blockage which gets in the way of the hot water as it makes its way to the taps or the central heating system. An airlock in the hot water system will prevent water from coming out of the affected tap.
Low Water Pressure: If the water pressure in your house is low, it can affect the performance of your water heater, leading to no hot water. Sediment Build-up: If you have an older water heater, sediment build-up can reduce its efficiency and cause no hot water.
Symptoms of a faulty diverter valve often include inconsistent hot water flow and radiators not heating properly. Homeowners may notice that radiators remain cold even when the boiler is on. Inconsistent water temperatures are common, with water switching from hot to lukewarm or cold unexpectedly.
When you use up all of the hot water in a traditional water heater, you will often need to wait anywhere from 30 minutes to two hours or more for the water in the tank to reheat. This is why many people opt to switch to tankless water heaters.
Air in the pipes
Pockets of air in your pipes can block hot water from passing through. In the case of boilers, clearing air from the system is often as simple as bleeding your radiators. Check the pressure of your boiler. Then, turn off your boiler and wait for your radiators to cool off completely.
Water heaters show several signs of going bad, including lack of hot water, temperature fluctuations, decreased water pressure, strange noises, rotten egg smell, visible leaks, rusty or discolored water, and increased energy bills. If your water heater is over 10 years old, it may be time to consider a replacement.
Possible Issue: One of the most common culprits for this predicament is a malfunctioning differential valve. This valve is responsible for controlling the flow of hot and cold water in your plumbing system. When it's not functioning correctly, it can lead to an imbalanced water supply, leaving you with cold water only.
Never pour hot water on frozen plumbing or try to thaw pipes with a blowtorch. The hot water usually freezes on the outside of the pipe, and the torch can cause frozen pipes to explode if steam is produced. Also, do not strike pipes in an attempt to break up the ice, as hammer blows can cause pipes to crack.
No water or a slow trickle when a faucet is used. Whistling and banging coming from pipes, or strange bubbling sounds when you flush a toilet. Dampness, puddles, or rings on drywall or ceilings—signs of an urgent issue.
Examine your boiler and hot water function
While there may be many reasons why your tap's hot water function isn't working, the most common problems are leaks, limescale build-up, and boiler pilot light malfunctions.
It may be due to a leak, a build-up of sediment or it could have become unplugged. Try to fix it by flushing the water heater. Allow it to heat up again and try using the faucet once more to see if the water comes out.
However, a tank water heater definitely can freeze if the power goes out and the unit stays off for more than a day or so. A tankless water heater will always be more at risk of freezing as it won't ever turn on and heat unless you're currently using hot water.
Sediment Build-Up
If you realize the water coming from your hot water heater is only lukewarm, it's likely due to sediment build-up somewhere in your unit.
Cold weather can freeze your pipes, causing overexpansion. This leads to burst pipes and forces your water heater to work harder than usual. In some instances, it results in smaller leaks that can go unchecked and cause major damage long term.