Water Heater Not Filling Up? Here’s 4 Likely Reasons & Fixes

Hot water is a modern luxury that has become essential to many household appliances, such as dishwashers and washing machines. There’s also nothing quite like a relaxing hot shower or bath. However, if you find your water heater isn’t filling up, it could significantly diminish the amount of hot water you have available in your home.

The most common causes of a water heater not filling up include leaks, air or pressure lock within the tank, continual filling and draining of the tank’s water supply, or a clogged cold feed pipe or valve. 

Read on to learn more about the common causes why a water heater isn’t filling up, along with some troubleshooting tactics so you can solve this issue yourself without spending hundreds of dollars on assistance from a plumbing service. Best of all, you can reduce wasted energy costs!

How to Tell Your Water Heater Isn’t Filling Fully

Most individuals don’t go peaking into their water heater tanks regularly to determine if it is filling up properly, especially if they aren’t familiar with the equipment.

Although this is the easiest way to determine whether your water heater has an issue filling up, many homeowners might not recognize the signs that there is even an issue or that their water heater is to blame. 

Here is a list of common issues that might indicate your water heater isn’t filling up and needs to be fixed. 

• Puddles of water around your water heater or outside near the tank
• Low hot water pressure
• Uncharacteristically limited hot water in your home
• Uncommon noises coming from your water heater, such as creaking, rattling, banging, knocking, or whining
• Uncommon lack of noise coming from your water heater (you can typically hear when your water heater is filling and functioning correctly. If you find you don’t hear this as often as you used to, it might indicate the tank isn’t filling all the way).
• Little to no hot water to bathtubs and sinks.

If you are experiencing some of these issues and you’ve determined that your water heater isn’t filling up, but you don’t know why to refer to the common reasons below to find the culprit and troubleshoot the issue quickly.

Reasons a Water Heater Won’t Fill Up

Fixing a water heater to fill correctly can be a simple solution you remedy yourself or require professional help. Sometimes, the underlying issue is so severe it might warrant replacing your entire water heater tank, especially if the equipment is 10+ years old.

Of course, the first step in resolving this issue is to find its cause.

Here are some of the most common reasons for your water heater not filling up.

Check for Any Leaks

Arguably the most common cause of your water heater refusing to fill is a leak somewhere in the tank or its connected pipes. First, check around your tank for any puddles or excessive liquid.

This would indicate that there is likely a leak somewhere in the bottom of the water heater tank (source).

If you don’t see any liquid here, check the walls near your water heater. Not all water heaters will have a nearby wall, but many are tucked away in a corner in basements or closets.

If you notice the adjacent walls are wet, then a pipe might be leaking towards the top of the water heater.

After checking these two areas and determining there is no leakage inside near the tank, take a trip outside before deciding a leak isn’t the culprit.

Some water heaters will have connected pipes that go underground. If this pipe has been damaged or leaks, you might find flooding or pools of water outside your home in the vicinity of your water heater. 

Resolution

The degree of troubleshooting with a leak will vary depending on what equipment is leaking and why. If an underground pipe is leaking, it is unlikely you can remedy this yourself. However, if you discover an exposed pipe or valve is leaking water, this can be a quick fix. 

To determine this, dry off your water tank’s exterior, have someone turn on a faucet somewhere in the house, and see if you can find what piece is causing the leak. If this piece is accessible, tighten it yourself. 

Check for An Airlock

An airlock occurs when a pocket of air is trapped within the pipes of your water heater system and prevents the tank from filling or hot water from traveling through your pipes to faucets throughout the house. 

The best way to determine if you have an airlock somewhere in your water heater is to go to one of the faucets in your home and turn on the hot water. You are more likely to see the airlock with hot water than cold because the hot water requires more pressure.

If you notice that the water coming out is cold or lukewarm rather than hot, you most likely have an airlock. Another sign would be if the water flow is significantly reduced and comes in small streams or even drips rather than a constant heavy flow when turned on fully. 

Resolution

There are two quick ways you could try to fix an airlock yourself. The first does not require any equipment, just your hands.

1. Check the hot and cold water valves on your water heater to make sure they are both on. 
2. Go to any faucet in your home. Tightly place the palm of your hand over the faucet opening (this is to prevent water from flowing out of the faucet in later steps).
3. Keep your hand tightly over the faucet and completely open your hot water valve.
4. Leave the hot water valve open and completely turn on the cold water.
5. Keep a tight hold on the faucet and ensure no water comes out. Hold this position for about 15 seconds.
6. Turn off your cold water after 15 seconds and release your hold on the faucet. The hot water should now be running at the appropriate pressure and temperature. 

Alternatively, you could try this method using a 1-foot-long garden hose and some duct tape. 

1. Check the hot and cold water valves on your water heater to make sure they are both on. 
2. Place one end of the garden hose around your hot water faucet end and the other around your cold water faucet end. 
3. Fasten the hose to the faucets tightly using the duct tape. When finished, the garden hose should connect the two taps so water can flow between them and not into your sink.
4. Open your hot water valve completely, then completely open your cold water valve.
5. Leave both valves open for 5 seconds.
6. Turn both valves off and go to a separate faucet in your home.
7. Turn on the hot water at this separate faucet to determine if the airlock has been removed.
8. Remove the duct tape and garden hose if the issue has been resolved. Repeat steps 4-7 two to three more times if the problem persists. 
9. Contact help if the issue remains after attempting this method three times. 

Another potential issue that could be inhibiting your water heater’s ability to fill is a pressure lock. Try opening the Temperature Pressure Relief valve (TPR valve) To remedy this. If you notice the tank filling up with you hold the TPR valve open, then a pressure lock is the culprit. 

Check if the Water Heater is Filling and Draining Continuously 

Your water heater might not be filling all the way because it is potentially filling and draining continuously, and therefore, maintaining the same water level. A key indicator of this is if you hear water constantly running in your water heater tank, even when connected household appliances and faucets aren’t in use. 

Resolution

The most likely explanation for this occurrence is if the TPR valve has been opened and somehow stuck in this open position. This would cause water to drain from the tank, and so, to prevent the tank from emptying, the system tries to continually resupply this draining water. 

A stuck TPR valve will most likely occur with an older water heater tank. You can most likely close the valve using simple tools. You could also replace this valve if it is damaged or too old to function properly.

Another, much simpler reason your water heater might be endlessly filling and draining would be if a faucet has been left on somewhere in the home. This is most likely a faucet that is rarely used and has stayed on unnoticed. The issue should be resolved once you shut the faucet off. 

Check if the Pipes are Clogged

It can be hard to tell if the pipes in your water heater system are clogged just by glancing around the tank. Unfortunately, the best way to determine if this is the underlying issue would be to empty your tank and check the bottom. If you see any excessive sediment at the bottom of your tank, then it is likely more of that sediment is actually clogging your feed pipe or some valves, effectively preventing water from filling or draining from the tank.

Resolution

In addition to draining your water heater, you could remove any clogs caused by debris or sediment by backflushing your water heater. Here’s how:

1. Turn off the power to your electric water heater at the circuit breaker. For gas models, turn the unit’s thermostat to “pilot position.”
2. Connect one end of a garden hose to the drain valve located at the bottom of your water heater tank. 
3. Place the other end of the garden hose into a bucket that can hold at least five gallons, into a floor drain, or somewhere safe outdoors. 
4. Turn off any recirculation pump your system might have (not included in all systems).
5.  Shut off the supply valve for the cold water inlet. 
6. Go to the sink faucet closest to your water heater and turn the hot water on.
7. Go to the sink faucet furthers from your water heater (opposite end of the home or on the highest level) and turn the hot water on. 
8. Lift the TPR valve lever at the top of the tank, so it is open.
9. Open the drain valve and drain approximately one to two gallons of water from the tank. CAUTION: water will be hot.
10. Continue to drain water until you no longer see sediment in the stream.
11. Close the drain valve and remove the garden hose.
12. Close the TPR valve at the top of the tank.
13. Open the valve for the cold water inlet. 
14. Wait for a steady stream to flow from the two faucets you opened throughout the house. When this occurs, shut them both off. 
15. Turn your water heater system back on.

If your pipes are still clogged after backflushing the system, you will need to contact a professional for additional assistance (source).

Final Thoughts

A water heater that doesn’t fill properly can be extremely inconvenient to home life, especially if you enjoy the relaxing qualities and health benefits of hot water. Luckily, you can remedy many of these common causes without professional help. 

However, if your water heater is extremely old and has damaged or clogged parts, you will likely want to invest in professional assistance rather than trying to fix the issue yourself. 

Related Reading: Can A Tankless Water Heater Be Installed Outside?