Find out why a Tankless Water Heater needs a Filter

It's a common misconception that because a tankless water heater doesn't store a large volume of water, it doesn't need a filter, and that is not true. If you live in a hard water area or are pulling your supply from a well, it's even more critical. The bottom line is your tankless water heater needs a filter.

Like all homes, the incoming water supply should be safe to drink, but it still contains microscopic particles, such as mineral deposits, that can cause issues with your tankless water heater. These circumstances make a water filtration system essential.

The intense heat generated by a tankless heater means that those particles can fuse onto the heat exchangers inside the unit and build up, just like plaque on your teeth. As the layer thickens, it acts as a barrier between the heater and the water, so it has to work harder to do its job correctly. Or, the sensitive elements can just fail.

Increased energy costs are the result, which is bad enough; however, over time, the buildup of sediment will significantly shorten the life of your heater and make it less efficient.

If you notice your hot water isn't as hot as usual or there is a reduced flow, it could indicate a problem with sediment buildup in your tankless heater. It's time to investigate.

Tankless water heaterTankless water heater

What can I do if my water is cooler than it should be?

Here are four quick checks you can carry out to establish what's going on and what you can do to remedy it before calling a plumber.

Check the thermostat

Check your water heater thermostat. It could be a little low, and it just needs a tweak upwards, particularly if the outside temperature has dropped. The setting may be right for warmer months but too low to deal with cold water temperatures in the winter. Coldwater varies in temperature over the year, so it could be several degrees lower in the winter as it enters your heater.

Check the water screen

Check the water screen on the cold water supply side of your heater and clean it monthly. These are easily accessed and removed for simple cleaning. The little filters catch a lot of debris and can be easily blocked, which will cause low flow into your water heater and, in turn, restrict the delivery of hot water to your faucets.

Check the plumbing

Check the pipes coming from the water heater. If the hot water supply pipe is leaking, there will be lower pressure and a reduced amount of hot water. Check the route of the hot water supply pipe and look for visible leaks and puddles, or if you hear dripping, you need to identify where the leak is and fix it. It's essential to prevent the loss of expensive hot water.

Check for discolored water

If your hot water is discolored, there may be a buildup of residue inside the water heater on the heat exchanger, which will lower the heater's efficiency. Check, and if you see limescale, that will prevent your water from heating. Flush the heater out with clean water and household white vinegar to clear the problem.

What causes tankless heater issues?

 Hard water

Suspended minerals in the water supply are the leading cause of hard water. The usual suspects are magnesium, lime, and calcium. They can aggravate sensitive skin and leave streaks on your tiles and glassware.


Anything that handles running water in your home can be affected by a buildup of scale. The scale is a combination of lime or calcium, and it takes only a small amount to affect the efficiency of your heater. An effective way of dealing with scale is a filter which will control the amount of scale getting inside your heater and system.


Sediment is the problem that can wreck a water heater. It comes in via your local authority water supply, but it is far more widespread with well water. As tankless water heaters don't store water, any sediment coming in travels through as you draw water. The sedimentation on the vital and sensitive elements can damage the heater and heat exchangers and may even block your drains.

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What can I do?

If your local water supply is hard or derived from a well, here are two possible solutions:

  1. Fit an inline filter to catch sediment before it gets inside your heater or,
  2. Fit a water softener on the supply side of your system.


It's a relatively easy task to fit a simple inline cartridge filter to your incoming water supply and your tankless water heater. The cartridge will remove most of the minerals coming in.

The cost will depend on the style of filter you buy, and fitting is straightforward by any experienced DIYer. It is essential to ensure the filter is capable of delivering the correct gallons per minute (GPM) volume of water, or your heater won't work. The cartridges should be checked regularly and changed out when they are dirty.

Water softener

A water softener is a more expensive project but will not only supply soft water to your water heater but your whole home. The ion exchange system involves two tanks – one filled with resin beads that draw the contaminants out of the water supply and hold them, delivering clean, soft water throughout your home.

The system has a second tank where salt crystals mix with water and backflush the resin beads to remove the contaminants they have trapped. They are flushed away down your wastewater drain. The resin beads are washed again with clean water, and the job complete. The system runs automatically; all you have to do is check the salt level regularly and top up as needed.

A water softener system will deliver soft clean water to your home and will prevent all of the issues that go along with hard water and ruin your appliances.

Regular maintenance

Regular maintenance of your tankless heater will ensure you avoid serious problems that can shorten its life and cost you more to run.

Check the water filter screen on the incoming side of your heater and clean it monthly. These little filters catch a lot of debris and can block up quickly, which will cause low water flow into your heater and restrict the delivery of hot water to your faucets.

An inline filter fitted to your water supply will prevent the small screens from blocking up every couple of weeks, especially with well water. The only maintenance an inline filter needs is every couple of months. Check and change the filter cartridge if it's dirty.

Check the heat exchanger, and if it appears contaminated with sediment, it needs to be cleaned and de-scaled. If it builds up, it will take the burner longer to heat your water, which will cost you more in energy and shorten the life of your heater, and they are not cheap to replace.

It would help if you flush your tankless water heater at least once a year for around an hour with water and white vinegar. All of these basic procedures will increase the lifespan of your heater. Cleaner water coming through the heater will also help it work more efficiently and save you money.

Check your pressure relief valve once a month to ensure it's working correctly. If it becomes blocked with sediment, it could stop it from functioning in an emergency, which is potentially dangerous.

It's an excellent plan to have the unit (mainly gas), professionally checked to ensure it's running efficiently; however, flushing is a job you can do yourself and save money.


Your tankless water heater does need a filter. The choice is yours, of course, however, if you are using well water as your supply, an inline filter at least is essential, and a full water softener system would be best, but it depends on your budget. A cheap inline filter will go a long way to saving your tankless water heater from an early failure.

A water softener system will deliver clean, soft, and contaminant-free water to your home. It will lengthen the life of your tankless water heater and every other appliance in your home that uses water; plus, it's good for your family's health.

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