The 7 Most Common Tankless Water Heater Problems; And How To Fix Them

You’ve probably already heard many of the benefits of tankless water heaters; the reasons for getting one in the first place. Things like saving significantly on your energy bill, helping the environment and never missing out on hot water when you need it.

But what about when things don’t go according to plan?

Tankless water heaters are certainly growing in popularity, but they aren’t perfect–at least, they’re not indestructible. This post will run you through some of the issues that may arise with your tankless water heater, how to solve them, and some tips for making sure it doesn’t happen again.

These are the 7 most common tankless water heater problems:

  1. No ignition message
  2. Mineral Build up
  3. Flame failure message
  4. Hot-cold-hot dilemma
  5. System Overload
  6. Air Supply/Exhaust Blockage
  7. Minimum flow rate

Let’s find out what went wrong, and how to fix it.

My tankless water heater won’t ignite

Ignition problems are a hassle. Depending on your model of tankless water heater, it should display a message which reads something like, ‘no ignition’. There are a few reasons this might happen.

The first culprit is your gas supply, so naturally before going any further you’ll want to make sure that you haven’t simply run out gas: Check that your propane tank is full.

Your ignition may also fail if your gas or water valves aren’t fully opened.

The Solution

After checking your propane tank, gas and water valves, it’s possible that your ignition pack has failed (or there might even be some bigger problem lurking behind the scenes). In most cases, the first three checking steps will catch your ignition problem–if you’re still stuck after that, it will be necessary to call technical support or a plumber to discuss the problem and possibly order replacement parts.

My tankless water heater is growing something…

99% of the time, this is nothing we can’t fix: a mixture of calcium and magnesium that causes a scaly build-up in your tankless unit. If something else is growing in your tankless water heater, I’m afraid I can’t help you there.

But seriously, mineral build-up is something to keep an eye on. All water heating systems will be exposed to large amounts of water on a daily basis. Depending on your area and local water supply, this can range from soft to hard water (or anything between); where ‘hard water’ describes water sources that are high in minerals, and tend to feel a bit sharper on your skin. The harder the water, the more likely your tankless water unit is to suffer from some significant mineral build-up.

Mineral build-up can impair the functioning of your tankless unit, blocking the valves or otherwise making the components defunct.

The Solution

Again, it’s nothing we can’t fix. The good thing is, mineral build-up is one of the issues you can solve yourself, so long as you stay on top of it. I would suggest cleaning or flushing your unit every six months with a descaling cleanint kit. Many water heater brands will offer their own specific descaler, but in my experience, they work perfectly fine across brands.

Flame Failure message

This is rather similar to your ignition failure problem. Other causes that are more specific to flame failure issues are a lack of ventilation, a gas line that is too narrow or small to produce a consistent flame, or regulator failures.

The Solution

Again, be sure to cover the basics before contacting support. Make sure your propane tank is full (or that your gas bill has been paid this month!). Otherwise, check to see that the venting in your tankless unit hasn’t been blocked or stopped off (which can kill the flame). If these steps fail, then it may be time to reach out for help.

My shower goes from hot to cold to hot again

This is most common in family households–maybe you’ve noticed that it tends to happen when you shower right after another family member or spouse? There’s a simple explanation for it, and it is something of a downfall in many tankless water heating units.

For those of you who don’t know what I’m talking about, it’s called the ‘cold water sandwich’ in water heater circles. Say a family member has just finished in the shower, and now you’re up. When you turn the shower on, you’ll be met with warm/hot water immediately (this is great, no waiting time), only to be followed by six or seven seconds of cold water again.

What’s going on here?

Well, it’s pretty simple really. Whoever showers first will need to put in the initial waiting time as the water in the unit heats up. Then, once it’s up and running, they will shower to their heart’s content and turn off the shower while the hot water is still blasting. This leaves some remaining hot water in your heating unit. So when you come along and turn on the shower, you will be hit with this remaining hot water, but if the unit has switched off then the heating process will need to start over… this means cold water will run through the unit once again, and you’ll be faced with the same initial waiting period that always happens.

The Solution

While tankless water heaters are much faster at heating water than tank models, they aren’t actually instantaneous. Hate to break it to you, but the six to seven seconds of cold water isn’t a flaw–it’s just part of how the system works, and there’s no ‘fixing it’.

The solution, then, is to wait for the initial gush of warm water and cold water to pass by before hopping into the shower (unless you enjoy the brief cold rush!).

My tankless water heater shuts down when too many taps are running

This is also known as a system overload. Every tankless water heater will have a capacity–when it exceeds this figure, it may not be able to handle the load. This could happen when you are running multiple showers at once; using hot water through the kitchen tap while running a bath in the other room; or maybe just running two taps at once is enough for your unit to freak out. It can vary from your heating unit shutting down entirely, to simply not being able to produce enough hot water to satisfy all the necessary outlets.

The Solution

Depending on the severity of the issue, you can fix this by either:

  1. Limiting simultaneous use of showers/taps/water outlets;
  2. Upgrading your tankless water heater’s capacity; or
  3. Invest in a second heating unit.

If you are noticing this problem only on rare occasions, then the first solution will be enough to fix the issue on its own. Simply be aware of running more than one shower at the same time, and be sure to let your spouse know if you’re running hot water in the kitchen while they’re taking a shower!

For more severe cases, you may be able to cover the load by upgrading your existing unit. Many modern tankless water heaters are able to provide ready-hot water for an entire household these days. But as another solution, you might simply invest in a second unit. This sounds a bit extreme (I thought the point of tankless water heaters was to save time and money), but if you follow the logic through you’ll find it is still worth your effort.

By continuing with a water heater that doesn’t handle your household’s water needs, you will be frequently wasting water and gas as you wait for the unit to produce hot water where it’s needed. By installing a second unit, you will only use it when needed–it’s not a constant cost, just a one-time purchase which will save you in the long run.

My tankless water heater’s air supply or exhaust is blocked

You may have received a message on your unit telling you that the air supply is blocked–this may not have been very useful to you, though! When the error code is displayed, the machine is telling you that it’s having problems with combustion air or venting.

The Solution

The first thing to do is to check for obvious hazards or obstructions that might be blocking your vents. This could be something from your own household (a piece of laundry that found its way jammed in behind the unit) or something foreign like a wasps’ or rodents’ nest. Yuck, I know.

Next, you’ll want to check your vent pipes; make sure they haven’t come loose, are connected properly to the unit and that they are free from punctures or holes. If you find they are punctured, you can try taping them up, but I would suggest ordering replacements.

Lastly, you should double check with your product manual to be sure that it has been installed properly. If your tankless heating unit has been installed too close to other objects, this can cause unwanted blockage and the problem won’t go away–even if you fix everything else.

The hot water shuts off in the middle of my shower

If you’ve ever experienced this with your tankless water heater, I feel your frustration. It’s a painful moment to hear your heater switch off mid-shower, then feel the rush of cold water take hold.

This is most common in older models of tankless water heaters–designs that were made ten or more years ago. It has to do with the minimum flow rate of your water heater being too high. Minimum flow rates describe how much water needs to be traveling through your tankless water heater in order for it to actually work; for it to receive the ‘heating signal’.

This problem is perhaps most likely to occur when you have an old school tankless unit combined with a newly installed, eco-friendly showerhead. These showerheads are great for saving water and helping the environment, but they may not be using enough water for your tankless unit to fire up. As a safety precaution, tankless water heaters shut off when there isn’t enough water running through the unit. Without enough cold water running through the unit, there is a big risk of overheating–which is why manufacturers tend to add in this safety precaution.

Useful for not burning down the house–not so useful when you’re just trying to warm up in the shower.

The Solution

There are two ways to approach this problem, but sadly (unlike some of the others on this list) there is no simple solution.

The first pathway is to change your showerhead to something with a more substantial water flow. However, this would of course defeat the purpose of having installed an eco-friendly shower head in the first place. I mention this solution just in case your showerhead isn’t particularly eco-friendly, it’s just weak. Replacing a weak shower head with a stronger one may be enough to get more consistent water flowing through your tankless unit–so it won’t need to turn itself off.

Pathway number two requires professional help. Ask a plumber to upgrade your unit, so that the minimum flow rate meets modern standards. Old units typically come in at ½ to ¾ gallons per minute (gpm) for minimum flow, while newer designs allow you to go as low as ¼ gpm.

Summing Up

Many of the most common problems can be cut short in the early phases. Things like mineral build-up can be prevented with regular maintenance, while other common breakdowns can be fixed without calling in the plumber.

If you do meet a dead end, however, don’t stress. Talk with a support crew and try to pinpoint the exact tankless water heater problem with their help–it’ll be much cheaper to find the precise issue and replace the problematic part than it will be to call in the plumber or repair service.

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