Tankless Water Heaters 101
Basic Tankless 101
A tutorial for the homeowner and the professional
The Basics of Tankless Water Heaters
Tankless water heaters have been sweeping the nation with a huge growth over the last 5 years or so. These are water heaters that do not have a tank. Many of the larger tankless companies have seen their sales grow by hundreds of percent each year! These compact units mount on a wall either inside or even outside the house and supply hot water on demand literally without end! The Europeans have been heating water with tankless heaters for years. Many people believe that within the next 5 to 10 years 50% or more of all American homes will have a tankless water heater in them! Even the big tank water heater companies have tried to jump on board by partnering with larger Japanese companies to have units private labeled for them (more on this later).
Tankless water heaters are available in electric, natural gas and propane fired models. The electric tankless water heaters have advantages over tank type electric models but very few provide enough capacity to serve multiple fixtures with only one unit and may require a larger electrical service to operate them. This has kept most builders from using them in new construction. Although they do tend work well in small home, condo or apartment applications where gas is not an option. For the purpose of this tutorial we are going to talk about the gas fired units. Watch for a future article containing information on electric tankless heaters.
Tankless water heaters work on demand by using sensors and computer boards to monitor the flow of water and change the rate of firing to supply just the amount of hot water required for the current demand. (They are also called on demand heaters) This means that they burn less gas to supply hot water to something like a sink than they would if you are using multiple fixtures at the same time. This modulating firing rate also makes them very efficient to operate as you are only using the exact amount of fuel needed at that time.
A term that should be avoided is "instantaneous". Tankless water heaters are not instantaneous. It does take them about 2 seconds to go from their at rest "off" mode to producing hot water at the set point temperature. This is not a big issue however. The problem is if a consumer thinks by hearing the term "instantaneous" that they will get water at every outlet in the house instantly if they get a tankless heater, they will be disappointed. Most homes have many feet of piping between the water heater and the outlets and do not have a recirculating system. The amount of time it takes from when a faucet or other hot water fixture opens to when the set point water gets to that point is called "Lag Time". In today's large homes with low flow fixtures it is not uncommon to see a lag time of over 3 minutes to get hot water to remote fixtures in a home. Changing the type of water heater will not improve the speed of the delivery of water unless the location of the heater is altered or if a recirculating system is installed. Because of their small size of course, many times when a tankless heater replaces a tank, it can be moved to a more central location or nearer to the fixtures it is to serve. This may cut down on the lag time considerably
Tankless water heaters save space in a home because they take up NO floor space. They also do not require protection from vehicles if installed in a garage and are so small they can be installed in a crawl space or attic as well. If you really need space, many can be installed outdoors giving you all of your interior space back. Just be sure to choose a model designed for outdoor installation and with freeze protection for your area. (More on this later)
Energy Savings of Tankless Water Heaters
Tankless water heaters save fuel because they do not have to maintain a supply of hot water in a tank and are typically "always off". Tank type heaters fire on and off all the time to maintain the temperature of water in the tank within about 10* of the thermostat setting. (This is called "Stand-by heat loss") This also can result in some noticeable temperature difference. Tankless water heaters provide hot water to the set point temperature plus or minus 2*.
Another thing that makes a tankless hot water heater more fuel-efficient is that they are "fully modulating". In other words they only use the fuel needed to heat the water to the set point at the current flow rate. If you are washing your hands using under 1 gallon per minute (GPM) you will be at a lower firing rate than you would be if you are filling your tub at 3 GPM. This works much like your car. When you are sitting still the car is idling. When you want to go or go faster, you give it more gas and when you get where you are going you turn it off. With their "always off" condition and their modulating capabilities it is common to see up to a 50% reduction in fuel use when changing from a tank type heater to a tankless unit.
If you are going to change from a 50-gallon gas heater to a tankless you are probably not going to realize quite that much energy savings. In fact a 50-gallon tank water heater does not use much more fuel at all compared to most tankless heaters. However a 50-gallon tank heater only can really give you about 40 gallons of hot water per use before you begin running out of hot water. The tankless heater will deliver more than 300 gallons per hour for most of the year and you can never run out! If it is an electric tank water that you are replacing, your savings may be higher than 50% depending on the size of the tank. If you really want to know what your savings may be, look for the yellow "Energy Guide" sticker on your existing heater and look for the one on the tankless unit you are considering. This will give you a good idea of what to expect. Of course your personal use will effect this as well. If you have a family of 6 that has never had enough hot water with a 50 gallon tank heater, your bill might just go up because now your family will not be taking cold showers or have to shorten them. If you have a vacation home that is occupied only on weekends or using the tankless for something like a school locker room, your savings will be greater because the tankless unit is "always off" eliminating a lot more stand-by heat losses.
Part of the decision making process is; What do you want? Endless hot water may be worth the additional investment to you even without an energy payback.
Location of Tankless Water Heaters
Builders like the tankless water heaters for several reasons, not the least of which is space savings. When you charge by the square foot for a home, saving space means that home is worth more. A tank type heater installed in garage requires a floor stand, a pipe to protect against vehicle impact and normally venting all the way to the roof. In a two-story home, this means more framing, drywall and paint to enclose it. A tankless water heater is wall mounted and can be sidewall vented, keeping the cost of venting to a minimum. It also does not take up any of the garage floor space. Being able to install tankless heaters outside or build them into a wall gives even more options that the builder never had before. This means even less venting cost (practically none) and even more space savings. Some builders will locate them centrally in a crawl space to cut down on lag time. Others will locate them near a master bath or kitchen. Some will install them in attics or outside to free up more space. Since there is no tank to burst, installing a tankless heater in an attic is not as risky as installing a tank there. Even with a drain pan, a tank water heater in an attic is a catastrophe waiting to happen! A drain pan that is 3" deep will not do much good if the bottom blows out of a 50-gallon or larger tank water heater. (This is not an uncommon occurrence!)
On a large home, some builders will have two tankless water heaters systems. One will serve one part of the home and the other will serve the rest. This way the heaters can be located closer to the points of use and cut down on lag time without installing a recirculating system.
Outdoor Installation of Tankless Water Heaters
Many of the better tankless companies have models that can be installed outdoors. This frees up all of the interior space and does away with venting costs or combustion air issues. These units will have their own freeze prevention systems however you will need to protect your water piping from freezing. This can be done with a pipe cover kit or recessed wall box, which can be insulated. It is also recommended that you install self-regulating heating cable on the piping to keep it above freezing. Outdoor units require power at all times to operate their freeze prevention system. In the event of a power outage in freezing weather, you will need to prevent you tankless from freezing by leaving a faucet dripping or draining the unit until the power comes back on. A "back-up" power supply or generator is also an option.
Performance of Tankless Water Heaters
Another reason that builders like tankless water heaters is that they are able to provide hot water to todays popular large tubs. A standard bathtub holds about 35 gallons to the overflow. The popular soaking tubs hold anywhere from 45 gallons to over 80 and just filling up the tub leaves most without any hot water with a tank type heater for a period of time. A tankless can fill all the tubs of a home and then provide back to back showers, do the dishes, and wash the clothes. A tank type heater has to be very large to do all of these things without running out. We are only limited to the flow rate our tankless unit can provide. Choosing a tankless heater with the proper capacity for our house makes it possible to handle multiple hot water needs at once normally, without the worry of running out of hot water.
Until now, most people made "water rules" to determine who showers when, or when they could do the clothes or dishes. This goes away with a tankless water heater.
Some people mistakenly think that they will only be able to run one fixture at a time with tankless heater. While this may be true of the "Home Center" models, this is far from accurate when speaking about the professional grade heaters from Noritz, Rinnai and Takagi. These models have the capacity to operate 3 showers or more at the same time! Some will correctly claim that tankless water heaters limit the flow rate to make sure you get the setpoint temperature and say that this means you will not be able to do multiple things within the home using hot water. This is simply not true. Choosing the right unit is important and will be discussed a little later, but making this claim is like comparing all tank water heaters to the old 30-gallon tank heaters that would run out after every use. Today's tankless water heaters provide more than enough capacity to meet any hot water need from a one-bathroom house to a hotel. You just need to choose the correct system for your application just like any other hot water system.
Here is something else to consider when choosing your tankless heater. Asking it to operate three showers, the kitchen sink, the washing machine, the dishwasher and a laundry sink at the same time is not only unrealistic....your water pipes can't carry that much water! Most homes only have a 3/4" hot water main and most are now in PEX or CPVC materials. These piping system can not carry more than about 8 - 10 GPM total, including cold water. Also, many new homes typically see less than 2 GPM at a shower head due to pressure looses in the piping. In other words if you choose a tankless heater that can deliver between 6-8GPM in the warm months and 4+ GPM in the winter months you will be quite happy in a typical 3-1/2 bath or less home. You should avoid the tendency of some to oversize a tankless system based on unrealistic system demands. If in doubt, contact the manufacture for help.
Capacities of these water heaters have improved greatly over the first tankless models that showed up about 10 years ago in the US. The largest of these tankless companies, Noritz, has eight models currently available in the US that range from 6.3 GPM to the largest output model available in the world at 13.2GPM. Rennai models produce the same flow rates as the residential Noritz heaters and Takagi also has units in this range. Noritz even has commercial grade models that can produce up to 13.2 gallons per minute (752 gallons per hour from one unit!). Most of these products can be installed in multi-unit installations for high flow rate demands like luxury homes, large shower system with body sprays, locker rooms or hotels.
Many tankless water heaters are also installed with a remote control unit that makes it easy to change the set point temperature of the unit. One manufacturer, Noritz, has a standard remote that lets you set an alarm to the capacity of your tub. You then set the temperature you would like and fill with just the hot water. When the unit measures the gallons set an alarm sounds to remind you shut off the water. Another manufacturer, Rennai, is able to provide multiple remotes to serve the same heater to provide for multiple locations to change the temperature of the hot water if you like. (You still only get one water temperature at a time, you just have more location from which to choose the temperature.) These digital remote control panels also provide diagnostics for the water heater in the event that there is a problem. They flash a fault code to help service personnel find and fix any problems that may come up quickly.
The better tankless heaters by Noritz, Rennai and Takagi totally control outlet temperature so they can NOT be "overshot" giving you less than the set point temperature. You always get setpoint temperature plus or minus 1 or 2 * (Unlike a tank which is + or - about 10*) Electric units and home center models do not have this ability. I would advise avoiding the home center models completely as they lack the technology and BTU's to give you good performance. In units that can not control their outlet flow, you will need to "throttle" the flow rate yourself at the outlet. This also means that if you are taking a shower and someone else turns on another fixture needing hot water, you may get a big surprise as the water temperature drops considerably in your shower!
Serviceability of Tankless Water Heaters
The better tankless heaters are very easy to work on. (I say the better ones because I have not had to work on a home center unit yet) Like anything else, training is preferred but any good contractor with a cell phone and a Philips screw driver should be able to take care of a problem using the tech support lines of the big 3 listed above. A manometer (Device used to measure gas pressure) is also a very handy tool to have.
How long do they last? About 20 years on average. Compare that to the 12-year average of tank heaters. This also contributes to the "total value" of the product since this means lower cost of ownership when factoring in replacement cost and life span of the product. Add in the long-term energy savings and your tankless heater just might pay for itself and in some cases it's replacement!
Keep in mind however that installing these units outside in cold climates if there were a power outage, there is no freeze protection to these units until power is restored. You should protect the unit as detailed above. Most homeowner's policies do not cover the water heaters themselves so it is a good idea to provide a generator in these installations or just put the unit indoors.
Choosing The Right Sized Tankless Water Heater Unit
The first thing we need to do is establish the peak hot water demand for the job. In a home this is usually the number of showerheads, X the flow rate X 80%. Example: 3 showers @ 2.5GPM each = 7.5GPM X 80% = 6GPM peak demand.
You want to choose a tankless unit that can meet or at least get close to this demand during the warm half of the year. (Remember it is VERY uncommon to have a demand like this actually happen in a home, plus we are not going to install a unit that can not control the flow rate anyway.) If you are within 1 GPM of this rate you will be happy. It is not possible for most people, even plumbers to tell the difference between 1 GPM and 3 GPM without a direct caparison next to it.
Considerations for Professionals
Please talk to your local wholesaler about attending a training class on these products. They must be installed properly to work well and installing them improperly may result in damage to the tankless heater, poor performance, premature failure or injury to the homeowner.
What about the tank manufacturers?
Good question. In the United States, over 9.5 MILLION tank type water heaters are sold every year! About half of them are electric. That is a huge market. Every time a tankless company sells a tankless water heater, the tank companies loose a little bit of their market share. (They don't sell a tank) It did not take very long for this to get the attention of the big tank companies.
These companies quickly looked into the market. They had some serious questions to ask and decisions to make. The questions would be along these lines: Is this a fad that will go away? Do these things really do what they say they can do? How long have these products been in us in other parts of the world and what is their track record? How long will i t take us to produce a market-viable unit?
These great companies are full of very smart people. They got their answers and had a group "Uh-oh" moment. They made a conscious decision that they would not roll over and let the "invaders" take over their market without a fight.
They found out that over 25 years of research and development went into these products. With estimates of 5 years before half of the market flipped to tankless, they had to act fast.
They knew they could "Reverse Engineer" the products but too takes a lot of time and they would have to be careful to keep from violating patents.
The first step by a few of them was to attempt to slow things down. They did this by putting out letters, and articles touting the "negatives" of tankless heaters. While this was going on on the surface, they were behind closed doors with the lawyers striking deals with the tankless companies to have units private labeled for them in an effort to slow down their shrinking market share until they can catch up with the technology.
The results so far are this: Bradford White has Rennai manufacturing its "Everhot" tankless water heater line. It IS the Rennai line with the BW name on it. State and A. O. Smith have Noritz manufacturing theirs.
Now don't think that the very savvy Japanese companies were putting the cart before the horse. They know what is going on and they know that they own the technology for now. They simply set the whole thing up so their original products still have better pricing on the street. They knew that the tank companies would find someone willing to get the quick sale. By doing this they at least kept them from partnering with companies making lower quality units. (That could have started a whole new problem for tankless.)
One very large company even went to the trouble of taking "their" new tankless water heater and putting against their tried and true gas fired tank water heaters in a "test". The parameters of the test were set up to with high levels of water hardness. They knew that the tankless would require cleaning because of it and their tanks would not. (Because you can't!) One thing they did not bother to tell anyone or include in their "short term" test was that had this test been allowed to continue, the hardness would have certainly ruined the tanks and required them to be replaced a lot sooner than the tankless which could simply be cleaned. This was done also to try to slow down the onslaught of tankless while they tried to figure out what to do next.
Most of these tank companies are still producing papers trying to slow the growth of tankless. (Even the ones with their "private labeled" products!) They will sometimes make claims using the lower quality of the available units to try to show them as "point of use" or call them a "niche product" even eluding to tankless as a fad at times. They will make statements claiming that they don't believe in tankless water heaters as "whole house" units. These are just attempts to "stem the tide" as long as they can until they can catch up. Some will print more than others will but it is all for the same reason. They need to get people to NOT consider tankless water heaters for as long as they can to protect their own interests.
Rumor has it (and they are just rumors) that they are all reverse engineering at this moment and will have units ready sometime in the next 5 years. We'll have to wait and see on that one. At least for now, they are trying to keep up with the private labeled Japanese units and they are participating in the market.
So, the big tank companies are worried, as they should be. They are doing what they can, and they are not going away. Tankless is to water heating what indoor toilets have been to bathrooms! I don't think tank heaters will ever completely be replaced with tankless. However, the battle lines have been drawn and it is going to be very interesting for those of us watching this market develop.
Warranties of Tankless Water Heaters
Tankless water heaters usually have a warranty that covers the heat exchanger and the parts separately. The heat exchanger is the main part, much like the tank in a tank water hater. Normally the warranty is for about 10 years on the heat exchanger and 3-5 years on the rest of the parts. This average warranty also reflects on the average life expectancy of tankless heaters. (20 years!)
A brief word about warranties. Manufacturers (Of all Products) tend to set warranties at about half of the life expectancy. Your cars, your dishwasher, your water heater, all have a warranty that is about one half of the average life expectancy of that product.
The nice thing is that with a tankless, even if the main part (the heat exchanger) fails after warranty, you have the choice. Replace the heat exchanger or replace the whole unit. You do not have this luxury if your tank fails.
Advanced Tankless 101
Venting Tankless Tankless Water Heaters
Venting is very important to gas fired tankless water heaters. If these products are not vented properly, many bad things can happen. The least of these is the unit may fail very soon in its life span due to condensate being allowed to enter the product. At the very worse, Carbon Monoxide poisoning could occur. "Shortcuts" should never be taken in regards to the venting system on this or any other piece of equipment that burns gas, oil, wood or other products. All manufacturers' instructions should be followed and you should always make sure that your heater is vented properly.
Tankless water heaters can have their venting go either out a side wall with horizontal venting or up through the roof. Keeping the venting run as short as possible is both good for the heater and will help keep costs down. In fact, many times it makes sense to move the location of the heater closer to an outside wall and run the water lines to it rather than run longer venting. (Copper, PEX and CPVC are relatively cheap compared to Stainless Steel vent piping!) This also may give you a reason to get some needed space back within the home.
Most tankless water heaters require a special stainless steel vent piping material. This material is known as "Category III" and is typically AL29-4C Stainless steel. This is required because the combustion efficiency of the heaters make it very likely that condensate will be formed within the venting system. This condensate, although there is not a lot of it, is highly acidic and will destroy standard vent material in a short time. Type "B" gas vent can not be used on most of these heaters for this reason. Another reason this vent is required is that the vent systems are under pressure from the fan within the heater. This is known as "Positive Pressure" venting and requires that the vent system be UL listed as positive pressure and sealed to prevent carbon monoxide from leaking out into the occupied space. Tankless water heaters as well as other products also have maximum lengths that you can run the venting. The number of elbows required in the system shortens these distances. Always consult your manufacturer instructions and never exceed these distances!
It is also not allowed to tie these vents together or tie them into an existing masonry chimney. The condensing gasses would quickly begin to cause damage to the masonry and result in structural damage to the property as well as a very unsafe condition with carbon monoxide. If you can run the proper vent product up through an existing chimney to the outlet and provide the proper condensate drain to protect the unit, you can use the existing chimney as a "chase" to run your new vent pipe in. (Consult individual manufacturer's instructions!)
Most manufacturers require that you either slope horizontal venting away from the heater, or provide some type of condensate drain within 3' of the vent connection to protect the unit from damage caused by condensate. The condensate in the venting will destroy the heat exchanger if allowed to run back to the unit. Units not vented properly will have their heat exchangers ruined within a few short years. Most vent manufacturer's now have the ability to provide a drain tee even when venting is installed straight up to properly protect the unit.
Venting is probably the most important part of a tankless heater installation. To recap this very important section: No "B" vent! No Common Venting! No connecting to masonry chimney's without lining it with the proper vent pipe. Use Stainless steel UL listed positive pressure venting made for these products. Allow for proper removal of condensate within the vent system. Keep venting runs to a minimum length. Never exceed manufacturer's venting lengths. Read and obey your owner's manual and manufacturers installation instructions!
Gas Piping of Tankless Water Heaters
Tankless water heaters save energy because they are always off and they modulate their firing rate to the demand. However just because they save energy does not mean your existing gas line is large enough. When these products have to go to high fire to meet a large demand you must be able to provide enough gas for the unit to function properly. Do not assume that if your existing gas line is the same size as the connection to your tankless heater that your pipe is large enough. In an existing home it probably is not!
Most homes with tank water heaters do not have a gas line sized properly for a tankless water heater, especially if other equipment is connected to the gas piping system. The best solution for this is to run a separate gas line to the tankless from the meter without re-running the entire gas main. There are many good flexible gas piping systems that can make this job simple and limit the number of joints and installation time of the new gas line.
Needless to say, gas piping is not something the average DIY'er should be attempting. The money saved on running a gas line yourself is not worth the risk of your home and family. Gas piping should always be checked or installed by a licensed and insured contractor trained for gas piping.
Water Pipe Connections to Tankless Water Heaters
Tankless water heaters do not come with relief valves like tank water heaters. Most of the world does not require them on tankless systems but the US does. These relief valves should be installed on the hot water piping leaving the tankless heater. An easy way to do this is to use a tankless water heater valve set like the EXP made by Webstone. This valve set gives you only 5 joints to assemble. It has union connections at the heater, which will greatly speed up replacement later. It has ball isolation valves on each side, drain valves that give you the ability to flush the tankless system later or drain the unit easily without draining the whole house and it comes with the proper relief valve. All of this is in a very compact kit and makes hooking up the waterside of your heater fast and easy. This one pipe kit saves about 16 joints at the heater! Make sure you get the one with the relief valve sized for your heater.
Minimum Flow Rates of Tankless Water Heaters
A very important consideration when deciding on your tankless water heaters is minimum flow rate. All tankless heaters need a minimum flow rate and pressure to work properly. You should look for a model that has a minimum flow rate of .5 GPM for a residential application and one that will operate well down to about 30 PSI system pressure.
Even with a minimum flow rate of .5 GPM it is possible to have a flow related problem but it is a lot easier to solve. If you have single handle bathroom (lavatory) faucets you are going to need to open them all the way to get the minimum flow rate to fire the heater, especially in the summer time. This is why I advise to not use tankless water heaters with high minimum flow rates in homes. Commercial units can have minimum flow rates of .75GPM . That is about as high a minimum flow rate as you would want in a home. Most homes requiring these units (Large luxury homes with "carwash" shower systems) do not have very low flow fixtures and typically do not need to worry about minimum flow rates.
Also, debris in faucet aerators and showerheads can cut their flow rates down to a point that will keep the tankless from firing. Make sure your fixtures are free from debris.
Most 2.5 GPM showerheads will not supply 2.5GPM of flow in a new home. This is due to system pressure loses. Every foot of pipe and each fitting in the water main has a pressure loss. At far ends of the home these add up and can cause lower flow rates at fixtures like showerheads. This is not a big problem though and rarely causes issues with the better tankless water heaters. Most people never know the difference and as stated above, it is almost impossible for most people, even plumbers to tell the difference between 1.5GPM and 2.5 GPM in today's showerheads without measuring the flow. As long as the velocity of the water is acceptable most people are quite happy and of course some showerheads are better than others.
Most tankless water heaters have an inlet water filter. This should be checked and cleaned regularly to make sure that flow is not slowed or stopped by this filter. This is the first place to look whenever there is a problem with your tankless water heater. Look for it in your owner's manual.
Hard or Acidic Water Concerns
Hard and acidic water can affect tankless water heaters. What tank manufacturers don't want you to know is it effects ALL water heaters. These water quality issues also effect everything connected to your domestic water system. All faucets, fixtures and appliances connected to a water system distributing bad water will have problems associated with these water conditions. These problems can be staining, scale build up and erosion of the metal. (Including drainage piping!) Water with excessive hardness causes scale buildup. Water that is acidic is harmful to copper and brass and most other metallic components in or connected to your water system.
Hard water contains particulate that is dissolved in it. (dissolved solids) These are microscopic particles that are held "in suspension" in the water. They will build up on surfaces over time causing calcium or lime scale buildup. What makes matters worse is that when water is heated, these particles bind together and become heavier than the water and settle out faster than they do if just flowing through your pipes.
This means that in a tank heater, where the water movement is very limited and the water is heated and reheated while there is little or no flow, this hardness drops out of suspension and settles in the bottom of the tank where it is very difficult to remove. This hardness builds up on the bottom of the tank and insulates it from the burner below causing a less efficient heat transfer to the water. This makes a tank heater loose its recovery ability over time and eventually will result in overheating of the steel at the bottom of the heater. This is why many gas fired tank water heaters develop leaking tanks and why many of them fail when the bottom of the tank is burned through, flooding the house. This is known as a catastrophic tank failure and results in many thousands of insurance claims each year. When a tank water heater ruptures in this way, many times families are displaced for months while the home is dried and repairs made to water damaged property. Even a tank heater set in a drain pan is going to flood the space if the bottom blows out dumping 50+ gallons of water under system pressure into the shallow pan all at once! Electric tank water heaters are not as likely to have this kind of failure. With an electric tank heater, hardness builds up both on the bottom of the tank and on the elements. This also makes them begin loosing their efficiency from the very first day they are installed!
A tankless water heater is also effected by hard water just not quite as quickly. In a tankless water heater, it is only heating water when the water is moving through the heat exchanger coil. As particulate drops out of suspension in the heat exchanger most of it is whisked away unnoticed. Some will bond to the interior wall of the copper tubing, but at a slower rate since it is being flushed as it is being used.
Over time, this scale buildup will result in the overheating of the heat exchanger. This will cause and error code and the tankless water heater will lock out. This means it will not operate until the unit is reset and the problem corrected. Resetting a tankless heater without solving the problem will simply make the unit lock out again. It will protect itself. The error code flashing on the remote or computer board will help the technician figure out what is wrong so it can be fixed.
When this occurs you will need to call your service technician or plumber to fix it. They will hook up a flush kit that will circulate a solution through your tankless water heater which removes the scale buildup. This can be a product like CLR or a vinegar/water solution of one of the commercially available cleaners that boiler companies use to do the same thing with tankless coils in boilers. If your tankless heater has one of the water piping kits installed like the Webstone EXP kit mentioned above, this process is very easy and fast. In about 45 minutes your tankless water heater is like new again! This makes tankless heaters a much better investment than a tank heater that can not be repaired or cleaned.
Acidic water is water with low Ph level. A Ph of 7 is considered neutral. Chances are if you are on a public water supply your water will not be acidic. (Although it may be hard) If you are on a well you should have your water tested to determine both hardness and Ph.
If you have acidic water it will attack all metal in the plumbing system including your tankless water heater. Acidic water will cause "pinholing" in copper piping (and copper heat exchangers) and can also cause blue or green staining of clothes as metal is eroded from your plumbing system or appliances. How fast acidic water will damage a tankless water heater or other appliance is dependant on the level of Ph in the water. In short, yes, acid water will damage a tankless water heater and anything else metal connected to the water system over time.
Because product manufacturers have no control over your water system or condition all water heater manufacturers have warranty clauses that state that their product is not covered if damage is caused by aggressive water conditions or hardness. It is worded differently form one manufacturer to the next, but all of them, including the tank manufacturers do not cover their products against bad water.
So what should you do if you suspect that you might have a bad water condition? Simply have the water tested and most importantly treat your water! We have already explained that bad water affects everything in your plumbing system including appliances connected to it. The cost of a properly designed and installed water treatment system will pay for itself by making all of your appliances last much longer and require less maintenance.