Venting 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. The professionals of Dallas Plumbing Masters are very aware how critical propper venting is when it comes to installing tankless water heaters in Texas. “Shortcuts” are never  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 are followed. Dallas Plumbing always makes 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.

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 water heater unit.

Venting is probably the most important part of a tankless heater installation.

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 in Orange County with tank type 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 limit the number of joints and installation time of the new gas line.

Needless to say, gas piping is not something the average do it yourselfer 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. Always call Dallas Plumbing Masters 1st and have it done right. This will not only save you time, it may save your life, or the life of a loved one.
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 looses.  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 to consider with tankless water heaters:

Hard and acidic water can effect 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 effects 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.