As someone who
is both a home inspector and a developer, I've seen
"the good,the bad, and the ugly" when it comes to
the installation of whole-house "tankless"(AKA
"demand') water heaters.
When they are installed
correctly I've been favorably impressed with the performance
and convenience of tankless units - I have one installed in
my own home, and have installed them in several of my
Tankless heaters, though, have to be properly installed to
produce satisfactory results, and experience has taught me that
there are considerable differences between the installation
requirements of tank and tankless water heaters. This is
especially true when a tankless water heater is being
installed to replace an existing tank type unit.
This page is not intended as a comprehensive manual on
tankless water heater installation or trouble shooting, or
as a substitute for careful reading of manufacturers'
installation instructions and a through understanding
of the building codes.
Rather, it's a list of real-world problems and
issues intended to assist installers, inspectors and
homeowners to archive satisfactory results the first time out by
avoiding some of the problems I
have encountered on my own projects and during
inspections at homes, condominiums and rental
Tankless Water Heater Installation and Problems
word about efficiency
This page isn't about comparing the
efficiency of tank type and tankless water heaters - there are
many sources for such information.
But since I'm so often asked the question, I thought a brief
comment and at least one suggested link was in
The payback period for tankless water heaters is considerable.
test simulating normal household usage a gas fired
tankless heater saved around $97 a year compared with a
convention tank type heater. As the installed cost
difference between and tank and tankless
unit often exceeds $500 and can be two to three times that,
under some conditions the expected payback time of a
tankless unit can exceed its expected operating life!
So why consider a tankless unit?
In the case of my own home we installed a large commercial
tankless water heater because we have very wide demand
swings: most of the time my wife and I are only
occupants, but when we have house guests we can have two
tubs (including a hydro-therapy tub) or showers running at the
same time as the laundry, the dishwasher and the kitchen
Our water heater was relatively expensive to install but it
provides what we require: essentially unlimited hot water
in periods of high demand without the cost or floor
space required to keep 100+ gallons of water hot
In the case of our rental rehab properties we could not make an
"economic" case for tankless units on the basis of
energy savings alone. However in out tight urban environment
(Evanston IL) a tankless heater's smaller footprint
recovers a few precious square feet of rentable floor space in
each apartment and increases rental value of each by around $225
a year, greatly shortening the payback period.
So there are valid reasons other than "average
energy-efficiency" to consider tankless heaters.
Tankless units really
The installation requirements and
procedures for tankless hot water heaters differ in important
respects from tank type units.
In my experience many installers are not fully aware of the
differences, but "don't know what they
don't know". When this happens they can make
mistakes which result in poor performance, increased
costs, create fire and safety hazards, and
sometimes require expensive changes to correct such problems
"after the fact".
It has also been my experience that some local building code
officials are not familiar with the differing installation
requirements of tankless water heaters, and that these units
can pass municipal inspections when installed in incorrect
and even dangerous ways. top
Planning the installation
Tankless units have requirements
significantly different from those familiar
to installers of conventional tank type water
heaters. These include differences in gas supply
plumbing, venting, and other aspects of
installation. As a result, it is easy for even experienced
installers to make installation mistakes if
they assumed that understanding of tank type
systems will be sufficient to allow them to
properly install a tankless unit.
Based on my experience, I recommend the
following guidelines to reduce the likelihood
of this sort of problem.
Attempts to mix different types of vent materials are a common
installation mistake when installing tankless units. Tankless
water heater manufactures may require materials such as vent
pipes, fittings and terminators which are different from those
used with common venting materials for tank-type water
- Have the general contractor or whoever is going to have
overall responsibility for the installation thoroughly review the
manufacturer's installation instructions and completely plan
the installation *before* ordering materials, or better yet
before construction begins. Following links at
these sites will provide installation
Bosch AquaStar, Noritz, AO Smith, Rheem, Rinnai, and
This picture shows a correct wall ventilation terminator for a
Takagi T-K3 configured for direct vent operation.
- Consider ordering the system as
a "package". If the installer is not
thoroughly familiar with the entire system - heater, mounting
brackets, back-draft protection requirements, plumbing,
valves, safety devices, venting materials and the like - it is
best to order the entire system as a package from a wholesaler or
heating supply house familiar with these products.
- Someone needs to be responsible for coordinating the work
of all trades involved in the install.
Installation of a tankless system will often require
coordinated work by at least four different
trades: carpentry, HVAC, plumbing and electrical.
It is important that all installers involved clearly
understand how the requirements of the tankless
installation may differ from those that they are familiar
with based on their experience with tank type water heaters.
For example the wall framing required to accommodate
the venting clearances requirements of tankless
units may be different from that which framing
carpenters are used to, the gas supply piping required
may be larger than the plumbers expect, the vent materials
may be different from those the HVAC installers are
familiar with, and local regulations my require a
dedicated electrical circuit for the heater even if
only a 120V circuit is required. top
Size the unit
Tankless water heaters do not store hot
water, instead they heat water on demand. As
a result tankless units are sensitive to the input
temperature of the water they are heating.
For a given make or model of tankless
heater the installation manual and/or sales
literature will list the capacity of the unit in gallons per minute
flow for a given temperature rise - the colder the inlet
water the less hot water the heater can provide at
a given output temperature. So a tankless
unit must be sized with climate in mind.
For example here in Chicago I've encountered water inlet
temperatures as low as 38 F, which is 30 degrees or
more colder than in warmer parts of the US. This means
that in comparison to those in a warmer area of the
country I need a water heater with greater heating
capacity to provide the same amount of hot water.
Fortunately many modern tankless water heaters have
"modulated" burners or variable output heating
elements - they monitor water flow and inlet and outlet
temperatures and increase heating as required to keep up
with demand while using only as much gas or electricity as
is needed to heat the required quantity of water.
This means that usually there is not a large efficiency
loss for "oversizeing" the capacity of a tankless
heater - a higher capacity tankless unit will have a
higher initial cost, but not a substantially higher
operating cost, than a smaller one.
For this reason my recommendation is that if you have a choice
of a unit calculated to be on the low
side of your peak demand and one calculated to be on the
high side, that you opt for the higher capacity unit. top
Use an experienced installer
Whenever possible use an installer
familiar with the specific brand and model of product being
installed, preferably someone who has received some
"factory training" in the installation
of such units. top
of vendor technical support
If necessary, don't hesitate to avail yourself of the
technical support available from vendors. On the first few
installations of tankless heaters in my projects we obtained
invaluable advice from factory field representatives -there
is no substitute for having someone thoroughly familiar with the
product and its installation on-site to evaluate how to best fit
it in to a job, especially if it's a rehab where time,
space and money area all tight. top
To help with tankless installation, planning and trouble
shooting, I've listed below some of the
"gotchas" I've encountered in my own projects
or during inspections for buyers or
Insufficient Combustion Air For Gas Fired Units
Tankless water heaters can require a *lot* of combustion air
- a gas fired tankless water heater large enough to
serve the needs of a modern three bathroom house may require
a quantity of combustion air considerably greater than is used by
a gas furnace or boiler heating the same structure!
One result of these high combustion air requirements is
that when a tankless unit is installed to replace
a tank-type water heater, the existing method of
providing combustion air may not be sufficient.
If sufficient combustion air is not available the
efficiency of all the gas appliances may be reduced. More
important, it is possible that one or more appliances
will "backdraft", which can cause toxic exhaust gases
(carbon monoxide) to be spill back into the house.
Combustion air to this direct vent water heater is
supplied from the exterior. Note that though this is a direct
vent installation this is not a CAT IV appliance and cannot be
vented through PVC. Because of the high temperature of the
exhaust vent and it's proximity to the intake, the intake
also had to be in non-combustible material.
The brackets spacing the unit out from the wall must be ordered
separately, missing spacers are another example of the sort of
installation mistake made by installers not familiar with
The amount of combustion air required should be calculated
based on the input BTUs of all appliances
requiring combustion air, and a
determination made whether the
available combustion air is sufficient.
It is important that the heating contractor or plumber
perform these calculations, and not
just "guesstimate" if sufficient combustion air
is available . A basic combustion air
requirements calculator is available here, but you
should consult the manufacture's installation
instructions and research local requirements when designing
the vent system.
If additional combustion
air is required it can be provided by increasing the size of the
interior space from which combustion air is drawn or by arranging
for additional combustion air to be brought to the appliances
from outside the structure. Building codes and utility company
requirements in your area provide a set of rules as to how
this can be done, and it is important that these rules be
If it is not possible to provide the required combustion air
by these methods, or if it is not considered desirable to do so
(perhaps because you do not want to introduce cold outside air
within the conditioned space of the building) an alternative
is to utilize a "direct vent" tankless water heater
which brings in outside combustion air through a pipe without
mixing it with air within the house.
Such direct vent units will initially be more expensive,
but will be more efficient as they are not sending heated
air from within the house "up the chimney". top
Incorrect Exhaust Venting
Most gas fired tankless water heaters require special
provisions for their venting. If the vent system is not
properly designed and installed there is a real
possibility that the high temperatures involved may
damage vent components, produce conditions that
allow exhaust gases to enter the structure, or create a
It is important to note that in many cases you cannot just
attach a tankless unit to the vent pipes previously used by the
conventional hot water heater it is replacing. Each tankless
heater's manufacturer provides detailed installation
instructions specifying the units vent requirements. Often
these requirements include the use of special stainless steel
vent pipes designed to resist high temperatures and
corrosive exhausts, specified means of attaching them and spacing
them away from other building components, and often the use of
specifically designed portions of the venting system such
as vent terminators (the portion of the exhaust vent located
outside of the house at the end of the exhaust system).
Always check clearance requirements when planning
installation. This venting material for a CAT III unit can
require as much as 8" of horizontal or 12" of vertical
clearance from combustible material when enclosed in a soffit or
Tankless heaters are often installed using materials and
methods unfamiliar to some plumbers, HVAC contractors and
homeowners. Installation errors are a likely result, in this
example single wall vent pipe has been placed too close to
electrical wiring and wood structural members - so close floor
joists above the vent were showing visible evidence of
These differences between tank and tankless venting
requirements can be large, and can cause significant problems if
planning for the installation has not anticipated the increased
One problem I have encountered is that most plumbers and HVAC
contractors are used to running such vents in "B-Vent"
material which typically has a clearance of 1 inch from
In addition some tankless water heaters can be vented in
"unconventional" ways. Even an experienced contractor
or inspector encountering this auxiliary power vent might assume
that "This can't be right..." - but this
method of horizontal power venting via an auxiliary external fan
is a factory option for this Bosch water heater. Photo (c)
Scott A. Figert, Figert Building Inspections Ltd. Cecil, OH used by
Some types of vent pipes specified by manufacturers of
tankless hot water heaters require clearances as great as eight
to twelve inches from combustible materials to the top and sides.
If this requirement was not been taken into account when planning
the installation, it can prove difficult and expensive to provide
the required clearances "after the fact".
Another potential problem is that manufacturers of tankless
heaters, or the manufacturer of the vent pipe they specify,
may prohibit mixing vent components from different manufacturers
or may specify the exact circumstances in which components from
various manufacturers can be mixed.
This can add considerable complexity, cost, and time to
projects, especially if additional material and fittings cannot
be obtained "off-the-shelf" from a local supplier. top
Insufficient Gas Supply
The symptoms of an undersized gas supply include water heaters
which are not supplying the expected quantity of hot water and
burners which intermittently turn off when they should be
providing heat, or both. According to my local Takagi
representative 80% of the service call on newly installed units
are the result of undersized gas supplies; and because of the
high energy requirements of tankless water heaters it is likely
that the gas supply to tankless units replacing conventional
tank type water heaters will have to be upgrade and problem
may be encountered in new construction as well.
It is important that gas supply sizes be accurately calculated
based on the manufacturer's installation instructions, the
NFPA. or local requirements rather than based on
"guesstimates" - at one of my rental rehab projects the
plumbers ended up tearing out and replacing the entire gas
distribution system because they had failed to perform these
calculations - an expensive mistake as the walls and ceilings had
been finished in the meantime. As a guideline only, here
are typical gas supply pipe
sizing tables for gas and propane appliances.
When replacing an existing tank-type unit keep in mind that most
exiting residential tank type units employ a 1/2" supply
pipe for at least a portion of their gas supply run, but many
higher capacity tankless units specify a minimum 3/4" gas
supply line running continuously all the way to the heater,
including valves and any flexible connectors. If you are installing a unit for which the manufacturer specifies a 3/4" gas or propane supply, consider installing a high capacity flexible gas connector and a valve rated for the required flow rate.
Note that in addition to the branch line from the main gas
line within the house to the water heater, the high combined
demand of the gas tankless water heater and a gas forced air
furnace or boiler may require resizing some or all of the
other gas lines running within the house.
The gas supply components outside a building can also cause
problems for tankless heaters.
To translate meter cubic/ft/hour into BTUs multiply by 1020.
If the result is less than 105-110% of your maximum demand
and you are on a low pressure gas distribution system my
experience is that you may experience intermittent problems which
can be corrected by increasing the capacity of the meter.
If the current gas meter is "on the edge" of
being able to supply the quantity of gas calculated to be your
maximum demand (the input BTU ratings of the gas appliances -
water heater, furnace or boiler, clothes dryer, oven and cook
tops - you expect to operate at the same time) it may be
necessary to replace the meter with a higher capacity unit.
In some cases, it may also be necessary to upgrade the
underground gas line from the gas main at the street to the
house. Such problems are most likely to
occur if the building is on a "low pressure"
gas system at the street (the utility company will be able to
tell you if this is the case).
Gas supply problems caused by meter capacity can be
frustrating to solve as they often only be
diagnosed by "trial-and-error" and the utility
company may be skeptical that a bottleneck at the meter is
causing the problem. In two cases I've had to pay the
gas company to replace the a meter with a larger
size; in both cases this solved the
problem, but in neither was the gas company
willing to refund the cost of the upgrade even though they had
originally specified the meter size. top
In areas where freezing conditions are not expected, tankless
heaters can be installed (local codes permitting) on the exterior
of the structure, reducing the complexity and length of vent
runs, and thus reducing the cost of the installation.
In areas where freezing temperatures are
possible tankless units must be mounted in a conditioned
(heated) space, and the manufacturer may specify a back draft
preventer (to control the movement of cold air back into the unit
through the vent), a unit with a thermostatically controlled
electric heating element, or both.
It important that the manufacturer's installation
instructions be carefully reviewed and followed with regard to
freeze prevention requirements. Failure to do so can cause severe
damage to the water heater and will meanwhile have voided the
manufacturer's warranty. top
Tankless water heaters follow the same general plumbing code
provisions for their installation as conventional tank type
heaters, however a few points bear mentioning.
Some tankless water heaters use a pressure release
valve different from the temperature-pressure release valves
commonly installed on tank type water heaters. (The type of
safety device required can also depend on local code). The
manufacturer's installation instructions should be consulted
to determine the proper type of relief valve.
Many tankless heaters have a debris tap screen on the cold
water inlet, which needs to cleaned after initial startup
and periodically thereafter. Maintenance of these can be
simplified by the installation of unions and valves specifically
designed for use with such heaters, such as the
Webstone Webstone and Dormont water heater isolation valves. top
Takagi auxiliary control unit
Some tankless water heaters (for example Takagis) require a
separately purchased controller if you want to set the output
temperature above or below the factory preset.
Some of these controllers also provide additional information
such as diagnostic codes or input water temperature.
Note that as a safety feature some controllers do not
automatically reset to ON after electrical power has been
interrupted and restored, in this case the heater will be
inoperative after a power outage until the controller has been
Also note that in new construction if you want to install the
controller remotely from the heater you will need to prewire the
connection between them. top
Since I wrote the first version of this FAQ, I've run across
some additional resources
you may find useful:
This page from Puget Sound
Energy puts some numbers on the issues addressed above.
page at Houseneeds.com has direct links to
installation manuals for many brands
This page was written for Paragon Property Services Inc.,
Evanston, Illinois by Michael Thomas. I am always interested in
readers comments on all aspects of property inspection. If you
have questions or comments about this article please feel free to
contact me by
or at 847-475-5668.