Gas tankless water heaters are sized according to the hot water output (in Gallons Per Minute - GPM) they are capable of providing with respect to difference between the ground water temperature and desired hot water output temperature (by default around 120°F) – a term commonly known as "Temperature Rise" or simply "Rise".
Most manufacturers provide a chart (ground water temperature map) which allows to select a model based on rise and desired GPM output (or number of fixtures). For more precise calculations, we recommend using cold water temperature data from your local municipal water supply company (where available).
Difference between Condensing and Non-Condensing Tankless
Typical gas tankless water heater burns gas to heat the water in the coil and force-vents the hot exhaust gases outdoors. A condensing tankless water heater is designed to extract as much heat as possible from exhaust gases, resulting in a much higher efficiency. Due to lower exhaust temperature in condensing THW’s, condensation of exhaust gases occurs and hence the name.
Condensing technology, while more expensive, can deliver energy efficiencies as high as 95-99%, while non-condensing produces about 82-84%.
Tankless Venting Options
Non-condensing tankless water heaters require Category III stainless steel venting (such as Z-Vent made by Z-Flex
Condensing tankless hot water heaters require Category IV venting (such as Innoflue PolyPropylene pipe and fittings
made by Centrotherm). While some manufacturers allow PVC and CPVC piping for venting (flue gases specifically), we advise against this practice, since neither of these pipes was designed for handling of hot flue gases and they pose potential health and safety risks even when properly installed.
Points to keep in mind:
- Get the right adapter to connect your tankless unit to the vent pipe of your choice (may not be required on some models).
- The easiest and least expensive way to vent an indoor tankless unit is to install and vent it through an outdoor wall.
- On condensing tankless WH’s, outdoor vent components must be UV-resistant (Innoflue is 100% UV resistant, PVC & CPVC are not resistant, other PolyPropylene brands may have both).
- Provide adequate air intake. Some installers may be tempted to convert the unit to use indoor air for combustion purposes (less holes to drill and less piping to use), but it may not always be sufficient to supply the burner needs and may cause the water heater to shut down or malfunction.
Indoor or Outdoor?
Outdoor units are generally designed for use only in warm climates, where temperatures do not fall below freezing. Depending on the brand, model or design, a THW may also require an outdoor conversion kit or recess box. For example, Takagi makes both indoor and outdoor-ready units which do not require conversion kits, while some Bosch indoor units require a conversion kit when installed outdoors.
An outdoor tankless water heater will generally require a roof or suitable cover above it to provide protection from the elements, while allowing for proper venting to take place.
Frequently Asked Questions
Q: Is it possible to change the hot water output temperature on a TWH unit?
A: Most models allow for either direct (inside/outside the unit) or remote (wired or wireless) temperature control, either at certain presets or precise (to the degree) adjustments. Check the manual, product description or with the manufacturer for details.
Q: Can I use a gas tankless water heater for heating (i.e. radiant) purposes?
A: Some manufacturers allow this practice, but this generally lowers the duration of warranty. Be sure to check the manual or with manufacturer and if allowed, avoid using any cast iron or malleable iron components in the heating system, since these produce a lot of sediment and can potentially clog the tankless coil. Alternatively, use a dirt separator
(to filter and collect the sediment) or a heat exchanger
(to completely isolate the tankless side from the heating system)
Q: Do I need a install a condensate neutralizer for a condensing TWH?
A: If a tankless unit has a condensate drain, it will produce condensate which must be properly disposed of. We recommend reading more about condensate neutralizers here
to make an informed decision.
Q: Do I need an expansion tank on a tankless water heater?
A: Check the manual. In most cases, where TWH is used solely for the purpose of producing domestic hot water, the expansion tank is not needed since water heater will produce hot water only when the tap is opened and burner will shut down automatically when tap is closed (so no expansion occurs in the system).