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Tankless Water Heater Buying Guide
A graphic showing the benefits of a tankless water heaterTired of running out of hot water in the shower (or anywhere else, for that matter)? Is your current water heater on the fritz, ready to succumb to years of constant use? Whatever your situation, when it concerns a new water heater, there's a big choice to make: tank or no tank? Tankless is becoming more and more popular, but how do you know if it’s right for you?

Unlike conventional tank water heaters, tankless water heaters heat water only as it is used, or "on demand". Tankless units have a powerful burner or heating element that's activated by the flow of water when a hot water fixture is used. Free of the limitations of the tank, hot water can be produced in perpetuity. They're touted as water and energy savers, but as always, it depends on the situation. To choose the best water heater for your home, you'll need to consider your usage habits, your plumbing fixtures, and the limitations imposed by the house and its existing plumbing.

What Is A Tankless Water Heater?

We're all familiar with standard tank water heaters, which work well but aren't all that efficient. No matter how great the tank's insulation is, the hot water inside will give up its heat over time - that's just how the universe works. The only way to keep that water hot is to regularly warm it up - a constant cycle that consumes energy and costs you money. Standby heat loss typically accounts for 10-20% of total annual water heating costs. In some homes with standard tank heaters, the energy consumed for heating water is even greater than that used for heating the home itself!

Tankless water heaters were designed to avoid the shortcomings of standard heaters, providing a more energy-efficient way to have a steady hot water supply. Utilizing powerful gas burners or electrical elements, they're able to rapidly heat water at the time of use, as opposed to storing (and perpetually reheating) it for eventual use. Heating water only when you need it makes sense, and eliminating standby heat loss almost always results in lower energy bills.

In addition to avoiding standby heat loss, tankless heaters are also smarter in how they use fuel (be it gas or electrical energy). By modulating their fuel use, these units use only what's needed to heat water at the flow rate actually being experienced. If you open up a sink faucet that's drawing 1gpm, less fuel is used to heat that water than would be used for a tub spout running at 5gpm. As a result, it's not uncommon to see fuel-use reductions of up to 50% compared to a tank heater.

There are two varieties of tankless water heaters: centrally-located whole house heaters, and point-of-use (POU) heaters for individual fixtures (or rooms). What you choose depends on the water habits of your household, and the demands of the fixtures involved - all of which we'll get into below. Some will opt to install point-of-use heaters to supplement a central unit, typically at fixtures that use a lot of hot water or are far away from the central water heater (resulting in long waits for hot water). Whole house heaters work best in smaller to average-sized homes, but multiple units can be installed to meet the demands of larger homes.

Figuring Out What You Need

If you're leaning towards tankless, you'll need to determine the hot water needs of your household before you can start (seriously) looking at units. How much hot water will you need at peak demand (say when someone's showering, you're doing dishes, and there's a load in the wash)? And just how much will the incoming cold water need to be heated?

Figuring out hot water demand is fairly simple: list all of the fixtures that will be using hot water from the tankless heater, and add up their flow rates. How do you find out the flow rate? If you bought the fixture, you can check the documentation it came with or go online to find out. If you have no idea, use the following assumptions:

  • Faucets: 0.75 gallons to 2.5 gallons per minute
  • Low-flow shower heads: 1.2 gallons to 2 gallons per minute
  • Older standard shower heads: 2.5 gallons to 3.5 gallons per minute
  • Clothes washers and dishwashers: 1 gallon to 2 gallons per minute

For older fixtures, use the larger of the figures; for new fixtures, use the smaller number. If you're at all unsure of the age of the fixture, always go with the higher flow rate to avoid purchasing an undersized unit that will be unable to meet your needs. For the greatest accuracy, you can time how long it takes a given fixture to fill a gallon jug.

You'll also need to ascertain the temperature of incoming water. While actual temperatures will of course vary, this rough guide should be enough to help you pick out an adequate unit. For greater accuracy, contact your water provider.

Once you know the incoming water temperature, you can calculate the temperature rise - how much the heater will need to heat the cold water. Most homes will be satisfied with 120°F water, and most units will heat up to 160°F. To calculate the temperature rise, simply subtract your incoming water temperature from the desired output temperature. For example, if your incoming water temperature is 50°F and you want 120°F hot water, the temperature rise would be 70°F.

Figuring out your home's hot water demand and the temperature rise required allows you to focus on tankless units that can actually meet your household's needs. A table or chart will be included on item listings, spec sheets, or even on the heater packaging itself indicating the maximum flow rate at various temperature rises. Your goal is to find a unit that provides the required flow rate (as determined above by adding up the flow rates of the fixtures to be used) at the necessary temperature rise. The greater the flow rate, the lower the temperature rise.

Let's look at an example:

  • Your incoming water temperature is 50°F and you want 120°F hot water delivered to a low-flow showerhead (1.5 gpm), and two lavatory sinks (1.0 gpm each). The total flow rate of the three fixtures equals 3.5 gpm.
  • If you were interested in the gas-powered Takagi TK-110U-I water heater, you would take a look at its flow rate chart and see that at a 70°F temperature rise, it supplies just over 3.0 gpm. While you may be able to get away with this, you're much better served upgrading to the TK-310U-I, whose flow rate chart shows a flow rate of about 4.5 gpm at a 70°F rise.
  • If you were instead looking at an electric model like the Eemax Home Advantage II, you would consult the handy table on that page that shows the temperature rise at several flow rates for each model in the Home Advantage II family. There's unfortunately no column for 3.5 gpm, but 3.0 gpm and 4.0 gpm are represented. Going down the 3.0 gpm column, you can see there's only one model that comes close to the required 70°F rise: the HA036240, which gives a temperature rise of 82°F at 3.0 gpm and 62°F at 4.0 gpm.

Determining the Best Power Source

Once you know the temperature rise and flow rate you need from your new water heater, you'll be able to decide whether an electric or gas-powered model will work best for you. For anything beyond small point-of-use heaters, it's likely that you'll need to upgrade either your electrical system or the gas supply line and water heater venting.


With electric tankless water heaters, the primary concerns are with the load they place on your home's electrical system, and the wire size required to power the unit safely and sufficiently. Electric units are fine for point of use, but are generally avoided for whole-house applications due to the cost of electrical upgrades that would likely be necessary.

  • Electric models will specify the number and amperage of breakers required for the unit's operation. Most homes will have 100 or 200 amp electrical service; depending on the other electrical demands of the household, you may be able to get away with a point-of-use heater or small whole-house unit without having to upgrade the electrical service. If you aren't familiar with your home's electrical system or the demand placed on it, speak with an electrician before you buy.
  • At minimum, all but the smallest POU units will likely require a new breaker or two, as well as new (thicker gauge) wire. DIYers with electrical experience may be able to save a few bucks by making these relatively simple upgrades themselves, but for everyone else an electrician should handle things.
  • If your breaker panel is all filled up, a subpanel may need to be installed to accommodate the new water heater. This will entail an additional cost of at least several hundred dollars, and is not a project for most DIYers.
  • If your home's electrical service is underpowered, you'll need to decide whether the cost of upgrading or adding a second service is worth it. Installing a second service or upgrading from 100 to 200 amp service will typically run around $1000; going up to 400 amps about double that. Depending on your home's gas supply, it may be a wiser move to invest in a gas-powered unit, or reconsider standard tank water heaters.
  • On the plus side, electric units don't require venting! This saves time, money and space - and allows for a far greater number of installation locations throughout the home.

Natural Gas or Propane

Gas (and liquid propane) tankless water heaters are more powerful than electric, and are the type most commonly used as whole-house water heaters. But there are two things you must know before deciding to go with gas: is your gas supply large enough to power the unit, and how will you get rid of the dangerous exhaust the heater creates?

  • What's the size of your home's existing gas supply line, and how far from the meter will the new tankless unit be located? Is the pipe size reduced at all over the course of its run to the heater? How much gas is used in the rest of the home? It may be easy enough to investigate these things yourself, but when it comes to using that information to decide whether or how much you need to upgrade, things can get really complicated. Call your gas provider or a plumber certified to deal with gas systems to schedule a visit and discuss your options with you.
  • Homes with undersized gas supplies may need larger-diameter pipes to deliver more fuel, or the provider may need to increase the supply pressure to the home (standard residential pressure for natural gas is 1/4 to 1/2 psi; 2 psi may be necessary in homes with high demand). The cost of each will likely be substantial: gas repiping involves a lot of labor, and upgrading the gas service calls for a new meter and regulator. Prepare to spend a minimum of several hundred dollars for a meter/regulator change, and potentially much more for repiping.
  • Be sure that the gas-powered models you're looking into use electronic ignition devices, which avoid the constant fuel consumption of a standing (always on) pilot light. The overwhelming majority of newly manufactured tankless heaters utilize this technology, so you shouldn't have much difficulty.
  • If you think you can save some money by re-using the venting from your old water heater, think again! Tankless gas units require large metal flues that can deal with very high temperatures - category III stainless steel vent pipe being the preferred choice. Vent pipe should never be mixed and matched, so you'll need to factor in the additional cost of new venting.
  • Gas water heaters have two venting methods: direct-vent and power-vent. The specifics and code requirements of tankless venting can present significant challenges, so professional installation is always recommended (many manufacturers will even void warranties if a unit has not been professionally installed, venting included).
    • Direct-vent units typically require two separate vents: intake and exhaust (some heaters can utilize concentric vents that combine both in a single unit, saving time and space). Intake air from outside the home is drawn in through the ventilation pipe; hot exhaust leaves through a different one. Because they're fed air from outside, direct-vent units can be installed in very cramped locations. Some direct-vent systems will allow for exhaust to be vented horizontally out a side wall, making installation that much easier.
    • Power-vent water heaters take in the air around them to fuel combustion, and use only a single ventilation pipe (for exhaust). Whatever room they're in will need to be large enough to supply sufficient air to the unit; the air must be clean and free of fumes (especially from corrosive chemicals like chlorine, sulfur or fluorine). If the desired installation location can't provide the fresh air necessary, the unit must be moved or (in some cases) converted to direct-vent. The "power" in these systems refers to a fan that actively pushes exhaust out of the unit through the vent pipe. Since exhaust gases are being forced out, a power-vent system can be vented through a wall rather than the ceiling.
    • Whatever venting method is used, there are strict codes that govern how and where venting can be installed - and it's not the same everywhere. Some things make perfect sense (the exhaust can't be placed too close to a window), but other requirements may not be so obvious. Be sure to hire a fully licensed and experienced local professional to install the heater - it's their job to know how to do things right!
  • Outdoor Tankless Water Heaters: If you live in a temperate climate, the issue of venting can be made moot with a water heater designed for outdoor installation. These gas or propane powered units are designed to weather the elements, and usually feature a freeze-protection device that's good down to single digit temperatures. Installing an outdoor unit frees up valuable space inside the house, and saves money by cutting out the cost of venting entirely. Many outdoor tankless units can be installed in a recess box, keeping your home's exterior clean and free of distractions.
  • Condensing Tankless Water Heaters: Condensing units are the ultimate in efficiency. These gas/propane models feature a heat exchanger before the vent pipe that captures thermal energy from the hot exhaust, using it to preheat the cold water coming into the heater. This mean less "fresh" energy is being used, and it also results in a cooler exhaust temperature. Because the exhaust isn't insanely hot, some water heater manufacturers state their condensing units can be vented via PVC pipe, rather than the more expensive steel or aluminum venting required with non-condensing tankless units. We do not recommend this - and neither do PVC manufacturers, who rate their pipe only for venting sewer gases, not combustion gases. There's always a chance PVC could melt in such a situation, so you're much better off using metal venting.

Tankless Water Heater Maintenance

There's no anode rod to worry about replacing, nor is there a huge tank that needs to be flushed out, but tankless water heaters do require a bit of maintenance every now and then. The build-up of limescale (hard water deposits) in a tankless unit's heat exchanger can severely impact performance, and eventually break it. You'll need to delime/descale the heater regularly to ensure maximum performance, with the regularity totally dependent on the hardness of water in your area: it could be every 6 months, every year, or even every few years.

How will you know when to descale your water heater? Some newer units can detect scale and alert you when it's time to clean, making things incredibly easy. Models that don't have such fancy features should be inspected by a professional within a year of installation (or sooner, if you've noticed a decrease in performance). They'll be able to tell you how much scale has built up, and what your maintenance schedule should be.
The actual descaling procedure is usually outlined in the water heater documentation, and may vary between models. Depending on the severity of the buildup, the process can take from 45 minutes to over an hour. A pump, some valves, fittings and a bucket are the typical tools required. It may seem a bit pricey (mostly due to the pump), but if you're confident in your skills and understanding of the descaling procedure, doing it yourself is the way to go: consider how one or two maintenance appointments with a plumber will already equal the cost of the kit!

Source: https://www.plumbingsupply.com/tankless-water-heater-buying-guide.html
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