Replacing Water Heaters

Our approach to improving water heating system performance is to make better use of what is already present, unless the water heater happens to be really outdated.  Water heating equipment needs to be changed frequently enough that it may be more cost-effective to wait.

When the water heater is replaced, comply with current building codes.  Installers either ignore or fail to realize replacing water heaters is not a repair, so a building permit is normally required.  Too often, we find things that were meant to fixed when the water heater was changed out has been left undone.

Topics discussed in this article include . . .

  • Water Heater Performance Targets
  • Preparing the House for a New Water Heater
  • Choosing & Installing the Water Heater
  • -- Storage Water Heaters
  • -- Tankless Water Heaters
  • Post Water Heater Installation Testing

Customer  Performance Targets for Water Heaters

  • Design water heating system with energy-efficiency in mind
  • Provide just enough hot water for peak-hour use
  • Install an energy-efficient model that makes financial sense
  • Add most efficient model when energy is the primary consideration

Prepare House for Water Heater Improvements

Review Recommendations. Before installing a new water heater, consider whether it needs to be relocated in the house.  It’s also the time to consider other improvements to the water heating system.  Specific recommendations should be in your report.  General recommendations are discussed in the Water Heating Systems article.

Combustion Safety. If it’s a gas, oil, or propane water heater, ensure the air pressure in the area it is installed in won’t cause it to backdraft under worst-case conditions.  See the article on Combustion Safety.

Energy Supply.  When larger water heaters are installed, or storage tanks are replaced by tankless water heaters, a bigger energy supply may be needed.  The electrical service may need to be upgraded or the gas supply lines made larger.
Implement Water Heater Improvements

Water Heater Types.  There are several types of water heaters: storage, tankless, heat pump, solar, and integrated with the heating system.  To complicate things, water heating systems can be created by combining types.

Storage vs. Tankless. Rather than exploring the range of water heater types and possible combinations, two types of water heaters are discussed in this article: storage (conventional) and tankless.  

These are the common types found in most houses.  They also use two different water supply methods.  Understanding both methods will help evaluate all other types of water heaters and combinations available.

Decision Factors. Water heating decisions come down to affordability, water use, and availability.   Energy efficiency should be considered after these factors. 

To be sure, there are very efficient water heaters, but the life cycle cost is prohibitive.  If high energy efficiency is a major consideration, we are prepared to discuss other types.

Affordability.  The life-cycle cost of a water heating system is the cost of equipment, installation, utility bills, and maintenance.  Most of us are willing to close attention to energy efficiency after knowing we are able to pay for it.

Water Use. Hot water use is dependent on availability, climate, economic status, work schedule, and age. As such, it is the most variable class of energy consumption.  Families with small children use far more hot water than senior citizens.

Availability.  Acceptable hot water supply is having it available when and where needed.  Most people expect hot water is available regardless of how many plumbing fixtures and appliances happen to be in use at the time.  You should be able to get a shower and run the washing machine and dishwasher at the same time.  At least they do when I clean house.

Storage Water Heaters

The most familiar water heater is the conventional storage tank.  They permit a large volume of water to be used at flow rates greater than the water heater’s ability to produce more hot water.

Selection Factors.  The variables for deciding which storage water heater to buy depends on water use, tank size, and recovery rate.  Once these needs are met, energy efficiency is considered to compare models.

Water Use = Stored Hot Water + Recovery Rate

Water Use. A storage water heater needs to be installed capable of supply all the hot needed during the part of the day when the heaviest amount of water will be used.  This is called the First Hour Rating.  It’s on the upper left-hand corner of the yellow Energy Guide label.

Calculating Water Use. The Federal Housing Administration (FHA) has guidelines for deciding the minimum size storage water heater to install.  The first decision is the peak hour water use.  It’s determined as follows:

Water Use Factor          Gallons
Each Person 10
Each Full Bath 10
Clothes Washer 20
Dishwasher 10
Hand Dish Washing 5

When a house is built, it’s unlikely the actual number residents and use of washers and dishwashers are known.  The building code determines the number of occupants based on bedrooms: # of bedrooms +1.  Two people are assumed to share the master bedroom.  Houses are also built with laundry facilities.  Most houses have a dishwasher.

An additional 3 gallons is added to the total.  We call it a universal fudge factor.  It’s probably the water needed to purge the cold water out of the hot water lines.

According to this formula, a 3 bedroom, 2 bath house has a peak hour hot water use of 73 gallons.

Smallest Choice. To pick a water heater, select one with a First Hour Rating that meets or just exceeds calculated water use.  Choose the minimum size that will just meet needs to avoid heating water needlessly along with its fuel cost.  Going smaller potentially causes a shortage of hot water.

Opportunity!  In the United States, we tend to live in big houses.  It’s not uncommon for 2 people to live in a 4 bedroom house with 3.5 bathrooms.  The FHA formula requires an 88 gallon peak hour use water heater for the house.  The real need is closer to 53 gallons. 

As long as you’re living in the house, by all means, downsize the system!  However, when you sell the house, plan to reinstall a larger water heater consistent with the size of the house.  Doing so will avoid needlessly heating 33 gallons of water every day.

Recovery Capacity.  Recovery capacity is the amount of water produced at a given energy efficiency and energy input in one hour.

Gas water heaters produce hot water faster than electric ones.  Therefore, gas water heaters need to store less water to meet the peak hour water use.

Tank Size.  Water heater storage tank size is determined by subtracting the Recovery Capacity from Water Use.  The other consideration is to choose a shape that fits were it is to be installed.  Short, fat and tall, skinny models are available.

Energy Efficiency.   For existing houses, the type of fuel used for water heating will probably be whatever the current one is using.  The hard part of selecting a water heater comes down to making choices about energy efficiency ratings.  This is where the Energy Guide label helps.

Our preference is to choose the one with the highest affordable energy efficiency.   Find out what tax credits and rebates are available from governments, utility companies, and manufacturers.

As with all things, the law of diminishing returns applies.  It does not make financial sense to purchase a highly energy-efficient water heater that has a higher life-cycle cost.


Tankless Water Heaters

Many households are considering switching to a whole-house tankless water heater.  Advertisements claiming up to 50% savings in water heating bills is compelling. 

Tankless water heaters provide hot water on demand without storing any.  They should deliver all the hot water desired to all the plumbing fixtures calling for it. Storage water heaters provide hot water at the desired flow rate by having some in reserve.  Tankless water heaters must heat the water to the desired temperature at required flow rates for various plumbing fixtures.

Selection Factors. The variables for deciding which tankless water heater to buy depends on water use, flow rate, and cold water temperature.  Once these needs are met, energy efficiency is considered to compare models.

Water Use.  The number of hot water plumbing fixtures that can be used at the same time is the limiting factor of tankless water system.

Tankless water heaters are able to supply water in the 1.2 – 6 gallons per minute (gpm) range.  The Plumbing code dictates what the flow rate for each plumbing fixture is.  The formula for water use is as follows:

Unit Max Flow Rate = Flow Rate of Fixture 1 + Fixture 2 + . . . + Fixture n

It’s true some cold water is mixed with hot water, so the argument could be made that there is more hot water available than this formula allows.  Be careful! Hot water is defined at 110-120 degrees—a temperature a tankless water heater may find challenging to meet anyway.  At these temperatures, I don’t bother to turn the cold water on!

To be on the safe side, ignore tempering with hot water.  Just assume the entire flow is hot water.

Flow Rate.  To figure out what combination of plumbing fixtures can be used at the same time, know what the flow rates for various hot water plumbing fixtures are.



Plumbing Fixture Plumbing
Bathtub 4 2-4
Clothes Washer 4 ---
Shower 3 1.5 - 3.0
Dishwasher 2.75 1.0 - 3.0
Kitchen Sink 2.5 1.0 - 2.5
Bathroom Sink 2 ---
Laundry Sink --- 2.5 - 3.0

From the information in this table, the best a maximum flow-rate tankless water heater can do is reliably deliver hot water to 2 plumbing fixtures at the same time.  It may be possible to stretch it to 4 by accepting cooler water temperatures.

One way to overcome these limitations is by adding additional tankless water heaters.  Otherwise, there are real limits to available water occupants have to plan around.

Ambient Water Temperature.  The minimum temperature rise a tankless water heater must meet is raising the temperature to 110 degrees—the Plumbing Code definition of hot water.

Some of the better gas tankless water heaters are able to raise the temperature 90 degrees.  They could start with ice water to just meet the definition of hot water—for a few gallons per minute.  Expect the temperature to drop considerably after 4 gpm though.

Choosing a Tankless Water Heater.  To replace a storage tank, a whole-house model is needed (6 gpm).  To have the same availability of hot water as provided by a storage tank, add more tankless water heaters.

Electric Tankless Water Heaters.  Don’t even bother with an electric whole-house water heater.  They may not even be available any more.  First, the temperature rise is around 50 degrees, rather than 90, so warmer water is needed.  Second, the ability to heat water decreases rapidly as flow rate increases.  Basically, they can’t heat water fast enough to replace conventional water heaters when the ground water is cold.

Gas Tankless Water Heaters.  Currently, only gas tankless water heaters are capable of delivering 6 gallons per minute for a whole-house unit.  The energy input is much higher than required for a gas storage water heater, so larger gas lines are needed to supply it.

Energy Efficiency.  What about the claims for saving up to 50% of the water heating bill?

For homes using less than 41 gallons per day (the average is 64), the savings rate is 24-34%.    If 86 gallons are used per day, the savings rate is 8-14%.  The savings rate of 27-50% is possible if one is installed at each hot water plumbing fixture.  However, whatever is saved in energy costs is more than consumed in equipment and installation costs.

Consumer Reports says it best.  Tankless water heaters are energy efficient but they’re not necessarily economical.  They recommend storage type water heaters.  If energy efficiency is a primary consideration, tankless water heaters may be worth it.

Other Considerations.  Besides being limited to the number of hot water plumbing fixtures that can be used at the same time, there are a number of other considerations as well. 

First, expect a shot of cold water!  The last batch of hot water will be purged by cold water before newly heated water is available.

Second, when only a trickle of water is needed, such as for shaving, the water heater may never come on because the flow rate is too low.

Third, the units are high maintenance to keep them functioning well over their life.

Finally, installation costs are much higher too.

Post Water Heater Installation Testing

When the contractor is done installing the water heater, final testing is needed for gas, propane, or oil water heaters, sometimes by a third-party who didn’t do the work.

For home performance contractors, final testing is mandatory when there are combustion appliances are present.  The concern is ensuring the areas fireplaces, water heaters, and furnaces are in are not depressurized to the point of causing smoke to backdraft into the house.

Where there are incentives or certificates offered, the sponsor usually requires final testing too.  Their considerations are to verify better equipment was installed, duct leakage meets or exceeds performance targets, or system performance is improved.

We are ready, willing, and able to perform third-party testing.

In Conclusion . . .

Better water heating system energy performance is available by making improvements just short of replacing the water heater.   They are low-cost and make a big difference.

Water heaters don’t last forever.  A more energy-efficient model should be available when it’s time to replace one.   Significant energy improvements are usually made to newer models since the current water heater was installed.  Storage water heaters typically last 10-15 years.  Tankless water heaters last 20 years.

New water heaters should just meet household needs at the peak hour of use.  Multiple hot water plumbing fixtures will probably be used during that time.  The water heater needs to meet all the demands.

The most affordable energy-efficient model should be installed.  The highest energy efficiency model should be installed only when the primary consideration is energy efficiency as these models will not make financial sense in terms of life-cycle cost.

Here’s to a more comfortable and energy efficient house!



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