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A brief look at Residential Water heaters!


April 20th, 2012
Bhaskar Ale- MSME, E.I.T

Almost all houses have some type of water heating system in the US. Domestic hot water is used for various applications in a home such as washing, shower, cleaning etc.The main components of a water heater are; heat source, a heat exchanger, a piping system and flue vent (if gas, propane or oil) and plumbing fixtures like showers.

There are basically 3 types of heat sources for hot water heater:  gas, electric and oil. Since they are a major part of your utility bills, it can consume 15% of electricity and 25% of natural gas in residential home.

There are several ways that energy is lost in a water heater and they are:

  1. Demand – the energy used to heat up the incoming cold water up to a temperature setpoint as hot water in the tank. It depends upon the efficiency, occupant behavior, consumption, fixtures, schedules etc.
  2. Standby – the heat loss through the storage tank walls mostly due to conduction
  3. Distribution – the heat lost through the pipe and fixtures

Most houses have storage capacities of 30-65 gallons per unit. The water heater efficiency is measured based on energy factor which basically goes from 0.5 -1, where 1 being 100% efficient. The calculation assumes that the average use is 64 gallons of water per day. It is basically the fraction of water heater energy input that actually remains in the water leaving storage tank. The energy factor is 0.9 for electric heaters and around 0.59 for gas and oil.

There are basically 2 types of water heaters:

  1. Direct Storage water heaters
  2. Indirect Storage water heaters

Direct storage water heaters

Most houses in the US, especially single family homes use this type of system. They are usually less efficient going upto 80% efficiency. It is a combination of a heat exchanger and a storage tank. It can basically hold upto 30-80 gallons of water. It is generally insulated upto R7-R25 with fiberglass insulation on the outside. The higher the insulation the better the energy savings. Generally the combustion air enters through the bottom, rises up the flue which is surrounded by water which absorbs the heat from the gas. Draft diverter is attached to allow dilution air to enter flue to relieve pressure that would otherwise pull directly on the burner causing burner flame out.
Electric storage gas heaters on the other hand donot have any flue pipes hence there are no losses through flue. It is low maintenance and easy to install but the cost of operation is 1.5 to 2 times more expensive than gas or oil heaters due to the pricing of electricity.

More efficient models which have induced draft mechanism have also been developed recently. It basically uses a fan to pull the combustion gases through the flue. It doesn’t have a draft diverter and hence uses less air indoor air for more reliable combustion. Another system which is also frequently seen are the sealed combustion water heaters. Here the combustion air is drawn from the outdoors and fed into the burner. It is mostly found in mobile homes.

Indirect Storage water heaters

These are most common and most efficient types of water heaters available in the market today. They are very common in multi family homes. It is used for dual purposes i.e space heating and water heating. Here the principle is to heat water as it flows through the heater. There are no storage tanks. It is generally more expensive than storage tanks. It has a modulating gas valve to control the gas input required to heat the water in demand.

There are mainly 2 types of indirect water heaters:

  1. Immersed coil – here the boiler or solar water is circulated in the tank heating domestic water in the tank
  2. Tankless coil – are heat exchangers installed within a larger boiler for heating domestic water

Besides these systems there are also solar water heaters which are mostly classified as active or passive depending upon pump use.

After choosing the heater you must also learn how to use it efficiently and maximize energy savings. Here are a couple of methods to reduce the use of hot water :

  • fix leaks i.e if the tank leaks then it needs to be replaced
  • use flow controls such as low flow shower heads
  • insulate the tank to R-35 with fiberglass
  • use heat traps, that stop convection of hot water into the hot and cold water pipe above the storage water heater or external storage tank by the use of floating ball valves or delicate check valves.
  • insulate the pipes
  • have automatic flow controls for gas and water supply i.e supply per required demand
  • some maintenance techniques as well
    • flushing of storage water heater using curved tip tube and ball valves to drain and remove sediments from the bottom of the tank
    • prevent corrosion by installing a sacrificial anode that gives water something to corrode. It needs to be replaced every 2-3 yrs.

Hence I hope the brief description and solutions presented above gave you a glimpse of some water heater systems for the home and some basic maintenance and solutions that can be used to maximize savings. There are also various prescriptive rebates available from MassSave for high efficiency Water Heating systems. For more information you can also visit Masssave website. For more information on different types of heaters, also visit the DOE website.