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You wake up early in the morning about to take a shower; you turn the hot water knob and it’s… cold? While a cold shower is certainly one way to wake up, you're probably more concerned with the source of that icy irrigation. The culprit is probably your hot water heater. With the numerous types of hot water heaters available today, it’s helpful to get a primer on each type and know how to diagnose issues and when to call in some help. Get to know the world of water heaters and its components in depth so you can make the right decisions about maintenance or replacement.


Burners, vents and flues for hot water or steam systems can be the same as for warm air systems. Inspect burners, vents and flues the same way you would inspect warm air systems.

Check the altitude pressure gauge to determine whether or not there is water in the boiler. 1.0 PSI will lift water 28 inches; 4.34 PSI will lift water 10 feet. If you are inspecting a two-story house, the height of the radiators or convector baseboard heat will be 15 feet to 20 feet above the boiler. Assuming this is true, you need about 10 pounds of pressure in the system to lift water to the top of the radiators or convectors. Add a few pounds for air or water losses over the months or for bleeding.

A three-story house would demand more pressure. If there is no pressure in the system, the boiler could be damaged. The bottom side of the gauge, which shows altitude pressure, is a water temperature gauge. This gauge is one of the first things you should check. Inspect the area around the bottom of the boiler inside and out for visible leaks or previous stains. If the heater has been off for a long period of time, you may see considerable water develop when you turn the unit on. This may be condensation, and not a defect.

Check the metal heater cabinet. Rusting and failure of the metal may indicate a failure of the sealing material between the boiler tubes. When the heat escapes from the boiler area and comes in contact with the cooler metal cabinet around the heater, condensation forms and eventually rusts out the cabinet. Repairs/sealing are possible, however, they tend to be temporary.

Pressure Relief Valve
Inspect the pressure relief valve for seepage or corrosion, present or past. This indicates excessive pressure in the system.

Possible Causes:

  • Failed or waterlogged expansion tank
  • Failed pressure relief valve
  • Too much water in the system; leaking water feed
  • Failed domestic hot water coil
  • Limit Controls

Most of the systems we see will have a combination control with a high and low setting. If the system has a domestic hot water coil in the boiler, there will be a differential dial with a 15-degree range.

A typical heating cycle would be as follows:

  • Thermostat calls for heat; burner comes on.
  • When the temperature of the water in the boiler reaches the low limit, the circulator will come on.
  • When the temperature in the boiler reaches the high limit, the burner turns off.
  • The circulator will continue to run until the water temperature drops below the low limit range.
  • If the thermostat is still calling for heat, the burner refires and starts another cycle until the thermostat has reached the desired temperature.
  • The domestic hot water control operates at the low limit and within the range set at the differential dial. Assuming the low limit is set at 150 degrees and the differential dial is set at 15 degrees. The water temperature in the boiler will be maintained between 135 and 150 degrees.

This also assumes the thermostat is not calling for heat at which time the controls would respond to the heating temperatures. There are numerous limit controls on the market and just as many electrical control variations. Most newer limit controls have the burner and circulator start at the same time.

Zone Valves
Zone valves are installed in the line or path of a heating loop. A thermostat operates a low voltage relay, which controls a plunger or gate valve. A single circulator can effectively service multiple zone valves. Each zone valve and heating loop needs a thermostat to control its operation. Most zone valves have a manual control lever, which physically lifts the plunger or gate out of the heating loop. This will supply uninhibited heat in case of zone valve failure.

To inspect zone valves, each thermostat has to be operated separately to make sure the burner comes on and heated water is delivered to its receptive loop.

Expansion Tanks

  • Open system tank -- installed at the top of the system.
  • Closed system tank -- installed above the boiler.
  • Diaphragm tank -- installed above the boiler.

In open systems, the tanks at the top of the system are at zero atmospheric pressure. If too much water is added or if too much pressure is developed in the boiler, the water is discharged harmlessly onto the roof.

In closed systems, the tanks installed above the boiler have system pressure in the tank. When the system is filled, water is pushed into the tank, compressing the air in the tank. Air is highly compressible and acts as a cushion, which controls moderate pressures.

These tanks can become waterlogged or fail. If they become water logged, they or the system can be drained to re-establish the air cushion. Failed tanks rust out at the top of the tank. Oxidation will occur when you have air and moisture. This condition is present at the top of the tank where oxygen is present. Water covers the bottom of the tank, where oxidation is less likely to occur.

When the metal fails at the top of the tank, the warm, moist air in the tank escapes. When this air comes in contact with the cooler, dryer basement air, it will have the tendency to condense. When condensation occurs, a water droplet develops on the top of the tank. When the drop is large enough, it rolls down the tank. When this water evaporates, it leaves stains on the tank. The failures in the tank may let air out but may not be large enough to leak. The average life expectancy of a tank in a closed system is 40 to 60 years.

Diaphragm tanks have a bladder or membrane, which keeps air and water separated. These tanks are relatively stable, but the air, which is separate from the water, has no place to go. This air provides a cushion for the pressure in the system.


  • Cast iron radiators
  • Cast iron baseboard
  • Copper tubing with aluminum fin baseboard convectors

Radiant heat
Radiators, baseboard and radiant heat deliver heat mostly by convection. Objects absorb the energy and transform it into heat.

Radiant heat is a uniform pattern of piping, usually copper in a sand bed below a concrete floor. Hot water moving through the piping warms the concrete floor and the room above. Concern with such a system is that it may leak. Because the pipes are below the floor, leaks may be undetected for months and possibly years if the heating system has automatic water feed.

When inspecting this type of system, pay close attention to the altitude pressure gauge. If you do not have proper pressure, you should slow down and investigate. If the system has an automatic water feed, you should manually turn the water off and watch the pressure. Allow ample time so you are relatively certain there is no pressure loss. Typical life expectancy of radiant distribution systems below concrete floors is 25 to 30 years.

Modern radiant distribution systems are PEX piping. There are versions of this piping emerging every few years. It appears to have the attributes needed for long life.

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Basic Components

Cold Water Inlet Pipe
The inlet pipe (usually copper) is connected to the main cold water supply pipe and provides water to the tank.  The average temperature of the incoming cold water is 50° F.

Cold Water Control Valve
The cold water control valve allows the incoming cold water to be turned on and off.  The valve may be turned off when the water heater is not in use, in an emergency when the water heater leaks, or when the water heater is being replaced or breaks down .

Cold Water Dip Tube
To prevent the incoming cold water from cooling the hot water leaving the tank (at the top),  the cold water is introduced at the bottom of the tank (which is also closer to the heat source). When the cold water inlet pipe is located at the top of the tank instead of at the bottom of the tank on the sidewall, a dip tube is used to direct the cold water to the bottom of the tank.

Hot Water Outlet Pipe
The outlet pipe is a short pipe that extends into the top of the tank.  Since the water at the top of the tank is generally hotter than the bottom (heat rises), the hot water is always taken from the top.  The average temperature of the hot water is 140° F,  however, for safety and energy conservation reasons, water heaters are often set to 120° F.

Anti-Corrosions or Sacrificial Anode
A rod placed in glass-lined metal water heater tanks to help prevent the tank from rusting (oxidizing).  The anode rod, usually made of magnesium, attracts the corrosion-causing oxygen in the water.  The rod protects the tank from eroding, and eventually wears away.  The rod must be replaced to prevent the metal tank corrosion from accelerating.  Replace the anode every year or as necessary.

Drain Valve
The drain valve at the bottom of the water heater allows the tank to be drained.  Sludge or sediment from the water or the sacrificial anode will collect at the bottom of the tank over time.  Excessive sludge/sediment may lower the water pressure or reduce the water heating efficiency.  Recommended maintenance includes draining the sludge once a year.

Temperature/Pressure Relief (TPR) Valve (TPRV)
The temperature pressure relief (TPR) valve is a safety device that is designed to:

  • Prevent the water in the tank from exceeding 212° F, and
  • Prevent the water pressure in the tank from exceeding 150 pounds/square inch (psi).

TPRV Discharge Tube
A tube or pipe that is attached to the TPR valve to direct the superheated water down to the floor and away from anyone in the discharge area in order to prevent scalding or burning.

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The electric water heater uses two immersion heating elements, one near the top of the tank and one near the bottom to heat the water.  The heating elements are electrical-resistant elements that operate in a similar fashion to an electric stove-heating element.  The unit is powered by a 220-volt circuit.

Heating Elements and Controls
Each heating element has a thermostatic switch that turns the individual heating element on and off.  Both thermostatic switches are usually set to the same temperature.  When the water temperature (at the top or bottom) drops below the preset limit, a control switch closes to create an electrical circuit through the heating element.  The electric current heats the element, thereby heating the water.  When the water temperature reaches its preset limit again, the heating element will turn off.  Thermostats are adjustable and are set between 120°F-130F for increased energy savings and scald protection.

Lower and Upper Heating Elements
The lower element maintains the water temperature in the tank (when idle) because the water at the bottom of the tank will cool first (heat rises).  In addition, the lower element heats the cold water entering the tank through the dip tube (when hot water is being drawn out of the top of the tank).

When the lower heating element cannot keep the water being drawn of the top of the tank hot (e.g. high hot water demand), the upper element activates and will try to heat the warmer water at the top tank as it is drawn.  Note: most electric water heaters are designed so that both elements will NOT work at the same time.  The lower element will shut off when the upper element is active.

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Gas-fired water heaters have a gas burner located below the tank, a thermostatic switch, a gas valve and a pilot light. The thermostat will open the gas valve when it senses the water temperature at the bottom of the tank has dropped below the present limit. Gas flows to the burner and is ignited by the pilot light, heating the tank from below. In addition, the hot exhaust gases are vented through a hollow core in the center of the tank to help heat the water. The exhaust is then passed into the draft hood and to the vent connector where it is discharged to a chimney flue.

Gas piping in residential applications is typically black steel. Galvanized steel can be used, however, it is more expensive and not necessary. Seamless steel, copper and aluminum alloy tubing are allowed with gases and are not corrosive to the piping materials.

Aluminum alloy tubing is not to be used on the exterior or underground. Corrugated stainless steel tubing (CSST) should comply with performance requirements of Standard Fuel Piping Systems.

Plastic pipe, tubing, and fittings are for underground use only and should conform to Standard Specification for Thermoplastic Gas Pressure Pipe and Fittings. Piping should be marked “gas” and “ASTM D 2513.”

Interior gas piping should not be installed through a circulating air duct, clothes chute, gas vent, ventilating duct, or dumbwaiter or elevator shaft.

An individual gas shut off valve should be present at each appliance, or no further than 6' maximum, visible and accessible.

Sediment Traps, sometimes referred to as drip legs, are commonly required to be present and immediately upstream of the shut off valve. The idea being any debris will be trapped in the 'leg' before it's able to reach the gas shut off valve.

The above ground portion of the gas piping system, upstream from the equipment shutoff should be bonded to a grounding electrode. The gas piping is not to be used as a grounding electrode.

Home inspectors do not check gas pressure, however, testing should not be more than 1 ½ times the maximum working pressure and not less than 3 psig for 10 minutes.

Unvented/ventless room heaters are not to be installed in bathrooms or bedrooms. Check with your local government, as many communities have restrictions in place and do not allow any unvented or ventless type heating systems to be present inside a residence.

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Oil-fired water heaters use an oil burner and a combustion chamber (similar to an oil-fired furnace), which is located below the tank, as the heat source to heat the water.  When the thermostat (at the bottom of the tank) senses that the water temperature has fallen below the preset limit, the oil burner will ignite.  A jet-like flame is created in the combustion chamber to heat the bottom of the tank.  In addition, the hot exhaust gases from the combustion chamber are vented either through a hollow core at the center of the tank (like a gas-fired system) or around the tank to help heat the water.  The exhaust is then passed into the vent connector and directed to a chimney flue.

Once the water temperature reaches the set limit of 120° F to 140° F, the oil burner is turned off.

Note:  Fuel-fired systems heat the water by heating the tank. As a result, the tank experiences more wear and tear than with an internal electric heat source.  In addition, proper venting of the exhaust is always important to prevent carbon monoxide poisoning.  Be sure that the vent/exhaust system is properly installed. 

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Coil Systems
Homes with a hydronic (hot water or steam) home heating system can use the tankless or coil system. A pipe coil built into the boiler and the water from the the cold water supply is heated as it passes through the coil. To accommodate periods of high demand, small storage tanks are sometimes connected to increase hot water availability. A cold water mixing valve is usually installed as a safety device to prevent scalding. Due to the extremely high temperature of a boiler, the hot water leaving the coil is beyond the safe limit of 120° F to 140° F. The mixing valve tempers the hot water by adding cold water to the hot water leaving the coil.

Generally, these systems are advantageous in regions where the furnace is on most of the year. There is no cost for maintaining and heating a large volume of stored hot water, and by sharing the boiler, no separate heat source and fuel is needed. However, during warm seasons, the boiler will have to generate the same amount of heat used to heat the house but only to heat the water. As a result, the the fuel saving is lost.

Instantaneous Hot Water Heaters
In addition to boiler-mounted tankless heaters, stand-alone units are available. Gas-fired instantaneous water heaters have a heat exchanger with a built-in coil. Like boiler-mounted systems, water from the the cold water supply is heated as it passes through the coil. Likewise, there is no cost for maintaining and heating a large, stored volume of hot water. However, stand-alone units typically have a lower flow rate than boiler-mounted systems and may fall short during periods of high demand.

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TPR Valve and Discharge

The temperature pressure relief (TPR) valve is a safety device that is designed to:

  • Prevent the water in the tank from exceeding 212° F, and
  • Prevent the water pressure in the tank from exceeding 150 pounds/square inch (psi).

Basic Physics (Temperature and Pressure of Water)
Water boils at 212° F when at sea level or under 14.7 psi of pressure, however, the boiling point increases when under pressure, which allows the water to superheat beyond 212° F.  Typical residential water pressure ranges between 40 and 80 psi and at 50 psi, the water can reach 300° F.

Potential Problem
If the water heater's thermostat, which controls the heat source, malfunctions, the pressurized water in the tank could continue to heat and superheat (beyond 212°F).  This will cause two problems:

First, since water expands when heated, the water pressure in the tank will increase as the water is superheated.  If the pressure exceeds the maximum pressure threshold (approximately 300 psi) the tank could rupture or even explode. Second, the release of superheated water (now significantly above 212°F) would immediately return to atmospheric pressure (the boiling point of water is 212°F) and flash into steam, causing a sudden increase in volume and release of energy.  The force of the flash steam would be tremendous.

How it Works
If  the water reaches a temperature of 210°F (before superheating occurs) or the water pressure exceeds 150 psi, the valve will open and release the overheated water.  This will allow cold water to enter, lowering the temperature of the water in the tank and consequently the water pressure.  The valve will remain open as long as the temperature or pressure exceeds the preset limit.  The valve must be located on the tank at the top or on the side near the top, where the water is generally the hottest.  TPR valves are not included with the water heater, so be sure that the TPR valve matches the BTU (heat rating) rating of the water heater.  A Discharge Tube must be attached to all TPR valves.

Discharge Tube
A discharge tube is a tube or pipe that is attached to the TPR valve that directs the superheated water down to the floor and away from anyone in the discharge area to prevent scalding or burning.

The pipe itself must be made of a material that is rated for both high temperature and pressure, which includes most rigid wall copper or iron.  Also, the size of the pipe must match the opening size of the TPR valve discharge (usually ¾ inch).

The drain/drip tube must extend to within 6" to 12" of a flush type floor drain. With bowl type floor drains the drain/drip tube can over hang the side of the floor drain. An air gap must be maintained between the end of the drain/drip line and the floor drain to prevent backflow. Thus, any hot water escaping from the TPR relief valve will be directed safely into the floor drain.


If the discharge tube is routed to the exterior, the pipe must discharge 6 to 24 inches from grade, with a downward slope to prevent the pipe from clogging or forming a trap.  Blocked discharge tubes will prevent the superheated water from discharging and will burst.  Though counterintuitive, it is often recommended that the discharge tube terminate next to the water heater so that any malfunction of the water heater will be more readily noticed.

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Safety and Efficiency

The following are general safety and installation requirements for a standard water heater.

  • All water heaters placed in the garage must be 18 inches off the floor to prevent accidental combustion of gasoline fumes in the garage.
  • All water heaters in the garage must have a physical barrier to prevent impact damage from automobiles.
  • In some locations, water heaters must be mechanically secured (i.e. earthquake straps) to prevent the system from being dislodged.
  • Gas or oil-fired water heaters should not be located in sleeping areas.

Heaters are generally set to 120 ° F to help prevent accidental scalding.  However, with some dishwashers that do not have a heating element to raise the temperature of the water, spotting of glassware and dishes may occur.  Check the dishwasher's manufacturer information for the recommended temperature setting.
Require properly installed temperature and pressure relief valve and discharge tube.

Recovery Rate
Recovery rate is a measure of the number of gallons water that can be heated (from approximately 50° F) to 100° F in one hour. The faster the recovery rate, the more water can be used without running out of hot water. Generally, oiled-fired systems create the most heat and have the fastest recovery rate followed by gas-fired systems and then electric systems. Since fuel-fired heaters have a higher recovery rate, a large tank may not always be needed.  Conversely, larger tanks that hold more usable hot water, do not require a faster recovery rate and can be sufficient for intermittent use. Either way, when choosing a water heater, select one with an appropriate capacity and recovery rate to match the daily hot water demands of the home.

An external insulation blanket wraps around the sides of the water heater.  Insulation blankets prevent heat from radiating from the tank and help reduce the fuel cost associated with maintaining the 120° F to 130° F storage temperature. To determine if one is needed, feel the water heater jacket. If it is warm, heat is escaping and the tank should be insulated. If it is cool, no insulation is needed.

Newer heaters do not require and actually have warnings posted not to add an insulation blanket. With gas or oil-fired systems, the insulation should be kept away from the burner area. No insulation should be installed on the top of the heater

Pipe insulation of the hot water line will also help increase efficiency.

Peak Timers
Peak timers are installed on electric heaters and allow the water heater to turn on during specific times of the day. However, this requires that all activities using hot water (e.g. bathing, laundry, dishwashing, etc.) be accomplished at generally the same time of the day.

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