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HVAC Hot Water Heating Systems

Parts of this story: Introduction >> Fuel burning units, general >> Forced warm air heating systems >> Steam heating systems >> Electric resistance heating >>Air conditioning systems

++CTA++Hot water heating systems, like warm air systems, are of two types, forced or hydronic and gravity.

Gravity systems are sometimes found in older single-family houses, but in most cases such systems have been replaced or converted to a forced hot water system. Gravity systems have no water pump and use larger piping. They tend to heat unevenly, are slow to respond, and can only heat spaces above the level of their boiler. Like gravity warm air systems, they are considered inefficient and normally should be replaced during the rehabilitation process.


The hot water boiler on left is gas fired. The one on right is oil fired. Both have standard pressure and temperature gauges.

Forced hot water systems are usually heated by gas- or oil-fired boilers. Occasionally they may use immersion-type electric resistance heating coils. These coils replace the burner found in gas and oil-fired boilers. The hot water pump and distribution piping for electrically heated systems are similar to those of gas- and oil-fired hot water systems and should be checked as described below.

Assess the condition of forced hot water heating systems as follows:

Boiler. Most hot water and steam heating systems have steel boilers with a service life of about 20 years. Cast iron boilers, which are less common, have a service life of about 30 years. Old cast iron boilers converted from coal fired units may last much longer but are usually quite inefficient. Inspect all boilers for signs of corrosion and leakage.


Run the boiler for one-half hour or longer and check for leaks. Occasionally a boiler fitting will leak slightly before it warms up, expands, and returns to a watertight fit. Dont confuse condensation droplets on a cold boiler with water leaks.

Expansion tank. The expansion tank is usually located above the boiler (although it may be in the attic) and is connected to the hot water distribution piping. Most tanks are compression-type tanks that are designed to permit heated water to expand against a cushion of pressurized air within the tank. When the tank loses air, it becomes waterlogged and expansion cannot be accommodated. Instead, water discharges from the boilers pressure relief valve each time the system heats up. Check for such a condition.

Waterlogged expansion tanks should be drained and re-pressurized. This should be done by a heating or plumbing contractor.

expansion tank

This hot water expansion tank is located above the boiler. Look for signs of leakage in expansion tanks.

Boiler controls. All boilers should be equipped with a pressure gauge, a pressure relief valve, and a pressure reducing valve. The pressure gauge indicates the water pressure within the boiler, which should normally be between 12 and 22 psi (83 to 153 kPa). A temperature gauge may be included in the pressure gauge.

The pressure-reducing valve (actually a water make-up valve) adds water to the system from the domestic water supply when the boiler pressure drops below 12 psi (83 kPa). Pressure readings lower than 12 psi indicate a faulty valve that should be adjusted or replaced.

The pressure relief valve should discharge water from the system when the boiler pressure reaches 30 psi (207 kPa). Look for signs of water near the valve or below it on the floor. High pressure conditions are usually due to a waterlogged expansion tank. If the boiler also generates domestic hot water, high pressure may be caused by cracks in the coils of the water heater, since the domestic water supply pressure usually exceeds 30 psi (207 kpa). The pressure relief valve should be mounted on the boiler.


As a last resort, the pressure relief valve may be tested by a service technician. But since it may be old or clogged and become stuck in the open position, the test should not be performed without having a replacement valve on hand and the proper tools for removing and reinstalling the valve and extension pipe.

Hot water boilers should have a high-temperature limit control or aquastat that shuts off the burner if the boiler gets too hot. Check for such a control.

Circulating pump and controls. The circulating pump forces hot water through the system at a constant flow rate, usually stated in gallons per minute (gpm). It should be located adjacent to the boiler on the return pipe near the boiler. The pump may be operated in one of the following four ways:

  • Constant-running circulator, in which the pump is controlled by a manual switch. The pump is usually turned on at the beginning of the heating season and runs constantly until it is turned off at the end of the heating season. The boiler is independently activated by the thermostat as heat is required.
  • Aquastat-controlled circulator, which turns the pump on and off at a preset boiler temperature (normally 120 F [49 C]). Like the constantly running circulator, the burner is independently activated by the thermostat as heat is required.
  • Thermostat-controlled circulator, in which water is maintained at a constant temperature in the boiler by an aquastat.
  • Relay-controlled circulator, in which the pump is activated (via a relay switch) whenever the boiler is activated by the thermostat.


Determine which kind of device controls the pump and check its operation. Inspect the condition and operation of the pump itself. Listen for smooth operation; a loud pump may have bad bearings or a faulty motor. Inspect the seal between the motor and the pump housing for signs of leakage. Examine the condition of all electrical wiring and connections. Feel the return line after the system has been operating for a short time; it should be warm. If it isnt, the pump may be faulty.

In heating systems used for generating domestic hot water, the thermostat will control the circulating pump and an aquastat will control the burner.

Distribution piping. The forced hot water distribution system consists of distribution piping, radiators, and control valves. Distribution piping may be one of three types: series loop,one-pipe, and two-pipe.

In a series-loop system, radiators are connected by one pipe directly in a series. Since the last radiator will receive cooler water than the first, downstream radiators should be progressively larger. Alternatively, series-loop systems may be divided in small zones to overcome this problem.

One-pipe systems differ from series-loop systems in that their radiators are not connected in series. Instead, each radiator is separately attached to the water distribution pipe with a diverter fitting, which is used to regulate the amount of hot water entering it.

one pipe heating system

One-pipe forced hot water heating system

Two-pipe systems use separate pipes for supply and return water, which ensures a small temperature differential between radiators, regardless of their location. Individual room control is possible with both one-pipe and two-pipe systems, although a change in the valve adjustment on one radiator will affect the performance of others downstream.

Distribution piping should be checked for leaks at valves and connections. Make sure pipes are properly insulated in unheated basements, attics, and crawl spaces.

When the distribution piping is divided in zones, each zone will have either a separate circulation pump or a separate electrically operated valve.


Check the operation of all zone valves by activating each thermostat, one at a time. If hot water is being distributed properly to each zone, the radiators in that zone should be warm to the touch within several minutes. Locate all valves, inspect their electrical wiring and connections, and look for signs of leakage.

Radiators and control valves. Radiators are of three types: cast iron (which in most cases are free standing but sometimes are hung from the ceiling or wall, convector (which may have a circulating fan), and baseboard.

Older residential buildings usually have cast iron radiators that are extremely durable and normally can be reused, although they are less efficient than convectors.

Baseboard radiators are considered the most desirable for residential use because they are the least conspicuous and distribute heat most evenly throughout the room.

Radiators should be located on outside walls whenever possible.


Activate the system and look for signs of water leakage. Feel the surfaces of all radiators to ensure that they are heating uniformly; if they are not, bleed them to remove entrapped air. Examine the fins of all convectors for dirt and damage. These fins can be combed straight. Check the condition of all radiator control, safety, and bleed valves and make sure they are operational. Often the valves need tightening or their packing needs replacing.

Radiant panel heating. Hot water distribution piping may be embedded in floors, walls, and ceilings to provide radiant heating. Because the piping is embedded, it can only be inspected by looking for signs of water leakage or rust on the floor or wall surfaces that cover the piping. Such heating is normally trouble free, unless there are major structural problems that damage distribution piping and joints, or unless piping includes copper pipe and steel pipe that are not separated by a dielectric coupling.


Activate the system and look for signs of water leakage. Feel the surfaces of all radiators to ensure that they are heating uniformly; if they are not, bleed them to remove entrapped air. Examine the fins of all convectors for dirt and damage. These fins can be combed straight. Check the condition of all radiator control, safety, and bleed valves and make sure they are operational. Often the valves need tightening or their packing needs replacing.

Operate radiant heating systems to determine their functional adequacy. Radiant surfaces should be warm to the touch within several minutes. Check the condition of the shutoff valves for each distribution zone and the main balancing valves near the boiler and look for signs of leakage.

Inspect the expansion tank and air vent (if any) in accordance with the subsection on expansion tanks (above). If the system appears to be in good condition but heating is not adequate, consider having a service technician pressure test the distribution piping for a period of up to 24 hours. If a drop in pressure occurs, there is a leak. When a leak is detected, have the service technician flush the piping to check for galvanic corrosion.

Editor's note: This story is adapted from the U.S. Department of Housing and Urban Development's Residential Rehabilitation Inspection Guide, 2000.Click here for other stories in this series.

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