Steam And Hydronic Line | Heater Service & Troubleshooting

Electric Zone Valve

Some hydronic heating systems use electrically operated valves to control the flow of water into each zone. In these zoned hydronic systems, the valves operate in conjunction with a single circulator. The construction details of a typical electric zone valve are illustrated in Figure 10-62.

When the room thermostat calls for heat, an electrical current is sent to the valve operator. The current flows through a normally closed switch and around a coil called a heat motor. The heat created in the heat motor causes a piston to move out and push against a spring-loaded lever that normally holds the valve closed. This action opens the zone valve. At the same time, the piston extends a bit further and trips an end switch that sends a signal through a relay back to the circulator, turning it on and sending water into the zone. The piston extends a bit further and reverses the sequence. This in-and-out movement of the valve piston will continue for as long as the room thermostat calls for heat.

Flow Control Valve

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A flow control valve is used (1) to prevent gravity circulation in a hydronic heating system when the circulator is not operating and (2) to permit the summer–winter operation of an indirect water heater. As shown in Figure 10-61, a flow control valve can be installed in either a vertical or horizontal position. Either location will require that it be installed with the arrow on the valve body facing the direction of flow.

When the circulator is operating, water passes through the flow control valve. When the circulator stops operating, the flow control valve remains closed and blocks the gravity flow of the water. A knob on top of the valve allows it to be manually opened to drain the system or to bypass it if there is a loss of electricity and only partial heating is possible. The knob is turned clockwise for the normal position and counterclockwise for the manual-bypass position.

Flow control valves do not require service or maintenance. A defective flow control valve should be replaced.

Hot-Water Heating Control

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A hot-water heating control consists of an outdoor liquid expansion type bulb connected by a capillary system to a double-ported three-way automatic mixing valve (see Figure 10-59). It is designed to blend the hot water from the boiler with the cooler return water in inverse proportion to the outside water and deliver the blended water to the circulating system.

In operation, the outdoor bulb reacts to changes in temperature and creates pressure. This pressure is transferred through a capillary system to the indoor bulb and then to the mixing valve, which is positioned to increase or decrease the amount of hot water from the boiler. Temperature-range adjustments can be made by turning an adjustment on top of the valve. A typical installation in which a hot-water heating control is used is shown in Figure 10-60.

Water-Tempering Valves

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Water-tempering valves are used in hot-water space heating systems where it is necessary to supply a domestic hot-water supply at temperatures considerably lower than those of the water in the supply mains. The water-tempering valve automatically mixes hot and cold water to a desired temperature, thus preventing scalding at the fixtures. These valves are designed for use with hot-water space heating boilers equipped with tankless heaters, boiler coils, or high temperature water heaters.

The A.W. Cash Type TMA-2 valve illustrated in Figure 10-50 is a thermostatic water-tempering valve that is shipped from the factory preset to operate at 140°F . These valves can also be field adjusted to change the temperature of the mixed water leaving the valve by loosening the adjustment nut and turning the adjustment screw either clockwise (for colder water) or counterclockwise (for hotter water).

If turning the adjustment screw does not produce the desired mixed-water temperature, carefully touch the hot-water inlet on the valve to make sure hot water is being delivered. If you are certain hot water is getting into the valve and a temperature adjustment still fails to produce the desired results, the problem most likely lies inside the valve. Problems with these valves can usually be caused by one of the following:

• Binding of bonnet to push-rod
• Sticking of push-rod to O-ring
• Binding of piston
• Sticking of body O-ring

Before attempting to service or repair these valves, close off the hot, cold, and mixed water connections. Water must not be allowed to enter the valve when the bonnet has been removed.

Access to the internal parts of the hot-water tempering valve illustrated in Figure 10-50 is gained by unscrewing the bonnet (i.e., turning it counterclockwise) and removing it. If the bonnet is binding to the push-rod, pull the push-rod out of the bonnet and wipe it off with a crocus cloth. Do the same with the inside of the bonnet. If the push-rod O-ring is sticking, it should be removed and replaced. Reassembly is in reverse order; place the pushrod O-ring, reinsert the push-rod, and then screw the bonnet back on.

A binding piston should be removed, cleaned (with a crocus cloth), and lubricated. Access to the piston is also gained by unscrewing the bonnet. A body O-ring that is sticking should be removed and replaced. Access to the body O-ring is gained by unscrewing and removing the bonnet (leaving the push-rod in the bonnet). Push the piston and piston spring up and out through the top of the tempering valve. Lift out the post thermostat assembly. When reassembling, be sure to lubricate both the body O-ring and piston.

The design of the Watts No. N170 Series water-tempering valve differs from the one described above in that the discharge or mixed water orifice is located in the bonnet (see Figures 10-51 and 10-52). The water temperature can be changed by loosening the locknut and turning the adjustment screw. Each full turn of the adjustment screw is equal to approximately a 10°F change in temperature.

A typical installation in which a Watts No. N170 valve is used is shown in Figure 10-53. Tempered water at 140°F can be delivered to the system. The thermostat in the valve makes trapping unnecessary except in extreme cases.

A two-temperature recirculating hot-water supply system is shown in Figure 10-54. In this system, a water-tempering valve and recirculating line are used to maintain approximate fixture water temperatures of 140°F in the mains at all times. A relatively small capacity recirculator is used in the recirculating return piping, because very little hot water is required to maintain the low temperature in the mains. Long runs of recirculating piping should be insulated to reduce the heat loss from the piping.

Tempering valves cannot compensate for rapid pressure fluctuations in the system. Where such water pressure fluctuations are expected to occur, a pressure equalizing valve should be installed.

The Watts 70A Series tempering valve, shown in Figure 10-55, is designed for small domestic water-supply systems and tankless heater installations. Piping connections for these applications are shown in Figure 10-56. A balancing valve should be installed below the tempering valve in the cold-water line to compensate for the pressure drop through the heater.

This valve is available with both threaded and sweat connections. It is also available in both high- (120–160°F) and low- (100–130°F) temperature models. Temperature changes are made by turning the dial-type adjustment cap on the valve (see Figure 10-57).

The Spirax Sarco Type MB water blender (see Figure 10-58) has a 55°F adjustment range for supplying tempered water to a system. It is a three-way double-ported balancing valve, essentially resembling the Watts and A.W. Cash valves in construction, except for an extended bonnet containing spirals. This type of construction allows a certain degree of pressure fluctuation between the hot- and cold-water inlets without disturbing the control of the tempered water.

Electric Control Valves

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An electric control valve (regulator) is designed to provide remote electric on-off control in heating systems and steam process applications (see Figure 10-49).

The solenoid pilot at the top of the valve is connected to a room thermostat, automatic time clock, or some similar device from which it can receive an electrical signal. When the solenoid pilot is electrically energized, the pilot valve opens, and pressure builds up in the diaphragm chamber. As a result, control pressure is applied to the bottom of the main valve diaphragm, and the main valve is opened. The pilot valve closes when the solenoid pilot is de-energized, and control pressure is relieved through the bleed orifice. Steam pressure acting in conjunction with the force of the main valve-return spring combine to close the main valve.

Temperature Regulators

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Temperature regulators are used for many heating and cooling applications, including small-flow instantaneous heaters or coolers (shell-and-tube or shell-and-coil heat exchangers), small storage or tank heaters, and similar installations.

The Spirax Sarco Type 25T temperature regulator shown in Figure 10-48 is a diaphragm-operated valve used for regulating temperature in a variety of different process applications.

Before start-up, the main valve is normally in a closed position and the pilot valve is held open by spring force. The steam enters the orifice inlet, passes through the pilot valve and into the diaphragm chamber, and then out the control orifice. Control pressure builds up in the diaphragm chamber when the flow through the pilot valve exceeds the flow through the control orifice. This build-up in pressure opens the main valve.

The bulb of the temperature regulator is immersed in the medium being heated. At a predetermined temperature setting, the liquid in the bulb expands through the capillary tubing into the bellows and throttles the pilot valve. The main valve will deliver the required steam flow as long as the control pressure is maintained in the diaphragm chamber. The main valve closes when heat is no longer required.

Air Eliminators

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Sometimes air pockets will form in the pipelines of steam or hot water heating and cooling systems and retard circulation. One method of eliminating these air pockets is to install one or more air eliminators at suitable locations in the pipeline.

An air eliminator (or air vent) is a device designed to permit automatic venting of air. These automatic venting devices are available in a number of sizes, shapes, and designs. Not only are air eliminators used for venting convectors, baseboard radiators, and other heat-emitting devices; they are also frequently used for this purpose on overhead mains and circulating lines.

Three types of air eliminators (air vents) used in steam or hot water heating and cooling systems are:

• Float-type air vents
• Thermostatic air vents
• Combination float and thermostatic air vents

A float-type air vent (see Figure 10-45) consists of a chamber (body) containing a float attached to a discharge valve by a lever assembly. The float-controlled discharge valve vents air through the large orifice at the top. The float action prevents the escape of any fluid, because the float closes the valve tightly when it rises. When the float drops, the lever assembly pulls the valve from its seat, and the unit discharges air.

Float-type air vents are available for hot-water heating and cooling systems to 300 psi and low-pressure steam heating systems to 15 psi. A float-type air vent used in a steam heating system should be equipped with a check valve, which prevents air return under vacuum.

Steam cannot be maintained at its saturated temperature when air is present in the system. As shown in Table 10-4, the temperature of the steam decreases as the percentage of air increases. A thermostatic air vent is specifically designed for removing air from a steam system. The one shown in Figure 10-46 consists of a valve head attached to a bellows, operating in conjunction with a thermostatic element. Its operating principle resembles that of a thermostatic steam trap. When air is present, the temperature of the steam drops. The temperature drop is sensed by the thermostatic element, which causes the valve in the vent to open and discharge the air. When the air has been discharged, the temperature of the steam rises, and the valve closes tightly.

In some special applications, it is necessary to use an air eliminator that will close when the vent body contains steam or water and opens when it contains air or gases. Combination float and thermostatic air vents have been designed for this purpose.

A combination float and thermostatic air vent (see Figure 10-47) consists of a vent body or chamber containing a float attached to a valve assembly. The float rests on a thermostatic element that responds to the temperature of the steam. The operation of this element is similar to the one used in a thermostatic steam trap. When the vent body is filled with air or gas, the float is at its lowest point, causing the thermostatic bellows to contract. Because the float is at a low point in the vent body, the head is moved off the valve seat and the vent discharges the air or gas. The head moves up and closes the valve when either water or steam enters the vent body. The entry of water into the vent body forces the float upward and eventually closes off the valve. The entry of steam, on the other hand, causes the thermostatic bellows to expand and force the float upward, closing the valve.

Troubleshooting Expansion Tanks

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An undersized expansion tank or one that is completely filled up with water will cause the boiler pressure to increase when the water heats. Because the expansion tank is too small or too filled with water to absorb the excess pressure, the relief valve will begin to drip. The dripping relief valve is only symptomatic of the real problem, and replacing the valve will in no way solve it.

There is not much you can do about an undersized expansion tank except replace it. As a rule-of-thumb, expansion tanks should be sized at 1 gal. for every 23 ft2 of radiation, or 1 gal. for every 3500 Btu of radiation installed on the job. In Table 10-3, the allowance is slightly higher.

If the problem is a completely filled tank, it should be partially drained so that there is enough space to permit future expansion under pressure. The first step in draining an expansion tank is to open the drain valve. The water will gush out at first in a heavy flow and then tend to gurgle out because a vacuum is building up inside the tank. Inserting a tube into the drain valve opening will admit air and break the vacuum, and the water will return to its normal rate of flow. After a sufficient amount of water has been removed, the drain valve can be closed.

Diaphragm Expansion Tanks

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The air in a diaphragm-type expansion tank is separated from the water by a flexible rubber membrane (see Figure 10-44). These tanks are smaller than the closed steel tanks and come from the manufacturer precharged with compressed air. When the tank arrives at the site, the diaphragm is fully expanded against its inside surfaces. When the tank is installed and connected to the system piping, water enters the other side of the tank chamber and presses down on the diaphragm.

As a rule, diaphragm tank manufacturers will precharge their tanks to 12 psi, which is sufficient to match the water-fill pressure requirements of the typical house or small commercial building.

Closed Steel Expansion Tanks

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The closed steel expansion tank has no moving parts (see Figure 10-43). It is normally two-thirds filled with water and one-third with air. As heated water expands and its excess volume enters the tank, it compresses the air at the top of the tank. The compression of the air in the tank results in an increase of system pressure, which is indicated on the boiler pressure gauge.

When the system water cools down, its volume contracts, and the air in the tank expands back to its original volume, causing system pressure to fall. To sum it all up, the rise and fall of system pressure is created by the expansion and contraction of the air in the expansion tank.

One problem encountered with a closed steel expansion tank directly connected into the system is that the system water can absorb the air and send it to the radiators and convectors by gravity circulation. Installing a gravity-flow check valve on the expansion tank will prevent gravity circulation.

Category: Steam and Hydronic Line