How Water Unit Heater in a mechanical room

Mechanical rooms have very important equipment that needs to be protected. Often mechanical rooms are not conditioned and subject to outside temperatures and freezing conditions that can freeze piping inside the mechanical. For that reason mechanical rooms often have unit heaters located there to protect piping and other things that cannot be frozen or subject to freezing conditions. The components and sequence of operation are fairly simple. The main components in a hot water unit heater include a fan, a valve, and of course the piping and coils along with the cabinet.

Hot Water Unit Heater Sequence of Operation

Each Hot Water Unit Heater shall be controlled by a unit mounted thermostat. Sometimes this will refer to as a wall mounted thermostat. Upon a drop in space temperature below the heating set point of 55° F the fan shall start and the hot water valve shall open. When the temperature rises above the set point, the fan shall stop and the valve shall close.

The location of the thermostat is dependent upon the design engineers specifications.  Typically it is located on the wall next to the door of the mechanical room especially if the door has access to the outdoors.

The valve is usually located on the piping near the unit heater and is controlled by thermostat and monitored by the building automation system. Sometimes the unit heater is controlled and monitored by the building automation system so any alarms from the unit heater by the BAS especially in freezing conditions should not be ignored.

Motor Disconnect and Starter Motor is used for turning the power off and on the an electric motor used in HVAC applications. It also has overloads built into the starter so if one of the motor windings begin to pull too many amps the overloads open the circuit and the motor stops operating.

Motor starters are commonly used for 3-phase motors and are used to kill power to the circuit if there is an overload or short circuit. There are solid state motor starters unlike the one pictured that use electronics to detect an overload and other possible motor problems. The one pictured above controls the starting and stopping of the motor via a relay which is located in the bottom right corner of the photo. The relay has a red light on it indicating the motor is on and running. The relay is controlled by direct digital control which offers precision computer control of building systems including HVAC.

Motor Disconnect and Starter Motor

Chiller Evaporator Barrel

A chiller evaporator barrel exchanges heat between one refrigerant and another. Usually the primary refrigerant is R-22, R-134A, or some other blend or alternative HFC. The secondary refrigerant is water. This water is circulated throughout the building where it passes through piping and coils. This is generally in most cases a forced air coil where the heat is removed from the air by a heat exchange process with the water and the coil. The cooler air is then forced through duct work and into the space to be conditioned.

Small Residential Chiller

The water is then pumped back through the chiller barrel evaporator so that the heat absorbed can be removed in the chiller barrel evaporator where another heat exchange process is done. The heat in the water is absorbed into the refrigerant and the water makes another trip to the building to absorb more heat. The refrigerant is then pumped to the compressor where it is compressed and then it travels through a condenser coil or a condenser barrel where the heat it absorbed from the water is rejected. This is the basic process for a chiller. There are many different types of chillers which use different refrigerants but that is the basics of a chillers operation. Chillers range from small residential units to very large multi-tonnage systems which serve very large buildings and process refrigeration in industry.

Smaller chillers are typically air cooled while larger systems use a cooling tower. If the heat in the condenser is rejected vie air then the chiller will have several fans that will pull air through the condenser coils and the heat is rejected into the atmosphere allowing the refrigerant in the condenser coils to cool and then return to the metering device and the evaporator barrel. If the chiller uses a condenser barrel then the heat is rejected into water that is circulated through the same condenser chiller barrel where the refrigerant. Heat is exchanged from the refrigerant to the water. The water is then pumped to a cooling tower where the cooling tower rejects the heat from the water into the atmosphere using the heat exchange process of evaporation.

HVAC Air Conditioning Chiller Evaporator Barrel

There are some facilities including campus like complexes that use a centralized steam plant for various reasons. The steam is generated from a large steam boiler and piped to various buildings around the campus or whatever facility is used. Typically several buildings utilized the steam for heating and hot water for domestic uses including showers, washing dishes, washing clothes and other domestic uses. The steam is usually high pressure and for this reason it is safer to deliver the steam to a mechanical room, reduce the pressure, and run the steam through a steam to hot water generator. A steam to hot water generator is nothing more than a big heat exchanger where at one end steam is delivered and at the other end water is delivered. As the steam runs through the exchanger it exchanges heat with the water heating the water. Thus, producing hot water for domestic use and heating use in the winter.

It is important that the process is controlled so that the hot water generated does not get too hot. The hot water produced can be used in hot water coils and pumped into storage tanks for use later for domestic purposes. Again, it is important to control the temperature as closely as possible to prevent the water from becoming too hot especially for domestic uses where scalding water can be inadvertently delivered to showers and sinks where people can be burned. The steam pumped into the generator is usually superheated and can produced water with very high temperatures.

Steam to Hot Water Generator

Newly Installed VAV Box

Variable Air Volume boxes are typically installed in commercial buildings and provide heating and cooling for occupants. There are different types of VAV boxes but how a VAV box works is fundamentally the same from one VAV box type to another.

In many cases a VAV box is nothing but a sheet metal box with a damper and controls inside a control panel. A thermostat connects to the controller while the controller positions the damper based on programmed inputs and variables that include air flow and temperature. Some VAV boxes also have a heat source inside the box. The heat source in many VAV boxes is typically electric but can also be a hot water coil. The thermostat and the controller will also turn on the heat whenever the temperature in the space falls below the set point as determined by the thermostat and pre-programmed temperature limitations in the programming. Usually the building engineer or maintenance staff controls the temperature limitations in the program. This prevents occupants from running the thermostat above certain settings that are unreasonable and possibly overload the HVAC system. These limited set points are usually within reasonable ranges for most peoples’ comfort factor.

Flow Ring inside a VAV Box

The air that is supplied to a VAV box comes from a large air handler and central duct work. The air handler must have the ability to supply the VAV boxes with a variable amount of air as the dampers in the VAV boxes will modulate to different positions based on set point requirements. When the box is in cooling mode the damper will modulate to a nearly fully open position as air conditioning or cooling requires more air flow. Heating requires less air flow so the damper will modulate to a minimum position.

The air handler fan needs to modulate to vary the air flow depending on demand. Typical VAV boxes have a flow sensor inside them to measure air flow. The air flow and temperature variables control damper position based on temperature demands and C.F.M. demands. Another air pressure sensor inside the main duct that supplies air to the boxes measures the pressure inside the duct. There is a pressure set point inside the program of the air handler control. The air handler will ramp up and ramp down the air handler blower to maintain the duct set point requirement in the program.

Variable air volume boxes provide comfort for many people working in commercial buildings. The system is different than the air conditioning and heating systems in our homes but these systems work well and provide good comfort.

How Do VAV Boxes Work?