HVAC523

Basic Subsystems

Basic Subsystems

In a hydronic heating system, water is heated by the heat source and conveyed by means of a distribution system to heat emitters where it is released to the building.

A control system regulates these elements in attempt to keep the rate of heat delivery very close to the rate of building heat loss.

Basic Subsystems

The hydronic system contains four subsystems

The Heat Source Distribution System The Heat Emitters The control System

Basic Loop This simplest hydronic system can be

best described as a loop, or piping circuit.

If the circuit is sealed off from the atmosphere at all locations, as with most hydronic systems, it is called a closed loop system.

If the circuit is opened to the atmosphere, as in domestic hot water, it is known as an open loop.

Basic Closed Loop System

Temperature controls

The basic loop system requires two simple control devices.

These two devices are the room thermostat and the temperature-limiting device which is known as the aquastat.

Temperature controls

The room thermostat determines when the building requires heat based on its setpoint temperature and the current indoor temperature.

The aquastat ensures that the water temperature within the heat source remains within a predetermined range while the demand for heat is present.

Expansion Tank

As water is heated, it expands. This increase in volume is an extremely powerful but predictable characteristic that must be accommodated in any type of closed-loop hydronic system.

Most modern hydronic systems use a diaphragm type expansion tank.

Expansion Tank

Expansion Tank

As the heated water expands, it pushes into the tank and slightly compresses the captive air volume.

As a result, the system pressure rises slightly.

As the water cools, its volume decreases. This process repeats itself each time the system operates.

Expansion Tank

Older hydronic systems used expansion tanks without diaphragms.

These tanks had to be larger and also had to be mounted higher than the heat source.

Steel Expansion Tank (no diaphragm)

Pressure Relief Devices

Consider the fate of a closed loop system in which a defective control fails to turn off the heat source after its upper limit has been reached.

As the water gets hotter and hotter, system pressure steadily increases due the waters expansion.

Pressure Relief Devices

This pressure could eventually exceed the pressure rating of the weakest components in the system.

Most residential system components have a pressure test rating of at least 60 psi and may withstand two or more times that pressure before bursting.

Pressure Relief Devices

The consequences of a system component bursting at such a high pressure and temperature could be devastating.

For this reason, all closed loop systems must be protected by a pressure relief valve.

This is a universal requirement of all mechanical codes in North America.

Pressure Relief Devices

Pressure relief valves are designed and labeled to open at a specific pressure.

Most residential and light commercial systems typically have pressure relief settings of 30 psi

Pressure Relief Devices

Pressure Relief Devices

DO NOT IN, ANY CIRCUMSTANCE, PLUG, CAP OR USE A PRESSURE RELIEF VALVE OTHER THAN WHICH IS RECCOMENDED BY THE MANUFACTURER.

Pressure Relief Devices

Make Up Water System

Most closed loop systems experience water minor losses over time due to evaporation from valve packings, pump seals and other components.

The common method to replacing this water is through a make up water system which consists of a pressure reducing valve (water feed valve) backflow preventer, pressure gauge and shut off valves.

Make Up Water System

The water feed valve is designed to maintain the minimum water pressure in the system.

The water feed valve allows water into the system whenever the pressure on the outlet side of the valve drops below the valves pressure setting.

The water feed valve has an adjustable pressure setting.

Water Feed Valve

Back Flow Preventer

The back flow preventer does just what its name implies.

It stops any water that has entered the system from returning and possibly contaminating the potable water system supply.

Most municipal codes require such a device on any heating system connected to a public water supply

Back Flow Preventer

Flow Check Valves

Another component commonly used in hydronic systems is a flow check valve.

This valve can serve one or two purposes depending on the system it is installed in.

Flow Check Valves

Flow Check Valves

In a single-loop system, the flow check valve prevents hot water in the boiler from slowly circulating through the distribution system when the circulator is off.

Whenever a device containing hot water is part of an unblocked piping path, the potential for such flow exists.

Flow Check Valves

If not prevented, such thermosiphoning allows heat to “leak” away from system in an uncontrolled manner, often ending up where it is not desired.

In a system using circulators, a flow check valve must be installed in each zone circuit.

Air Separator

An air separator is designed to separate air from water and eject it from the system.

Modern air separators create a region of low pressure as water passes through.

The lowered pressure causes dissolved gasses in the water to form bubbles.

Air Separator

One formed , these bubbles are guided upward into a collection chamber where an automatic air vent expels them from the system.

The process of separating air from water is enhanced as water is heated.

Air Separator

For best results the air separator should be installed where water temperatures are the highest.

Therefore, the air separator should be installed in the supply pipe off of the heat source.

Air Separator

Spirovent brand Taco Brand