What do you understand by hydronics?

As we know the word “Hydro” means "water" and when we use liquid water or steam or a water solution (mostly, glycol with water) as a heat-transfer medium in heating and cooling systems, it is being referred to as Hydronics. This name is used to differentiate the systems using water as a heat transfer medium from oil and steam systems. Hydronics can be used to provide both heating and cooling effect. Thus we can use Hydronics where both heating and cooling are desired like HVAC systems.

What does a hydronics system mean?

A system that uses Hydronics for heating and cooling by recirculating it through pipes in a building, is termed a Hydronics system. Hydronics can be either hot or cold and radiates or absorbs heat energy, depending upon the desired conditions, as it is circulated throughout the network of pipes.

Types of hydronics system

There are two types of Hydronics systems:

  • Hot water
  • Chilled water

Hot water hydronics system

Hydronics heating systems comprise of the following equipment as discussed below:

Tankless water heater

Tankless water heaters work by heating water passing through their pipes. These heaters take power either from an electric or a gas source. They do not have any storage tanks for retaining water. They deliver hot water the very instant you switch it on. They occupy less space but are capable of delivering as much as 20 gallons per minute of hot water.

However, there are two drawbacks also. Firstly, the tankless water heater gives inconsistent water temperatures. And secondly, it won't turn on if your hydronic heating system's demand for hot water is below the heater's actuation level.

Combi Boiler

A combi boiler as the name suggests is a combination of a central heating boiler with a high-efficiency water heater. Combi boilers can be referred to as the advanced version of tankless water heaters as they too lack a water storage tank. They work in the same manner as tankless water heaters, that is, once you open a tap, they deliver cold water, and when you raise its temperature, they send hot water through the pipes. The only difference is that it is more efficient compared to tankless water heaters.

Heat pump

A Heat pump is a heat engine running in the reverse direction, that is, it transfers heat from a cold body to a hot body at the expense of some external work done on it by an external agent. A heat pump generally works on the principle of the Reverse Brayton Cycle or Bell Coleman cycle or Joule cycle. It comprises four key components which include an evaporator, a compressor, a condenser, and an expansion valve. The working substance used in these components is called refrigerant.

The diagram represents a heat pump comprising of evaporator, compressor, condenser and expansion valve.
CC BY-SA 4.0 | Image credits: https://commons.wikimedia.org/ | Timpet

Types of heat pump

Geothermal heat pumps (GHPs)

Geothermal heat pumps (GHPs), are sometimes referred to as geo-exchange systems. A ground source heat pump or a geothermal heat pump is a heating or cooling system for buildings that use a type of heat pump that transfers heat to or from the ground. As we all know the Earth acts as a thermal reservoir with infinite capacity. A geothermal heat pump utilizes this. In winter geothermal heat pump absorbs heat from the ground and transfers it to the room while in summer it rejects the heat from the room to the ground, thus providing the most energy-efficient HVAC ( Heating, Ventilation, and Air Conditioning). Other than HVAC it is also used for water heating. As it uses far less energy than can be achieved by burning fuel in a boiler/furnace) or by use of resistive electric heaters, it is widely used for residential and commercial purposes all over the world.

Geothermal cooling and geothermal heating are described through two diagrams.
CC BY-SA 4.0 | Image credits: https://commons.wikimedia.org/ | Goran tek-en 

Air source heat pumps

Geothermal heat pumps have high setup costs and that’s why air-source heat pumps started coming into use widely. Air source heat pumps look similar to air-conditioning units. Their size depends on how much heat they'll need to generate for your home, for more absorption of heat a bigger heat pump is used. The air source heat system takes heat from the surrounding air and transfers it to the hydronic. The hydronic then transfers this heat to the room air. Similarly, the heat transfer takes place between the surrounding air and the water that has to be heated.

The efficiency and performance of today's air-source heat pumps is a result of technical advances such as the following:

  • Improved design of the coil.
  • Thermostatic expansion valves for more precise control of the refrigerant flow to the indoor coil.
  • Improved designs of electric motor and two-speed compressor.
  • Copper tubing, having internal grooves to increase surface area.
  • Variable speed blowers, which are more efficient and can compensate for some of the adverse effects of restricted ducts, dirty coils, and dirty filters.

Example of hydronics heating system

Radiant floor heating

Radiant floor heating works by supplying heat directly to the panels, the floor, or the ceiling of the house. Either electrically heated coils or water-heated tubing are responsible for the heat transfer from the source to your room. NOTE: A good radiant floor heating thermostat is desirable to meet maximum efficiency.

Advantages of radiant floor heating

  • Minimizes heat losses
  • Zero maintenance cost
  • More space efficiency

Disadvantages of radiant floor heating

  • Takes a long time to heat up
  • Steep Installation cost

Chilled water hydronic systems

Chilled water hydronic systems are mainly used in large commercial HVAC systems and industrial cooling applications. Although they can also be used in some residential and light commercial HVAC. Chilled water systems are popular because they use water as a refrigerant. Water is far less expensive and also is not hazardous like many other refrigerants. This makes them cost-effective, and efficient, especially in commercial HVAC applications.

There are two types of chillers, mainly,

The compression chiller

These are the most common types of chillers used nowadays. The compression chiller, as the name suggests, uses a compressor in the refrigeration process and requires all the major components of the refrigeration process. Depending upon the size and load, compression chillers use different types of compressors for the compression process. Semi-hermetic, screw, scroll, and centrifugal compressors are the different types of compressors used in chillers.

The compression chiller can use multi-compressor staging and cylinder unloading to reach higher levels of efficiency. Under low load applications, a multi-compressor chiller may only run one compressor to meet demand. While a single compressor chiller that has unloading capabilities may unload on the cylinder to meet low demand and load up that additional cylinder in higher demand situations. This makes them more efficient than having the chiller running in full blasting conditions all the time.

The absorption chiller

The absorption chiller does not use a compressor for the refrigeration process. They are also called Vapor Absorption Refrigeration System. These systems use either a chemical process to raise the temperature of the refrigerant and then use another chemical process to absorb the heat from the refrigerant. Water is used as a refrigerant, and lithium bromide is used as an absorber. Lithium Bromide is more common because it is safer and non-toxic.

Common Mistakes

Hydronics and hydroponics sound similar but they are different terms. Hydroponics is a type of agriculture or gardening method that doesn’t use soil which involves growing plants, usually, crops without soil, by using mineral nutrient solutions in an aqueous solvent. The term comes from the Greek words “hudor” for water and “ponos” for work, so in translation, it essentially means “water-working”. Terrestrial plants may grow with their roots exposed to the nutritious liquid, or, in addition, the roots may be physically supported by an inert medium such as perlite, gravel, or other substrates.

Context and Applications

This topic is significant in the professional exams for both graduate and postgraduate courses, especially for

  • Bachelor of Technology in Mechanical Engineering
  • Master of Technology in HVAC
  • Master of Technology in RAC
  • Compressor
  • Expansion valve
  • Air-vents
  • Hot-water heating

Practice Problems

Q1. The refrigerant used in steam jet refrigeration is ___________.

  1. Brine
  2. Water
  3. Ammonia
  4. Freon

    Correct option: (b)

    Explanation: The refrigerant is a working substance used in refrigerators. Water is used as a refrigerant in steam jet refrigeration because it is readily available, cheap, non-flammable, eco-friendly, and safe to use.

    Q2. An Electrolux refrigerator works on ______________.

    1. Vortex tube
    2. Absorption refrigeration
    3. Vapor compression
    4. Steam refrigeration

    Correct option: (b)

    Explanation: Electrolux refrigeration is a refrigerator of absorption type, and it is also termed as a three fluid absorption system. In this absorption system, ammonia is employed as a working fluid.

    Q3. What is used as an absorber when water is being used as a refrigerant?

    1. Ammonia
    2. Water
    3. Lithium bromide
    4. Freon

    Correct option: (c)

    Explanation: In a lithium bromide absorption refrigeration system, lithium bromide solution can be used in the water. Water is used as a working fluid (refrigerant), and lithium bromide is a type of salt and can be used as an absorbent.

    Q4. The C.O.P of absorber chiller in comparison to compressor chiller is ___________.

    1. Less
    2. More
    3. Equal
    4. Unpredictable

    Correct option: (a)

    Explanation: Absorber chillers have less coefficient of performance (C.O.P) than the compression chiller (rotary compressor chillers or electric chillers). The range of absorber chillers C.O.P can vary between 0.54 to 1.1, and electric chillers may vary between 1.0 to 8.0.

    Q5. Bell Coleman cycle is a reversed ____________ cycle.

    1. Rankine
    2. Otto
    3. Joule
    4. Kelvin

    Correct option: (c)

    Explanation: Bell Coleman cycle is also termed as reversed Joule cycle or reversed Carnot cycle. In this cycle, air can be employed as a working fluid. Bell Coleman cycle is used in aircraft, and others.

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