August 24, 2016 by Rob Waters
Glycol is a mystery fluid to many in the HVAC industry, but it is an essential tool for designing many different types of HVAC systems. In Canada, where glycol is essential to protect HVAC systems from freezing, most contractors have a basic understanding of why glycol is used. However, many may not realize all of the implications and issues that must be considered when the decision is made to use glycol in a system.
Glycol in a hydronic system impacts the way that many circuit components such as pumps, pipes, air eliminators and boilers work. If the wrong choices are made there can be serious consequences to the efficiency, performance and longevity of the system. Glycol must be accounted for early in the system design, as using glycol with its lower heat transfer capability will impact the sizing of many of the system’s components.
USING THE RIGHT GLYCOL
Water is a better heat transfer media than glycol–you can see that when you compare the specific heat of the fluids. At 60F/15.5C water has a specific heat of 1.0 Btu/(lb*oF). Compare this to 50 per cent glycol at 0.84 Btu/(lb*oF) and you can see that glycol has 16 per cent less heat carrying capacity. The specific heat also changes with temperature and it only gets worse for glycol at lower temperatures.
Glycol is also thicker and more viscous than water, making it more difficult to push through the pipes. If the system is originally designed for water, and then at the last minute changed over to glycol, you will most likely have problems as the initial sizing of components are no longer sufficient for the glycol system.
Using the correct type and concentration of glycol is very important, as you only want to use as much as required to do the job. Too much glycol adds expense, impedes heat transfer, and affects pumping capacity. Not enough glycol can lead to damaging and expensive freeze-ups.
The type and concentration of glycol used is dependent on the project location and the type and specific requirements of the system. There are two basic types of glycols used in HVAC systems, propylene glycol and ethylene glycol.
Both of these fluids have similar freeze protection and heat transfer characteristics with the main difference being that propylene glycol has a lower toxicity level. Due to its lower toxicity, propylene glycol is more commonly used in residential and small commercial HVAC systems.
Either type of glycol will always include added corrosion inhibitors to protect pipes and components. There are many different types of inhibitors used that are specific to different applications. Using the correct type of glycol – inhibitor mix is crucial to providing long-term reliable system operation. Climatic conditions are very different in Toronto and Yellowknife, so the levels of freeze protection required and the resulting concentration of glycol will vary for the same type of system installed in different locations.
All glycol manufacturers provide charts (see Figure 1), which show the percentage of glycol required to provide a certain level of protection against freezing or burst protection. Typically a 50 per cent concentration of glycol will give you freeze protection down to -30F/-34C. However to provide pipe burst protection to the same temperatures, only 33 per cent glycol is required. Burst protection means that the fluid can no longer be pumped, but it has not expanded to a point where it will burst pipes.
The HVAC applications that most commonly use glycol are snow and ice melting systems, ground source heat pumps, solar water heating systems, chilled water cooling systems, and in the hydronic systems that require pipe burst protection due to their location or activity level. Some of these applications require high levels of freeze protection, while others only need a lower level of burst protection as a safety measure.
Snowmelt systems by their nature require a high level of freeze protection because all the components are located outside the building envelope and are exposed to the ambient conditions. This means that snowmelt systems require quite high levels of glycol concentration in the range of 50 to 60 per cent.
Solar water heating systems also require a very high level of freeze protection as solar collectors and piping are located outside the building. Typically glycol concentration in the range of 45 to 60 per cent will be required, with most applications in Canada using 50 per cent glycol. Installations in the far north usually need to increase the concentration to account for colder winters.
Solar systems also have the unique characteristic of producing very high fluid temperatures during summer stagnation conditions. This leads to very specialized glycol requirements that use special high temperature corrosion inhibitors to protect the fluid from breaking down rapidly. If the wrong glycol is used in a solar water heating system, the fluid can break down and turn nasty very rapidly. This can result in plugged collectors, blocked pumps, and in extreme situations systems that must be abandoned entirely. There are special glycols made exclusively for solar water heating systems and they are highly recommended for these types of systems.
Ground source heat pumps often use glycol in their earth loops. As these loops are often much deeper in the ground, they are typically not exposed to the same kind of extreme conditions and therefore require a lower concentration of glycol, usually in the range 15 to 25 per cent. Fluid toxicity is certainly an important issue in the ground source industry so therefore propylene glycol is usually essential for these types of applications.
Many hydronic heating systems, especially those that incorporate radiant floor heating will utilize glycol to provide a level of safety against burst pipes and the resulting damages that can occur to the building and the system. In these applications a lower concentration of glycol in the range of 25 to 30 per cent is usually utilized just to provide peace of mind against bursting pipes during unoccupied periods. Commercial buildings that use radiant floor heating systems and that may be unoccupied for periods of time are certainly candidates for this protection. Pipes that burst in a concrete slab are a serious problem that can result in very expensive and disruptive repairs.
When glycol is used in the system there are certain installation, service and maintenance factors that must be considered to maintain a long reliable lifespan of the fluid and the system components. Here are a few of the things to look out for:
Robert Waters is president of Solar Water Services Inc., which provides training, education and support services to the hydronic industry. He is a mechanical engineering technologist graduate of Humber College and has over 30 years experience in the hydronic and solar water heating industry.