HPAC Magazine

14 years of hydronics

November 1, 2012 | By John Siegenthaler

A comparison of the hot topics of the past, the present and, most importantly, the future.

I started writing for HPAC almost 14 years ago. Back then, each column was typed up on a boxy little Macintosh computer with a nine-inch monochrome screen, printed on a dot matrix printer, and saved on a 3.5-inch floppy disc. Each month, a printed copy of the column, along with the floppy disc, were mailed to HPAC’s editor. After working some magic, the editor would fax me a copy of the completed layout. We then discussed any final changes over the phone.

At the time, hydronic heating was enjoying a resurgence of interest driven by the red-hot topic of radiant floor heating. In the years that followed, radiant floor heating continued to be the tail wagging the hydronics dog. In the late nineties, many of us became accustomed to radiant heating market growth rates of 25-plus per cent per year. Many newbies were drawn into hydronics through a growing interest in radiant heating.

Other hydronic-related topics that generated strong interest back then included:

• Mixing strategies and devices

• Protecting boilers against flue gas condensation

• Primary/secondary piping

• Differential pressure bypass valves

• Outdoor reset control

• Radiant specific thermostats

• Customized controls using ladder diagrams and programmable logic controllers

• Oxygen barrier tubing versus non-barrier tubing

• The energy savings of radiantly heated buildings versus those heated by other means

• How comfort was attained at lower air temperatures in radiantly heated buildings

• High capacity domestic water heating

• Snowmelting – steps, patios, walkways, driveways, and entire parking lots.

Conventional fuel was relatively cheap during these times, and the economy was growing at a healthy rate. These circumstances, in combination with advancing building technology, set the stage for a boom in large homes that needed very customized hydronic heating systems.

The North American hydronics industry benefited from this trend. A growing number of hydronic “artisans” acquired the skills needed to assemble complex hydronic systems that often contained thousands of dollars worth of electronic controls, walls full of circulators, and miles of PEX tubing. Many of these residential systems were arguably more complex than systems in much larger commercial buildings. The market was awash with devices that allowed the hydronic artisans to sculpt increasingly complex and intricate designs. I know, because I was one of them.

Most of these high-end systems worked, but many required several hours of “tweaking” to weed out unexpected control, pumping, or balancing issues. I recall one installer telling me how he “camped out” overnight in a certain mechanical room trying to tame the beast he had created.

Some of this is still happening today. Mansions are still being constructed, although in lower numbers due to a struggling economy. Those that are being built often aspire to be “green” by including renewable energy heat sources (i.e., eight heat pumps instead of four boilers) along with thousands of other components assembled on site by those accomplished artisans who live and breathe hydronics.


The entire HVAC market got a slap in the face when the bottom fell out of the real estate market in 2007. Double digit growth gave way to trying to limit losses. Marketing terms like green, sustainable, renewable, and LEED became the industry’s new vocabulary, and inevitably set the trend that has brought us to the present. Today, the topics that consistently generate strong interest in the hydronics industry include:

• Solar thermal combisystems

• Ground source heat pumps systems

• Use of intelligent ECM-based circulators

• Using methods other than primary/secondary piping to achieve hydraulic separation in multiple circulator systems

• Dealing with short cycling of mod con boilers

• Post PVC boiler venting systems

Lets take a brief look at each of them.

Interest in both solar thermal systems and ground source heat pump systems is currently quite strong throughout North America. It is the result of saturation level eco-marketing combined with government subsidies that reward those who use these heat sources.

Appropriate use of renewable energy heat sources is something I strongly support. The fact that hydronics enables the operation of nearly all thermally-based renewable energy technologies could be a tremendous stimulus for the North American hydronics industry. However, building a business model based on the currently available government subsidies really worries me.

I worked in the solar thermal industry during the early 1980s when the U.S. government pulled the plug on renewable energy. I was one of the first to be laid off when it became clear that the ride provided by the tax credits was over. This experience convinced me that renewable energy systems will only gain “mainstream” acceptance when deployed in configurations that are reliable, easily repeatable, and can create immediate monthly savings without government subsidies.

Those who have read my HPAC articles over the last few years know that I am a big proponent of ECM-based intelligent circulators. I think they represent the single biggest advancement in hydronics technology over the last two decades. I encourage people to think of these circulators as the compact fluorescent light bulbs of our industry.

When they are eventually implemented by the millions, these circulators will have a profound positive effect on the industry, further widening the gap between what is possible using water-based rather than air-based distribution systems. Given that European legislation will be mandating use of ECM-based circulators in new installations as of January 1, 2013, it seems inevitable that they will soon become the norm in North America.

Primary/secondary piping remains as a viable design technique. However, more designers are learning how to use the broader concept of hydraulic separation to provide the benefits of primary/secondary piping without the need of constructing primary loops, or incurring the installation and operating cost of primary loop circulators. Retaining the benefits while using simpler and less expensive approaches is value engineering at its best. Look for this trend to continue.

Shift in zoning

Zoning has been and continues to be a major benefit of using hydronic heating and cooling. However, zoning is slowly shifting from the extensive use of small wet rotor circulators, to valve-based techniques (e.g. either zone valve or “wireless” thermostatic radiator valves). There are strong economic arguments for this evolution. This trend will also continue as energy costs increase, and more heating professionals learn to market the advantages of these devices.

The use of PVC venting for mod/con boilers is at a crossroads. While PVC remains acceptable to some boiler manufacturers, other manufacturers have specifically banned it from use with their products. The issues in question include long term integrity of the pipe and its joining methods at higher than expected flue gas temperatures, leaching of potentially corrosive chlorides from the pipe, and the lack of specific approvals from some PVC pipe manufacturers regarding use of their pipe for venting heat sources. I think it’s inevitable that polypropylene and AL29-4C stainless steel will take over as the preferred venting materials for all condensing heat sources.

Snowmelting continues to be an option the hydronics industry offers. However, increasing energy prices, growing altruism about reducing energy use, and increasingly restrictive energy codes will inevitably restrict when and where snowmelting is deployed. And, just in case you’re wondering, solar powered snowmelting systems are NOT a good idea.

The issue of
short-cycling has become an Achilles Heel to those who work with low mass mod/con boilers. I often ask contractors at seminars about this issue, and find it to be a pervasive problem. It stems from the fact that it is easy to “microzone” hydronic systems. The ability to create a six- or 12-zone system, in combination with low thermal mass boilers that can only reduce their firing rate to about 20 per cent of rated output, is the underlying cause of short cycling. This situation was not very common when residential boilers contained 400+ pounds of cast-iron, 10 or more gallons of water, and served one to three zone distribution systems.

We did not appreciate the “self buffering” characteristics of these older boilers, at least not until we began using low mass boilers with ever-increasing numbers of zones. I think the solution lies in restoring thermal mass to the heat source, preferably as site-added water rather than manufacturer-added metal. I predict that future combustion-based hydronic heat sources will rely on increased water mass as their stabilizing element.


There are several developing situations that could significantly impact the future growth of our industry – IF we recognize the need, and respond with appropriate solutions. These include:

• Ultra-low energy use buildings that need “drop in” heating and cooling solutions.

• Homes requiring heat generation capacities no higher than 30000 Btuh, along with heat sources that can remain in stable operation while delivering five per cent or less of this capacity.

• Appropriate integration of renewable energy heat sources where they can deliver positive monthly cash flows, from the day they are first turned on.

• Increased demand for very low mass heat emitters that can quickly respond to changing loads due to internal and solar heat gains.

• The advantages of central heat generation along with heat metering in multiple occupancy buildings (condos, apartments, and leased office space).

• Increasing awareness that the distribution system, and not just the heat source, plays a significant role in determining system operating cost.

• Smart grid technology, and time-of-use electrical rates that need on-site thermal energy storage. Water is the best stuff on earth for storing sensible heat.

• Net zero buildings with solar photovoltaic arrays, and therefore a strong preference for electrically-driven high efficiency heat sources (e.g. heat pumps), along with low energy use distribution systems (e.g. hydronics).

• Gas-fired absorption heat pumps that can supply heating water at temperatures up to 140F, equivalent combustion efficiencies up to 140 per cent, and chilled water for cooling.


In the months ahead, I will be writing more about these trends. I will do so on an iMac with a 27-inch LED display, e-mail them to Kerry Turner at HPAC, and very seldom print anything to paper.

The computer that I used to write my first HPAC article has long since evolved into a device that is now indispensable to my business, and has undoubtedly improved the quality of my life. This happened because a certain computer manufacturer recognized the need to provide solutions rather than boxes classified by megahertz and gigabytes.

Hydronics technology, working behind the scenes, and without the glory of eco acentuated headlines, holds the potential to improve the quality of our lives, and reduce both Canadian and global energy demand. This potential teeters on the ability of our industry to deliver solutions rather than hardware. I think it can be done.  <>

John Siegenthaler, P.E. is the author of Modern Hydronic Heating (the third edition of this book is now available). For reference information and software to assist in hydronic system design visit www.hydronicpros.com.



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