In North America, we do not heat or cool buildings; we heat or air-condition them. Is the temperature too cold? Crank up the heat. Is it too hot? Crank up the air-conditioning.

This method of temperature control is ingrained in our language and in the names of our systems, our companies, and our organizations. It is ingrained in our thought process. Few in the United States can even conceive of other ways of making interior spaces comfortable than by being blasted by refrigerated air.

Heating in the U.S. is treated as an afterthought—for instance, variable air volume with perimeter heat and packaged terminal air-conditioning with electric heat. Similarly, ventilation is always lumped in with heating and cooling. This seems strange, since ventilation is a health issue, while heating and cooling are primarily comfort issues. The flow of fresh air into a space really shouldn’t depend on a space’s thermal requirements, but again, this is how things historically have been done.

Thus, we call engineered building systems related to space comfort “HVAC engineering” instead of heating, cooling, and ventilation. The remaining mechanical design tasks concerning piping systems are lumped under the title of plumbing engineering. This has led us to divide the engineering tasks of designing mechanical systems into separate disciplines: HVAC engineering and plumbing engineering.

This unfortunate division has had a profound impact on the resulting energy use of U.S. buildings and our failure to adopt more efficient technologies. When a client asks us to engineer a comfortable space, we give him a forced-air air-conditioning system.

This article focuses on the idea of fluid comfort or hydronics, and serves as an open call for engineers to take a leadership role in engineering energy-efficient space comfort systems based on hydronics and associated technologies. (Roughly half of the comfort level in a space is determined by the radiant effects around you and the other half is from the temperature, humidity, and the motion of the air, so why shouldn’t we all embrace comfort engineering and not just plumbing or HVAC engineering?)

Learning from Experience
According to a very knowledgeable German engineer, 100% of new commercial projects in Germany use hydronics.  In the U.S., the number is closer to 6%.  Over 60% of cooled commercial buildings in Germany use radiant cooling.  Over here it is less than 1%.

Why would German engineers provide the extra work of designing hydronic and radiant systems when it is easier to slap a packaged A/C unit through the wall or on the roof?  Well if you think that German engineers are too backward to select a packaged rooftop unit, think again.  They know what they are doing.  And by learning from their smart moves, we could save ourselves several decades of work.

Radiant cooling has been dismissed as impractical in the United States because of high humidity levels in many parts of the country. Also, Americans are accustomed to using cold blasts of air to cool interior spaces.

Figure 1 – Geothermal Heating and Cooling
(Photo courtesy of Priest-Zimmerman, Denver, CO)

One could just assume that the Europeans don’t get it, except that in the building industry many of the latest innovations come from Europe. Consider condensing boilers, dual-flush toilets, PEX and polypropylene-random (PP-R) piping systems, and in-floor radiant heating. Clearly they get it, and they are good engineers.

Heating, Cooling, and Ventilation Requirements
A Fluid Comfort System (F/C) is not a standard A/C system.   One of the biggest differences between F/C and A/C is the way ventilation air is delivered to the space.   In A/C, ventilation air is mixed with return air from the space and all the air is then cooled (or heated).   With F/C, there is no central air distribution system to bring in fresh air, so a dedicated outside air system (DOAS) is needed.

a.     A little fresh air – Every occupied space in a building needs fresh outside air to keep the occupants healthy and happy.   The currently recommended quantity is easily calculated using the most recent version of ASHRAE Standard 62.  Historically this has been done by a large damper in the return section of the A/C unit which draws in X% of outside air.   This may over or under ventilate some areas, but “close enough” was the thinking.  But today, many A/C systems vary the air flow to a zone based on demand.   So if a room needs no cooling, then a damper shuts, cutting off the fresh air.   So these systems now must have minimum settings that keep the air flowing even if it over cools the space.  To overcome this new problem of the space being over cooled, these A/C systems usually include a reheat coil (often electric resistance heat), that heats the cooled air back up before it goes to the space.   This explains why a DOAS is preferable.   Another reason is it makes energy recovery very simple.  You see, for every cubic foot of fresh air brought into the building, a cubic foot must be removed.  By designing the fresh air and exhaust systems to cross paths it is easy to transfer the heating and cooling energy from the exhaust air to the incoming fresh air.   It is possible to recover 60% or more of the energy that would be exhausted to the fresh air.   This is called enthalphy recovery and it saves operating costs and substantially reduces the size of the building’s heating and cooling plant, saving capital cost.

b.     It’s the humidity –  One of the single biggest cooling loads can be removing humidity from outside air.   As mentioned above, using enthalpy recovery with the outside air can meet 60% of this load directly by transferring the humidity to the exhaust air stream.   If additional dehumidifying is needed, a fresh air supply unit can further dehumidify the outside air to the point where it will offset any humidity gains in the space.

c.     Delivering the goods – Actually getting the right amount of fresh air to each space is simple.   A small duct (4 or 5 inch dia.) will usually do it, and an automatic constant airflow regulator ensures that each space gets its proper ventilation.   Spaces needing large volumes of fresh air can be served by their own enthalpy recovery unit.   Ideally the DOAS system will satisfy both the ventilation requirements and the latent (humidity) space loads.  There is often a positive relationship between the need for ventilation and the need to remove humidity (called the latent load).

d.     The sensible thing – The heating and cooling loads not related to humidity are called the sensible loads, meaning these loads will change the space temperature and therefore will be sensed or measured by a thermometer in the space.  Since the DOAS substantially reduces the latent load, this allows the F/C system to focus on sensible cooling at the space level.  In the winter, this is the heat that must be added to replace the British Thermal Units (Btus) that flow out of the space due to heat loss to the outdoors.  In the summer, it is the thermal energy that must be removed to offset external and internal heat gains, and includes solar heat gain through the windows and heat generated by lights and equipment.   These loads are expressed in Btus per hour or reduced to MBH (1000s of BTU per hour - BTUH).    Moving these BTUs (sensible loads) around a building is a main function of a mechanical comfort system and it can be done much more cost effectively and energy efficiently using water than air.

Figure 2 - Outdoor heat pump/chiller

Space comfort using F/C
Delivery – Piping a fluid around a building to transport heating or cooling is nothing new.  It actually predates forced air.   Remember the old steam heat systems?  Or 180ºF hot water systems?   But thanks to the cheap energy during the last century, these systems yielded to lower first cost systems like forced air, electric heat, though-the-wall A/C, and split systems.

Figure 3 - In floor heating and cooling

These low first cost systems do not lend themselves to alternative energy, waste heat recovery, thermal storage and solar or geothermal energy, so in Europe the trend has been toward hydronics.

Hydronic systems can be designed two ways: A system that heats or cools directly, or a system that uses water-to-air heat pumps to transfer energy from the hydronic loop to the space. For direct cooling, the loop temperature must be below the space temperature and for direct heating the loop temperature must be above the space temperature.

So normally direct systems require two loops or can only provide one function at a time.  The advantage of the heat pump is that one hydronic loop can perform both heating and cooling.   And heat pumps make it simple to transfer excess heat from a warm zone to a cold room.   A disadvantage can be that when free heating or cooling energy is available, a compressor must run to transfer the energy.   Plus there are multiple refrigeration systems to install and maintain.   Heat pump systems almost always use forced air at the zone level.  Direct systems can use forced air, convection, or radiation.

Distribution – Whether the decision to go with a chilled water loop (approximately 45 to 55º), a heat pump loop (approximately 50 to 95º) or a heating loop, (typically anywhere from 95 to 180º), the same design rules hold true;

i.            Plan the layout – Centering the central plant will cut piping and pumping costs.  Make the most direct route to the furthest terminal unit.

ii.            Select – Distribution options like 4-pipe, 2-pipe change over, single-pipe with zone pumps (e.g.Taco’s LoadMatch™ system), pipe material, variable flow, etc.

iii.            Equipment selection including Delta T – The larger the temperature drop (or rise) of the working fluid, the smaller the pipes and the lower the pumping energy. This decision is affected by the capabilities of the equipment to perform at low flows.

iv.            Size the Pipes – Start from the most remote location and work toward the central plant. Oversize the main run to save pumping energy.

Central plant – There are numerous reasons to have one or more central plants in your facility.

i.            First, in some types of equipment, higher efficiencies are offered in larger sizes.

ii.            Second, central plants tend to be better maintained than decentralized equipment.

iii.            More options are available, like waste heat recovery, solar thermal, co-generation, thermal storage, water-cooled chillers, geothermal heat pumps, air-source heat pump chillers, etc…

Plus it may be possible to transfer the cooling heat rejection to be the heating source.  The economics of these energy options improve with the use of cooler heating water.   It is possible to heat with only 95º heating water using either radiant or fan coils. With fan coils this may mean using the same coil for both heating and cooling.   At 95º there are lots of potential energy sources.  In fact this 95º loop could even double as a heat rejection loop for refrigeration equipment.

An important energy tip to consider as part of the central plant design is to think about part load performance.  It often pays to have a small boiler or chiller to handle light loads.  Also pumps should be designed for variable flow using a VFD.

Make it Green
Buildings leave a large footprint on our planet.  It is not just the size of the foundation.  It is also the building’s energy impact.  How much coal or natural gas will this building need over the next three or four decades?   How many tons of CO2 will be added to our atmosphere?  These are important questions.  But here’s an important key to look for in minimizing the footprint; the lower the connected load, the smaller the footprint.   An optimized F/C building can have a very small connected load. 

Figure 4a & b - Generator producing waste heat for heating and cooling

The Road to Net Zero
A goal in the green building movement is to see if buildings can generate as much power over the year (through photovoltaic, wind energy, etc.) as they consume.  This is called net zero.   The first step will be to have a minimum connected load, and to be flexible in using different energy sources.   In my opinion, and based on the European experience in this area, hydronics will be a key part of getting there.

About the Author
Steve Clark is a Professional Engineer in the United States and Canada. He has worked as a development and applications engineer for the Trane Company and as an HVAC and energy engineer for consulting engineering companies, including his own firms, with an emphasis on building energy efficiency. His building system designs have won energy-efficiency awards, including First Place for Commercial Buildings from ASHRAE. He holds several international patents on HVAC and piping systems.  Steve’s 30 years of experience in building energy optimization has led him to believe that hydronics are a key component to efficient building design and that selecting the right pipes is key to an efficient and reliable piping system design. To help fill this need in North America, he now serves as President for North America for the German-made plastic piping manufacturer, Aquatherm.