Comparing Water vs. Air

Comparing Water vs. Air Water Vs. Air

In the HVAC world there are two principal delivery systems employed to provide indoor air comfort – water (hydronics) and air (direct expansion systems or DX for short) – and each has its proponents and schools of thought. But is one delivery system better than the other and, if so, how and why? Is one better than the other in terms of costs, comfort levels, indoor air quality, energy use, and even safety? That’s the discussion we want to develop via this blog, and we’d like you to weigh in. Naturally, we think that a hydronic system is a better offering than an air system, especially for air conditioning, and we’ll be offering a number of reasons in support of that view.

Traditional hydronic chilled water systems have long provided very comfortable and reliable air conditioning, and they come with several major advantages: excellent indoor comfort (especially with multiple zones), lower energy costs, and improved IAQ. Plus, there are new developments occurring with hydronic-based chilled water systems: the growing popularity and use of radiant heat systems that now include a radiant cooling and chilled beam options, and the emergence of a new single pipe system that can dramatically lower the usually higher first costs of a hydronic system in comparison to a DX system.

DX systems have been around for a long time but larger commercial DX systems – VRV (variable refrigerant volume) or VRF (variable refrigerant flow) – are now on the market.. VRV has been called “another refinement of DX refrigerant split systems.” Unlike earlier versions of refrigerant split systems that employed multiple condensers, evaporators and refrigerant lines, VRV/VRF systems use only one large outdoor condenser and one set of refrigerant pipes for the entire building, with a separate evaporator for each building zone, similar to a hydronic chilled water system.

In essence, VRV/VRF systems are made up of a central chilled water system with refrigerant pipe. In doing so, the cost of a DX system has come down in price. That’s important because chilled water systems are typically more expensive to install than DX systems, resulting in higher first costs. However, to select a VRV DX system over a chilled water system based on first costs alone would be shortsighted, considering our contention that chilled water systems provide more benefits overall.  More important chilled water systems are safer since occupants are not exposed to an unsafe concentration of refrigerant if a leak occurs

In citing those benefits, none is as valuable as the end result: optimal indoor air comfort, regardless of a building’s size, configuration or climate setting. Hydronic systems provide superior comfort to all other available options for comfort air conditioning – VRV systems included.

With a chilled water system it’s easier and less costly to provide multiple zones of temperature control, particularly in larger buildings, because multiple terminal units (fan coils or heat pumps) are linked to one set of central generation equipment. One of the traditional concerns with chilled water systems has been balancing the system upon start-up and commissioning. However, the development of automatic flow control valves that can automatically adjust for differences in pressure and flow rates has improved balancing of hydronic systems. And with the advent of single pipe systems that self-balance, system start-up and commissioning has become that much simpler.

Next time: more on the innovative single pipe system and its benefits. In the meantime, let us know your system delivery preference and why.

Posted by Greg Cunniff, Applications Engineering Manager, Taco, Inc.

One Response

  1. Chilled water trumps direct-expansion and VRF for many reasons. From an environmental view it’s better due to substantially lower refrigerant volume. In fact, the refrigerant volume of some chillers has been reduced to unbelievable lows by implementing Micro -Channel condensers. Space is the second big advantage; ducting is a necessary evil of a DX system, chilled and hot water piping takes much less space, leaving room for other building mechanical systems.
    Load tracking is the third advantage; most chillers have much better capacity turn-down ratios than DX, allowing the chiller to track the building load more closely, especially when combined with variable flow pumps. Redundancy is the fourth advantage; having backup rooftop units is impractical at best, but having multiple circuited chillers along with back-up chilled water pumps is not only practical, it’s already an industry standard. Add chilled beam technology and chilled water wins hands down.

    Patrick Peterson

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