Chilled Water vs. DX-Type Systems for Commercial Buildings Bob Mayoh for Taco, Inc. Interviews Greg Cunniff, Senior Applications Engineer for Taco, Inc.
In this wide ranging interview Greg Cunniff examines the features, benefits and differences between air conditioning with chilled water on the hydronics side and a DX-type system (VRV/VRF) on the air side.
In Part One Greg compares the two systems in light of the design engineer’s objective of providing the most comfort for the least dollars in both first-costs and operating costs as well as providing a safe environment for building occupants. He also examines dehumidification, variable speed drives, and discusses the emergence of radiant cooling on the hydronics side of the equation.
Chilled Water vs. DX-Type Systems for Commercial Buildings: Part One
Bob Mayoh: My name is Bob Mayoh, and welcome to this special podcast, Taco Podcast, with Greg Cunniff, who is a Senior Applications Engineer with Taco. And our subject today is chilled water vs. DX Systems. Greg, let's talk about air-conditioning for large commercial buildings and the two different delivery systems, which is our subject here: a hydronic-based chilled water vs. a direct expansion or DX system. Now, in doing so, we're going to pose a question that Greg's going to help us with: Is one system better than the other? And, if so, why? What are the features and benefits of both?
We hope that we will demonstrate that a chilled water system is the preferable system to go with for a number of reasons. So to begin, Greg, let's compare the two systems: the chilled water System (which is hydronic in nature, using water) and the DX-type system using refrigerant that is gaining ground in the marketplace. The manufacturers are claiming double-digit growth. Apparently the U.S marketplace is the next big target for these manufacturers. The application that we're talking about here is for larger buildings than these systems have been previously used in. Again, it's a variable refrigerant flow, or VRF, or VRV for variable refrigerant volume. First, chilled water, Greg. Talk to us about a basic chilled waters air-conditioning system.
Greg Cunniff: Well, Bob, you've brought up some good questions. In commercial buildings, there's basically two ways to provide comfort and move BTUs around the building. You can do it with water, and you can do it with refrigerant. We call those airside systems or DX vs. Hydronic, our water-based systems. Both systems certainly have their advantages, however, we feel that hydronics has the most advantages in terms of comfort, in terms of efficiency and safety. We in the hydronic business have been providing very comfortable, very reliable, and very efficient systems for a number of years. The DX or VRF systems have come to the forefront in the last few years. We're starting to see the emergence of that particularly in the United States, although they have been readily available in other parts of the world.
What we're interested in mostly is which system can give you, as the owner or as the user of that system, the most comfort for the least dollars of both first cost as well as operating cost. And, again, provide a safe environment for the occupants of the building. We think that hydronics is by far the better choice, and there are a number of good reasons for that. Hydronics, in particular, use a fluid that is non-toxic; it uses water-readily available. DX systems use refrigerant, and refrigerant, interestingly enough, is toxic; it's a toxic fluid. It's toxic enough that the building codes are now requiring alarm systems if there happens to be a release of the refrigerant into an occupied space or an occupied mechanical room, as an example.
So, there's certainly some questions that need to be answered or at least asked, and if you're designing or installing those kinds of systems, you at least need to be aware of the liabilities that you could ultimately be faced with.
Bob: All right. Let's talk back on the subject of chilled water systems. They have a long record of reliability of these chilled water-based systems. Let's talk about indoor air quality as well as lower energy costs as benefits that you get out of a chilled water system. Can you elaborate a little bit on those?
Greg: First of all, to begin with, water is a denser fluid, and it has a higher specific heat than air. So you're going to be able to move BTUs around the building with less horse power than you are with air systems. In fact, it takes about half the horsepower to push BTUs around the building with water than it does with air. Now, certainly, hydronics systems have been well-proven. They've existed for a long period of time, and they provide an easy and very reliable way to move these BTUs. The newer VRF or VRV systems do avoid some of the problems with straight air systems, where you're moving large quantities of air from a central air handling system. You're still having to have local air handling units or fan coils to distribute those BTUs in the building. But you can move BTUs through smaller pipes from the condensing units out to the individual evaporators. However, you're still going to have to have air moving in that space.
Hydronics have advanced to the point now where we can literally distribute BTUs without any air movement. We've had hydronic heating systems in place for a number of years that use radiation as the principal means of heat transfervery comfortable systems. We're putting piping in the floor. We're running hot water through that. The heat transfer is radiation from that warm floor to your cold bodyvery little air movement. The problem with air movement from a comfort standpoint is it creates evaporative cooling, and you're uncomfortable when air is blowing over your body. With radiant heat systems, there is no air movement. It's all radiation heat transfer.
The Europeans have been working on that technology in reverse for about 20 years with radiant cooling systems, where they're installing piping in the ceiling that has chilled water in it, and then that ceiling is able to absorb radiant heat from your body and cool you down without moving air. Again, the reverse of radiant heating systems with radiant cooling. There have been further advances with that technology with chilled beams, which are basically more efficient radiant cooling devices, and, now, active chilled beams which combine some convection with radiationvery comfortable systems. Again, moving BTUs with a lot less horsepower than you are with air. So the comfort levels with hydronics systems are really the best that you can achieve.
Bob: Now, Taco has a couple of products...recently developed products for the radiant cooling market. Can you give us a brief overview of those two products?
Greg: Sure. Interestingly enough, when you're dealing with radiation as the principal means of heat transfer, you want to be able to radiate heating or cooling in such a manner that you feel comfortable with that heat transfer. As an example, if I pump 180 degree boiler water through a floor, yes, I have radiation that's heating my body, but my feet are uncomfortable...because the water is too warm. When you deal with radiant cooling, it's not a problem with the ceiling being too cold from a comfort standpoint. But if it's too cold in terms of the humidity, then you'll have condensation on the underside of that panel, and it will literally rain in the room.
Bob: [Laughs] So not a good thing?
Greg: Not a good thing, no. So we have to provide chilled water above the dew points, so we don't rain in the room. So you could think of it this way: In a heating system, we're trying to mix down the high temperature boiler water from 180 let's say, to 110 or 120 in the floor. In a chilled water system, we're trying to mix up the low temperature chilled water, 42 or 45, to the temperature that you would use in that radiant ceiling or chilled beamupper 50's to low 60's. So how do we achieve that? We'd like to be able to use conventional chillers and boilers, and high temperature hot water and low temperature chill water to give us small pipes and low pumping horse power to move all those BTUs around. So we at Taco have developed some technology that'll allow you to utilize standard equipment at standard distribution temperatures, but still provide the lower temperatures for radiant floor and the higher temperature for radiant ceilings. We do that through injection pumping.
Greg: We've had our radiant mixing block product out on the market for some period of time-seven or eight years for radiant heating systems. We've just introduced a new product called the Low Flow Mixing Block that allows you to mix up the low temperature chill water to the higher temperature needed by the chilled beams. It consists of an injection mixing circulator and a zone circulator. The ejection circulator being a variable speed to maintain the temperature that you need to the chilled beam. Then, a constant speed zone circulator controlled by the room thermostat.
Bob: And this is all in one unit?
Greg: This is a package unit, yes. Correct.
Bob: Mmm-hmm. And Taco has a single pipe distribution system that works along with this low-flow injection pumping. It's part of the...
Greg: Yes. Our injection pumping systems, whether it's the radiant mixing block for heating or the low-flow mixing block for cooling, all utilize a single pipe distribution system. That's correct. That is a way for hydronics systems to be more competitive on a first cost basis because we're reducing the quantity of piping that it takes to move the BTUs around the building. Interestingly enough, that system is also self-balancing because we're using a decoupled primary/secondary piping system to distribute the BTUs from the primary to the secondary or terminal unit circuit. Big advantage in terms of commissioning startup and simplicity.
One of the...I'm not going to say knocks, but one of the...difficulties with hydronics systems has been balancing and achieving the correct distribution of water to all the terminal units. What we have been working on as way to eliminate those kinds of startup problems, and the single-0pipe distribution does in fact do that. They are essentially self-balancing.
Bob: OK. Let's talk...let's define the actuall physical setup of a VRV or a VRF system. It employs a...you called it another refinement of a DX refrigerant split system. It employs apparently only one large condenser, one set of refrigerant pipes, with separate evaporators for each building unit. Are those the equivalent of zones? These things...
Greg: Essentially, Bob, yes.
Bob: Zoning. OK.
Greg: What the VRF and VRV systems are doing is utilizing some technology that has been around for a long time, but hasn't necessarily been applied to that particular system. That's variable speed. We've had variable speed fans around for at least 25 or 30 years with variable air volume systems. In the hydronic industry, we've had variable speed pumps. They've been out on the market for a number of years. Probably 15 years at least, maybe even longer.
The DX systems have essentially been constant volume systems where you have one evaporator coupled up with one air-cooled condenser. And then one set of refrigerant pipes between those two devices. The DX industry or DX manufacturers have finally figured out how to incorporate the variable speed technology within their systems as well. What they've done is they've now found a way to use multiple evaporators on a single condensing unit. Essentially, it's a chilled water refrigerant system. All right? To make the work, you have to have a variable volume of refrigerant. You can't have a constant volume system pumping the same volume of refrigerant around and then trying to somehow modulate that amount of refrigerant going to each evaporator.
So the good news here is that that increases efficiencies, just like it has increased efficiencies of all air systems, variable air volume and our variable water volume systems, that we've had around for a long time. It's all good things in terms of being able to bribe more efficient systems.
Bob: One of the things that obviously is a key consideration with any air conditioning system is the matter of dehumidification, regardless of the delivery means. How does the chilled water, and a VRF, VRV system, how do they handleseparatelyhow do they handle the matter of dehumidification?
Greg: In certain parts of the United States, dehumidification is a big issue, and it's a major portion of providing comfortcomfort cooling. The two systems work somewhat differently. The biggest difficulty you've got with dehumidification is being able to remove the moisture on a more or less continuous basis for optimum comfort. We're all familiar with theI'm going to call them the 'through-the-wall'we call the PTech Units, package terminal and air-conditioning units that you find in the hospitality industry. OK? [Laughs]
Greg: If you compare that kind of technology, that kind of equipment with a fan coil doing cooling, there is a marked difference in comfort. If you walk into a room that's cooled with a DX PTech system, it'll be cool, but it'll be clammy, cold and clammy. If you go into a room that's cooled with a hydronic fan coil, it will be cool, but not clammy.
Bob: I've been in a lot of hotel and motel rooms. There cold and clammy.
Greg: Yes. [Laughs] The problem comes in that the DX unit is typically on/off. And it's supplying cold refrigerant, and so you satisfy the sensible cooling load in the room fairly quickly, and the unit shuts off. Unfortunately, in those climates, that doesn't remove the moisture. You have to look at something that we call the sensible heat ratio. The sensible heat ratio is the ratio of the sensible to the total heat. OK. Typical loads will have sensible heat ratiosin the low to mid .7, 70%, let's say. OK? That's in the Southeast, Midwest parts of our country. The sensible heat ratios of the coils, meanwhile, are not that low. They're .8 to .9. So what happens is that when that unit shuts off from the sensible cooling, we can't maintain the dehumidification.
The hydronic system, on the other hand, you can provide modulation. So you can modulate the amount of cooling that we're providing to keep that unit on sensibly and still continue to remove the latent heat. That's why hydronics systems will provide better comfort than DX systems. Now, the variable volume systems are better in that you can maintain those systems on for a longer period of time, but you still cannot modulate them as well, as low a flow of fluid, as you can with a hydronics system. So a hydronic system is always going to provide you better humidification than a direct expansion system, whether it's constant volume or variable volume.