Going With The (Variable) Flow
Considering a new HVAC system? A variable refrigerant flow system (VRF) is one additional alternative that should be added to the option list. VRF is most widely used in air or water cooled heat pumps or condensing units with heat recovery. The technology has been around for some time but has not been a common HVAC option for most owners and engineers in this country. Only recently have we seen more and more engineers look toward VRF as a viable option to consider. Some significant potential energy savings can be realized as well.
Variable OptionsFor the air cooled option, VRF systems utilize a single or ganged-together configuration of multiple outdoor units that use a single set of refrigerant pipes serving multiple indoor zones. The indoor units are fancoils that come in many different configurations, including cooling-only or heating and cooling, either concealed or exposed.
VRF systems derive their efficiency from the use of variable-speed compressors that load and unload to deliver only the necessary quantity of refrigerant to the indoor units.
A heat recovery option is available for designs requiring simultaneous heating and cooling, which can be a great advantage to laying out an entire floor plate. With the heat recovery option, heat is rejected from the indoor units in cooling mode, recovered, and then utilized as the heat absorption source for the indoor units in heating mode.
This is somewhat similar to a central water source heat pump system where the heat pumps reject their heat or cooling into the condenser water loop depending on their operating mode, except that in VRF, the piped refrigerant is transferring the heat. Significantly different though is that a phase change is occurring such that two indoor units (one in cooling and one in heating) can complete a refrigeration cycle along with the compressor.
Another way to consider the VRF heat recovery operation is that a zone in heating can make liquid for a zone in cooling. In this way, the heat recovery operation can reduce the duty of the compressor as it only has to pump the refrigerant and facilitate making up any difference. When heat recovery is available, compressor loading is greatly reduced and will benefit overall performance and your energy bill over the life cycle. Per one manufacturer’s published data, if you had a 50% demand for full cooling and a 50% demand for full heating (equaling the full load of the condensing unit), in heat recovery mode the compressor would only be loaded at 48%.
Other advancements that have added to even greater increases in energy performance of VRF systems include advanced compressor design, advanced inverter technology for tighter and more efficient power control, variable-speed condenser fans, and sophisticated control logic.
Using VRF In PracticeAs a case in point, a recent remodeling project for an entire floor of a mid-1930s medical office building forced the design team to look toward alternate design possibilities. From a mechanical engineer’s perspective, the opportunities looked challenging, to say the least. The roof was cluttered with existing equipment, the existing air cooled chiller was at capacity and scheduled for daytime operation only, and the building’s structural capacity to carry large loads was limited. Architecturally, we were to minimize ductwork and piping by using the existing horizontal and vertical penetrations while keeping equipment above the only area that was to have a ceiling (the core) - and, of course, we were to keep costs down.
Our first thought was, do nothing and use natural ventilation with steam convectors at the perimeter. The immediate response was negative - the new occupants would not go for that, and we could not touch the exterior of the building or change out the single-pane glass, which has a window to wall ratio over 60%. It was back to the drawing board.
After looking at small packaged units, increasing chiller capacity with fancoil units, separately zoned heat pumps, and water-sourced heat pumps, we were introduced to a VRF system. The outdoor unit had a very small footprint, which we could shoehorn into our cluttered roof and did not weigh much, making our structural engineers smile. The network of piping was much easier to deal with than ductwork, and we could connect our 24/7 technology closet A/C units to the same condensing units. All looked good so far, but we had not evaluated the installed costs of this system vs. the other options.
The system we selected utilized refrigerant 410-A. R-410A is a 50/50 blend of two hydrofluorocarbon refrigerants: R-32 and R-125. R-410A is slowly replacing R-22 as the refrigerant of choice in a number of DX applications.
There are some outright advantages to R-410A: not to downplay the required phaseout of R-22, but R-410A has a higher refrigeration efficiency (approximately 5%), an ozone depleting potential of 0.0, and a global warming potential (GWP) of 1997 (GWP is similar to R-22). However, for practical purposes in design, the refrigerant operates at a higher pressure, thus allowing for much longer piping runs. For our case, the additional piping lengths allowed us to connect that unplanned technology closet in the basement to the same condensing unit, providing a surprise benefit.
First-Cost PlusHaving compared refrigerants, we turned to sizing up hardware and impact on size. For comparison, we chose to evaluate the costs of the VRF as compared to what would traditionally be the least expensive option, the single-zone heat pump. With 12 zones, the heat pump solution would have required separate electrical service to each unit, a multitude of roof supports and penetrations, and refrigerant piping runs to each outdoor unit.
In comparison, the VRF system from the factory was more costly on a tonnage basis but only required one electrical connection, one roof support location, and one location for pipe penetrations. The result was a first-cost savings to install the VRF system. Our analysis has resulted in our current rule of thumb, which is when we are evaluating VRF against the single-zone heat pumps, the breakeven cost point to go to VRF is four or more zones.
In conclusion, VRF will continue to be in our arsenal of design options. One additional application we are considering uses VRF on larger projects to serve the IT closets that require 24/7 cooling, allowing shutdown of the central systems. We are somewhat hesitant to start using these on whole-building applications due to current available capacities, but as we gain more knowledge and experience, and the commercial market expands their product lines, VRF may prove to provide a benefit to our clients and building owners in those circumstances as well. ES
Sidebar: VRF: A Quick LookThe additional energy benefits to using VRF include:
- R-410A’s higher working pressure allows for dramatic increase in refrigerant piping runs over more common R-22 machines
- Energy savings: full load EER 11.2: with heat recovery at 50% cooling mode and 50% heating mode compressor load at 48%
- Diversity loading up to 130% of the connected indoor units to the condensing unit