ASHRAE Journal:
ASHRAE Journal presents.
Drew Champlin:
Welcome to the latest ASHRAE Journal Podcast episode. I'm your host, Drew Champlin, ASHRAE Journal editor. We'll be speaking with Kelley Cramm, a regular contributor to ASHRAE Journal about variable refrigerant flow systems, how to properly install these VRF systems and how to avoid some installation mistakes.
Kelley, how are you doing today?
Kelley Cramm:
Hey, I'm doing well. Thank you, Drew. It's good to be here.
Drew Champlin:
Thanks for joining us. Before we get into the nitty-gritty of VRF systems, why don't you tell us about your background and any work you've done with ASHRAE and anything else you want to share?
Kelley Cramm:
Okay. Well, I'm Kelley Cramm. I'm a mechanical engineer with Henderson Engineers and I'm in the Kansas City area. I have been a practicing consulting engineer in the industry for over 40 years. I've also been active in ASHRAE for most of my career. I served on my local chapter board through all the chairs and have served on many ASHRAE committees over the years. I am a past director at large on the board, and I'm also active on two TCs, technical committees, 9.1 and 9.10.
Drew Champlin:
Well, let's get into the topic at hand, the background of VRF. Tell us what it is and why is it important?
Kelley Cramm:
Okay, so variable refrigerant flow, or VRF systems, are all electric heat pumps and they may incorporate heat recovery. They're very flexible systems. The indoor fan coil units are offered as drop-in ceiling cassettes or compact wall mounted units, or you can get concealed ducted units and then multiple indoor units may be piped to a single set of outdoor condensing units so that reduces your condensing unit footprint. They offer a really cost-effective way to provide individual space temperature control. And this can be particularly helpful for occupancies such as K-12 schools or maybe buildings with multiple enclosed offices where you want individual temperature control. VRF is also inexpensive compared to, say, a hydronic fan coil system or a classic VAV with reheat system. And it also allows for heat recovery. So if a building has internal zones that need cooling while perimeter zones need heat, this system can accommodate that. The heat rejection from the cooling zones can be used as a source for the zones that need heat so it's a really efficient system.
Drew Champlin:
What are people saying about VRF these days and why might they be saying these things?
Kelley Cramm:
Well, unfortunately, compressor failures has been an ongoing problem with VRF, and this has contributed to a negative view of VRF in the market. Some owners have become very frustrated with the need to routinely replace compressors. Manufacturers have tended to replace failed compressors that are under warranty without really looking deeper into why they're failing. Manufacturers typically work really hard to maintain good relationships with their customers, so they've been willing to replace compressors without asking questions. Unfortunately, not getting to the heart of the problem has shaped the industry view of VRF over time.
Drew Champlin:
All right, well, Kelley, why are things going wrong with installation issues here?
Kelley Cramm:
Well, most of the ongoing problems with VRF can be traced back to two issues, improper installation and a lack of routine maintenance. It could be either or a combination of both of those. What do I mean by that? So some of the things that on the maintenance side are filters on indoor units need to be changed regularly, and this can present a challenge for buildings that have a lot of indoor fan coil units, especially if the maintenance staff is already stretched thin. The need to go around and change all those filters can be a real challenge, but if the filters aren't changed, airflow can be reduced at the indoor unit and that can potentially lead to a compressor failure. Also, condensing unit coils, the outdoor units need to be cleaned at least twice a year. Most VRF systems use micro fin condensers, which tend to trap dirt and debris a lot more readily than standard old-fashioned fin tube condensers.
So those condensers need to be hosed down at least seasonally. Not doing this can also lead to condenser failures because you have reduced airflow across the condenser due to the fouling of the coil. So the other thing to remember is that these are heat pumps, so the condensing units run year round as opposed to DX cooling units, which only run during cooling season. So they get a lot more hours of operation, and if the owner does not have sufficient staff to perform this kind of routine maintenance, they really need to engage with a qualified service contractor to ensure that it gets done.
Drew Champlin:
Okay. Well, what installation practices would you say are important for VRF?
Kelley Cramm:
Well, the number one item is that the pipe has to be purged with an inner gas, usually argon or nitrogen during brazing. And this is critical to prevent slag and poor joints. The brazers also need to be trained and certified by the VRF manufacturer. And the other thing is that if the pipe is not installed per the manufacturer's shop drawing, the installer must create an as-built piping drawing and have the manufacturer rerun their software using the revised layout to determine the proper refrigerant charge. Refrigerant charge is informed by the length and volume of the pipe, and if the layout changes in the field, that refrigerant charge has to be updated.
Failure to do this may result in the system being overcharged, which can lead to premature compressor failures. One thing that's different about VRF than traditional DX or heat pump split systems is that VRF compressors are routinely operated at high speeds. So if a compressor fails and the owner is not diligent on checking error codes, they may not know they've lost a compressor since the remaining compressors will speed up to compensate. Over speeding compressors for long periods of time can result in additional failures. This is very common where an owner will get an error code and just silence the error and go on and not check it. So owners need to be diligent about checking the error codes.
Drew Champlin:
Well with refrigerant line lengths and isolation valves, what do we need to know about those?
Kelley Cramm:
Okay, so first item is installing suction line side stream filter assemblies, that's a mouthful there, with fine mesh filter elements and isolation valves is also critical. The purpose of these is they're used during startup to filter out fine particles that may be generated during brazing. Even the best qualified and conscientious brazer, you're going to have a little bit of slag in there that needs to be filtered out. If they don't do that, and that debris migrates to a compressor that can cause premature failure.
So these are used during startup, and then once the system is operational, it's been filtered, you valve those bypasses closed and they're not needed any longer. Refrigerant line lengths and sizes, like I said before, are used to inform the refrigerant charge. So any field changes to the shop drawings requires a new charge calculation. And another thing about the piping that I wanted to mention is that the engineer is responsible for ensuring that the design complies with ASHRAE Standard 15. Standard 15, and also the International Mechanical Code have some limitations on where you can put refrigerant pipe, how it's installed, and maybe the potential need for openings between spaces to increase volume.
Drew Champlin:
Can you speak on some of those limitations?
Kelley Cramm:
Well, yeah, I can. There are a number of limitations. For example, refrigerant pipe is not allowed to be installed in egress corridors or paths of egress due to the potential leakage. And so for example, if you have an egress corridor that doesn't have a ceiling and you need to cross it with refrigerant pipe, you have to come up with a way to enclose that pipe so it's not exposed in the egress corridor. That's one example and there are a few others in the International Mechanical Code that are important to be aware of.
Drew Champlin:
Okay. Well, Kelley, how can engineers help owners ensure that they are getting a good installation?
Kelley Cramm:
Well, I would say during design, one design issue that can lead to problems is relying on the VRF system to condition ventilation air. This is especially true in humid climates. VRF fan coil units have a limited ability to pick up latent load. So I always recommend decoupling the ventilation air from the VRF units, especially in humid climates. There have been many articles published over the years in the ASHRAE Journal on the benefit of using dedicated outdoor air systems, otherwise called DOAS and delivering conditioned ventilation air directly to the spaces. This allows you to size the DOAS to dehumidify the outdoor air and then supply it at a low enough dew point to the space to pick up the space latent load. There's an ASHRAE publication on dedicated outdoor air systems that talks specifically about this, and goes through the details of how you do that calculation.
It's fairly straightforward. If you do that, the VRF essentially becomes a sensible heat unit and you also eliminate the problem of condensate in the drain pans because it's just sensible cooling only. Other things the engineer can do is the engineer's specification really should require the installing contractor to submit proof that the installers have been factory trained and certified by the VRF manufacturer. Unqualified installers can cause big problems because they may not understand how to properly do the brazing and the installation. The engineers also, once you get to construction engineers need to visit the site during construction. This is really important to assess the quality of the installation. Some of the things that you should look for when you're on site is be sure that you see inert gas cylinders on site because that tells you whether or not the inert gas is being used during brazing.
If you don't see those inert gas cylinders, nitrogen or argon, that should raise a red flag. Check on those suction line side stream filter assemblies we talked about and make sure those have been installed properly. And most importantly, crosscheck the installation against the manufacturer's shop drawing and make a note if there are deviations. So take that shop drawing with you either on a tablet or in paper and just spot check to make sure it's going in as the shop drawing notes. You don't have to note all the deviations, that's not necessary, but if you observe some note in your site report that they need to have the manufacturer adjust the refrigerant charge based on the revised layout. The other thing is I would suggest checking on site to make sure that those ASHRAE 15 compliance items are included in the installation that we talked about earlier, that the piping is installed, it's not in an egress path exposed. If you have small rooms where you need connections to adjacent spaces that those are installed. It's important to look for all that in the field.
Drew Champlin:
On the other side, how can contractors help owners ensure that they're getting a good installation?
Kelley Cramm:
Yeah, I think the most important thing that contractors can do is to ensure the labor that's on site is qualified and trained to install VRF piping and equipment. Some contractors will have one installer on their staff who's certified to install, but that person may not be on site because if they have three or four VRF jobs going at once, they only got one person. So it's really important to make sure that the onsite labor is qualified, trained, and certified to do that installation. And I think that falls both on the mechanical contractor and on the general contractor or the construction manager to ensure that they've got qualified labor doing that work.
Drew Champlin:
All right, well Kelley, what do we need to know about systems integration here?
Kelley Cramm:
All right, well system integration, we're talking about integration from the VRF controller to the building management system. That really continues to be a challenge. Each VRF manufacturer has their own proprietary software and control systems that only work with their equipment. So those VRF controls tend to be what we call black boxes. They're not accessible and you can't get into them. So this is changing slowly over time, but we still have a ways to go to get true integration to the BMS.
Drew Champlin:
How does climate factor in here?
Kelley Cramm:
Well, it's important to remember that VRF systems are heat pumps. Heating capacity with a heat pump is dramatically reduced as outside air temperature falls. So this can present a real challenge in cold climates because when you need the heat the most, you have the least amount of heat with the heat pump. So depending on the location and the climate on the project, you should always consider providing a source of backup heat. I always like to consider putting backup heat in the DOAS. That's an easy way to provide some additional heat in case you need it when it gets really cold outside. Another thing that the engineers should consider doing in cold climates is all of the manufacturers offer what they call wind guards for the outdoor condensing units and for cold climates, that's really important because these can mitigate capacity reduction that might come with cold wind.
Drew Champlin:
Interesting. Well Kelley, what other conclusions and recommendations do you have for VRF?
Kelley Cramm:
Well, VRF systems are a great cost-effective option if you need individual temperature control for multiple spaces. I would say that design engineers should educate themselves on these systems before attempting a project. There's a lot to them and like I said, there's a lot of technical literature out there. A lot has been written in the ASHRAE Journal about these systems, so that's really important. Also, I recommend decoupling the ventilation air and use a dedicated outdoor air system as opposed to having the VRF condition that ventilation air. Do the ASHRAE 15 calculations and observe the do's and don'ts of refrigerant volume in occupied spaces and pay attention to those code requirements regarding refrigerant pipe in egress paths in particular. Keep a close eye on installation and quality and compliance with the shop drawing during construction. And also be skeptical of manufacturers published heating capacities and low ambient temperatures.
In my experience, those capacities can be overstated, so provide a source of backup heat for cold climates. Also for the engineers, VRF systems are proprietary to the manufacturer and each manufacturer does things a little differently. They may not have the same piping layout requirements across different manufacturers so if your project is going to be bid on the open market, you may need a way to allow other bidders than the basis of design. So you have to be aware that there may be differences and your specification needs to require the bidders and manufacturers to be aware of and address any changes that may be required if they're going to be the successful bidder.
Drew Champlin:
All right, well, Kelley Cramm, thank you so much for your time on the ASHRAE Journal podcast.
Kelley Cramm:
You're welcome. It's been great, Drew. Appreciate it.
Drew Champlin:
That will wrap up this episode of ASHRAE Journal podcast. I'm Drew Champlin, ASHRAE Journal editor. Thank you so much for listening.
ASHRAE Journal:
The ASHRAE Journal Podcast team is editor, Drew Champlin; managing editor, Kelley Barraza; producer and associate editor, Chadd Jones; assistant editor, Kaitlyn Baich; associate editor, Tani Palefski; creative designer, Teresa Carboni; and technical editor, Rebecca Matyasovski. Copyright ASHRAE. The views expressed in this podcast are those of individuals only and not of ASHRAE, its sponsors or advertisers. Please refer to ashrae.org/podcast for the full disclaimer.