Mini-Split, Multi-Split, and VRF Systems

Unless you have been living under a rock, you are sure to have noticed the rise of mini-split, multi-split, and variable refrigerant flow systems. This segment of the HVAC market has spawned a whole new vocabulary. The discussions about this equipment and even the technical literature are replete with this new jargon. So I thought I would spend a few moments to discuss some of the more common terms.

Unitary
This is not a new term, but you may often hear traditional systems referred to as unitary. Really, this just means that the system is a manufactured unit, instead of a field built system. However, it is often used to designate traditional HVAC systems: split systems as well as packaged units.

Mini-Split Systems
Mini-split systems are indeed split systems, with an outdoor condensing unit and an indoor blower-coil, which is often referred to as a head. Some common heads are high wall mount, ceiling mount, floor mount, and compact cassette – which tucks away above the finished ceiling. As the name mini-split implies, both the outdoor and indoor units are much smaller than found in traditional unitary systems.  For the most part, the indoor units are designed to be mounted in the space they are conditioning, with no ductwork. For this reason, many people also refer to mini-split systems as ductless systems. However, mini-split systems are actually available with both low static and high static ducted indoor heads which are designed to be installed in a concealed area. Static refers to the static air pressure  difference the blower is designed operate against while moving the air. Low static units are designed for very short, single runs – not complete duct systems. High static indoor units are designed to be used with complete duct systems. They are used more in commercial applications than residential applications.

Multi-Split Systems
Multi-split units are designed to be used with multiple indoor heads. The idea is to install one outdoor unit to supply from two to four indoor units. On typical multi-split units, the refrigerant line sets for all heads run back to the outdoor unit. The outdoor unit has connections for up to four indoor units. Another type of system which is often referred to as multi-split uses a small refrigerant network for up to nine heads. This type of system has one line set with branches that wye off at each indoor unit. These are really more like a small variable refrigerant flow (VRF) system.

Variable Refrigerant Flow
The term variable refrigerant flow (VRF) has two uses within the industry. The more global definition simply refers to any system which varies the flow or refrigerant to match the load. This is usually accomplished by changing the compressor speed. Truthfully, most mini-split and multi-split systems fit the global definition of variable refrigerant flow. There are now several unitary systems which incorporate variable refrigerant flow. Most of these systems use variable speed compressors to match the compressor capacity and refrigerant flow to the load.

However, variable refrigerant flow (VRF) also has a more specific meaning which refers to a specific type of system. These VRF systems distribute refrigerant via a network of refrigerant piping that supplies multiple indoor heads. Heating and cooling is distributed using refrigerant lines instead of ductwork.

One of the difficulties in studying this segment of the industry is the lack of uniform terms. For example, Daikin refers to their VRF system as  VRV for variable refrigerant volume. Each manufacturer tends to have their own name for the different network components. The components used to branch off from the main refrigerant line to feed an individual head have several names, depending on the manufacturer. They are called refnet joints (Daikin), branch joints (Mitsubishi), separation tubes (Fujitsu), or Y-branch fittings (LG). Most manufacturers have boxes that take a single refrigerant flow and divide it up among several heads. Common names include branch selectors (Daikin), branch controllers (Toshiba), branch boxes (Mitsubishi), flow selectors (Toshiba), or heat recovery units (LG).

Heat Recovery Systems
Again, the term heat recovery has several meanings within the field. The global definition simply refers to any system which finds a productive use for the condenser heat, such as heating hot water, or dehumidification reheat. In the VRF world, heat recovery systems refer specifically to a VRF system which can heat and cool simultaneously. This is accomplished by moving heat from rooms which need to be cooled to rooms which need to be heated. This way the heat is recovered and reused, rather than being discarded.

Hopefully this discussion will help you make sense of the mini-multi jargon. For more details, refer to Unit 46 Mini-Split, Multi-Split, and VRF Systems in Fundamentals of HVACR, 3rd edition.

Belt Basics

Belts are seldom used today in residential applications, but they are common in commercial applications. Understanding a few belt basics can make the commercial technician’s life much easier. The main type of belt used in HVACR is the V-belt, named for the V cross-section shape. The sides of the belt that form the V are what should be riding on the pulley, not the bottom of the belt. Belts are made of layers of cords encased inside rubber, similar to car tires. The cords provide strength.

Belt Identification
There are two predominant naming conventions for HVACR belts. One uses 3L, 4L, and 5L to designate belt widths of approximately 3/8”, 1/2”, and 5/8”. A number follows the with designation which gives the belt circumference in inches. So a 5L-440 is approximately 5/8” in width and 44 inches in circumference. The other common convention is A, B, and C for 1/2”, 5/8”, and 7/8” widths respectively.

Matched Belts
Large commercial applications may use multiple belts on the same drive. You should always ask for a matched set – not just two or three of the same size. This is because there can be small differences in length, which are of no consequence when using one belt. However, when using several on the same sheaves, they must be identical or some of the belts will be loose. A matched set is much closer to identical than just two belts with the same name size. This also means you should always replace all of the belts, even if only one of them needs replacing.

Installing a Belt
A common (and incorrect) way to install a belt is to roll it over the edge of the pulley. This saves time, but damages the cords in the belt – leading to shortened life. The correct way is to loosen the belt tensioner or motor mount so that the belt fits easily over the motor and fan pulleys. Then tension the belt using the belt tensioner or by adjusting the motor, depending on the design.

How Tight Should it Be
Many techs make the belt as tight as they possibly can get it. However, a belt only needs to be tight enough so that it does not slip at maximum torque. For fan belts, that is at start-up. Belt tension is measured by pressing in the middle of the belt while measuring both the force used and the amount of belt deflection. Browning recommends a deflection of 1/64 the belt span. The span is the center to center distance between the pulleys. So a belt with a 64" span should have a 1" deflection. The amount of force varies with the particular belt and span. Browning publishes a chart for their belts. A belt tension gauge is used to measure the deflection force. There are a couple of different ones out there. Again, Browning makes one.

Curing Belt Jitters
Many techs mistake misaligned belts as being loose. A belt that vibrates and jumps up and down usually indicates alignment issues. Remember that the shafts and pulleys must be aligned in three dimensions. The shafts should be parallel to each other. If you extend an imaginary line out from the shafts they should not intersect each other. You should be able to lay a straight edge across the pulley faces. If one pulley is farther forward than the other the belt will jump.

Worn Belts
Worn belts typically have shiny, glazed sides. If you see this, replace the belt. The hard, slick sides can’t grab the pulley wall, and so it slips even if the belt is correctly tensioned.

Belt Lubrication
Belts do NOT NEED to be lubricated. An old trick is to spray some WD-40 on a slipping and squealing belt to quiet it down. This basically just makes it slip quietly – just long enough for you to leave. It does not fix the problem. If anything, it makes the belt slip worse. It also does not last very long and leads to early belt failure.

I just got home a few days ago from the 2016 AHR Expo. That is the place to be if you want to see what is happening in HVACR. There were 2000 exhibitors and 60,000 visitors. It really was impossible to see all the displays in one day. Companies and people involved in all aspects of HVACR were there.  I was particularly impressed by how well run and organized the show was. With that many people, I expected long lines for everything. The only lines were in the morning before the show opened. Even then, once the show opened people got in the show quickly. I arrived over an hour early the first day, expecting a long line at registration. Instead, it took me about 5 minutes. There were multiple free shuttle busses taking people to all the local hotels. The longest wait to get on a bus was less than 30 minutes. If you missed this year’s show make sure to plan to attend next year in Las Vegas. I guarantee you will not be disappointed.  

Fundamentals of HVACR, 3rd edition is HERE!

The third edition of Fundamentals of HVACR is here! There are four new units in all. They include Motor Controls, Motor Application and Troubleshooting, Electrical Troubleshooting, and a unit on Mini-split, Multi-split, and Variable Refrigerant Flow Systems. The electrical section now boasts twelve chapters! The addition of the unit on Mini-split, Multi-split, and Variable Refrigerant Flow Systems has detailed coverage of the function, application, and installation for this critical and growing segment of the HVACR industry. Lots of updated art throughout with 2900 figures and 1540 pages. 

See details at http://www.pearsonhighered.com/educator/product/Fundamentals-of-HVACR-3E/9780134016160.page

Don't Lose Your Students in Techno-Speak

Many students find the large number of technical terms used in HVAC/R confusing. To the uninitiated, HVACR has its own language of technical jargon that can become a barrier to learning. Confusion over terminology can lead to not clearly understanding crucial concepts. This problem is compounded by the use of acronyms and abbreviations that are frequently used when discussing common topics. For example, one major valve manufacturer uses the acronym TEV to represent “thermostatic expansion valve,” while another uses the acronym TXV for the same thing. Instructors should try to keep techno-speak to a minimum. Instead try using, common, easily understood language whenever possible. Making a relatively simple concept seem highly technical by using an overabundance of jargon does not help students. There are plenty of truly difficult things as it is, we don’t need to make simple things complex.

Of course, students must still learn the HVACR language. Having a solid grasp of the terminology is necessary to make use of essential technical literature produced by equipment manufacturers. To encourage students to learn the language, use the complete word or phrase before introducing an acronym. It helps to explain concepts plainly, and then introduce the technical terminology that is used to refer to the concept. The students are more likely to remember the terminology if it is logically connected to something they understand. For example, I have found that students just starting to learn the operation of the refrigeration cycle often can recite the order of components, but they have not made the connection between the component names and what they do. It is much easier to remember “TEV” if you know that “TEV” is an acronym for “thermostatic expansion valve” and you know that the refrigerant expands as it goes through the thermostatic expansion valve. Most of the unique terminology in HVACR describes something and understanding what it is describing helps students retain it. I am not a proponent of giving students a long list of terms to memorize – I believe they should learn the terms as the topics come up so they can learn them in context. For example, to really understand what a transcritical system is, you need to understand a bit about the critical point and preferably refrigerant pressure-enthalpy diagrams. Then it is easy to point out that the high side of a transcritical system is above the critical point while the low side is below it. So the system operates across the critical point – thus; transcritical. Understanding where the term comes from is really important to remembering it and using it correctly.

Be Part of the Solution

My father lost his battle with cancer early on the morning of December 21, 2015. When someone dies there begins a very busy period where family and friends come together to take care of all the details associated with final arrangements as well as to figure out how everyone whose lives were closely knitted to the deceased will regroup and continue with the business of living. In a peculiar twist, a house that has just been overcome by the shadow of death overflows with life. A neighbor called in the midst of this activity. I answered the phone and was mistaken for dad. The neighbor did not know dad had died, and I did not correct her. Instead, I just listened because I could tell she was upset and needed help. The mail carrier had already been by her house and had not come up to the house to bring the mail. She had two letters she needed to mail and feared that she had missed her chance. She is legally blind, and so tries to void walking down to the street. I explained that I was not dad, but his son. However, I said that I would be glad to take her letters and see that they were mailed today. I walked down to the neighbor’s house, got the letters, found the mail carrier, and handed over the letters. You see, dad’s neighbors had gotten used to turning to him when they needed a solution. One of dad’s mantras was “be part of the solution, not part of the problem.” I was glad to help dad be part of the solution one more time.  

Funky Flames

Funky flame patterns may be an indication of a dirty or restricted secondary heat exchanger in 90% furnaces. Most furnaces today use draft inducer blowers which draw the vent gasses through the heat exchanger. In the case of furnaces with efficiencies of 90% or greater, there are two heat exchangers – a primary and a secondary or recuperative heat exchanger. The secondary heat exchanger condenses water out of the flue gas. Most secondary heat exchangers look something like a tube and fin coil made out of stainless steel. Many have turbuators inserted into the tubes to increase the heat transfer by making the gasses swirl as they pass through the tubes (see photo). Flue gasses could never make it through without the induced draft blower drawing them through.

If the water does not drain properly out of the secondary heat exchanger, flue gases cannot travel though easily, and the combustion gases back up. This causes the flue gas created by the flames to find another exit – usually out the front where the gas and air entered. The errant flue gasses disturb the flames creating yellow coloring, dancing, extinguishing and relighting, or rolling out of the combustion chamber. The resulting flames often create a lot of carbon, which can further clog up the heat exchanger.



If you see these symptoms BEFORE the indoor blower is energized, you should first check to see that the induced draft blower is operating and that it is actually producing a draft. Sometimes the blower wheel comes apart or slips off the shaft – causing the motor to turn without actually moving any air. Next, you may want to inspect the secondary heat exchanger for obstruction. This varies between units, but most often you will need to remove the draft inducer to see the secondary heat exchanger. If the secondary heat exchanger is filled with water – you need to solve the drainage problem.

If it is clogged with black soot, the secondary heat exchanger will need to be changed. If that is the case, you will also need to look for the cause of the soot. Double check the orifice and operating manifold gas pressure. An oversized orifice or incorrect manifold pressure can cause soot. If the furnace is located more than 1000 feet above sea level, remember that it must be de-rated – which can mean a lower manifold pressure and/or a smaller orifice. Check with the manufacturer for orifice and manifold pressure recommendations. If the furnace uses sealed combustion (combustion air coming in through its own pipe) check to see that the combustion air pipe is not restricted. Birds and rodents are common problems. If the flame pattern is fine up until the indoor blower comes on, and THEN the flames get wacky, you probably have a cracked primary heat exchanger. In this case, the primary heat exchanger definitely needs to be changed.