Monday, October 20, 2014

NOx Nox Who's There?

Cracked. Cracked who? Cracked heat exchanger. OK – not the funniest knock knock joke – especially if it is YOUR furnace. Low NOx emission inserts are causing pre-mature furnace heat exchanger failures. You can read more about this at http://ncidavid.blogspot.com/


I want to talk a bit about what NOx is, why an insert can help reduce NOX emissions, and why NOx inserts can cause trouble. NOx is a generic term for nitrogen compounds NO and NO2 which are formed when nitrogen is present during combustion at high temperatures. These are major contributors to air pollution and acid rain. Laws requiring reduction of NOx emissions in areas of particularly high air pollution (California) have required the development of low NOx furnaces. Since the air is 78% nitrogen, any combustion process that uses air for its oxygen source has lots of nitrogen present. NOx will only form at high temperatures, but the burners in gas furnaces are definitely hot enough to form NOx. One quick and easy solution is to put something in the flame to cool it down. NOx rods and NOx inserts work by cooling the burner flame temperature, thus reducing the NOx emissions. These NOx rods or inserts are typically metal. Stick some metal in the flame, cool it down, and you have a low NOx furnace. The problem is that metal held directly in a flame eventually burns up. It does not burn quickly, but it does burn. That is why we don’t want the furnace flames actually touching the heat exchanger. The heat exchanger material cannot withstand direct flame contact, called impingement. NOx inserts stick inside a tubular heat exchanger. Unfortunately, the insert eventually degrades from being exposed to the flames. In some cases, it can twist or deform and change the flame pattern. This can cause the burner flames to touch the sides of the heat exchanger, and the heat exchanger fails prematurely. Before the heat exchanger fails, the furnace will be producing higher levels of carbon monoxide and carbon particulates because of the change in flame pattern. So you trade one type of pollution for another. If you work on furnaces with NOx inserts, be sure to check them – and watch the flames to see that they are not touching the heat exchanger. If you work in an area that does not require the NOx inserts, you can solve the problem by removing them. However, now you are increasing the amount of air pollution the furnace produces. Low NOx burner technology does exist – where NOx reduction is achieved through burner design, but I don’t know if there are any true low NOx burners for residential appliances. Keep an eye out for furnaces with low NOx inserts. It might be worth a few extra moments to wath the flame pattern and measure the CO in the flue gas. Abnormally high CO might indicate a developing problem. 

Sunday, October 12, 2014

Playing with Your Phone on the Job



Many companies now issue company cell phones to their service techs. The phones are intended for communication, so the company can find the techs and talk with them virtually anywhere, anytime. Now those phones may need to be considered part of the technician’s toolbox. There are already many great apps, including free pressure-temperature apps – usually from refrigerant companies. Equipment and parts manufacturers have started producing apps for looking up parts and equipment service specs. Now you can use your phone or tablet to read the system pressures, line temperatures, superheat, and subcooling. Stride Tools (Imperial) released the i-manifold last year. It is a four port manifold without a display. It sends the information wirelessly to your phone or tablet, and your device becomes the display. Since the display is literally on a small computer, the app can do all kinds of cool things. For one, they have wireless thermometers that can work with the i-manifold to determine system operating conditions, capacity superheat, and subcooling. But wait – there’s more! Now Yellow Jacket has come out with a small device they call the mantooth. It connects to the system and has a tethered temperature probe. It also has no display, but sends the information to your smart device via Bluetooth. So you can see system pressures, temperature, superheat, and subcooling all on your phone. The advantage of this device is that you don’t have to fill up a hose with refrigerant to get a reading – so refrigerant loss is minimal. The “short” gauge concept is perfect for simply testing system performance. Another well known manufacturer is also testing “short” gauges that connect to smart phones and tablets. Their devices are also “short” gauges – eliminating the refrigerant loss from connecting hoses. Their gauges do have their own display as well as connecting to your device through Bluetooth. They are developing wireless thermometers to work with these gauges. Again, you will be able to see all relevant system operating conditions on your smart device and you won’t have to lose several ounces of refrigerant just to perform a check-up. You may soon be playing with your phone on the job – to improve your productivity!

Sunday, October 5, 2014

Condenser Subcooling



What happens to the refrigerant subcooling when condenser airflow or water flow are reduced? Many people instinctively say that the subcooling would decrease if the airflow across the condenser decreased. After all, the air is what is cooling the refrigerant – if you have less of it, the refrigerant will not be cooled as well – so you might expect subcooling to decrease when airflow decreases. The only problem with this is that it is exactly backwards. In fact, decreased airflow in an air cooled condenser usually causes an increase in condenser subcooling. Remember subcooling is just telling you the difference between the condenser saturation and the liquid temperature leaving the condenser. Condenser pressure and saturation temperature both increase with reduced airflow. Most techs understand that. While the condenser saturation temperature increases a lot, the liquid temperature does not rise as quickly. The increased condenser pressure also contributes to increasing the compression ratio, so the compressor moves less refrigerant. The increased compression ratio and the increased saturation temperature cause the condenser to hold more liquid refrigerant than normal. The liquid sits longer in the condenser and has longer to be cooled below saturation temperature. Since the liquid is starting at a higher temperature compared to the air circulating over it, it tends to lose temperature faster than it normally would. All this adds up to increased subcooling. 

This is easy to verify. Start a unit, let it run a while, and check the subcooling. Now block the condenser airflow and watch the condenser pressure and liquid line temperature. You should see a big increase in the high side pressure while the liquid line temperature stays about the same or increases just a little. One caveat – don’t let the unit run too long with a blocked condenser – unless you want to run the “what happens when the compressor overheats” experiment. Monitor the compressor temperature to be on the safe side. I am not responsible for you destroying your unit while checking this out. Better to use one at school. (After asking the instructors of course)

Sunday, September 28, 2014

Manual J Resources



Are you looking for materials to teach load calculations? ACCA has free excel spread sheets they call speed sheets. You can download them from their web site. They also offer a curriculum for studying Manual J8, also free from ACCA. You do have to buy either Manual J8 or Manual J8 abridged to take full advantage of these. If you are looking for load studies coverage in a text, “Fundamentals of HVACR, 2nd ed” has a complete Unit on residential load studies. It includes examples using Manual J8 while working on a complete block load for a small house. Other Resources include Wright-Soft’s Right-J program or Elite Software’s RHVAC program. These are two very full featured commercial programs which are used a good deal in the industry. The good news is that you can download and use demo versions of them for free. Typically, there are a few limitations on the free download versions, such as not saving the files. However, you get a chance to try them out before purchase.

For teaching, I really like the ACCA Manual J Speedsheets. They are not too hard to learn, and I believe that students get a better understanding of Manual J because they do have to use the manual some to use the software. I also like the Manual D Speedsheet for ductwork. Both of these are a little limited – they are mainly for use in residential systems that are not too complex. But that is exactly where our students should be starting. We typically have our students do a block load on a small cabin and a room by room load on a 3 bedroom ranch style house. They then do a radial duct design on the cabin and an extended plenum on the ranch house. You will want to spend some time with the speedsheets before giving them to students. Although they are not difficult, there you do need to go through one a couple of times to feel completely comfortable. The instructions are in the sheets and there are ample examples on every tab. Links to the  resources mentioned are


Sunday, September 21, 2014

Thermostatic Expansion Valve Failures



There have been a run of TXV problems in new units stemming from a chemical used as a rust inhibitor on some of the compressor bearings in Copeland Scrolls. A reaction between the rust inhibitor and the POE oil used in the compressor causes the POE to coagulate and collect in the TXV. This appears to be an industry wide problem, not limited to a single equipment manufacturer. The chemical causes the valves to get gunked up and stick, usually resulting in a restriction at the TXV. The symptoms can look very similar to an undercharged system – very low suction pressure, low head pressure, and high superheat. Normally, the way I differentiate low charge from a restriction is by subcooling. A restriction typically has a normal to high subcooling. However, this is often not the case if the compressor in the system is a newer Copeland scroll. The Copeland scroll compressors designed for R-410A unload when the suction pressure drops below 55 psig. This is to keep them from operating at pressures which will damage the compressor. If the restriction is severe enough, the suction pressure on an R-410a system can drop below 55 psig. The compressor unloads, limiting how much refrigerant is pumped and holding down the head pressure.  You have low suction pressure, low head pressure, high superheat, and low subcooling: classic undercharge symptoms. However, adding charge does nothing unless you really go whole hog and grossly overcharge the system. Then you can build some head pressure, but it won’t fix the restriction or make the system cool. If the compressor is unloading, you can often hear it and even feel it in the piping. If you have what looks like an undercharged system with no detectable leak and adding refrigerant has no effect on the pressures, I bet you have a restriction. These days the restriction is often in the TXV. Several equipment manufacturers have service bulletins out regarding this issue. Below are links to a bulletin from Nordyne and a link to a YouTube video showing the bulleting from Emerson.



Saturday, September 13, 2014

Keep the Condensnot Moving!



The evaporator condensate drain is often one of the last things run when installing an air conditioner and sometimes is not given a lot of thought. The trouble is it can demand your attention if it does not work. A check list for constructing a working condensate drain that will continue to work and be easily maintained would be
1. The drain should have cleanouts near the unit and the trap to allow for routine maintenance.
2. Draw through coils should always have a trap after the drain pan.
3. The trap should be 4” deep and hold 2” of water.
4. The leaving side of the trap should be at least 2 inches lower than the drain pan outlet.
5. After leaving the trap, the condensate line should slope at least 1/8” per foot.
6. Except for the one trap at the unit, the condensate line should NEVER run up.
7. Long condensate lines may require a vent. If you install a vent, you MUST install a trap and the vent must be AFTER the trap, never before the trap.
8. In humid areas the parts of the condensate line closest to the unit may need to be insulated to prevent sweating.

Cleanout
Drains are getting a little more attention these days because the newer mechanical codes require cleanouts. In the past condensate drains were often cleaned by cutting out the old trap, blowing out the line, and installing a new trap. Now you are required to leave a way to access and clean the drain without cutting into it. Some people use tees or crosses at places where elbows would normally go. You can also purchase some fittings to install in the drain line that make accessing and cleaning the drain line easy. One is called “ALL-Access” Link http://www.allaccessdevice.com/
Another is offered by Rectorseal http://www.rectorseal.com/index.php/easy-klear/
Rectorseal also makes a manufactured trap which is clear, allowing you to see when the trap is full of “condensot.” It has service access built in as well. http://www.rectorseal.com/index.php/ez-trap/

Trap
A trap is always required with a draw through coil, otherwise, water will be held in by the negative pressure and the air flowing in through the drain. Many people don’t trap positive pressure coils, such as a coil installed on a furnace. Usually, they will drain because the pressure is pushing the water out. One argument for leaving the trap off on positive pressure coils is that traps tend to get clogged more quickly with “condensnot,” the slimy brown algae that grows in drain pans and drain lines. However, if the air velocity on a blow through coil is high enough, it may still have a negative pressure at the drain outlet. Adding a trap on a blow through coil also provides an air seal between the coil and the outside Manufacturers often specify traps even on blow through coils. Packaged units often specifically warn NOT to trap because they sometimes have a built in trap, and adding another would amount to double trapping. The trap needs to hold enough water to offset the negative pressure of the coil. A common specification calls for a 4” deep trap with a 2” riser so that the trap holds 2” of water and the outlet is 2 “ below the drain pan outlet.

Vent
Ever notice that plumbing systems have vents? The vent keeps the draining water and effluent from creating a suction which can slow or even stop drainage. On longer condensate lines, a vent may be required to insure drainage. Vents can also help on lines with minimal slope. The vent is just a tee with a riser which is open to the air. When used, there MUST be a trap and the vent should be AFTER the trap. Putting a vent before the trap pretty much undoes any good the trap would do by allowing air to pass through the vent. The vent should be at the start of the condensate line (but after the trap) and the riser should go above the rim of the drain pan.  

Slope
One of the laws of plumbing is that waste products run down hill. After the line leaves the trap at the unit it should never go back up. Running the line up after dropping down creates a second trap. Remember that the line is not under pressure, it is gravity flow. It is easy for “condensnot” to accumulate at this second trap and cause the drain to overflow at the unit. I am embarrassed to admit that I have personal knowledge of this. An unintended second trap is a messy service call waiting to happen.

Insulation
If your unit runs a lot and the coil is located in a humid area, the condensate line can sweat. Usually, the PVC is enough insulation to prevent this, but in some areas people have had problems with dripping condensate lines from sweating. Insulating the trap and all the drain between the trap and the unit with armaflex takes care of this.

Instructions
Finally, always follow the manufacturer’s instructions. They usually are pretty specific about how the drain is to be run. Trapped or not trapped, vented or not vented – those things are all typically in the instructions. Here is a link to another online posting that shows one manufacturer’s instructions and a drain run according to those instructions.