Four Freedoms

I have been enjoying a thanksgiving weekend with my family. We spent a day looking at all the wonderful paintings in the Art Institute of Chicago. We saw the original Norman Rockwell painting titled “Freedom from Want.” I am sure you have seen it reproduced many places – a family is seated in anticipation of a thanksgiving meal and the turkey is just being placed on the table. It is one of a series of four paintings Rockwell did – each portraying an essential human freedom we often take for granted. They are Freedom of Speech, Freedom of Worship, Freedom from Want, and Freedom from Fear. I realized that I enjoy these on a daily basis, but seldom stop to give thanks or even realize how privileged I am to enjoy them without a second thought. So this Black Friday I plan to take some time out from chasing after the next great deal to give thanks for the basic human freedoms I enjoy daily – and thank God I have the freedom to do it.

Layered Learning

One common problem with teaching and learning HVACR is that there is just so much material to master. It is easy to drown the students in details if you are not careful. When I was using a text which presents enthalpy diagrams in its early chapters on basic refrigeration, students would come up to me and ask for drop slips after reading the chapter. They looked at the PH diagrams and concluded that they just were not smart enough for HVACR. It was just way too much information for them to comprehend while they were still wrestling with refrigerant boiling while it is cold! The real skill in presenting HVACR subject matter is to take inherently complex material and break it down into a series of comprehensible lessons, without leaving out material. Presenting information in stages, at a rate which most students can more easily digest is preferable to feeding them the whole enchilada at once and causing cognitive indigestion. You can still reach the peak, but you have to get to base camp first.

Electricity is a perfect example. We start with the basic concepts of voltage, current, resistance, and simple circuits. If you think of a simple balance beam with voltage in the middle – if resistance goes up, current goes down. Students can in fact understand how current, voltage and resistance are related without using any formulas. Similarly, you don’t have to bring in Ohm’s Law to understand the concepts of source, path, and load. In fact, I believe circuits are best approached at first without discussing Ohm’s Law. Students can build circuits and operate them to get a mental concept of how switches  and loads behave. They can even build series and parallel circuits to see how they differ: all without formulas. Then AFTER the students understand the basic concept of an electrical circuit, you introduce Ohm’s Law as a mathematical description of a circuit. Now they have something real to relate to the parts of the formula and it is less abstract. You can even build circuits with heaters and take measurements to demonstrate how Ohms Law works.

After students have learned to solve Ohm’s Law problems, then we tell them that Ohm’s Law does not work in most AC circuits. At first, we don’t discuss the effects of alternating current, inductive, and capacitive reactance. It is just too much to take in at once. However, since these concepts have far more to do with most air conditioning circuits than Ohm’s Law, we can’t really afford to ignore them. The effect of inductive reactance can be demonstrated by operating a small motor in a circuit and comparing its resistance, current, and voltage. You can have students build circuits that demonstrate the effect of inductive and capacitive reactance and how they relate to each other. Depending on how much AC electricity you teach, you can put an oscilloscope on the circuits to show what happens in capacitive and inductive circuits (the teacher would be doing this.) Finally, you can bring in reactance and impedance calculations, although I must admit we do not have our Air Conditioning classes at Athens Tech doing LCR calculations. It’s a little like boiling a frog – you do it a little at a time. Before the students know it, they have learned some complex subject matter that would have sent them running for the exits if it had been presented all at once.

Electricity for HVACR

Pearson has released a new Electricity book by Joe Moravek titled "Electricity for HVACR." It shares the same overall structure as my book - lots of pictures and drawings with easy to follow text. It has short units and easy to follow organization. I have not read it end to end yet, but I have looked through several chapters - especially the ones on motors. I am impressed with what I have seen. If you are looking for a text to cover the electrical aspects of HVACR, you might take a look at this new book. You can  learn more about it at the Pearson web site CLICK HERE

Furnace Drains

One important aspect of condensing furnace installation is sometimes overlooked – the furnace drain. A 90% condensing furnace makes a lot of water – comparable to an air conditioner evaporator. All that water needs somewhere to go. In most installations, there will also be an air conditioning evaporator and a drain for it. It is natural to just run the furnace drain into the air conditioning drain, but that can lead to problems. The positive air pressure inside the evaporator cabinet can create a slightly positive pressure on the evaporator drain. If the furnace drain is connected directly to it, this positive pressure can push back on the furnace drain. On some furnaces, this can create a positive pressure at the vent draft switch, causing it to open and shut off the furnace. One solution is to run completely separate drains for the furnace and air conditioner. If the air conditioner and furnace are connected to a common drain, you need to leave an air gap between the furnace drain and the common drain so that pressure cannot back up into the furnace drain and vent. If you are using a condensate pump, make sure it is rated to handle furnace condensate. Because furnace condensate is slightly acidic, some condensate pumps are specifically NOT approved for use with furnace condensate. In case you are concerned about the acidity, you can breathe easy. It is about the same as a carbonated beverage. However, some codes early on required that the condensate from condensing furnaces be neutralized before going into a sewer system. So manufacturers produced acid neutralizing filters which were basically large PVC containers filled with rocks (usually limestone or marble). The acid reacts with the rocks, which have a basic ph that counters the acid. I don’t believe most places require this any longer, but I am not certain. Another issue to watch is the drain outlet. If a gravity drain ices up or is blocked with snow, the draft switch can trip and shut off the furnace. Also, remember that the furnace really does make a lot of water. Try not to dump this water in a place that will cause problems – like right over a sidewalk.

RSES November 2013 Article

I want to brag a bit. The November 2013 issue of the RSES Journal has an article taken from one of my blogs. It discusses reading the external static pressure on a blower to determine the airflow. I get a kick out of seeing my stuff published in the RSES Journal because of the great respect I have for that organization. It is said that you can tell a lot about a person by the company they keep. I am happy to be in pretty good company this month. If you have a chance, you might want to check out the November 2013 issue - or better yet - subscribe. Looking for an easy way to pick up continuing education units?  Answer the questions at the end of the magazine.

Gas Furnace Firing Rate

How do you know the rate at which your furnace is actually firing? You really can’t assume that it is firing at the manufacturer’s published rate just because the manifold pressure is at 3.5”, even if that is what is specified on the nameplate. To determine the Btuh firing rate of your furnace you must first determine the cubic feet of gas per hour being burned (CFH) and multiply that times the heat content of the gas. To determine the CFH, you need to clock the gas meter. First, you want to turn off all other gas appliances. Then clock the number of seconds it takes for the smallest dial on the gas meter to make one revolution. For more accurate results, clock the time for two or three revolutions. The CFH can be calculated by dividing the time clocked into 3600 (the number of seconds in an hour). Multiply this answer times the value of the dial. If you clocked for multiple revolutions, multiply by the number of revolutions. The formula looks something like this

(3600/time clocked) x dial value x revolutions = CFH
So if the a 1/2 ft3 dial took 90 seconds to make three revolutions
(3600/90) x 1/2 x 3 = 40 x 1/2 x 3 = 60 CFH

Next, you need to know the heat value of your gas. You get this from your local gas supplier. If you can’t get that you can get a good estimate from the EIA. The US Energy Information Administration (EIA) has a table online that shows historical values for every state. GAS HEATING VALUES.

In our example, if the furnace is in Mississippi in 2011, the gas heat content is somewhere around 1010 Btu/ft3. So our firing rate would be 60 ft3/hr x 1010 BTU/ft3 = 60,600 BTU/hr

The same results in West Virginia in 2011 would mean a firing rate of 60 ft3/hr x 1083 BTU/ft3 = 64,980 BTU/hr

If the furnace in West Virginia is rated for 60,000 Btuh, you would need to reduce the manifold pressure slightly to reduce the firing rate. Another complication is altitude. Furnaces need to be de-rated approximately 4% for every 1,000 ft of altitude. In West Virginia, it is quite possible to be 3,000 ft above sea level, necessitating a 12% reduction in firing rate. So the new firing rate should be 60,000 Btuh x 88% = 52,800 Btuh. Now you are looking at firing a furnace at 64,980 Btuh which should only be fired at 52,800 Btuh. You most likely will need to change orifices as well as adjust the manifold pressure. After making any changes to the manifold pressure or orifices, make certain to clock the meter again and recalculate the gas firing rate to insure your adjustments brought the firing rate into line. Over firing a furnace can increase the production of CO in the flue gas. In extreme examples, it can cause soot formation.  

Ghost Voltages

I know everyone who has used a digital meter has taken some measurements that don't always make sense. I believe we just learn to ignore them, recognizing the "real readings" from the junk. But when students see the odd readings, they become confused. Here is a great article by Fluke explaining what causes strange voltage readings when there really is no voltage.

GHOST READINGS