The Difference Between VA and Watts

What is the difference in VA and watts? Both are expressions of electrical power. In a purely resistive circuit, both are calculated by multiplying volts times amps. That is where the VA name comes from. In fact, there really is no difference between VA and watts in a purely resistive circuit. A 5000 watt electric resistance heater is also a 5000 VA electric heater, but you would probably not see a VA rating on a resistive device.

The term VA is commonly used to describe the total amount of apparent power of an inductive device, such as a transformer or motor. An inductive device is one that uses alternating current to create electromagnetism to operate the device. A funny thing happens inside inductive devices – they actually generate voltage that is opposite the original voltage connected to them. The magnetic field created by the current moving through the wires changes directions when the alternating current changes directions. This has the effect of moving the magnetic field back and forth across the wire – which is pretty much what a generator does. This moving magnetic field generates a voltage in the inductive device that is opposite the original voltage, called counter electromotive force, or back emf for short. Since this voltage is pulling the current in the opposite direction, it dramatically reduces the amount of current traveling through the device. This property of magnetic devices operated on alternating current is called inductive reactance. That is why the physical resistance of inductive devices is so low – most of their opposition to current comes from inductive reactance.

Another effect of inductive reactance is to delay the rising and falling of the current, causing it to lag behind the rising and falling of the voltage. Since the voltage and current are not rising and falling together, they do not work together, so the true power of the device is less than you would get by just multiplying the volts times the amps, or the apparent power. So VA is a measure of the apparent power of an inductive device ( V x A ) while watts is a measure of the true power (V x A x power factor). Power factor is a measure of how out of whack the volts and amps are (note this is probably not the definition your instructors will want) It can be calculated using a bunch of really cool formulas, or you can measure it with a wattmeter, voltmeter, and ammeter. Then use the relatively simple formula of (volts x amps) divided by watts.  

PVC Vents for Condensing Furnaces

Since the advent of condensing furnaces, furnace manufacturers have been specifying  Schedule 40 PVC as the venting material they recommend. A good discussion of condensing furnace venting by Bob Formisano can be found at the HomeRepair site. Traditional metal vent material generally will not work because the vent gas on a 90%+ furnace is very wet and at a positive pressure. Other than stainless steel, metal vents would not last very long. Because the vent operates at a positive pressure, it must be sealed air tight. Traditional Type B vents are not air tight. With a negative pressure vent, the very small leaks in type B vents do not pose any problem. But if they were carrying pressurized vent gasses, even small leaks are a safety hazard. Since the flue gas temperature of a condensing furnace is much lower than a traditional furnace, plastic materials are feasible. So PVC seems like a good choice. It is definitely water proof and sealing it air tight is quite easy. The fact that PVC is inexpensive also makes it attractive. However, the standard referenced by many furnace manufacturers, ASTM D1785 for Schedule 40 PVC pipe, does not actually test PVC piping as a flue material. In fact, it specifically states that the standard does NOT cover its use as a vent for combustion appliances.

There have been concerns about PVC vent systems failing. Two sites I found that discuss failures and show pictures are  HeatingHelp, and Plumbing Engineering  Both sites discuss PVC venting systems on condensing water heaters and boilers where the PVC material turned brown, became brittle, and cracked or broke. In Canada, plastic vent materials must conform to ULC 636 – meaning no more “standard” PVC venting. It appears we may follow in the US. Some plastic materials made specifically for venting include UL 636 PVC, UL 636 CPVC, and a special polypropyylene pipe.

Here are a few links.
UL 636 PVC, CPVC
Polypropylene

How's It Flowing?

When air, water, refrigerant, or any fluid flows through a pipe, the flow can be characterized as laminar or fluid. I can hear you saying “what does this have to do with HVACR?” It applies to how refrigerant flows through piping, how air flows through ducts, how air flows across coils, and how refrigerant flows through those same coils. In short – it applies to pretty much everything in HVACR!

Laminar means in layers. Laminated furniture like you find in most schools has a thin layer, or laminate, on the surface to make it pretty and usually cardboard or glued together sawdust underneath to make it appear substantial. In laminar flow, the stuff on the top stays on the top, the stuff on the bottom stays on the bottom: there is no mixing. In turbulent flow, the fluid swirls all around mixing everything together and creating all sorts of eddies and whirlpools. If you turn on a water faucet that does not have an aerator on it, the stream of water is laminar. The water falls straight down without much mixing. After it hits the sink you have turbulent flow – swirling all around and mixing. Laminar flow is preferred in refrigerant piping, water piping, and ductwork. This is because the eddies, whirlpools, and swirling found in turbulent flow add a great deal of resistance to flow.

For example, take a bunch of people trying to leave a crowded room. If they file out in an orderly line moving in the same direction, they can all get out faster than if there is a lot of pushing and shoving with people going off in all directions and running into each other. To get fluid through a pipe or duct with the least amount of energy loss, laminar flow is the way to go.

High velocities tend to create turbulent flow while low velocities create laminar flow. That is why the resistance to flow increases so dramatically as pipes get smaller. Velocities increase and the flow goes from laminar to turbulent, increasing resistance to flow within the fluid. Its not just that the cross sectional area is smaller - the fluid is getting in its own way as it tires to go through the pipe.

In heat transfer coils such as evaporators and condensers, we need turbulent flow. In heat transfer, all the swirling becomes an asset. Turbulence makes more of the fluid contact the surface of the pipe and slows down the fluid, which helps it transfer more energy. This is why coils have features such as rifling, twisted tubing, or turbulators to encourage swirling and mixing.

Do You Have a Plan?

Soon, schools will be starting their Fall term classes, traditionally, the start of the academic year. Most programs have their highest enrollment at the beginning of the Fall term. So I have a question for you. Do you have a plan? I don’t mean any particular document or form, just, what is your plan for making it through the Fall term? Life is so much smoother when you think ahead just a bit. Even though I often use a GPS when traveling, I always look at maps and routes so I have a clue about where I am heading and how long it takes to get there. Things are bound to come up, something won’t go just right, and there will be issues to resolve. However, all of this is easier to handle if you have a plan.

One of the most valuable parts of planning is that it forces you to examine the details. What topics are you going to cover, how many tests will there be, how many labs, will there be time to do everything you want, and when will you eat lunch. Yes, eating lunch is important. You have to build in down time if you want all your uptime to be productive. The more thought through and detailed your plan is, the better it will serve you. Modifications to your plan can always be made when things come up, but having thought through what you want to accomplish and how you plan to do it is especially helpful when unforeseen circumstances occur. If your GPS loses satellite signal in the middle of some deserted location you have never before visited, knowing a bit about the route and having a map are invaluable.

I spend a full week every semester planning. I lay out the lecture topics, labs, homework, and tests for all 10 classes that we teach trying to make sure they the courses complement each other and the labs make sense for what the students know. I do a schedule for each class, determine lab assignment dates, determine test dates, and put all this in documents that both the instructors and students use. I make sure the online materials make sense and dovetail with the lectures and labs. Yes, it is a lot of work. However, it makes the rest of the semester so much easier – we just run the plan. If anyone would like a copy of any of my planning documents, I would be happy to send some examples to you. You need your own plan, not mine – but having something to work around can often help you get started.