- Tech Tips
Low pressures are often measured in inches of water column or “WC. Like most units of measure, it has a very simple origin, in a water manometer 1″ of water column is literally the amount of force it takes to raise the column of water by 1”. While some water manometers (water tube) are still in use the vast majority are either dial or digital gauges that still use the same scale.
1 psi is equal to 27.71 inches of water column; this is why water column is most often used to measure pressures under 1 psi. These low pressures are most often read using a manometer or a magnahelic gauge.
When we measure water column with our tools it is calibrated at atmospheric pressure or the gauge scale instead of the absolute scale. This means that for a manometer or magnahelic to be properly used they MUST be recalibrated before each (many auto calibrate to zero) to compensate for changes in elevation and barometric pressure. At altitudes over 2000′ above sea you will also need to follow manufacturer recommendations to adjust the gas valve and even change orifice sizes in some cases due to the effect the lower atmospheric pressure has on the gas.
Gas pressure is usually measured in “wc, most commonly we set single stage appliances to 3.5” wc on natural gas and 11″wc on propane. This varies based on manufacture specs, combustion analysis and meter clocking tests. Always read the manufacturer specs.
We also use “WC to check air static pressure on systems. Static pressure is pressure that is exerted in all directions in a contained space, it is not the directional force of the air.
We use a manometer or a magnahelic and measure the negative air pressure in the system return side before the blower (and after the filter whenever possible) and the positive pressure supply air side directly after the blower. By calculating the differential you come up with the total external static in water column. For example, if the return static is -0.3″wc and the supply static is +0.2″wc the total static is 0.5” wc.
Many manometers and all magnahelic gauges (to my knowledge) have two ports so you can read the differential pressure all at once. This also comes in handy when reading/testing differential pressure on many furnace air pressure switches to ensure they make and break at the proper pressure.
— Bryan Orr
I’ve always liked old books.
Think about an old printing press somewhere in Chicago or Boston or Scranton, Pennsylvania.
Imagine workers with their hands covered in ink up to their elbows, setting type while giant machines of iron, steel, and brass stamped out a book page by page. Then those pages went on to be bound, crafted in a way that few things are nowadays.
At that time the pages were new and crisp, fresh ink and fresh paper giving off a distinctive odor.
But that’s not the part I like the most.
The part that piques my imagination is the people who wrote it and the world they lived in. In most of my imaginations, the past is all in black and white, full of dull people, living dull lives.
But that’s just wrong.
When I open one of these old books they talk about problems we still face today, with information that still applies
Pretty quickly you begin to see the genius of these writers. You start to understand that their lives and work were often very similar to our own with many of the conveniences stripped away.
These people had to be resilient and resourceful. They had to memorize more and read more because access to information was rare and precious. They were more reliant on experimentation and discovery because much of what they knew they had to find out for themselves and pass on person to person.
Many of these books for the trades are written between the industrial revolution and the Second World War. A time in the world when anything seemed possible both good and evil.
Great leaps in technology and progress on the positive side. Abuse of workers at home and a looming enemy abroad seeking to tear the fabric of civil society apart on the other.
In reading the Building Trades Handbook from 1899 I learned that there was a booming correspondence school in Scranton Pennsylvania that educated thousands by mail correspondence in the trades and engineering one page at a time.
The books start with very simple skills like working with fractions that can be so daunting for Tradesmen Even today.
I learned in the American Electricians Handbook from 1921 that we knew so much about electrical motors and electrical engineering at that time. So much of it is well explained in that text using explanations that would make sense to the average Workman.
On the other hand, electricians from that era were not nearly as concerned with preventing electrical shock. The practices used to diagnose electrical circuits are laughable and frightening by modern standards. It does show that the Tradesmen that came before us were tough… even to the point of being a bit crazy.
While all of this is very interesting I’ve noticed something else. Most of the really great educators in our trade have gone back to old books to find answers.
Jim Bergmann told me that he went to old books to find answers about carbon luminous flame in old furnaces and boilers.
Dan Holohan always speaks about going back to books by “dead men” to learn about steam heating.
Joe Lstiburek (Building Science) talks about going back to very old construction books to learn about capillary action and capillary breaks to prevent moisture intrusion.
Why is that? Why do old books contain information that some of the new ones don’t?
Remember when you played that game of telephone as a kid where you say a phrase to one person and it’s repeated around the circle. By the time it gets back it’s either nothing like the original or a good portion of the information is missing.
That’s what often happens in education.
Those who make significant discoveries, invent practical machines and applications and work out the math are the first educators in a particular field.
Not only do they write about it, but they also LIVED IT.
The generations after that tend to get split, with the educators focusing mostly on the teaching and the field workers focusing mostly on the doing. They both have a piece of the puzzle, but over time the message gets diluted and breaks down until nobody REALLY understands the whole anymore.
We see this in our field today all the time –
Engineers know lots of theory and math but not what commonly goes wrong or the practical elements of the field.
Manufacturers understand their products but not necessarily the application.
Installers know how to assemble systems but not why or how to properly design.
Techs know how to fix “most” problems but really understand the why of a design? Forget about it!
For those of us who really want to understand the work we do we are left with going back to those people long dead who made the discoveries themselves.
The ones who worked in unsafe buildings and grabbed hot wires, and worked in sweltering labs before A/C existed and also the ones who wrote old books.
I just got in a book like that…
Never stop learning, never stop reading old books. Take a look at Ebay and Amazon and let me know the treasures you find.
You are probably all familiar with radiant barriers. Sometimes it is thin foil draped under the roof deck, sometimes it’s used on the inside of stud walls or over furring strips before drywall goes up and there is even plywood with a radiant barrier attached to one side that is used for roof decking.
The point of this article is to remind you that you eliminate the benefit of a radiant barrier when you sandwich it between materials in other words when there is no “air gap”, but I also want to help you understand why this is.
How Radiant Heat Transfers
Heat energy is the “force” that makes the atoms move and molecules jiggle and it’s in everything over absolute zero (-460°F). Heat is transferred or moved in one of three ways but heat itself isn’t these things, these are methods by which heat is moved like walking, flying in a plane or riding a surfboard.
So from a practical standpoint in a building we control conductive heat transfer with insulation, convective heat transfer by air sealing to the unconditioned spaces and radiation with low emissivity barrier with the shiny side facing an air gap, this is if you need a radiant barrier at all.
Radiant heat can only transfer when you have two surfaces pointed at one another that have a different temperatures. The rate at which heat will transfer between them is a function of the temperature difference, the distance between them and the emissivity of each surface. A suface with an emissivity of 1 is a so called “perfect black body” and is a theoretical perfect emitter and absorber of radiant heat.
A surface with an emissivity of of 0 is perfect reflector of radiant heat energy and neither absorbs or emits radiant heat. In practice we do not see 1 or zero but a fraction of 1 with a black dull surface being close to one and a shiny, reflective radiant barrier generally being around 0.10 meaning only 10% of the radiant energy is absorbed or emitted.
So why can’t we sandwich a radiant barrier?
Imagine getting a pan on a stove nice and hot and then hovering your hand over it, you would feel the radiant heat emitting from the pan. Now place a sheet of aluminum foil over the pan and hover your hand again, very little radiant will be absorbed and emitted by the foil and your hand will be much cooler.
Push your hand down on the foil and squeeze it into the pan…
NO DON’T DO IT! ARE YOU CRAZY?
Spoiler alert, it will burn you.
While aluminum foil has a low emissivity it is very thermally conductive and heat travels through it easily via conduction (molecule to molecule). This means that the only way it helps you block heat is when one shiny, low emissivity side faces an air gap (or vacuum or other fluid that allows the electromagnetic waves to pass easily through). This is why you see white radiant roofs on shopping centers that face the sky, or plywood for roof decking with a radiant layer that faces down into the attic.
If you press anything solid up against both sides of a radiant barrier you make it a conductive layer and it does NO GOOD.
Some of you may (incorrectly) assume that a radiant barrier must be pointed at a light source (like the sun) to do any good. Remember, you don’t need visible light to have radiant heat transfer just a temperature difference. So a radiant layer on the underside of roof decking will help block radiant heat from leaving that roof decking and entering the ceiling and trusses and whatever else is in that attic even if it is pitch black up there because the radiant barrier is bad at absorbing AND emitting radiant heat so even though the radiant barrier on the underside of the roof deck would be hot to “touch” (conductive) it does much less emitting then wood so more of the heat stays put.