Tag: heat

Air Flow and Ice – A Thought Experiment

Imagine a glass of ice sitting on a table.

Now imagine you place a lid on the glass so all the water and ice is contained in the glass.

If the ice and water are well mixed the water and ice will both be at 32°F because the ice is slowly changing state from ice to water which we call melting. Becasue this is happening at atmospheric pressure we can know what temperature this will occur at and the heat being transferred is going toward melting the ice rather than changing the water temperature which we call latent heat.

Let’s say the temperature in the room is 75°F. In this scenario, heat leaves the air molecules as they contact the exterior of the glass and heat moves through the glass into the water and ice. Becasue glass is a pretty good insulator this happens pretty slow but this heat still moves from hotter to colder.

This movement of heat from the air to the exterior of the glass transfers THROUGH the glass via conduction.

What happens if we blow air toward the glass? what changes?

If we move more air over the side of the glass we deliver more air molecules to the glass via convection but it doesn’t change the fact that the heat makes it through the walls and into the glass via conduction.

By delivering more air to the glass we warm the outside of the glass more which causes the water melt inside the glass faster, in other words more air over the glass means more heat transfer even though we didn’t change the temperature of the water or the air.

This same basic thing happens inside an evaporator and condenser coil, when we increase the flow of air we also increase the transfer of heat through the walls of the copper tubing in the coils. In the condenser more airflow increases the heat rejection out of the refrigerant and in the evaporator more heat is gained.

Because the refrigerant circuit is dynamic (refrigerant moving) and under pressure more or less heat entering or leaving the system impacts the process and changes the pressures of the refrigerant inside.

If we move less air over the condenser the pressure on the high side increases, if we reduce the air over the evaporator coil less heat enters the circuit, and pressures drop.

This is a basic picture for you to consider next time you see high or low system pressures and how coil airflow impacts heat transfer.

A more advanced but similar thought experiment is what would happen if the evaporator coil had no fins.

— Bryan

An Electric Heat Mistake

I started working as a tech when I was 17 years old, fresh out of tech school. My first winter out on my own I went to a service call in an older part of Orlando, a part of town I had never worked on before. It was an especially cold winter that year, and the service call was for insufficient heat.

When I arrived, I found the system was a really old GE straight cool system. After testing the system, I found the system had a 10kw heater, but only 5kw was working. After a closer look, it was discovered that 5 KW of the heat was disconnected. This was no problem for me; wiring was always my specialty! I grabbed some #12 stranded and had that puppy heating in no time.

Problem…

#1 – It smoked like a chimney and set off every alarm in the house

#2 – Once I got the doors and windows open and the smell cleared out as best I could it got me thinking… How long has it been since that second 5kw was connected?

When I looked closer I saw that the feed wire going to the air handler was only #10… then it dawned on me.

The REASON they had one-half of the heat disconnected was because the breaker and wire size were only rated for 5kw. Why did they a 10kw you might ask? Likely it’s what they had on the truck and they figured if they disconnected one-half it would be safe.

Lessons learned –

#1 – Never assume that a system was installed properly to begin with and keep an eye out for proper feed wire size.

#2 – Don’t use improperly rated heat strips or other rated parts and simply make an “alteration”. When the next technician arrives he likely won’t understand what you did. At best you confuse him, at worst you kill him.

— Bryan

The Good Old Heat Anticipator

As a technician you most likely know some customers who still have an oldie thermostat (you know, those old mercury bulb things, like the round Honeywell CT87 and such).  Keep in mind those usually have an adjustable heat anticipator.  If you’re newer in the field  you may not have seen or worked with those very much, or even not at all.  They can seem confusing at first (why is it set with amperage? What amperage? How am I actually adjusting this???) but actually are quite simple to work with.

First of all, I hear you thinking “do you actually need to adjust that?  I mean, is it going to make that much of a difference?”  Honestly, in most cases, no, it won’t make a big difference.  But it’s no reason to ignore it.  And when it actually does make a difference, you will want to know how to adjust it properly.

Here’s a hypothetical story: you just changed a system, let’s say converted from a 30 year old oil furnace to a brand new condensing gas furnace.  The homeowner just loves their old, simple, ‘’I-just-have-to-turn-it-up-or-down’’ thermostat and won’t upgrade it to a modern digital one.  And hey, it still works fine, so why bother.  Then, a few days later, you get this service call:  ‘’that new furnace you guys just put in, it doesn’t work right!  It keeps starting and stopping every 5 minutes! (or) It stays on for too long and overshoots the set temperature by a whole degree!’’  (and the line everybody loves to hear): ‘’It didn’t do that with my old furnace!  It’s that new one, you sold me defective garbage equipment!!!’’

Okay, it doesn’t happen like that all the time, but I’m sure you’ve heard of similar stories.  Now, to focus on the problem.  I’m writing this tip about heat anticipators, but please don’t assume that’s going to be the issue whenever you get this kind of service call.  I am merely reminding you that it is one of the many possible problems.  So let’s say everything else is normal, no faults occurring during furnace cycles, no airflow issues, proper system sizing, etc.  There’s a chance a very poorly adjusted heat anticipator will make a significant difference in cycle time.  After all, it’s what it’s designed to do.

In short, the anticipator is simply a resistor built in the thermostat that is in series with the heat call low voltage circuit, i.e. the “W” terminal.  That resistor generates a tiny amount of heat to preheat the bi-metal and end the furnace cycle a little bit earlier, anticipating the residual heat from the furnace and fan off delay to cover the gap in temperature and avoid overheating the space.  Now, even though it’s a resistor, you don’t set it by ohms.  You set it by amperage.  The amperage drawn by the heat control circuit.

Now it takes a little bit of effort to get that measurement properly, but it is quite simple.  First of all, you need to remove the anticipator itself from the circuit when checking the control’s amp draw!  All this means is you need to remove the thermostat from the circuit by twisting together the R and W wires at the thermostat.  This will, obviously, give you a constant call for heat.

Honeywell also gives a basic guideline for different heat types

To further adjust cycle times if the actual setting doesn’t seem to work quite right, you may change it accordingly: higher amperage setting = longer cycle time lower setting = shorter run time.  I wouldn’t stray too much from the ‘’proper’’ setting, however.

— Ben Mongeau

How “Hot” Does That Get?

I got in one of these tiny torches the other day to experiment with brazing aluminum in tight spots and one of the techs walked in and asked “what type of torch is that” to which I answered “It’s oxy/acetylene”, he picked it up and looked at it a bit then asked “Does that get hot enough to braze copper?”

It’s actually a tough question to answer simply

When we say does something get “hot enough” we often mean that the TEMPERATURE is high enough to melt solder or brazing rod, but heat is both an intensity (temperature) and a quantity (BTUs), so while this tiny torch is certainly high enough temperature to melt a rod, it may or may not be enough BTUs to heat up the base metal being joined.

The temperature of the flame is primarily dictated by the fuel or fuels being burned as well as the oxygen mixture. The BTUs depending on the pressure being used and the size/type of the tip.

Most torch manufacturers will list the size of the (copper) pipe that the tip is rated for as well as the proper oxygen and acetylene pressures for the task.

So, heat is both a quantity (BTU) and an intensity (Temperature) and when we say how hot or how cold it really depends on what you mean.  But to answer the question… Yes ,you can braze copper with the tiny torch, so long as it is small tubing.

— Bryan

Heat Doesn’t Rise

We’ve all heard some version of the phrase “heat rises” but is that really true? First, we need to remember that heat is energy not matter. Heat is a force not a thing, so while heat may result in changes to matter (stuff) it, isn’t matter itself. When we add heat to stuff the molecules inside move faster and when you remove heat molecules move more slowly.

So no, heat cannot rise because heat isn’t a thing. Hot air, on the other hand, does rise in colder air.

We see that when we heat the air in a balloon it will rise and float in the colder air around it, but why does this happen?

It all comes down to density and buoyancy and we see it all the time in water. When something is less dense than water it will float upward and when it is more dense than water it will sink down. Even water changes in density with changes in temperature and will sink or float depending on the differential of one mass of water compared to another.

Colder air is denser (more lbs per sq ft) than warmer air. So colder air “sinks” in warmer air and warmer air “floats” in colder air due to buoyancy just like hot air balloon floats in the air or a rubber duck floats in a bathtub.

When you add sensible heat to air the molecules in the air begin to move more quickly and they start to separate making warmer air less dense when the molecules are free to move. When you remove sensible heat from air the molecules slow down and the air becomes denser.

But that isn’t the force at play in air and heat movement.

We ALSO know that heat tends towards equilibrium or “hot goes to cold” so that when a cold air mass hits a warm air mass and they start to mix the heat from the warmer air will start to enter the colder air creating an equilibrium.

Then we also see that pressures also tend towards equilibrium or “high pressure goes to low pressure” which also impacts air movement.

Because air is relatively free to move in a building you will observe all of these forces at play at once with some of the dominant forces being stack effect in the winter and reverse stack effect in the summer.

When you increase the temperature of air in a space through a heater the density of that warmer air leaving the register will be lower than the colder air around it. This will result in the warmer air “floating” in colder air and the colder air “sinking” below the warmer air.

As that warmer air continues to rise it will naturally create a lower pressure near the floor which will tend to draw in cold air from outside through any gaps lower in the home. This is what we often refer to as “stack effect”.

In the Summer when the air is cooled, the cooler air will sink in the warmer air creating a lower pressure near the ceiling that will tend to bring in heat from gaps higher in the structure such as can lights.

Again… there are many factors that impact the movement of air in a space and stack effect is only one of them and is based on buoyancy.

For example, imagine a roaring open fireplace on the first floor of an old leaky home. As that fireplace heats the air, air begins to rush up the chimney. This creates a low-pressure area in front of the chimney and air from all over the home pulls into that area to fill the void (high pressure goes to low pressure). At the same time, the fireplace is heating the room it is in (mostly through radiant heat) and the heated air starts to float in the colder air. In the meantime, the entire house is going under negative pressure compared to the outdoors and cold air is being drawn in from gaps and cracks all over the home.

In other words…. Air is tumbling and mixing all over attempting to balance the forces of pressure, temperature and buoyancy due to the simultaneous increase in room temperature and a decrease in room pressure caused by the open fireplace.

So heat doesn’t rise, hot air floats in colder air and cold air sinks in warmer air and there are many other forces at play.

— Bryan

The Basics of Moving Heat (Thermodynamics)

In this episode of HVAC School we talk about thermodynamic basics, including:

• Heat & Temperature and the difference
• Fahrenheit, Celcius and Kelvin
• Absolute zero
• Molecular Motion
• Hot and cold
• British Thermal Units
• Energy Conversions