Tag: heat

 

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.

 

Now the amperage you need to measure is typically very low, no more than half an amp in most cases, sometimes much lower.  So, in order to get a more precise reading (unless you have a super sweet meter that gives you precise readings in the tenths to hundredths of an amp range, this would be done in series instead of with a clamp) you should proceed as follows:  get a nice very long piece of thermostat wire, which you will repeatedly wrap around your meter clamp, so it goes through it 10 times.  Then connect that wire to your W wire from the thermostat on one end and to the W terminal on your furnace control on the other end.  Simply put, just extend the W wire so that you have enough to wrap it around the clamp ten times.  Then turn the power on and let the furnace cycle begin.  Wait until all the relays and components are energized (on a typical gas furnace you will see the greatest amp draw coming when the gas valve is energized), then take your reading.  Divide it by 10, and you have your heat anticipator adjustment value.  Simple as that.  For example, you might read (completely arbitrarily) 2.40 Amps on your meter with ten wraps of wire.  Which means the control actually draws 0.24 Amps, so you will need to set your heat anticipator to 0.24.  It is recommended to insert the tip of a pen or something similar in the slot to gently slide the needle to the desired setting.  And this procedure, by the way, is still explained in modern install manuals.

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

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

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

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 to learn –

#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

P.S. – We released a new podcast on circuit boards today, you can listen here

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