Month: February 2020

I’m a big dummy when it comes to my own air conditioning maintenance. I talk about the importance of changing air filters to customers and techs but I never stay up on replacing my own.

Yesterday I walked into my mechanical room and my 2-ton air handler sounded like a vacuum cleaner about to implode.

My filter was nasty… nasty to the point that I wasn’t willing to leave the filter in. So I pulled it out and think to myself “I’ll just grab a filter from the office tomorrow”. well… I forgot and I live 35 minutes from my office.

So today I grab a filter from my nearby hardware store, a common brand and pull it out of the plastic wrap to install it. Sure it was a MERV 11, but that was the only option other than the cheap, spun fiberglass “bug catcher”.

I know what you’re thinking, I should have known better

I’ve got to give it to this filter manufacturer for actually printing the static pressure drop on the filter (shown above).

My system is setup for 350 CFM per ton so it’s required running at right around 700 CFM which means on my system this filter is going to add 0.26″wc of extra static to the return side of the blower.

With most systems being rated at 0.5″wc TESP (total external static Pressure) this makes up more than half of that, before any ductwork, grilles, registers, balancing dampers or coils in the case of furnace systems.

On a PSC blower motor this extra static from this filter would result in lower airflow, poor system performance and poor air distribution.

With an ECM motor this extra static can result in higher blower motor power consumption and condensate drainage issues/difficulty maintaining trap.

While some systems may be able to deal with the extra static at a cost, many will have issues ESPECIALLY on older systems that have PSC motors and furnaces with coils.

This is why larger filter cabinets with lower pressure drop filters often make sense or oversized filter back return grilles.

When choosing a filter remember that airflow (Pressure Drop) is just as important to consider as filtration (MERV rating) and just because a filter fits doesn’t mean it’s the best filter for the system

— Bryan

 

Every contractor is different, I get that.

We don’t all need to do everything the same way or include the same services with repairs but there are some “best practices” that can save you a lot of heartache before, during and after you make a big repair.

Catch it During Diagnosis

Let’s say you find a failed, shorted compressor on a 7-year-old system that still has manufacturer parts coverage. If you simply quote the compressor and leave you may be missing a lot of other maintenance-related issues that can affect operation once the compressor is replaced. A shortlist of items to check would be –

  • Look at the accumulator for signs of corrosion
  • Acid test to see if a burnout protocol should be employed
  • Check the air filter
  • Inspect the condenser coil cleanliness
  •  Look at the underside of the evaporator coil
  • Perform a static pressure test on the system to check for duct issues
  • Check the crankcase heater (if it has one)
  • Inspect the contactor
  • Check condenser fan and blower motor amps
  • Test all capacitors
  • Visually inspect wires and cap tubes
  • Check high voltage electrical connections

And this is just for cooling side issues. If the system is a fuel-burning appliance you would inspect every part of the furnace operation as well.

  • Venting
  • Condensate drainage
  • Burners
  • Flame proving
  • Safeties

And much more…

Testing all of these things is commonplace AFTER a repair, but it makes so much more sense to do it beforehand so that you can either charge appropriately for any of these items that need to be addressed or let the customer know you are including them to differentiate you from the competition.

Things to Do Along With Major Repairs 

There are a few things you need to do as a matter of course during major air conditioning or refrigeration repairs that just make good sense to prevent callbacks. You can include them in the price or not or not but either way, it will save you more than it costs to do it.

  • Clean the drain line and condensate pan (seriously…. do this)
  • Wash the condenser coil
  • Clean the blower wheel (if it is dirty)
  • Change the air filter
  • Test all modes of operation

Do these things along with all of the standards tests you perform to make sure that you have no issues and that whatever caused the fault in the system has been rectified and you will save a lot of problems. When the customer spends a lot of money getting a system fixed, they don’t want to turn around and have it fail for an “unrelated” reason.

While this list is clearly tailored to the residential and light commercial air conditioning market, every piece of equipment has its common maintenance items. So what do you do every time when you make a major repair?

— Bryan

 

When the quiz or the teacher asks what “latent” heat is there is generally some reference to it being hidden heat, which is what the word latent means. We then learn that it is heat energy transferred that results in a change of state rather than a change in temperature.

Later on, we hear a lot talk about how much more heat it takes to change the state of water than it does to change its temperature with a graph something like this.

So we learn pretty quick that a lot more energy gets moved when we are changing matter from one state to another and in HVACR we are going from vapor to liquid and back to vapor again in the refrigerant circuit.

In the condensing coil, we see latent heat rejected as the refrigerant changes from full vapor to full liquid at the condensing temperature.

In the evaporator coil, we see latent heat absorbed as refrigerant changes from mixed vapor / liquid flash gas to full vapor at the boiling temperature.

But there is another kind of latent heat we deal with in air conditioning that can leave people confused when we talk fast and loose about latent heat and the evaporator. This latent heat is the hidden heat it takes to change water vapor in the air passing over the evaporator to liquid water on the coil surface.

Which is Which? 

Inside the evaporator, there is latent heat of vaporization as heat conducts into the coil and boils the refrigerant. That internal temperature is fixed so long as the pressure remains the same across the coil and the refrigerant is a single component or azeotropic (no glide). There are refrigerant blends that do change increase in temperature through the coil through “glide” but set that aside for another article.

On the outside of the coil there is latent heat transfer out of the water vapor causing it to condense on the coil fins so long as the coil is below the dewpoint temperature of the air. This is why we call the ability an air conditioner has to remove moisture at certain conditions its latent capacity.

These two latent heat transfers impact one another indirectly but all the heat that the air moving over the coil imparts on the evaporator is done via conduction through the tubing (or microchannel) walls of the evaporator.

Let’s break that down a bit.

Convection is heat transferred through a moving fluid. So heat moving THROUGH the refrigerant is moving via convection. Heat moving THROUGH the air over the coil is also transferred via convection.

But there is no direct fluid connection between the refrigerant in the tubing and the air moving over the coil is there? (Unless you have a big coil leak).

The heat that moves out of the air and into the refrigerant has to move through the solid walls of the coil and the only kind of heat that can make it through a solid with any significance is conduction.

This means that the only way that the latent heat inside the refrigerant and the latent heat in the air connect is via SENSIBLE temperature difference across the metal walls.

When the air moving over the evaporator has more moisture in it and therefore a higher RH and dewpoint the surface temperature of the coil is increased so long as the coil temperature is below the air dewpoint.

When the surface temperature of the coil is held “higher” by more latent heat of condensation on the coil more heat enters the refrigerant inside the evaporator resulting in a higher evaporator pressure and higher boiling temperature inside the coil (especially in a TXV/EEV system).

This may sound like magic but the quantity of heat that can enter the evaporator is as simple as the temperature difference between the inside of the coil where the refrigerant is and the outside of the coil where the air is. Because we have the potential for latent heat transfer on each side this temperature difference has a lot of contributing factors that can make the math a bit confusing.

Just remember… There are two kinds of latent heat at play

Refrigerant boiling inside the evaporator

Water condensing on the outside of the evaporator

Heat interacting between the two moving from higher temperature to lower temperature via conduction.

Simple!

— Bryan

 

 

Products by Pros

Guest: Bryan Orr

What should technicians looks at when deciding on a micron gauge

Why should a tech even use a micron gauge

What role does fear or pain play in the use of micron gauges

What type of technicians don’t care about proper vacuum or micron gauges

What is the cost of adoption for high end tools

What are the pressures techs deal with

How can techs save time and relieve pressures

Proper evacuation

What are the incentives for techs to do things properly

What role does integrity play when it comes to business owners doing things properly

What micron gauge from Accutools would you suggest for technicians

Why do you consider Accutools micron gauges reliable

Looking to learn more ? https://www.hvacrschool.com/evac/

Want to have supplied near carry Accutools? Reach out to [email protected]

 

If you have an iPhone subscribe to the podcast HERE and if you have an Android phone subscribe HERE.

When we say that there is “flash gas” at a particular point in the system it can either be a bad thing or a good thing depending on where it is occurring.

Flash gas is just another term for boiling.

It is perfectly normal (and required) that refrigerant “flashes” or begins boiling directly after the metering device and as it moves through the evaporator coil. In order for the evaporator to transfer heat from the air into the refrigerant in large quantities, we leverage the “latent heat transfer of vaporization”. In other words, we transfer heat into the boiling refrigerant, or “flash gas”.

In a boiling pot of water, we create flash gas by increasing the temperature of the water until it hits the boiling temperature. At atmospheric pressure that occurs at 212°F which is the boiling or flashing point, we are most familiar with.

Inside of a refrigeration circuit we get flash gas when the pressure on the liquid refrigerant drops below the temperature/pressure saturation point or if the temperature of the refrigerant increases above the same point. In other words, either a drop-in pressure, an increase in temperature or both can result in flashing or boiling.

This “flashing” can occur in the liquid line when the liquid line is long or too small and also in cases with line kinks and clogged filter/driers. All of these instances result in a pressure drop and a drop in the saturation temperature.

This flashing can be prevented by keeping line lengths and tight bends to a minimum, insulating the liquid line where it runs through very hot spaces and keeping the refrigerant dry and clean with one properly sized filter/drier.

It can also be prevented in most cases by maintaining the proper levels of subcooling. A typical system that has 10°+ of subcooling will not experience flashing in the liquid line under normal conditions. Setting the proper level of subcooling acts as headroom against pressure drop in the liquid line due to long line lengths.

When you walk up to a liquid line near the evaporator and you hear that hissing/surging noise or when you look in a sight glass and see bubbles you are seeing refrigerant that is at saturation, meaning it is a mix of vapor and liquid. This doesn’t necessarily mean it is “flash gas”in the truest sense, it could very well be that the refrigerant was never fully condensed to liquid in the condenser in the first place. This can be due to low refrigerant charge and in these cases, the subcool will be at 0° Even when taken at the condenser.

The true liquid line “flash gas” issues are cases where you have measurable subcooling at the condenser coil outlet but still see, hear or measure boiling/flashing refrigerant in the liquid line before the metering device or see it in a sight glass.

— Bryan

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