Month: November 2018


Condensate overflow prevention devices or float switches as they are often called are such simple devices that you wouldn’t think there would be much room for controversy. In my experience there are few areas of the trade where technician and installer preferences and opinions vary greatly.

Let’s start with some float switch basics

Float (condensate) switches are designed so that they will remain closed when water is going down the drain like it’s supposed to and then open when an overflow condition occurs. In order for the switch to open it must be positioned in a location that is dry normally and will reliably fill with water when a drainage issue occurs.

There are very simple switches that just clip onto the edge of primary or secondary drain pans like the Rectorseal Safe-T-Switch SS3 shown above. When the primary drain overfills it begins leaking into the secondary pan below and a pan sitch like the SS3 will open when the water levels rise. This type of switch can also be used in the primary pan so that it only trips when the water level exceeds the normal levels. In both of these cases, it is important that the switch is firmly mounted and level so that it will function as designed when the time comes.

Then there are the more typical devices like the Safe-T-Switch SS1 and SS2 that are designed to be installed to an aux drain port or sometimes even in the primary drain. When it comes to the mechanical code, there is no “nationally adopted code” but most states and localities adopt the International Mechanical Code (IMC) is the most widely adopted code in the country and it states a few things that are important to consider in addition to the general requirements stating that an overflow device is required.

307.2.3.1: “On downflow units and all other coils that do not have a secondary drain or provisions to install a secondary or auxiliary drain pan, a water-level monitoring device shall be installed inside the primary drain pan. This device shall shut off the equipment served in the event that the primary drain becomes restricted. Devices installed in the drain line shall not be permitted.

307.2.5: “Condensate drain lines shall be configured to allow the clearing of blockages and performance of maintenance without having to cut the line.”

So while this may not apply to you depending on where you live, in general, the code says –

  1. You need to have overflow protection
  2. It needs to be in a secondary pan, in an overflow drain or in the primary pan UNLESS the unit also has an aux drain line that drains outside or a secondary pan under it. 
  3. The primary drain needs to have some sort of cleanout that doesn’t require taking the drain apart

While there are cases where installing in the main drain is permitted the cases are limited and there is also the issue of buildup collecting on the float itself when it is installed in the main drain. While some technicians swear by the practice of putting the float switch in the main drain line I generally advise against it.


So in most upflow applications, we will be installing the condensate switch to the aux port on the evaporator coil. When installing the switch you want to set in such a way that you accomplish the following –

  1. The float trips (rises) before the pan overflows
  2. Have free access to the access panels and filter door
  3. Water should flow freely to the float if an overflow occurs
  4. The float is installed level so it can rise and fall freely 
  5. The switch is installed so that it cannot be easily moved into an improper position 

Many technicians swear that if the aux drain is pitched down that water will flow into the float all the time and they pull the drain perfectly level or even pitched upwards. Others swear that the switch must be mounted directly to the front of the unit. The truth is that so long as you follow the 4 rules above the float could be positioned in many different locations. It is not the orientation of the PVC connected to the aux port that holds back the water, it is the level of the aux port opening that does that. The level that the water must rise and the total depth of the primary pan varies from model to model. This means that in some cases a practice that works just fine on one brand may not on another.


Many contractors have found that installing a float switch with a flat bottom like the SS2 right on the front of some shallow coils can result in the pans overflowing before the float trips. Others have found that this same practice can result in difficulty accessing air filters. In these cases it may make sense to route the aux drain around to the side, pitch the drain down and level it out with 45s and install the switch there. Like all drains, it must be pitched slightly down for the water to reliably reach its destination, but the switch itself should still be installed level.

Once again, this practice does not result in false tripping because it is the level of the aux drain port that holds back the water, not the pitch of the drain (unless the drain is improperly pitched upward).

Finally, there is a dispute over whether to break the “R” circuit or the “Y” circuit with the switch. It is generally recognized that breaking R is a better practice to prevent short cycling unless the system has another dedicated method for condensate switch connection.

— Bryan

I was fresh out of school working as an apprentice at my first real HVAC job and I was listening in on a shop conversation between a few techs.

They were talking about finding so many overcharged systems and one of the techs turns to me and says “I had a unit yesterday that was so overcharged it was running minus five degrees of superheat”. I don’t remember EXACTLY what I said in response to that but it started a miniature argument and set me on a crusade against misinformation that led me here all these years later.

When in doubt check your tools

Before we move on I want to mention something that Jeremy Smith pointed out to me. When working with a zeotropic refrigerant blend that has “glide” the change from liquid to vapor and vapor to liquid occurs over a range of temperatures and not at a single temperature. When calculating superheat we use the “Dewpoint” and when calculating subcool we use “Bubble Point” the saturation temperature is the range of temperatures between those two points meaning that it could be “interpreted” as negative superheat or subcool when it is actually just in the saturated range. In air conditioning, the traditional R22 and R410a refrigerants do not have any significant glide but newer blends do so it is something to watch out for.

Here is a list of things that if you observe them, it will be worth checking your tools to make sure they are set up correctly, connected correctly and properly calibrated BEFORE you start making an exotic diagnosis.

Negative Superheat   

Superheat is the temperature gained in the refrigerant once it is completely boiled into a vapor. When it is still in the process of boiling it will be in a mixed state and will be at saturation temperature for that given pressure. Zero superheat is something you will see often when a system has a flooded coil and liquid still boiling in the suction line. While this generally isn’t a good thing it is something that you will observe from time to time and will usually result in you as the tech taking corrective action.

Negative superheat goes by another name SUBCOOLING and the only way a substance can be in the subcooled range is if it is 100% liquid and has given off additional heat below the saturated (mixed) state. It is impossible in a running air conditioning system for the suction line to be 100% liquid subcooled below saturation, therefore it is impossible to have negative superheat both by definition or in practice.

So what happens when you measure negative superheat you may ask? Good question.

It is one of a few possibilities

  1. You are looking at the wrong refrigerant PT scale
  2. The refrigerant is mixed (somebody put something in on top of the original refrigerant)
  3. You are dealing with a blended refrigerant with “glide” like many of the new 4 series blends such as R407c
  4. Your suction gauge is reading too high
  5. Your line clamp thermometer is reading too low
  6. You do not have a good connection on the line / schrader core isn’t depressing / king valve isn’t open
  7. A combination of the items listed above

Negative Subcooling 

Just like we mentioned above, negative subcooling is superheating. There is no such thing as negative subcooling.

Is it possible for the liquid line to contain superheated vapor? It is THEORETICALLY possible but not practical. For example, if someone short circuit nearly the entire condensing coil and connected to the liquid line you could see superheated vapor…. but let’s be realistic.

When techs measure a negative subcooling (superheat) at the liquid line it could be

  1. You are looking at the wrong refrigerant PT scale
  2. The refrigerant is mixed (somebody put something in on top of the original refrigerant)
  3. You are dealing with a blended refrigerant with “glide” like many of the new 4 series blends such as R407c
  4. Your high side gauge is reading too low
  5. Your line clamp thermometer is reading too high
  6. You do not have a good connection on the line / schrader core isn’t depressing / king valve isn’t open
  7. A combination of the items listed above

Liquid Line Cooler than the Outdoor Air 

There are two cases where the liquid line can be cooler than the outdoor air when measured at the condenser outlet

  1. A Wet Coil
  2. A restriction inside the condenser cabinet in the liquid line, usually in a factory installed filter drier

Because the liquid line temperature will often be VERY close to the outdoor temperature on new, high-efficiency system this is often a point where you will measure a liquid line as colder than the outdoor air when that may not really be the case.

Often you may SEE a liquid line colder than outdoor ambient and it may be simply be

  1. Miscalibration of the line clamp or the ambient air thermometer
  2. Measurement of the ambient air in sunlight where the probe can be affected by sunlight
  3. The coil is still damp after cleaning or a rain (evaporative cooling)

It is always a good practice to have a backup set of thermometers and gauges so you can double check the calibration of your tools against one another. Whenever possible, test them under the conditions that you are using them.

If you have two clamps, place them on the same line right next to one another, when testing two air probes, stick them both in the same return air stream side by side. For temperature measurement you may also test in an ice bath just make sure that the water is pure and that the water and ice are fully mixed and circulating when you test for 32°F(0°C) degrees.

Also, keep in mind that every measurement device has “uncertainty” in the measurement of +/- a certain amount depending on the tool. Don’t expect your tools to provide a greater accuracy than what is published in their specifications.

— Bryan

I knew a tech when I was just starting who was hands down, no questions asked, the best technician at the company I worked.

EVERYONE, we are talking over 60 techs… we all knew it.

His name was Mike Gilford

Being the little brown nosing ladder climber I was, I made a complete study of Mike and found three things about him.

#1 – He read stuff. When a new product came out he would read up on it, data tags, product data, bulletins etc…

#2 – He didn’t talk much. There was no need. He proved what he knew by what he did more than what he said.

#3 – This is the big one. This is the one that I am confident is the trait that made him who he was. He was humble, willing to be wrong and willing to learn.

Let that sink in a minute…

Sure Mike had an ego like we all do. He didn’t like being corrected and he wanted to do things right.

But…

He never let his ego get in the way of growing and learning

If you had something to share that would benefit him and others, he kept an open mind and was willing to accept it.

That’s a hard pill to swallow.

I knew Mike when I was in my early twenties and I hadn’t seen him in over 12 years but I vividly remember things he taught me. I bumped into him at an ACCA meeting the other day and sure enough… He’s still the same guy.

I still want to be like Mike.

— Bryan

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