Improper condensate trapping can cause a lot of property damage. It’s necessary that a drain line is installed correctly to avoid damage to a residence or business.

Evaporators coils are installed to allow a blower to either push or pull air across the coil. The pressure, either positive or negative applied across the coil is also the same applied pressure to the drain line connected to the drain pan of the evaporator coil.

When air is pushed across the coil, it’s considered to be under positive pressure. Since the coil is under positive pressure, the drain line will also be under positive pressure.

When air is pulled across the coil, it’s considered to be under negative pressure. Since the coil is under negative pressure, the drain line will also be under negative pressure.

The idea of the condensate drain trap is to use the weight of the water to stop the flow of air produced by the blower from being sucked into (negative pressure) into the evaporator coil or from being pushed (positive pressure) out of the drain line during operation.

The positive pressure evaporator coil

When the blower blows across the evaporator coil, the coil would be considered to be under positive pressure.

As the blower creates a positive pressure moving air across the coil and through the ductwork, it creates a negative pressure pulling air through the return side.

The positive pressure coil is most commonly paired with a gas/oil furnace. Placing an evaporator coil upstream (before) of the furnace heat exchanger could cause premature failure of the heat exchanger due to moisture damage from the evaporator coil.

Since the positive pressure condensate line is under positive pressure, any condensate sitting in the pan is forced through the drain line with or without a trap.

The downside of not adding a trap to a positive pressure condensate drain line is the minor efficiency loss of air pushed through the drain line rather than being sent to the conditioned space. Adding a trap reduces those losses as the weight of the water acts as a barrier.

It’s important to note that the outlet side of the trap needs to be lower than the inlet otherwise water will backup once the trap is full.

The negative pressure evaporator coil

Using a drain trap becomes very important when you have a negative pressure evaporator coil. When a blower is pulling air across the evaporator, it creates a negative pressure across the coil and positive pressure on the ductwork.

The negative pressure evaporator coil is most familiar with heat pumps and cooling only systems usually found in attics.

The negative pressure applied to the coil will not allow the condensate in the pan to flow through the drain until the blower shuts off. By this time, there could be too much condensate in the pan for the drain line to handle or worse the pan could overflow before the blower even shuts down.

The air being sucked in through the drain can even splash condensate through the ductwork causing damage to the structure or nearby electrical components.

Adding a Vent

Adding a vent to your drain line is important done to prevent airlock caused by sagging drain lines, double trapping, or sharing a drain such as a high-efficiency furnace and AC evaporator coil.

It’s crucial the vent be installed after the trap or it’s like the trap doesn’t exist at all.

Drain piping should be sloped 1/4″ per foot towards discharge location without any sagging. Anytime you have multiple traps on the same drain line, you have a double trap, and this includes sagging drain lines as they can act as a drain trap.

Once water flows from the first trap to fill the second trap, a column of air is locked between the two. Air weighs less than water and rises which stops the flow of condensate from the first trap.

If you add a vent between the two traps, the air has an escape and condensate will flow.

Common Best Practices

• Dispose of condensate to an appropriate location. Don’t drain to a walkway, anywhere there’s a chance for the line to freeze or around the foundation of a structure.
• Using a 3/4″ drain line reduces the chance of a clogged drain which can cause property damage or damage to the unit.
• Add a clean-out that’s accessible without having to cut the drain line.
• Support the drain line with strapping to keep it from sagging or being knocked around.
• Add at least 1/4″ slope for horizontal runs to assist in drainage.
• Add a vent after the vent and before a common drain.

Clearing Drain Piping and Drain Traps

It’s important to note that any appliance equipt with a pressure switch must have the hoses removed from the switch to avoid damage to the diaphragm of the switch when blowing out or vacuuming the drain line or condensate trap.

There’s a number of high-pressure drain sprays or nitrogen that can be used to clear a drain quickly. The most common way is to simply blow the drain out manually or to use a vacuum to suck everything from the system.

Blowing out a drain will free up obstructions, but you should always flush the line after to clear out any gunk left or the line could plug again fairly quickly.

Blowing Out the Drain Line

Blowing out the drain line only requires a piece of 3/8″ condensate pump hose and a bit of electrical tape to make a seal inside the PVC.

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Once inserted, block off air from entering into the condensate trap of the appliance if applicable, and blow out debris in the drain line.

Thermistors are used to sense/measure temperature. You will find them in ductless splits to detect evaporator coil or condenser temperature, temperature measuring devices such as a digital thermostat, ambient air temperature, PTC start relay, and supply or return air temperature. The name thermistor is a combination of “thermal” and “resistor.”

A thermistor changes electrical resistance with a change in temperature. The difference in electrical resistance made is dependant on the type of thermistor used. There are two types of thermistors. PTC (positive temperature coefficient) and NTC (negative temperature coefficient), but measurements are usually in the thousands of Ohms.

A PTC thermistor increases resistance with an increase in temperature, whereas an NTC thermistor decreases resistance with an increase in temperature. NTC is most common in the HVAC industry.

Example: When used to sense freezing conditions on an air conditioning evaporator coil, as the coil freezes, the NTC thermistor would increase in resistance and not allow current to flow once the resistance is too high. The air conditioner would then not function.

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The plastic orifice shown in the image above is a snubber. Sometimes the appliance will experience sudden pressure changes such as wind against the exhaust or the burners igniting. The snubber is designed to only allow a designed amount of pressure to be pulled against the switch at one time.

The insert should not be removed to make the appliance function. If you remove the insert and the appliance works, you have another larger problem such as a plugged intake, plugged exhaust or drain.

Pressure switches open and close due to reacting to pressure. You will usually see them labeled as PF (pressure fall) or PR (pressure rise).

Example: A 0.65” WC PF would mean if pressure drops below 0.65” WC, the pressure switch will open and stop the furnace from running.

Pressure switch errors could be the reason for most service calls made.

The pressure switch is a NO (normally open) switch used as a safety device to ensure combustion blower operation with negative pressure through the heat exchanger(s).

Upon a call for heat, the combustion blower will startup. This is pulling a negative pressure through the air intake, heat exchanger, and a positive pressure through the exhaust venting. This negative pressure closes the normally open contacts to complete the safety circuit to start the combustion process.

Possible causes for a pressure switch error:

• A blocked drain could backup condensate from the combustion process into the secondary heat exchanger causing the pressure switch to fail open
• A broken combustion blower wheel
• A blockage in the exhaust venting or combustion air
• Sagging vent pipe will not allow condensate to drain out causing smaller venting. You’ll hear a gurgling sound when this happens
• Undersized or oversized venting
• Too many PVC elbow fittings

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Fan center controls generally are found in older standard furnaces. These are added to separate the heating side from the air conditioning side and added while air conditioning is installed. Newer furnaces have everything controlled via the control board.

Older atmospheric furnace fans are controlled by temperature and a temperature-controlled fan won’t work for air conditioning since the fan needs to run as soon as air conditioning is triggered on.

The fan center consists of a relay, transformer, and terminal strip for the thermostat wiring ran from the thermostat to the fan center control terminal strip.

The fan center control usually mounts inside the furnace on a 4″ x 4″ junction box.

The igniter is what’s going to ignite the gas in the combustion chamber.

Two of the most common igniters are the silicon carbide and silicon nitride.

Silicon Carbide

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Silicon carbide is among the older of the hot surface igniters. They come in different styles, and most of them run off 120v. They are very fragile and crack easily if not handled carefully. There’s a myth that the oils in your skin will cause damage. It’s not true.

The silicon carbide igniter lasts typically 3-5 years before failure, and the most common issue would be cracking. You can verify this by reading Ohm’s across the igniter electrical connections, and it would read OL or infinity when cracked. Remember to disconnect any electrical connecting to the igniter, so you’re only reading the igniter.

Silicon Nitride

Silicon nitride igniters are a far superior igniter when compared to the silicon carbide regarding quality and durability. Some packaging will say they last up to 3x the life of a silicon carbide igniter.

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DC can power silicon nitride igniters. Usually, around 80VDC and some will run off AC between 24V to 120V.

Honeywell makes a very popular general replacement silicon nitride igniter called the Honeywell Glowfly. It’s a 120VAC (AC voltage) igniter designed to replace over 110 OEM igniters including both silicon nitride igniters and silicon carbide igniters.

Inshot Burners

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There are many different kinds of burners. We’re mainly going to focus on the inshot type burner as seen above. The inshot burners are in most modern furnaces. A combustion blower is used to pull the flames through the burner tubes into the heat exchanger.

Burners are used to prepare the air:gas mixture for combustion. Gas is shot through one side, and as gas flows through at a relatively high speed, it pulls primary air into the burners with it. The quick flow of gas pulling air in with it is called the venturi effect.

Think of a spray-paint can. Venturi speeds up airflow by using a reduction in the size of a passage that gases pass through causing the gas to speed up.

A partially clogged orifice can cause a flashback of the flame as not enough primary air is being drawn in, so ignition happens in the mixing area.

After ignition happens correctly in the burners, the flames are pulled through the heat exchanger with the assistance of the inducer.

Furnace control boards can be very intimidating for newer technicians without much experience working with them. They’ll typically have many wires running to/from them, flashy lights, bulky built-in relays, resistors, and capacitors. It all looks very confusing, but it’s really not.

Ignition control boards

Ignition control boards are typical in older systems. They control the gas valve, igniter, flame sensor, and the damper (when used). With a call for heat, the board will start the sparker and light the pilot. When it senses the pilot, it will continue to light the burners.

Integrated Furnace Control (IFC)

An IFC, also known as the control board, controls everything an ignition control would and more. All safety controls, combustion blower, flame proving, gas valve, and the blower are all managed by the IFC.

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Watch for burn marks on the board as they could be an indication of possible failure. The examples below are the front and backside of a relay on a control board.

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Tip: Removing Plastic Mounting Pins Off a Control Board

The white plastic mounting pins holding a control board to the mounting plate can be a pain to remove easily without breaking anything or twisting back and forth with a pair of pliers over and over. Here’s a quick tip on how to remove most control board mounting pins. Below are the plastic pins that hold a control board in place.

The only thing you will need for this is a standard Bic clear pen.

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Step 1: Locate 4 pins on board

Step 2: Place Bic pen over the pin and push in on the Bic while also pulling control board off the pin

Step 3: Done

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Flame rollout switches are mounted around your burners. They exist to sense when/if the flame were ever to pose a risk to the homeowner.

These are usually bimetal switches that warp when heated to break the heat cycle when they sense a flame rollout situation. They’re traditionally manually reset by pressing the button in the middle, although some use a heat-activated fuse which would mean you’d need to replace the whole part if it were ever to sense rollout.

A few common causes of a tripped flame roll-out limit are:

• Blocked exhaust
• Blocked combustion air
• Plugged heat exchanger
• Cracked heat exchanger

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Above images are two of the more common high limits found in residential and lite commercial furnaces

A high-limit switch is a bi-metal snap disc switch that warps when heated to break a heat cycle and used to protect the heat exchanger from overheating and burning out due to insufficient indoor air passing over it. A dirty filter or squirrel cage, undersized ductwork, or many supply air vents closed are among some of the reasons to set this switch off.

Once a high limit senses the heat exchanger is in danger of overheating, it breaks the circuit shutting off gas flow and runs the furnace blower to cool the heat exchanger. Once the temperature is safe for the heat exchanger, the heating process continues.

These high limit switches are almost always automat reset meaning the user does not have to reset the switch. The most common in furnaces today are snap disc limits like the one above. If the heat exchanger is heated beyond the rating of the high-limit, a small disc inside the switch will warp and break the electrical flow of the circuit.

The high-limit will usually be marked with a rating similar to 160-40°F meaning at 160°F the high-limit will trip and reset when it drops 40°F down to 120°F.

Many things can cause these to trip, but they will, for the most part, be related to indoor air moving across the heat exchanger.

Common causes for high-limit to trip

• A dirty filter will not allow the proper amount of air to absorb heat from the heat exchanger and move it to your living space

• Too small of return air or not enough return air installed in ducting
• A dirty blower wheel will reduce the air that the blower wheel can move across your heat exchanger

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• Wrong blower speed will also move the wrong amount of air across the heat exchanger
• A broken belt will not allow for the proper movement of air on belt-driven motors. A broken belt will also make pretty loud banging noises

• A faulty capacitor
• A dirty evaporator coil
• A dirty secondary heat exchanger
• A cracked/damaged heat exchanger

• Incorrect gas pressure
• Incorrect burner orifices

Thermocouple

A thermocouple is a type of flame safety that prevents the gas valve from dispensing the gas into the burners without the presence of a flame. These are found mostly in older standard pilot appliances including (mostly older) furnaces, water heaters, and unit heaters. A typical thermocouple will produce upwards of about 30 millivolts. To give you an understanding of how small of a voltage that is, 1,000 millivolts is 1 volt.

One end sits in the flame, and the other threaded end is connected to the gas valve. With an applied flame to the hot junction, a small voltage (millivolts) is generated to power the electromagnet coil and hold the solenoid open to allow the flow of gas to continue. If the flame goes out, the solenoid will close and stop the flow of gas as the thermocouple will not generate the amount of voltage (millivolts) to hold it open. The thermocouple must shut de-energize the coil within 90 seconds. The thermocouple will power the safety solenoid, but the main gas valve will still need its 24v circuit to operate, which in the residential and light commercial space is usually 24v.

A thermocouple has two dissimilar metals. One end is known as the hot junction, and the other is the cold junction as seen above. The hot junction sits in the pilot flame no more than a half-inch. Since the other end does not touch the flame, there’s a temperature difference between the two, and it needs to be at least 400°F. The temperature difference causes something called the Seebeck effect.

The Seebeck effect is the temperature difference between two dissimilar conductors causing a voltage difference between the two. With heat from the pilot to one of the two conductors (hot junction), heated electrons move to the cooler one (cold junction). A small amount of mV (millivolts) is generated powering the pilot solenoid valve to stay open in the gas valve

There are two ways to test a thermocouple:

Open Circuit Test

The open-circuit test will show the potential millivoltage the thermocouple can generate with no load. Since this is with no load, I frequently stick with the closed-circuit test. An open circuit test on a proper thermocouple will yield 25mV to 30mV.

• Make sure your meter can read up-to 30mV
• Connect one of the meters leads to the copper sleeve of the thermocouple, and the other lead to the end contact of the thermocouple. With the flame hitting the hot junction of the thermocouple (first half-inch), you should read 25mV to 30mV

Closed Circuit Test

The closed-circuit test shows what millivoltage the thermocouple can keep while under load. The closed-circuit test on a proper thermocouple will yield 12mV to 15mV.

The main difference between the open circuit and closed circuit test is the thermocouple needs to be connected to the gas valve for this test with a test adapter.

• Make sure your meter can read at least 15mV
• Remove the thermocouple nut connector from the gas valve. Connect the thermocouple test adapter, but do not overtighten. The adapter just needs to be snug
• Connect one of the meters leads to the copper sleeve of the thermocouple, and the other lead to the contact on the side of the test adapter. Light the pilot as usual and watch the mV generated
• 12mV to 15mV would be a reliable rating. If you read below 12mV, replace the thermocouple

Thermopile (Powerpile)

A thermopile is also known as a powerpile. The thermopile is just multiple thermocouples connected in series to generate more millivoltage than you can with a single thermocouple. Most thermopiles are 10 or more thermocouples connected in series.

The most common issues with thermopile reading low millivoltage output are loose wiring connectors. With thermopile applications, you need to have good wiring connections since you’re dealing with such a small amount of voltage.

Your wiring connections need to be clean and free of corrosion.

Abnormally long wire lengths can cause an issue, but it’s uncommon to run into this issue.