Tag: furnace

This question was submitted on the site in response to the recent GFCI tip. It’s a good question with several possible answers. What do you think?

Hey Bryan,

I have had a few instances where we are firing off a furnace in a new build with a temporary power pole outside with gfci outlets installed that are tripping the gfci on blower motor startup. (In order to get temporary heat for drywall, we run an extension cord to the gfci pigtailed to the furnace). I’ve never had issues doing this with PSC motors, but every time I’ve tried to use a gfci with a variable speed or true ECM motor, it trips the gfci. I’ve tried multiple motors, different furnaces.

What gives?

Frustrated in Fort Collins

Howdy Frustrated (This feels like a Dear Abby article but I’m just gonna roll with it),

I haven’t tried this so I haven’t experienced it (No big call for temporary heat in Florida) but it does make some sense. The GFCI is just watching for a difference in current on hot and neutral. When you add in a variable frequency drive (Which is essentially all an ECM or X13 motor is) the circuitry/housing is exposed to a lot of “induced” electromagnetic fields due to the varying frequency/harmonics. Some of this induction (Magnetic Flux) may be routed to ground causing a tiny imbalance. Here is a thread in my buddy Mike Holt’s forum that talks about this very phenomenon HERE

Also… On a furnace with a flame rectifier, a very small amount of current (microamps) is purposely routed through the flame in order to prove the flame. That has nothing to do with it being ECM though.

Thanks for participating. Great stuff!

— Bryan

P.S. – What do you think is going on?

In order to maintain combustion (burning) you need three things, fuel, heat and oxygen. If you have all three in the proper proportion you can maintain a continuous state of combustion.

Remove one (or reduce one sufficiently) and the triangle of combustion can collapse.

In a common NG gas furnace the heat is the igniter, the fuel is Natural Gas and the oxygen is provided by combustion air.

Combustion air is literally just the air needed to provide a continuous supply of air for proper combustion (burning). In the case of burning fuels like natural gas our goal is to achieve complete combustion where the end products being vented are CO2 and H2O and this requires the right mix of air and fuel.

For perfect combustion you need about a 10:1 ratio of air to fuel with safe levels of extra air or “excess air” putting us more into the 13.5:1 to 15:1 range.

All gas fired appliances must have both a flue / chimney to exhaust the leftover products of combustion (outlet) as well as combustion air to provide the oxygen for burning (inlet).

In high efficiency furnaces the combustion air is generally piped in, directly from the outside straight into the combustion chamber. This creates a dedicated source of oxygen and also a cleaner install as no other provisions need to be make for combustion air.

In 80% furnaces the burners usually have “open” combustion and they rely on air being drawn into louvers on the furnace cabinet. In this design the space on which the furnace resides must have open communication to the outdoors or other “uncontained” space.

The International Fuel Gas Code requires the following combustion air openings for a room containing combustion appliances:

Vertical opening – One-inch free area for each 4,000 Btu/hr. input of gas burning appliances in the room.

Horizontal duct opening – One-inch free area for each 2,000 Btu/hr. input of gas burning appliances in the room.

Mechanical fan – One CFM of air for each 2,400 Btu/hr. input of gas burning appliances in the room.

Indoor air –  50 cu. ft. of area for each 1,000 Btu/hr. of the appliances.

Not to get into the specifics of code becasue there are lots of specifics that you need to pursue beyond a tip like this, but you must have a dedicated method to get significant air to the furnace to ensure safe and complete combustion.

If you do not, the real possibility exists that the furnace could begin burning improperly creating an unsafe condition for the occupants due to Carbon-monoxide.

Different parts of the country provide combustion air in different ways, but you MUST have some method of providing unlimited fresh air to a furnace or to the room in which the furnace is located. This means when a furnace is in a tight space, ensure you have some sort of significant combustion air.

— Bryan


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

This article was written by HVAC / Furnace technician Benoît Mongeau. Thank you Ben.



High efficiency (or 90%, or condensing) furnaces use a set of two heat exchangers in order to retrieve more heat from the combustion products than their mid-efficiency counterparts.  Because of this, they generate flue gases much colder than those of a mid-efficiency or natural draft unit.  This not only completely changes the way the furnace has to be vented (I will talk about venting specifically in a later tip) but also, and it’s what we’ll focus on, a lot of condensates is generated.  This water comes from two sources:  moisture which was already present in the combustion air, and the combustion process itself, as the hydrogen atoms from the natural gas molecules (methane, CH4) combine with oxygen to form water. Now as technicians you don’t need to know this part but if you’re a bit into chemistry, here’s the basic chemical equation:


CH4 + 2 O2 + heat = CO2 + 2 H2O


This means that in perfect combustion, for every molecule of CO2 you produce, there are also 2 water molecules produced. This adds up to a lot of water vapor.


In order for the furnace to work properly, that condensation needs to be drained out or else it would accumulate inside the heat exchanger, inducer and venting, impeding proper gas/combustion product flow.  Most furnaces will have at least 2 internal drains, typically one for the heat exchanger and one for the vent, usually at the inducer outlet or on the inducer housing.


The secondary heat exchanger outlet is sealed inside a plastic part called the collector box, which is designed to collect the condensate and drain it out.


All condensate drains go into a trap.  The condensate trap is absolutely mandatory for a high-efficiency gas furnace.  Since the drain taps into the exhaust system, leaving it open to the air would allow for a potential exhaust/flue gas leak in the living space, which is a big no-no.  Additionally, the inducer motor would suck air through the drain if it weren’t trapped, which could affect combustion, and would prevent proper drainage.  Keep that in mind, because if you ever add an extra drain (off a tee on the venting, for example), you will need to TRAP it, always.


The only downside to the trap is potential for blockage.  The trap needs to be cleaned out regularly, and that should be done every maintenance.  Rinse it out, make sure water flows through the trap properly from all its ports.  If there’s any poor flow, fill it up and blow through it a few times to get the dirt out.  Hotter water helps for stubborn blockages.  The need for regular cleaning also means that drains should be installed as much as possible in a way that allows for the trap to be easily removed.  I highly recommend using clamped flexible hoses for the drain, as close as possible to the trap.  Avoid hard-piping the whole drain, as it will be impossible to remove and clean out the trap.


To ensure proper drainage, here are the proper practices:

-Make sure every component that produces condensate is sloped towards the drain.  That means slope the venting down towards the furnace (typically a ¼’’ slope per foot of length, minimum), and also, slope the furnace itself!  Look in your install manual, most manufacturers will call for the furnace to be installed with a slight forward pitch to allow condensate to drain from the heat exchanger.

-Slope the drain line itself, obviously.  Avoid double trapping and vent the drain after the trap to prevent airlocks

-Avoid running the drain in an area where it could freeze.  That includes running it under the natural fresh air inlet if there is one.  


Finally, note that furnace condensate is acidic, and some states/provinces/countries may require the condensate to be neutralized prior to draining.

— Ben

This article was written by boiler technician Justin Skinner. Big thanks to Justin for being one of the rare techs who cares enough to take the time to write something like this. Thank you Justin

Pressure switches are used in a variety of applications. Generally, they are a normally open switch that closes with either a rise or fall in the pressure it is monitoring, They are used mostly as a safety device, but are also used for operation control, such as fan cycle controls for low ambient cooling, starting and stopping for steam boilers, and the list goes on. Today I am focusing on the pressure switch used in gas furnaces that prove inducer fan operation.

As noted above, the pressure switches used for gas furnaces are almost always normally open.

The board first checks the switch to ensure it is open prior to inducer call. If it is closed prior to call it will go into fault because the switch is clearly jumpered, the circuit shorted or the switch failed closed.

During preignition, when the inducer motor is running before the flame is established, it draws the heat exchanger into a negative pressure (draft). The pressure switch is used to prove this draft by a connection, typically rubber or vinyl tubing, directly to the heat exchanger or to the inducer draft motor housing. In the normal sequence of operations, if the pressure switch does not close during this preignition period, the control board will not allow the furnace to light. This is to ensure a few things. First, that the inducer motor is operational and not failed, inducer wheel broken, etc. Second, it proves that the exhaust pathways are clear. If the chimney is caving in, or Mr squirrel makes a nest in the top of the flue stack, the inducer motor will not be able to establish the draft (negative Pressure) required to close the switch. The switch should stay closed during the entire run cycle also.

If the control board determines that the inducer motor has been running for an allotted amount of time and the pressure switch has not closed, it will lock out the furnace and go into a fault situation. It also locks out into fault if the switch opens enough times while the flame is lit. We will use Carrier as the example. When the switch does not close, or opens during operation, and the furnace locks out, the led light on the control board flash a fault code 31 which indicates the switch did not close. If the switch does not open after the furnace satisfies the heating demand tries to start again, the board will flash a code 23. Code 23 would be seen as 2 short flashes, followed by 3 long flashes.

So what’s the proper procedure for troubleshooting if you come across a furnace displaying these fault codes? Most of the time the issue is pretty apparent after you reset the board and run the furnace. The inducer motor is failed, or a family of possums used the chimney base as their summer home, or most uncommon of all, the pressure switch is bad. But what about the when it’s not so apparent? When you reset the board and the furnace lights and runs fine and customer runs down and tells you how great you are? More than likely if you leave it at that, that same customer who praised you will be calling in again, but probably won’t be as happy with your service skills as before. So let’s dig in and check a few more things.

In the thousands of no heat calls I have run, I can count the number of pressure switches that failed on their own with no other factors on one hand. They just don’t fail very often. I would need a lot more hands to count the callbacks I’ve run where another tech condemned and replaced a pressure switch, only to have the same issue not long after.

Callbacks cost money and they always hurt my pride when they were mine, so the goal is to save both. So the first thing to do in this situation is slow down. This time of year (December) crazy busy for everyone, but the most important call you will run today is the run you are on right now. When I am in a hurry, I’m much more likely to miss things, and when that happens I normally end up coming back to the call anyway.

Next, Grab your trusty volt/amp meter and manometer. I prefer the digital version for both. Check incoming voltage to the furnace and make sure it is within spec of the voltage rating on the data tag. Check the amperage rating on the inducer motor, and make sure the actual amperage is in range. Let the inducer motor run for a while. If the amperage starts to go up substantially, or the motor gets noisy or too hot to touch, there is potentially a motor issue. Check the tubing that connects the pressure switch to the draft point. Look for cracks, brittleness, and crud or water inside the tubes.

If you see anything, clean it out. Hook a manometer to the tubing and check the draft from the heat exchanger in inches W.C. with a manometer or Magnahelic. The pressure switch has a pressure at which it closes, which is typically negative. Ensure the draft is above the required pressure. By this point on an 80% efficiency furnace, if you haven’t found the culprit, you will have to put on your best thinking cap, because the issue could be any number of things, including a bad board, loose or bad wire connections, or a bad pressure switch.

If you are working on a 90+% (condensing) furnace, move to the condensate drain. These furnaces produce water as a result of lower exhaust temperatures, and that water is removed from the furnace in a few ways. Carrier has a condensate trap that is a white box that mounts in the blower compartment on an upflow application. If this trap is partially clogged, water can back up in the secondary heat exchanger. This can prevent the necessary air movement required to produce enough draft to close the pressure switch. And if it’s only partially clogged, it may have drained when the furnace was locked out and not running. So when you get there and reset it, everything runs fine until the condensate starts to back up again. Also, where the condensate water goes after it leaves the furnace is important to note. Does it go to a condensate pump? Does that pump the water outside? Is it freezing outside? It could have been last night, which caused water to back up and the furnace to lockout, but today its 40 degrees, everything has melted, and running smooth.

These are just a few things I’ve seen with consistency over the years, but the potential number of causes for the fault codes listed above are almost limitless. And every heating season I run into an issue I haven’t seen before. This is in no way a comprehensive checklist. The goal of this is to prevent a technician from replacing too many parts that don’t need to be replaced, which I did early on more times than Id like to admit. Have a safe and merry heating season, and like we all hear from time to time, don’t be a hack!

— Justin Skinner


Does heating the air cause the humidity in the air to decrease? Yes and no

Heating air causes the RELATIVE humidity percentage to decrease but it does not change the overall moisture content in grains of moisture per lb of air.

Many old timers will swear a blue blaze that oversizing a furnace will directly result in lower humidity, cracking furniture etc…

The problem with that theory is that no matter how much you heat the the air you don’t change the overall moisture content and when you blow that air into the space it quickly acclimates with the room.

But there still may be some truth in this oversizing dries stuff out theory

In the Winter during cold climates the moisture content is very low outside regardless of the relative humidity. When you use a larger furnace than you need you also tend to move more air than you need to.

When you move more air there is often greater negative or positive pressurization of the conditioned space due to zonal imbalance and duct leakage. This pressure imbalance will tend to drive more dry air into the space or more of the inside air out resulting in lower humidity.

Neil Comparetto also pointed out that when the appliance takes its combustion air from the space this can cause significant negative pressures which also draws dry air in from outside. The larger the BTU output the greater volume of air that must come in for combustion.

The other factor is the supply air temperature itself. If the hotter supply air is blowing directly on an object it will tend to dry it out more quickly due to the increased temperature of the object itself.

In conclusion –

Furnaces don’t reduce air moisture quantity directly no matter how big or small

There are other reasons why oversizing can cause issues so don’t do it.

— Bryan

Every gas furnace data plate/tag has a specification for the temperature rise through that furnace.  It is shown in a range like 50-80° or 45-75 °. Those two numbers are the lowest recommended temperature rise and the highest allowed temperature rise through the furnace this is usually a 30° difference.  When possible we want to get the rise toward the center of that range through a combination of proper commissioning practices, primarily setting the airflow properly in heat.

Air temperature rise is the difference between the temperature of the return and the heated supply air from the furnace.  Temperature rise must be measured during installation and must be within the range on the furnace rating plate.  This is important not only for the longevity of the furnace but also for customer comfort.

So, what does temperature rise mean and how do we calculate it?  First of all, we need to verify that the furnace is firing on its full output (high fire) and that the manifold pressure is correct.  Then we need to let the appliance run for at least 15 to 20 minutes so that everything has a chance to acclimate.  You then take the temperature of the return air into the furnace and then take the air temperature of the supply air leaving the furnace.  The supply air temperature should be taken several feet away from the furnace so it isn’t affected by radiant heat from the heat exchanger.  You then subtract the return air temperature from the supply air temperature to get the air temperature rise through the furnace.

So what can we learn from this number? If you are too close to the low-end rating number with your rise, the air coming off the furnace is going to feel cool to the occupants and you may have complaints of “drafts” or not feeling warm.  If you are BELOW the low-end rise, you could even start to form condensation in the heat exchanger could shorten the life of it due to corrosion.  Low rise means you are moving too much air over the heat exchanger, this can (usually) be corrected by slowing the blower down.

More important is the high end of the rated range.  When we are near or over the high-end number, the furnace may begin cycling on the limit control.  the limit is a safety and not designed to shut the furnace on and off regularly and will eventually fail.  This is also a sign that you are over-heating the furnace and will cause problems with the heat exchanger and well as other components.  When the rise is too high you are not moving enough air over the heat exchanger.  This can (and must) be corrected with a blower adjustment or by resolving the source of low air flow in the system such as dirty filters, blower wheel etc….

Like usual, when you see signs of low air flow like high-temperature rise, look for the obvious maintenance related issues first.

— Bryan

In Florida, there are not many gas furnaces, At least not as many as up North. Sometimes we can look like real dummies compared to techs who work on them everyday.

One thing to know about 80% gas furnaces with cased evaporator coils is that you can often check the evaporator coil by removing the high limit and running an inspection camera up through the opening.

You may also be able to use a mirror and flashlight but you usually won’t see much due to the heat exchanger being in the way. Otherwise, you are stuck removing the entire blower assembly… and that’s no fun at all.

Another practice is benchmarking the static pressure drop across a new coil when it is dry and wet when installed or during the first service call. You can then easily watch coil loading over time without the need to look at the coil visually.

— Bryan

All fuel-burning appliances require oxygen to burn and sufficient oxygen to burn clean and safe, without soot and CO (Carbon Monoxide).

I live and work in Florida where most of our fuel-burning appliances are 80% efficient with open combustion that utilizes air and oxygen from the space for combustion.

With these low-efficiency appliances whether the appliance is forced vented or natural draft that combustion air is leaving the space, and exiting the flue.

This causes negative pressure that must be allowed to equalize as well as consumes oxygen from the space. It is because of this that these open combustion appliances must either be in a sufficiently large space or communicate with (be open to) a larger space or outdoors.

When you consider that other gas appliances also need to use oxygen and need to vent to outside you can see that without sufficient communication to outdoors that negative drafts can occur on natural draft appliances like water heaters.

This is why all open combustion appliances that utilize combustion air from inside the space must be in an “unconfined space” or connected to an unconfined space or the outdoors using an approved method.

I see many furnaces jammed into tight closets and mechanical rooms with little thought or planning regarding combustion air.

According to NFPA 31, 54 & 58 an unconfined space is a space that has at least 50 cubic feet of open area for every 1,000 Btu of input. This means that a 100,000 Btu furnace must be in a 5,000 cubic ft space to be considered unconfined.

If the appliance is not unconfined then additional combustion air must be made available to the space with one opening at the ceiling level and one near the floor.

If the air is coming from another unconfined space then the openings should be at least 1 square inch per 1,000 BTU and 1 square inch per 5,000 BTU if it is connected to the outdoors.

While these openings and are needed in many cases to allow for proper combustion and venting it helps illustrate why modern sealed combustion “direct vent” appliances that take all of their combustion air from outdoors make so much sense.

Not only are direct vent appliances more efficient on the fuel utilization side, they also prevent the negative home pressures and/or thermal losses associated with having vents in walls and ceilings.

So either make sure you have an unconfined space, you are bringing air in from an unconfined space or outdoors or you have a direct vented appliance.

— Bryan

I’ve heard a lot made of clocking gas meters over the years and honestly, in Florida there isn’t too much call for for heat and even fewer furnaces.

I was pleasantly surprised when I found out how easy it actually is. Here is how you do it, step by step.

#1 – Make sure all gas appliances are off other that the one you are clocking. Even shut off pilot lights or it can mess with your reading.
#2 – Make sure the appliance you are checking is running at high fire (max output)
#3 – Get a stopwatch (your phone has one)
#4 – Watch the smallest unit dial on the gas meter, it will often be 1/2 cubic ft
#5 – Time how long that dial takes to make one full revolution with the stopwatch
#6 – Multiply the dial size by 3600 (3600 is the # of seconds in an hr) so if it’s a 1/2 cu/ft dial it would be 1,800
#7 – divide that # by the # of seconds it took. So lets say it took 22 seconds that would be 1,800 / 22 = 81.82
#8 – Multiply that # by the BTU heat content of 1 Cu/Ft of gas provided by the utility. If it is 1,000 (which is common for NG) the total BTU per hr would be 81,820

The complete formula is Cubic Feet per Hour (CFH) = (3600 x Dial Size) / Time (seconds)

Then to get the ACTUAL device output in BTU’s you would multiply for the AFUE efficiency. In this case if it was an 80% furnace the input is 81,820 btu/hr and the output would be 65,456

Pretty cool huh?


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