Category Archives: Heaters

cfm Tech Tips – Operating and Troubleshooting Furnace Pressure Switches

Beginning January 1, 1992, all furnaces were manufactured with an integrated furnace control board, often referred to as FCB (Furnace Control Board). The FCB manages all operation requirements of the furnace. Fault and Information codes are displayed by one or more LEDs on the board. The FCBs used by York/JCI have the ability to store and retain the last 5 codes in ‘non-volatile memory’ – meaning that loss of power to the control doesn’t erase the stored codes – a very helpful tool when troubleshooting furnaces. This month’s topic is Pressure Switch Operation and Troubleshooting.

Pressure Switch Fault Codes:

2 Red Flashes – Pressure Switch is closed when it should be open

3 Red Flashes – Pressure Switch is open when it should be closed

6 Red Flashes – Pressure Switch has opened 4 times during a heating cycle

All furnaces now utilize ‘induced draft combustion’, meaning that every furnace has an inducer assembly to draw combustion air through the heat exchanger. To ensure that the inducer is operating, and moving the correct amount of air, a pressure switch is selected, for each furnace size, by the design engineers to monitor inducer operation. Pressure switches typically have the Pressure Setting (Pressure Measured in Inches of Water Column) stamped on the body of the switch. The Switch Setting is the predetermined point that the switch must open if combustion air falls too low.

Pressure switches have a ‘closing pressure’ – not listed on the switch body, but equal to about 0.15” greater than the pressure stated on the switch. This means that a pressure switch rated at 0.80” will not close until approximately 0.95” of pressure is applied. Once closed, it will remain closed until pressure falls below 0.80”. This is known as Hysteresis or Differential. Pressure switches also have a ‘tolerance’ rating of approximately 10%, so the 0.80” switch could open at 0.72” or 0.88” and be considered a correctly operating switch.

Furnace Sequence of Operation related to Pressure Switch Operation:

  1. Call for heat from the thermostat.
  2. FCB checks the Pressure Switch to make sure it’s open.
    1. If not, the FCB immediately stops the sequence of operation and flashes a Red Code 2.
  3. FCB starts the inducer.
  4. FCB checks the Pressure Switch to make sure it closed.
    1. If not, the FCB immediately stops the sequence of operation and flashes a Red Code 3.
  5. FCB continues the sequence of operation through the current heating cycle while monitoring the Pressure Switch, making sure it remains closed.
  6. If the Pressure Switch opens during the heating cycle, the heating cycle is interrupted and a Red Code 3 is flashed until the FCB repeats the above checks and restarts burner operation.
  7. If the Pressure Switch opens 4 times during a heating cycle, the FCB shuts down the burners and flashes a Red Code 6.

If an FCB is flashing a Pressure Switch fault code, it doesn’t mean that the pressure switch has failed. It simply means that the pressure switch is giving us valuable troubleshooting information. Don’t replace the pressure switch until the troubleshooting procedures indicate a failed switch. Troubleshooting a pressure switch will require two accurate test instruments that allow you to see what the switch is seeing.

  • A VOM (Volt, Ohm, Milliammeter) to check voltage and/or continuity across the switch contacts.
  • An accurate method of measuring inches of water column pressure in ‘tenths of an inch’. See examples below.
    • Digital Manometer
      • Available at a reasonable cost. Every Technician should own one.
    • Magnehelic Gauge
    • Incline Manometer

IE: You can’t troubleshoot a pressure switch problem without –

  • A tool to see the pressure that the switch is seeing
  • A tool to see the voltage/current that the switch is seeing.

Because pressure switches must monitor locations of extra high humidity – such as the condensate pan area of a condensing furnace – there is a risk of condensation collecting in the body of the pressure switch. To keep this from happening, a ‘bleed hole’ is designed into the body of the switch to allow dry air to bleed through the hole and travel through the connecting tube of the switch, keeping the switch body and tube dry. This means that if pressure is applied to the switch and then the connection is sealed off – such as pinching off the tube – because of the bleed hole, the switch will not remain closed. This is normal switch operation and doesn’t indicate a failed switch. So, if there is condensation in the body of the switch, it could indicate a plugged or missing bleed port. Because condensation in contact with the diaphragm of the pressure switch assembly nearly always results in incorrect operation, any switch with water in it should be replaced.

When troubleshooting Pressure Switches, it’s important to determine the pressure being applied to the switch during the time it is monitoring inducer operation. Because of the bleed hole in the switch body, a Technician must be able to connect his manometer into the switch tubing using a tee. If the manometer is inserted in place of the pressure switch – without the tee – it may lead a Technician astray.

Example: A pressure switch is monitoring pressure applied to a furnace condensate pan. There is corrosion in the connection of the condensate pan restricting the flow of air through it. The hole in the pan connection – because of the corrosion – is now smaller than the bleed hole of the pressure switch, resulting in a Red Code 3. Pressure inside the condensate pan is correct, but because of the corrosion in

the pan/tube connection and the bleed hole in the switch, when the switch is in place not enough pressure gets applied to it (more air is entering the bleed hole than can pass through the restricted port in the condensate pan), but when the manometer is applied directly to the condensate pan – without the bleed hole in play – pressure appears to be correct. This is not the fault of the switch – it is the fault of the corrosion in the condensate pan connection. A problem like this can only be diagnosed with an accurate manometer and would be missed if the tee (keeping the switch in the tube circuit) was not utilized.

Troubleshooting steps to resolve Fault 2.

  • Is the inducer operating?
  • Yes – If the inducer relay contacts are stuck closed, keeping the inducer on when it should be off, the pressure switch will be closed when it should be open.
  • Troubleshoot the FCB and/or inducer relay to determine why the inducer is running when it should not be.
    • No – If the inducer is not running, the pressure switch contacts must be checked with an ohmmeter to determine if they are stuck closed.
        • If so, it is a failed switch and must be replaced.
    • If not, troubleshoot the pressure switch wiring to determine if there is a short circuit, leading the FCB to believe the pressure switch is closed when it really isn’t.

Troubleshooting steps to resolve Fault 3.

  • Is the inducer operating?
    • No – Troubleshoot the inducer and related components to determine why it’s not operating.
    • Yes – Insert a tee into the pressure switch tubing and use a manometer to determine if pressure applied to the switch is greater than switch setpoint + differential.
  • If so, the switch must be replaced.
    • If not, troubleshoot the vent system and/or inducer to determine why pressure isn’t sufficient to make the switch.
        • Remember the example above and check the tubing for cracks and tubing connection points for restrictions.
        • Obstructions commonly found in vent systems – Mud Dauber material, bird nest material, children’s toys, condensate buildup (caused by pipe sags), etc.

Important Note – As the gas/air mixture burns and the hotter the vent gasses get, the flue gas density is reduced, resulting in reduced ability of the inducer fan to build pressure. If a pressure switch is operating close to its setpoint, it could open simply as a consequence of the warm flue gasses. The vent pressure of any furnace should be well above the switch setpoint, so if the pressure switch is cycling per the above information, investigate the vent system and/or inducer assembly to ensure adequate vent pressure before combustion occurs. If this occurs on an 80% furnace, remove the inducer assembly and check for debris in the inducer housing.

Troubleshooting to resolve Fault 6.

  1. Condensing Furnaces (90% to 98% efficient furnaces)
    • Check for low spots in the vent system that could be trapping condensate water. Water must flow back towards the furnace while the inducer is blowing air against water flow. This requires a minimum of ¼” slope per foot of pipe to ensure correct draining of the water in the vent.
    • If the vent pipe exits horizontally from the furnace cabinet; check for downward slope of the vent section between the inducer outlet and the first elbow. It is critical that all horizontal runs of the vent slope the necessary ¼” per foot back towards the furnace.
    • Check for debris in the condensate trap and line. A restricted condensate system can result in condensate buildup in the inducer assembly, resulting in pressure switch tripping during a long heating cycle.
        • Particulate matter (ash material from burned bugs, dust and other airborne matter) can build up in the condensate trap of a condensing furnace resulting in slow flow of condensate.
        • Condensing Furnaces now have a ‘blocked condensate’ pressure switch wired in series with the operating pressure switch. Divide and Conquer to determine which switch is tripping.
    • Check for double-trapping.
        • The built-in trap of the condensing furnace expects an open, sloping drain pipe to dispose of condensate water. Flexible tubing that is not lying flat on the floor can result in several traps in the line and cause condensate to back up into the furnace. All condensing furnaces should have a vent tee immediately outside the furnace to eliminate the air-locking that can occur with multiple traps.
    • Check for debris in the vent system.
        • Obstructions commonly found in vent systems – Mud Dauber material, bird nest material, children’s toys, etc.
  2. 80% Furnaces
  • Nearly all Fault 6 problems can be traced to debris in the inducer housing below the inducer wheel, or a restricted vent system.
    • Temporarily remove the vent from the furnace. Operate the furnace to observe if the pressure switch opens without the vent system in place.
        • If furnace works correctly, investigate the vent system.
        • If pressure switch cycles without vent attached, investigate the inducer assembly.
          • Items often found in the inducer housing – Bird Nest material, Mud Dauber material, sheet metal screws, etc.

Note: Debris in the inducer housing of 80% furnaces often results in a Code 7 or Code 8, not a Code 6 which can mislead a technician. This part will be covered in next month’s Tech Tips Letter.

Coming next month: Fault Codes 7, and 8 – Failure to prove flame, loss of flame after proof.

York’s Modulating Heat Pumps – The NEXT Thing for Ground Source Contractors! What?

What do YORK’s new Modulating Heat Pumps have to do with Ground Source? Quite a bit, it turns out. Here’s why – Ground Source Heat Pumps were successfully sold in areas where relatively inexpensive natural gas was unavailable and the only alternative was conventional air source heat pumps utilizing electric strip heat as the supplemental source of heat. Continue reading

Ductless, What is it Good For?

There are only two types of people in this world: Mac or PC users, MU or KU fans, people you drink with or people that make you want to drink, and those that do or do not consider hot dogs a sandwich. In recent years, I have found there are two types of HVAC contractors – those that do not use ductless and those that do.

To those of you that do not use ductless, I understand. When it sounds too good to be true we are all skeptics and hesitate to try something new, the HVAC industry is notorious for being slow adapters. You know what works, you have been installing the same equipment for years and we only continue to refine and improve it. So why use ductless?


Humans are creatures of comfort. We are usually more productive when comfortable. Ductless options can allow for increased comfort by offering zoned control, continuous operation by modulating capacity to the load requirements, and dehumidification from extended run times. For instance, the Daikin FTXS/RXS24LVJU can modulate down to 32.5% of its nominal capacity, or 7,800 BTUs!


If you prefer the racket of a window unit or PTAC in your room, ductless is not for you. If you need a quiet solution for a light sleeper or easily distracted office worker – ductless is your answer. I have had a contractor call and ask me if the outdoor unit was working because he couldn’t hear the compressor or condenser fan running outside (it was, by the time he got inside to check the indoor unit it had already lowered the room temp by two degrees). For reference, the Daikin wall mounted FTXS09LVJU has an output of 22 dB(A) at SL fan speed – 20 dB(A) is the equivalent to whispering at 5 feet away.


I have been on a roof in the cold and snow to startup HVAC equipment – it is not an easy process (for a salesman like me – I can only tell you how it should theoretically work ?). The best way to describe ductless was summed up by my service manager, plug & play. Installing a ductless system requires running power, a line set, and communication wire. You would only require two hands so you could hold a beer with one while turning it on with the other.

The only downside of this equipment that I can see is that it might run forever and not need to be replaced (I’m honestly not sure why we stock parts for them, they don’t break down)!


Now to the second group that already uses ductless – why should you use cfm? As a stocking distributor, we are your one-stop-shop to get everything you need for a proper installation. Can’t translate an installation manual from Japanese or Mandarin? No need, we employee three full time service managers that can! (Not really, but they can assist with any questions that you might have). Ductless systems are typically provided with HP operation, so the outdoor unit should be installed on padded risers or a wall bracket to keep it above any snowfall or condensate accumulation. Aesthetically, owners usually don’t want to see the line sets outside, so you can grab a Fortress line hide kit on your way out. And in case you need to brush up on making your own flare connections, just ask our full-service parts desk for a demonstration!

How can ductless simplify YOUR life? Why do YOU use ductless? The more you know.

Energy Analysis Case Study – Unit Heaters vs Tube Heaters – Part II

Earlier this spring we wrote a blog post discussing why you might choose unit heaters or tube heaters for your heating only project. The article detailed how each type of system worked, which applications were ideal for each style, and which applications were not ideal. But it was missing something. Where was the real world example? How do we actually determine which heater style is best not just in theory, but with numbers to back it up? Great questions! Let’s look at a real world example now.

A couple weeks ago we received a call from one of our really good customers who asked for some product selection assistance on a project he was working on; a 96′ by 78′ workshop with a multi-tiered roof line. We were asked to do a load calculation and help the contractor select the best type of system to heat the space based on the customer needs. Here’s how it all went down.

We met with the customer on the first cold morning of the year. It was about 40 degrees outside with a 20 MPH north wind, and the day before was 77. The regret for not meeting 24 hours earlier was sky high, but since you really don’t care we will keep moving on.

This is a large, 3 section barn under construction with most of the steel frame still exposed.

This barn will be the project site for our energy analysis.

The owner was not on site when we arrived, so we started to assess the half constructed building so we could get an idea on heat load. The structure was a butler style building with a metal shell, and the walls and ceiling had R-13 plastic wrapped insulation. The long side walls were 10′ tall, and as you moved to the center there was a pair of 18′ peaks, and then a 26′ peak in the center. The long north wall had six 10′ x 10′ overhead doors, and the east and west side each had one 15′ x 12′ and two 12′ x 10′ overhead doors.

The owner finally arrived as we started to put on our thinking caps. After doing hundreds of load calcs similar to this building, we estimated 45 btuh per square foot which came out to about 335,000 btuh of heat load. But before coming up with a solution, we better ask the owner what he wants. After all, it’s his building. Who cares what I think he should do.

cfm: “So what are you looking to do with your space?”
Owner: “Just want to keep it from freezing. We might have a gathering in here every once in a while, but we aren’t that particular on keeping it really warm”
cfm: “What temperature do you want to maintain on the coldest day of the year?”
Owner: (after a long drawn out story about all the examples he might run into) “There might be times we would like it to be 60 degrees”
cfm: What type of power do you have?
Owner: “We don’t have natural gas. We are bringing electricity over here next week”
cfm: “Would you be interested in using propane? It will save you a ton of money on utilities”
Owner: “Yes”

The conversation continued, and we got into what type of equipment he wanted. The owner had done some research and he was interested in tube heaters, although he wasn’t totally convinced yet. We all agreed with tube heaters (for now) and came up with a good product selection based on the 335,000 btuh load, and then mentioned it was subject to running the numbers. We of course checked our clearances based on the tables in the installation manual to make sure we weren’t going to be too close to any combustibles. We also discussed how the heat distribution patterns would work, with most of the heat coming from the burner boxes and then a decreasing intensity the further away you went. So far the owner liked what he was hearing.

Load Calculation & Energy Analysis Results

So here is the really fun part- now that we have all the information, what is the best solution for this customer? Infrared like he asked for? Or are unit heaters a better option? We certainly don’t want to try to “sell him” unit heaters when he asked for tube heaters, but one or two of you (being a little facetious here) may have an experience in the past with an owner “wanting” something, but then changing their mind once they see the price. So how do we quantify the right solution? We run a cost analysis showing payback and return on investment (ROI).

Here are screen shots from the load calculation and the energy analysis we ran. You might notice that the load ended up being about 315,000 btuh, close enough to the estimated 335,000 load from earlier that we did not make any selection changes.

Load calculation assumptions and building sensible loads:

Image showing the results of a load calculation for determining heat load.

Load calculation assumptions and building sensible loads.

Load calculation net loads:

This is a chart showing the load calculation net loads for the case study site.

Load Calculation Net Loads

Energy payback analysis:

This is a visual comparison between the two heating options and their cost of ownership.

*The price of propane we used was $1.75 per gallon
**Owner equipment cost is estimated and does NOT include installation labor, gas or flue piping, electrical, thermostat wire or hanging hardware for the unit heaters.

Analysis Results:

The tricky part about running an energy analysis on infrared is there are no published efficiencies. So what do we use? Well, we know the flue gases don’t condense, so our combustion efficiency can be no more than about 82 or 83%. But one thing we do know is infrared keeps the heat down low to the floor, which will reduce the heat load on the roof (our delta T between the ceiling and ambient air will be far less with tube heaters which will reduce overall load). And our air changes will also be reduced, especially since the ceiling is 26’ tall in the center. With forced air, the ceiling air will be warmer and it will want to leave the building much faster than the radiant solution. So to account for both of these factors, we reduced the roof load and the air changes per hour to come up with an effective “radiant heat” heat load of 270,000 btuhs.

Chart showing the energy analysis results for payback and ROI.

In this particular case, based on our results we can see that installing tube heaters would be the best option.

The moral of the story will be different for every job. But for this project, it probably makes sense for the owner to proceed with tube heaters. A 3.7 year payback is pretty good, and it’s hard to argue with a 26.7% ROI. You won’t find that kind of return in the stock market, unless your name is Warren Buffett. And in that case you’re probably not reading this article.

What has been your experience with tube heaters and their energy efficiency compared to 80% unit heaters? Do you have any jobs that we can help you look at? Just ask us in the comments below, or give us a call toll-free @ 1-800-322-9675

Blue image with text which reads the title of the blog article

Tube Heaters or Unit Heaters? Let the Debate Begin

This is one of the most common questions we get during the fall and winter seasons. Owners want to know what type of heating equipment they should use for their commercial warehouses and buildings, especially in their heating only applications. And for good reason; it is a tricky question. Before we get started, keep in mind that tube heaters and unit heaters aren’t the ONLY solutions. The application might be better served with makeup air units, air rotation units, packaged rooftop units, etc. But the majority of the owners are deciding between these two solutions, so for now let’s focus here. So, should you install tube heaters or unit heaters in your space? Well…? It depends.

The short answer is, if you have a lot of air infiltration from dock doors, or have a partially outdoor space (like we do at the cfm Distributors dock), tube heaters are typically a much better solution. Why? It is all in how the tube heater is engineered to deliver heat to the space. Infrared tube heaters consist of two main sections, the burner box and, typically, 4″ round metal tubes that are anywhere from roughly 20′-80′ long with polished reflectors. The burner box delivers hot gas to the tubes, making the tubes extremely hot, and the heat radiates from the tubes to the space. That radiation heat warms everything in the space, except the air. It heats the floor, it heats the walls, it heats the people and everything else in the space. So even if you have a rush of cold air coming in, it still feels warm with the radiation hitting you.

We experience this same exact radiation every day from the sun. Imagine it is a brisk 45 degrees outside, which would normally feel chilly, but if the sun is out and shinning on you, you might actually be warm. Or, when you are driving in your car and it is cold outside. You probably have a jacket on and the heater turned up; but, if the sun is out, you would start to get hot after a while. These are both examples of how infrared tube heaters work.

Shop Reznor Unit & Tube Heaters now on the cfm eStorefront @

Shop Reznor Tube & Unit Heaters now on the cfm eStorefront @

Tube Heater Layout Design

This is a lot easier than you might think. There are just a few things to keep in mind when designing a tube heater job. Let’s say the application you are working on is a 10,000 sq/ft warehouse, with 20 dock doors. This process of designing a tube heater job can be broken down into two steps:

  1. Load Calculation
  2. The first thing to do is a load calculation; which, if you need help with this, please call your territory manager for assistance. We would love to help! Remember too, it is important to know when you are running your load calculation if you have decided on tube heaters or unit heaters. Why? Because with tube heaters you can maintain a lower space temperature, and the “feels like” temperature will be the same as forced air. For example, with forced air equipment you might need the space to be at 70 degrees to be comfortable. But, if you are using tube heaters, a very conservative space design temp might be 65 (or a couple of degrees less). Since your space temperature design is lower, you now have a smaller delta T with the outside; therefore, your heat load is lower.
  3. Tube Heater Placement
  4. So after running your heat load you come up with 480,000 btuh required. Now we need to find the efficiency of the tube heater you are using and divide the capacity (btuhs) required by that number. For example- if the tube heater is 80% AFUE, you take 480,000 divided by 0.80 and you get 600,000 btuhs of input required. Now it is time to design the layout. How many tube heaters will you need? And where do you install them? In most cases, as long as you get the btuh’s to the space, and the heaters are evenly spaced, you will not have any issues. But here are a few things to consider when designing the layout:
    • Most of the heat comes out near the burner box.
    • The best way to visualize the heat distribution map of a tube heater with a straight tube, is to picture a Christmas tree laying on its side, and assuming the base of the tree is the burner box. You get a lot of heat at one end, and you slowly lose heat by the end of the tubes. Use this to your advantage by placing burner box ends near dock doors, outside walls, or the largest infiltration areas. If you need a more even distribution throughout the length of the tube, most manufactures have “U-Bends” that you can install. Adding a U bend will make the tube section into the shape of a U, and can even out your heat distribution.
    • Watch out for clearances
    • Check the installation manual for these distances before quoting your project. In certain capacities and tube lengths you might need 7-8′ of clearance to combustibles. Sometimes tilting the tube heater can reduce this clearance by a few feet, but double check the manual first!
    • Mounting height is critical
    • Since tube heat is radiative heat, it follows the inverse square law. When you double the height, you get 1/4 of the heat. So mounting height becomes a delicate balance between maintaining minimum clearances and being cognizant of losing effective heat when mounted too high.

But enough about tube heaters. When should you use forced air unit heaters?

When to Use Forced Air Unit Heaters

  • Minimal air infiltration applications
  • If your space will not have a lot of air infiltration, and you are looking for a more comfortable, even temperature distribution for creature comfort, unit heaters may be the best way to go. Since hot air rises, if you have really tall ceilings and you decide to go with unit heaters, consider adding some ceiling fans to keep the hot air near the floor.
  • Larger and more capacity options
  • You can also find unit heaters in much bigger sizes. Typically, the biggest tube heater you can find is 200,000 btuhs input, while standard unit heaters are available up to 400,000 btuh input. So on a 400,000 btuh application, there is a chance you could use one forced air unit heater, as opposed to two tube heaters. This will cut down on installation cost- less electrical, gas piping, roof penetrations etc.
  • Lower upfront cost
  • Upfront cost is another reason you might choose unit heaters over tube heaters. Unit heaters on average can give you up to DOUBLE the heat per dollar you spend on the equipment. So if the owner is on a very tight (upfront) budget, unit heaters will typically cost less to install. However, keep in mind that if the space is better suited for tube heaters, the unit heater option will more than likely cost more in utilities, and the owner might have to spend time warding off comfort complaints.

When it comes time to choose between tube heaters and unit heaters, the answer may not be as easy as you think. But that is why we are here, to help you make these challenging application decisions. Are you working on a project that we can help with today? Give us at cfm a call @ 1-800-322-9675, post a comment below, or message us on Facebook or Twitter @cfmdistributors. We’re always happy to assist our customers in project planning.