Tag: furnaces


Proving flame is an important part of the gas firing sequence. Without proof of flame you risk dumping unspent gas into the heat exchanger resulting in an explosion.

There are many ways to “prove flame” we are focusing on the flame sensing rod method here.

Here are the facts-

Flame sensing rods, also know as flame rectifier rods or flame rectification rods are commonplace in modern hot surface and ISI (intermittent spark ignition) gas fired appliances.

Flame sensing rods stick out into the flame and connect back to the furnace board. Once the board sends a call to the gas valve to open, it monitors current flow on the flame sensing rod. It does this by generating a potential (voltage) at the flame sensing terminal, this terminal is connected to the sensor with a conductor. When no flame is present there will be potential at the rod and no current, when a flame is present a small microamp DC current will be present as a path is made between the rod and the ions in the flame. This small DC current signals the board that flame exists and all is well with the world. If it does not sense this microamp DC current within a few seconds it will shut off the gas valve and try again.

The board outputs this potential (voltage) on the  flame sensing terminal right at the beginning of the sequence to confirm that the path is “open” with no flame. This ensures against false positives (sensing flame / current when there should be none) andonce it goes from 0 current to the rated micoamp current the board “knows” that flame is present.

These flame sensing rods are “dumb” devices. They do not generate potential (volts) or current (amps), their predecessor the thermocouple (seen in standing pilot systems) does generate a potential itself which is often the source of the confusion.

A flame sensing rod is a piece of metal with a ceramic insulator that keeps it from grounding out. That is all. However because it is conducting in the Millionths of an amp (microamp) a lot can go wrong with it that a normal electrical component wouldn’t have any issue with. Tolerances are tight so small factors make a big difference.

Flame sensors fail when:

  1. They short out due to a cracked insulator
  2. They Fail open because they are broken
  3. They don’t conduct because they are not properly placed in the flame
  4. They become coated in silica (glass) or carbon

Before I go any further I want to address a common question. Do flame sensors have a special coating that can be rubbed off with improper cleaning?

Well… If we are talking about a thermocouple or a thermopile then yes.. absolutely, but we aren’t discussing standing pilot systems here.

I have seen a lot of flame sensing rods, and I have done a good deal of research and I have found no evidence that most flame sensing rods have a special coating on them that can be rubbed off. Now, if you have real, quantifiable proof  from an manufacturer that says otherwise.. PLEASE provide it to me so I can retract this statement.

Here are the steps to test a flame sensor –

  • Ensure the furnace is properly grounded. You can do this by powering down the heater and taking an ohm reading between neutral and the burner assembly. You should read a few ohms of resistance max, the lower the ohm reading the better grounded it is.
  • Make sure your polarity is correct, incoming hot connected to hot, neutral to neutral.
  • Ensure the rod is positioned so it will be covered in flame
  • Get a meter that reads in the microamp scale with a .10 resolution minimum. Use a good QUALITY meter for this and make sure your leads are in the correct locations.
  • Connect your leads in SERIES. This means you have to disconnect lead from the rod, connect one lead to the rod and the other to the terminal to the board WITH THE CONNECTOR UNHOOKED FROM THE ROD
  • When the flame lights you should read between .5 and 10 microamps depending of the furnace. Readings between 2 and 6 are common.


If you do not have a proper microamp reading you can confirm the following

  • That the flame rod is not open. Ohm from tip to terminal on the rod. If the rod is open it is failed.
  • Check the insulator and make sure it isn’t cracked or grounded
  • Check for proper burner grounding and incoming power polarity (as mentioned)
  • Clean the rod… Now this is a controversial one. I suggest using a very fine steel wool or abrasive pad (magic erasers often work). remove and clean the rod and ensure you wipe it clean of any particles left over from cleaning. Handle very gently. Once complete perform an ohm test from tip to terminal again to ensure you haven’t damaged during cleaning. If you want to be real crazy, use some electrical contact cleaner on it after cleaning to help remove any residue… just nowhere near flame, unless you don’t want eyebrows.

Once you have established all of the above and you are still not getting the required microamps then you are left replacing the board.

Word of warning –

Test your tools regularly. If you are trusting your meter and you aren’t 100% sure your meter is working and set up properly you may end up with a misdiagnosis. Test and calibrate your tools regularly.

Do every possible test before replacing a board. Many techs advocate just replacing a flame senor if they suspect it isn’t conducting well. I am cool with that so long as

  1. You don’t charge the customer for it is there was nothing wrong with it
  2. You company is OK eating the cost of rods that were not needed

Or.. you just install a new one long enough to test. That is all fine and good if you have extra flame rods in your truck. Many techs do not have that luxury.


If flame rods are getting dirty / coated often, you will want to find out why. There is something in the environment or the combustion that is causing it.

In Summary flame rods should be

  1. In the flame
  2. Clean
  3. Not open
  4. Not shorted

Now is the part where the furnace techs from all over the world tear me apart.

— Bryan

I received an email from a podcast listener with some furnace related questions. Based on the nature of the questions I figured it would be better to ask an experienced furnace tech. Benoît Mongeau agreed to help by answering the questions. 

My name is Matt and I am a newer tech (fully licensed this September, have been doing the work for 2ish years) who lives in Northern Ontario, Canada. I really enjoy the HVACR school podcast. I don’t do any A/C stuff but I still enjoy listening and wrapping my brain around it. I have always struggled with the theory behind getting cold from hot. The bulk of my work is residential gas heating, mainly high-efficiency furnaces, and gas fireplaces. My questions for you are, (these are just ideas for your podcast though help is never turned down)

On a millivolt system (runs off of a thermopile)
– How to easily test for gas valve failure, what are the expected resistances across the solenoid in the gas valve?
– What expected readings should we consistently get from a properly working system (voltage of thermopile alone, with gas valve open, with thermostat closed etc)

On high efficiency
– What is the relationship between the pressures in the collector box of the secondary exchanger and the pressure switch?
– How does a clogged condensate trap lead to the pressure switch not closing?

Another Question
– Is it possible to check readings from the circuit board when the wires are in a harness? For example, I troubleshot a gas valve failure. It was either the board or the valve. The wires coming to the gas valve from the board are in a harness. How do I know which to check and what am I checking for. (Given that everything else was working I leaned toward a faulty gas valve and was right, just so you know!)

Thanks for your time and for doing the podcast.

All the best,

For the collector box/pressure switch:
During normal operation, the collector box is under a vacuum (negative pressure) when the inducer is running. That vacuum is what the pressure switch checks for. If the vacuum is sufficient the contacts will close and signal the board everything is good. If your condensate trap is blocked, the collector box will still be under a vacuum. That doesn’t change.

However, the pressure switch port (where the tube is attached on the collector box) should be at the bottom of the box, usually near the drain port. The backed up condensate will simply end up blocking that port and the switch will no longer be able to ”feel” the vacuum, the contacts won’t make and you will get an error (pressure switch not closing or stuck open).

What may also happen, but not always, is that the port will block during a cycle and the vacuum will remain stuck in the pressure tube. As your inducer comes off and normal pressure returns, the air can’t go in the pressure tubing because it’s blocked with condensate, and you’re basically trapping that vacuum inside. So the contacts will stay closed, until the next call for heat. When that call starts, the contacts will already be closed before the inducer starts, and that will also give you an error (pressure switch stuck closed).

Now if your exhaust is blocked, this will create back pressure and your collector box won’t be under the appropriate vacuum, and once again won’t close.

For millivolt systems:
Unfortunately, I can’t say what typical resistance values would be for a mV gas valve because I don’t know. I would say however that in three and a half years I haven’t had to replace a fireplace gas valve. They rarely go bad. In most cases the pilot tube/orifice is dirty, the thermopile is too weak, or, if it works with a wall switch, very very common: the switch is bad. Standard wall switches are meant for AC voltage.

Running millivolt DC thru them will work, but as soon as you have a bit of resistance in the switch contacts, that voltage will not get through. If it runs on a thermostat, usually it works better but you can still get the same problem.

For typical readings, I’d say between 450-650mV from the thermopile alone, open circuit. With the valve open (so, closed circuit) around 200-300mV. But this is very general, it may vary a lot between systems.

If your thermopile alone doesn’t produce enough mV’s, check your pilot flame. Make sure it hits the thermopile well. You might be able to adjust it (on some valves) to make it bigger. As I mentioned, the orifice or tubing may be blocked. That is relatively common especially if the pilot was kept off for a long time.
If your thermopile gives enough voltage but the valve won’t open, check your switch/tstat and even the wire itself for any significant resistance or short.

Isolate section by section and ohm it out. If everything is good and sufficient mV’s come back to the valve and it still won’t open, then yes, that valve might be bad. But I’d probably even replace a switch/tstat before I condemn the valve regardless, just to be sure, just because changing those valves in most cases is a total pain in the butt.

For the gas valve/board dilemma:
If your wires are all in a harness with a big fat connector on the board, there’s a good chance you won’t be able to pull it off and diagnose on the board pins, because by removing the connector you remove most or all of the safety circuits.

If you want to look at the gas valve, you need to hook your meter on the wires at the valve itself. If it’s just a standard 24v valve with 2 or 3 terminals (Common + hot or common + low and high solenoids) just pull the wires off (or connector) at the valve and you have to check for 24V on the wire across common and hot. Even with the valve disconnected if your board is OK it will still send 24V in that wire at the proper time in the sequence of operation (i.e. wait until the ignition sequence completes!!). If you don’t have 24 volts, the board is bad. If you have 24V, the gas valve is bad.

If it’s one of those Honeywell SmartValves, then that’s another story entirely. A good portion of the controls are actually inside that gas valve and it will have multiple wires going to it. They are a bit more difficult to diagnose. My best advice is to follow your electrical diagram. If there’s no way for you to disconnect wires at either end (which should never happen as far as I know…) you could always cut the wire and check your voltage in the wire itself. But try to avoid doing that.

— Ben



Let’s face it. It doesn’t get super cold in Central Florida regularly but when it does EVERYONE FREAKS.

In this episode we talk about:

  • Burning off heat strips
  • Heat Pump Defrost Mode
  • Gas Pool Heater
  • Heat Pump Pool Heaters
  • The thermodynamic principles of heat pump heating
  • Defining BTUs
  • LP vs. NG

And much more…

This podcast is in an unedited “meeting format” and is not usually the type of thing you will get on HVAC school, but I still think it may benefit junior techs and office staff.

As always if you have an iPhone subscribe HERE and if you have an Android phone subscribe HERE


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