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Fireplaces 101

Gas Fireplace Repair: How Fireplaces Work

 

There are basically 3 different types of gas fireplaces:

  • Millivolt System Gas Fireplaces
  • Standard System Gas Fireplaces
  • IPI System Gas Fireplaces

We categorize these by their control system.  This

When it comes to gas fireplace repair, it is important to know which type you have and how they work, so that you can have a better understanding of why they might be malfunctioning when they start giving you problems.

Probably the easiest way to tell which type of system your gas fireplace uses  is to determine how you would normally operate it.

With a milivolt or IPI system, you would normally turn the main burner flame on and off with a wall switch  where a standard system involves physically turning a knob or lever to turn the flame on and off (we will explain how each of these systems works later).

The difference between a millivolt or IPI system is whether you have a standing pilot light or not.  Millivolt systems have a pilot light (that tiny little flame that runs constantly) and would normally have to be re-lit if it were to go out.  IPI systems have no pilot light until you turn on your wall switch.  With IPI, your pilot will electronically light itself after you flip on your wall switch.  IPI is a relatively new system.  You can also look at your rating place (see our What Type of Gas Fireplace Do I Have? page to see where to find your rating plate) and most often it will tell you what type of gas fireplace you have.  The most common type of gas fireplace is a millivolt system.

 

How a Millivolt System Works

A millivolt system gets its power from the pilot light.  The anatomy of a pilot light will look something like this:

My Gas Fireplace Repair - Pilot Light Anatomy

To put it simply, the pilot light consists of three main parts.

  1. A Flame
  2. A Thermopile Sensor
  3. A Thermocouple Sensor

Most of the time gas fireplaces also have some sort of spark ignition system (piezo ignitor electrode as shown in the picture) to help assist you in relighting your pilot so you don’t have to light it with a match.  The piezo ignitor is usually connected to a button that will create a spark when the button is depressed.

The pilot flame will heat up both the thermocouple and thermopile sensors.  These sensors contain material that will create a small amount of voltage (under 1 volt) when they are heated up.

 

The Thermocouple

The thermocouple will create around 20-30 millivolts (.03 volts which is less that you can feel).  This sensor is connected to the main control valve.  This thermocouple will create enough electricity to keep the part of the gas valve open that controls gas flow to the pilot assembly.  This is why the pilot will stay lit on its own after initially lighting the pilot and heating up the thermocouple. This is also why it will stop releasing gas if your pilot light goes out.

 

The Thermopile

The thermopile is basically just a large thermocouple. But instead of creating 20-30 millivolts, it generates up to 900 millivolts. This is still under one volt but has enough power to turn on the main burner when you press your wall switch.

 

 

50 Comments
  1. In just fifteen minutes or less we fixed our gas fireplace. We spent $150 last year with a repair man trying to get it going again. The same repair man estimated it would take between $500 and 2500 to fix our fireplace. It was all in the switch!! Thanks you so much!!! Tamara

    • Awesome! Thank you for your comment. That’s great to hear. Please share this site with your friends!

  2. I have a superior d-500cmn the pilot would not stay lit after releasing the button I have now replaced the thermocouple and I still have the same problem. What now? Thanks

  3. I have a direct vent heat n glo gas fireplace I purchased about 10 years ago. From new the the fireplace has stayed lit at times for days, months, hours and even minutes and then goes out, main burner and pilot together. I then relight the pilot and start burner again wondering how long this particular session will last. I have payed for countless service calls and all is good when they leave, but sure enough after a day month or even longer the same thing happens. At this point I,m ready to just buy another fireplace. This is rather a important fireplace as it keeps the master bed room warm in the winter and is hooked to a thermostat. Thank-you John

    • Does the flame change in appearance right before the fireplace goes out? What has been replaced so far? How is the unit vented?

      • As far as I can see the flame does not change in appearance before shutting off, The flame has a nice blue color with whispers of orange on the top of flame.The thermocouple and the main valve has been changed. The only thing left to change is the thermophile.The unit is vented vertically with one short 90 towards the back wall and then another 90 straight backup. This is to offset the vent back so at the time a old style TV could be placed further up the wall in front of the fireplace. The venting from the top of the Fireplace to the termination cap outside above the roof is approx 16ft. From what I can see the fireplace has a excellent updraft, to the point that it will slightly pull away the pilot flame from the thermos. However I have never had the pilot go out when the main burner is not fired.I’m now thinking to put a shield around the back of the pilot flame and thermos so that the flame is not pulled to towards the center of the fireplace.The model is 7000XLT-NF Natural Gas. Thank-you John

        • I would try a vent restrictor. It sounds like once the fireplace establishes a good draft, it pulls the pilot flame away from the thermocouple sensor. You can usually order a restrictor that goes in the vent pipe that slows down this draft.

          Otherwise a pilot shield might work also.

    • I have the same problem with mine. It will stay good for an hour sometimes or 2 min other times. The pilot light then won’t light for an hour or so, then it starts all over. 2 service calls so far; the guy can’t figure it out.

      • When the phe pilot light won’t light for a hour or so, will it ignite and just not stay lit? Or does it not ignite at all? What is the make and model of the fireplace?

  4. I have a gas fireplace that only lights when it wants to. It has no pilot light it has a wall switch . It says Heatilator on it .

    • This sounds like an IPI or electronic ignition fireplace. What is the model number?

  5. Lots of great info here, thanks.
    Do you have any knowledge about “delayed ignition”?
    Is the cleaning that my local company suggests something I could do myself??

    • Delayed ignition can be caused by a poor pilot flame, plugged burner ports, or incorrect venting. If this is a relatively new issue, we can probably eliminate venting as a possible cause.

      Check your pilot flame. It should be directed over the main burner so that when the burner releases gas, it will ignite rather quickly.

      Sometimes burner ports can get plugged. Gas log sets with sand over the burner can have the sand melt and plug the burner ports over time. Otherwise spiders (they like the smell of the gas additives) can build nests inside burner orifices and burner tubes, slowing the release of gas to the burner.

  6. THANK YOU. We’ve been avoiding having a repair man come look at our fireplace and just happened to stumble upon your website. 15 minutes later, our fireplace is working perfectly! It was just the switch and you’ve saved us time and money. Sincerely appreciate it!!

  7. Our fireplace feels like the window is open. In the winter, we actually get frost on the hearth. We now have it covered in window plastic. It is about 10 years old and is a superior bri-36. The flue is closed. We are at a loss…Thanks..

    • Most of the time, cold air issues are related to how well the exterior wall behind the fireplace is insulated. When the fireplace is first installed, it gets framed in with 2×4 framing. The framing behind the fireplace should be insulated and have a vapor barrier on it. Many times, it may be insulated but not very well. Also, there should be a “ceiling” built above the fireplace that has insulation on the top of it. This is often called a firestop or draft-stop. The ways to check these things include either:

      1.) renting a snake camera and fish it behind the fireplace to see whats back there, or
      2.) taking the siding off behind the fireplace and cut a hole to look and see.

      I’m guessing this is the case.

  8. I have a buzzing coming from my fireplace and it does not shut off all the time when I turn it off from the switch. Could this be from a bad thermopile sensor?

  9. How do you become a local certified technician on your site?

    • Local certified technicians listed on this site have been certified through the NFI (National Fireplace Institute). They have proven knowledge of fireplaces, log sets and stoves as well as their venting systems. An NFI certification is required to be listed on this site.

  10. I have a SFC and not sure if a VF5-CMN-2, VF5-CMP-2, VF6-CMN-2, or a VF6-CMP-2 but all oare listed on the manual.

    The pilot light will stay running (only with the wall switch on) but the main burner will never light even pulling the logs and using a light a different spots on the burner assembly. I turn the low/high knob to several positions and wait and nothing ever happens. Only the pilot burns. Any suggestions?

  11. Fireplace lights easily when wall switch is turned on. Problem is that the flame pulsates when operating. Flame does not go out until wall switch is turned off.

    • Is this a standing pilot unit? Or electronic igniton? Whats the model number?

  12. I have 2 Desa NG fireplaces model VM42B and VM36B. Both will not come on with wall switch. If you jump TH/TP to TH the start with the switch on. Thermopiles are 700-900. If you try to bypass the switch and connect wires together nothing untill you jump TH/TP with TH. Any suggestions? Thanks.

    • I would start at the TP/TH and TH terminals then follow the wall switch wires back to where they go through the fireplace and back to the wall switch. Make sure that either wire does not go to another sort of sensor, spill switch, or rocker switch somewhere before it goes to the wall switch. Its not uncommon for there to be a saftey sensor that breaks the connection if it senses the fireplace is overheating or venting into the room. Many times this sensor will malfunction.

      If not, it must be in the wall switch wire in your wall.

  13. I checked the thermopiles fireplace 1 was 890 switch on 480. Fireplace 2 was 780 switch on 360.

  14. Just closed on a house. The area above the gas fireplace was kind of black, thought it might be a closed flue issue. Painter #1 says it is leaking and needs to be caulked with high heat caulking. Painter #2 thought it might need a new gasket. Shall I caulk it or call someone? Thanks.

    • This is a tricky one. It also could be a result of back puffing when opening the door too fast, or your house is very tightly insulated and there is an issue with sufficient draft up the chimney.

      You can check the gasket by using the “dollar bill test”. Take a dollar bill and close it in the door. When you try to pull it out you should feel some resistance. If it comes out without much effort, you may need to adjust the door or replace the gasket.

      If you want to be sure, I would hire a certified fireplace technician to do a thorough inspection of the fireplace.

  15. Just bought a house with a gas fireplace. The gas fireplace lights up using a light switch on the wall. When I flip the switch to turn the fireplace on it lights up okay, but when I turn it off there is a constant small flame coming from the line which supplies the gas in the firebox, is this normal?

    • Yes. This is called a pilot light. It will always stay lit even after you have turned your fireplace off

  16. The information that you have provided is very informative; however there is no information about the fan that blows the air out. The fan is control by an electrical switch, which is mounted on the wall, but when turn on nothing happens, if you can please advise or call me at (609)291-2789. Oh! by the way it use to work until the pilot light went out and we had to have it relit, we must have moved something or touch something that we should not have done.

    • Many times, the blower works off a temperature sensor and will not allow it to turn on without the fireplace heating up for a while.

      I would also check to make sure the fan is plugged in. When I do service on a fireplace I usually unplug the blower so I don’t accidentally shock myself. Flip open the lower grill/louvered area of the fireplace. On the very left or right side you should see an electrical outlet. Make sure the blower cord is plugged in.

      You can also check to see if there is a rheostat (speed control) knob underneath there as well. This could have been turned off or bumped so it is on a very low speed so you may not hear the blower running.

  17. I have a heat n glo sl-36gdv-cdn. My wall switch won’t work. My pilot stays on and if I switch it to on underneath it lights up and my wall switch won’t do anything. If. Turn it off underneath it turns off.

    I tested the thermopile and got 400mv on off and when I turned my wall switch no change and when I turned it on below I got like 175.

    Any idea what the problem can be?

    • Sounds like your wall switch is bad. Change it and it should work fine.

  18. Thankyou for this very informative site. I learned a great deal about gas control valves.

  19. I have a millivolt system that randomly turns the fireplace on and then off. Could this be the wall switch?

    • Most likely. Does it run on a wall mounted thermostat or is it just a switch?

  20. Having trouble with my gas fireplace…
    Two years ago my boyfriend try to turn it on and did not work, the year after my brother and boyfriend try again and they were able to turn it on I did use it all winter… now this year it does not want to turn on… my boyfriend try even to use a lighter directly on the piloy but it looks like if there is no gas coming out… what should we do? Can you help please…

    • If you turn your pilot light off in the summer, you may have blockage inside your pilot tube (spiders like the sulfur smell of gas). You would have to clean out your pilot tube. But first start by making sure your gas supply is turned on or your propane tank is full and fully on.

      To check for blockage, I start by unscrewing the pilot tube from the main control valve. Then push in your pilot knob like you normally do to light the pilot. You should hear gas coming out of the side of the valve where the pilot tube was screwed in. This would confirm blockage in your pilot tube. Sometimes you can clean it out by blowing through it, but sometimes you have to take it apart and run a small wire down it to clean it out.

      This can be a little tricky and may be worth contacting a service technician if you do not feel comfortable.

  21. I have a Warnock Hersey direct Vented Gas Fireplace WH-6540 which has been working fine for years, however this Fall when I tried to start it I got the Pilot Light going and after 30 seconds warm up when I turned the control from Pilot position to ‘ON’ the main Burner comes on for a moment and slowly goes out again. I’ve cleaned everything up including the Burners but the problem repeats itself. Could this be a fault with the Thermocoupler ???????

  22. I have a HeatnGlo SL-550TR, a basic system, that won’t shut off at the wall nor using the rocker switch in the located at the bottom of the unit. Only manually turning off the pilot/ignition dial will do it. Does this indicate that there is a short at the rocker switch?

  23. We just moved into a house that has a wood burning fireplace converted to gas, I can’t get it to light and I have no idea where to start

    • Start by diagnosing your pilot light. You may just need to light your pilot.

  24. We have a Majestic SRT36 direct vent gas fireplace. Early this year, we lost the flame on the main burner. We had the Energy company come to the house and they recommended we change the thermopile. Being that the summer was near, we decided to wait.
    We replaced the thermopile with no luck, so we change the thermostat (replaced with a Honeywell). Unfortunately, the main burner still will not light. I have been trying to self diagnosis this myself and stumbled onto this site.
    Reading through this thread, we decided to purchase a multimeter. The reading with the thermostat off is 387 and when turning the thermostat on it was also 387(we played with the temperature – same reading regardless of temperature). The pilot stays lit so we likely don’t need to change the thermocouple. The pilot flame is blue with an orange glow on the top perimeter. Any ideas as to what could be the issue? I am determined to figure this out without having to call a tech. :)

    • It sounds like the thermostat or wiring for the thermostat is bad. Did you try jumping the valve with a piece of wire to see if it comes on that way?

  25. I have an IPI Heatilator fireplace. At the end of last winter it was working fine. Mi went to start it this year and flipped the switch on the wall. I heard the clicking and saw the pilot light come on but the burner did not light. I checked the pilot light for corrosion and cleaned it but still isn’t working. Any ideas?

    • An IPI system is a little different. Check the main control valve. There should be a green wire connected to a green terminal and an orange wire connected to the orange terminal. Make sure these did not come off the valve and are connected securely.

  26. My problem is the burner will not shut off when I throw the wall switch that controls it. I have checked the switch and it checks out ok.

  27. Sometimes there is another toggle switch that connects to the valve. Disconnect everything from the terminals (including anything clipped on the backside) and reconnect the thermopile to the TP and TP/TH ports. Then try turning it on. If it still turns on without any switch connected, your valve is bad.

  28. Day 1
    —–—–
    Pilot light does not stay on, remove and clean component of the pilot system. Still not working.
    Remove copper tube that feeds the pilot system and discover some small piece of dirt/rust. Now pilot stay on flame large enough had to reduce the flame size with the pilot screw.
    First time in 2 years that the pilot flame is working. Yes I am happy.
    Repainted the inside of the gas fireplace. Looks good.

    Day 2
    —–—-
    New problem, the main burner does not come on.
    Pilot flame is burning.
    Thermopile output voltage is;
    540mv with the wall switch off / no load
    262mv with the wall switch on / loaded
    I can hear a picking noise when the wall switch goes on, I think the valve gets activated.
    Main burner is not on why?
    Also what are the two adjustments called in & out, these look like valves similar to the pilot light adjustment and they are closed. Was 2 years to long?

    Day3
    ——–
    OK I have learn a lot in the past few days on gas fireplace.
    I did a dumb thing I forgot to turn the knob from pilot to ON, now it works fine.

    The clicking sound is the relay engaging which is OK.
    The voltage are normal both loaded and unloaded.
    Still do not know what the in and out screws/valve are but they need to be fully close.

    I will get to bed a little smarter today.

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