It does not matter if your Heatilator won’t stay lit, your Heat N Glo Pilot won’t stay lit, your Montigo won’t stay lit, your Mendota won’t stay lit the basic troubleshooting principles from one brand of gas fireplace to another brand of gas fireplace are exactly the same! If you have a Standing Pilot Gas Fireplace Repair ahead of you, you should be able to troubleshoot it using this post.
Warning: This information is provided as is and is intended for professional use only. This information is provided to assist new fireplace technicians in fully understanding their training. If you are not a certified fireplace technician or training to be one you have no business attempting fireplace repair or maintenance.
First and foremost, if you are a professional in your field, you need professional tools. That does not mean you need the most expensive products on the market but it is really unprofessional to show up for a gas fireplace service call with a bottle of soap and water.
In addition to a lot of hand tools, here are some of the tools a professional gas fireplace service technical should have with him and use on each and every service call.
Electronic Gas Detector
Digital Multimeter
Digital Manometer
Jumper Wires
Do not forget a drop cloth and remember to put it down! Always use shoe covers or ask if they would like you to take your shoes off!
You need to talk to your customer before starting any trouble shooting procedures. Ask them what the problem is, when it started and if anything has changed. Is it all the time or just once in a while. Has anything changed? For example, we just had a customer with a B Vent unit that stopped working. Another company had spent four hours testing the unit and replacing parts including the valve. As it turned out the customer had installed new windows and doors. In a three minute phone call, we had the issue identified by simply asking the customer to open a window.
Before Proceeding to the next step, we have a very simple policy. As soon as you touch the fireplace, you test it for gas leaks. The very last thing you do before you leave is test the fireplace for gas leaks.
Now that you know the unit is safe, you ask the customer to show you what they do and what the fireplace does. Often it is operator error. Diagnosing the operator is just as important as diagnosing the fireplace. If you light the pilot you have lost the opportunity to see if your customer is making a mistake.
Now you have started with the basics and made sure the proper procedures are being followed you can diagnose the issue with the gas fireplace and begin the process of repairing it.
Pilot Light Wont Light
Assuming you have watched your customer try to light the pilot light and you have verified for yourself that the pilot light will not light.
Remove the glass.
Follow the Procedure for lighting the pilot while watching carefully.
Use your gas detector to insure gas is coming through. If not, check the shut off valves.
If you have Gas, clean around the piezo check all the connections and make sure the appropriate gap is in place. Make sure the piezo is grounded. If it is damaged, replace it. If you determine it needs to be replaced, carefully light the pilot manually so you can complete your testing.
THE NUMBER ONE CAUSE OF MULTIPLE SERVICE CALLS IS NOT COMPLETING ALL TESTS JUST BECAUSE ONE PROBLEM IS DISCOVERED.
IF YOU HAVE NO GAS
Make sure your shut off(s) are allowing gas to pass through. If it’s a propane unit make sure your tank is not empty. Check for clogs and make sure all connections are tight.
The Pilot Lights But Won’t Stay Lit
Check the Thermocouple For continuity as shown in the picture below. If the thermocouple has continuity move on the next step. If there is no continuity the thermocouple will need to be replaced.
Check the Millivolts at the thermocouple as shown in the picture below. If the Millivolts are within the manufacturers specification the thermocouple is good.
If after confirming everything else is in order and you still cant get the pilot light to stay lit, replace the valve.
If the Pilot works, Lets Move onto the Main Burner
Obviously lets make sure that the valve knob is set to the on position.
Then We always perform a few simple tests.
1. If the unit does not light, install a jumper wire across the TH and THTP connections on the gas valve as shown below. If it works, check all the wiring to the switches and remove any extra switches and splices.
2. Check the thermopile for continuity as shown below. In the first picture you do not want continuity. If you have continuity the thermopile is bad. In the second picture you should have continuity and if you don’t replace the thermopile.
Next test the thermopile for Millivolts. Check it with the unit off as well as with the unit on (if possible) See image below:
Check Gas Pressure on the IN and OUT sides of the valve and make sure they meet the specifications shown by the manufacturer. To do this you will use a digital manometer as shown below.
If while testing the pressure as shown above the IN pressure is not within range call the gas company or a licensed plumber to check the household gas pressure. If the out pressure is out of range, try turning the adjustment knob (sometimes there may be a screw under the knob as well). If you are unable to get the out pressure into range replace the valve.
Also check resistance across the valve. Make sure your meter is set to OHM. usually check it by placing one of my multimeter prongs on TH and the other on TP. Some manufacturers will ask you to also check TH to THTP. If the reading is out of range, replace the valve.
Again, recheck all wiring and that all gas lines and connections are free of blockage. If all else fails replace the valve.
If your valve is good and you are replacing a thermocouple, a thermopile or an ignitor consider all relevant factors including cost variation and age of the unit to determine if you would be better to replace the entire pilot assembly.
One more set of tests that should be done BEFORE replacing the valve as described above.
No discussion of any type of fireplace, with the possible exception of wood, is complete without the discussion of limit switches. To keep wood fireplaces from feeling excluded we will include snap disks in this discussion 
Open Versus Closed Switches and Circuits
This is perhaps the most confusing area for many people. A closed switch or circuit is one that allows electricity to flow through it. An open Circuit does not. Open circuits or switches and closed circuits or switches can be used to achieve the same function.
Normally Open Switches will have no continuity when tested as shown below. If they have continuity they need to be replaced.
Normally closed switches will have continuity. If they don’t have continuity they need to be replaced.
How can a normally open versus a normally closed switch accomplish the same function?
Lets take the example of a high limit switch used as a safety device to shut the fireplace down if it gets to hot.
Accomplished with a Normally Open Switch
One side of the switch is attached to the TP on the valve. The other is attached to the unit. Under normal circumstances, the switch is open and nothing happens. If the unit gets to hot the switch closes and the thermopile is grounded out (the small amount of power it generates takes the path of least resistance and travels to the ground. This means the valve is not getting any power so it shuts the gas off and the flame goes out.
Accomplished with a normally closed switch.
Using a normally closed switch the Thermopile is wired to one side of the switch and then connected to the TP part of the valve from the other side of the switch. Under normal circumstances the switch allows the current to pass through and the unit works. If the unit gets to hot, or if the switch malfunctions the switch opens depriving the valve of power and the unit shuts down.
Testing is simple. Set your MultiMeter to continuity. Attach one probe to each side of the switch. Your multimeter will beep if there is continuity and will not if there is not. THIS MEANS YOU MUST KNOW WHAT YOUR SWITCH SHOULD BE IN ORDER TO TEST IT.
Pilot Assembly:
Pilot Assembly, Above is located inside the fireplace. From left to right you have the thermocouple, heat source, pilot flame/hood and thermopile. The picture below shows the other ends which are connected to your valve.
Valve side of Pilot Assembly From Left to Right: Ignitor/heat source; thermocouple; thermopile; pilot light.
Gas Shut Off Valve in On Position
Millivolt SIT Brand Valve
So, there you have it. I hope you enjoyed the article and have a pretty solid idea about repairing gas fireplaces. Have any questions in mind? Ask out in the comment section below.