Sometimes your stove starts up but then after several minutes, the fire dies. This is because the auger motor stopped turning and your stove failed to go into run mode. Typically your stove will go into run mode 15-20 minutes after the is stove turned on. If it doesn't start up within this time frame, it usually means the igniter isn't working, causing the low limit switch not to detect a flame, the low limit switch itself isn't working therefore not detecting a fire, or there are no pellets in the hopper. If there are pellets in the hopper and the igniter is hot, then the problem could be a bad snap disc. The Low Limit Snap Disc also known as the Proof of Fire ( POF ) Switch typically serves two purposes:
- Activate the Convection (Room) Blower to heat the room. This takes air from the back of the stove and sends it through the heat exchanger tubes to blow heated air out the front of the stove.
- To tell the control panel to switch from startup mode to run mode, which means fire has started and the stove is heating .The typical low limit switch is Closed on Rise (or Normally Open, N/O) and the trip point is 120 or 140 degrees, thus the switch closes the circuit at these temperatures. This is what tells the stove to switch to run mode. The newer HHT as well as the old Danson's Pelpro, Glow, and Cheap Charlie pellet stoves are the exception. These switches are Normally Closed (Open on Rise.) In this case when the switch gets hot and opens, the stove switches to run mode. Knowing this is crucial for testing the switch.
Testing the Low Limit/ POF Switch for Proper Operation
There are 3 ways this Snap Disc can fail.
- The Bimetalic disc does not open or close the circuit.
This is most likely to be the problem so it's best to start here. This problem occurs if no heat from the fire is present to bend the Bimetalic switch and close the contacts or open the contacts in the case of a Pelpro Pellet Stove.
You can make a male to male jumper to bypass this switch temporarily to see if the stove continues to run with no error # 3 light. Attach to male spade lugs available at Home Depot, Lowes or any big box store and crimp them onto the a wire at least a few inches long. I use the stranded 18 gauge wire and blue insulated spade lugs so they don't short out. Blue lugs indicate the correct size to use. I recommend Tyco brand. They plug in to all other types of quick disconnects. Some brands will not. You should unplug the stove before performing this test. Even so this test uses electricity so you must be careful not to touch the wires to the body of the stove or they can short out potentially causing damage to the stove. On St. Croix Pellet Stoves, this jumper must only be inserted 60 seconds after the on button is pushed so the diagnostic startup test will still pass.
This is how you bench test a Normally Open/ Close On Rise Snap Disc. See Video below. Note: For Normally Closed/ Close on Rise Low Limit Switches, the switch will open when heated.
a. Put the low limit snap disc in a vice or holder that is heat resistant
b. To make it easier to connect an ohm meter or continuity tester, cut a set of meter leads, then crimp on male and female quick disconnects or spade lugs. Now it is easy to unclip the meter lead tips and clip the females to the Low Limit. If the low Limit has screw terminals, then prepare 2 wires with male clips on one end, stripping the other end to fasten under the screws.
c. Connect the meter leads to the low limit snap disc.
d. Set the multi-meter meter to ohms
e. Heat the low limit switch until a click is heard which means the switch has closed
f. The meter should read zero ohms (If it does not read zero ohms, then see number 2 below)
The video in the link below shows how to test a Low Limit or POF Safety Switch that stops power going to the auger that feeds pellets and shuts the stove down within the startup period of approximately 15-20 mins.
- The Contacts in the switch that complete the circuit are corroded or worn causing excessive resistance.
If the first test passes, then you'll want to test this next while you're still bench testing. This happens due to excessive oxidation and old age. The corrosion or wear is on the inside, so to see if this is the problem you will also need to use a barbecue lighter and multi-meter for this test. The Low Limit Switch also called POF (Proof of Fire) is located next to the exhaust blower or on the plenum that contains the hot exhaust. In a Whitfield stove that I worked on, the old switch closed when the stove heated, which is unusual because it typically won't close if it isn't working. I further tested it by igniting a barbecue lighter under the switch. I heard a click, and it closed again. There had to be something else wrong like the switch was worn or corroded. I used a multi-meter and discovered that the switch was more resistive, measuring 0.3 to 0.4 ohms when the switch was closed, when it should have read zero. So the contacts must have been worn. The switch and the stove stopped running because the voltage going through the switch was too low.
A brand new or good switch does measure resistance when heat first closes the switch but usually settles down to 0.0 ohms or 0.1 at the most. The voltage sent through the switch must be high enough to trigger the circuit to allow the stove to go to run mode from the initial start up mode and continue operating properly.
- The switch mounting is not making good metal to metal contact.
If it is neither of the first two problems, the switch is good, but the metal of the switch is not making good contact with the metal of the exhaust blower. In that case, the Heat Level # 3 light will start flashing when the pellet stove fire dies out. This can be caused by loose screws or a bad white gasket underneath. There should not be a gasket on the switch and it should be removed. The loose screws can be removed and can be replaced with #10 self tappers to assure a snug tight metal to metal connection. You can add thermal paste or a piece of aluminum foil for an even better metal to metal connection. Dielectric grease can be used for the wire connections. In St. Croix Stoves the low switch is mounted on a more rounded surface, which can cause this problem.
- Cleaning and Lubricating the Snap Disc for the Best Operation
Even when your snap disc is working well, there are things to maintain it and make it work even better.
a. The snap disc can be removed by unclipping or unscrewing the 2 wires and then removing the 2 screws. It can be cleaned with Acetone and a paper towel or cloth.
b. There are 2 types of compounds for improved operation:
i. Thermal Paste also known as Heat Sink Compound which is sold as Super Lube at Home Depot.This helps transfer the heat from the stove to the snap disc to trigger the bimetalic switch to turn on the convection blower and activate run mode.
ii. Dielectric grease to insure the best electrical connection and prevent terminal oxidation or corrosion