Safeties, Safeties, and More Safeties
December 8, 2020 Jeff Rosenblum
In modern furnaces you may have noticed a lot of safeties. I’ve heard numerous times "boy, these new furnaces are too complicated". Well, there is good reason for these safeties. The safeties in modern furnaces are designed to prevent damage to the furnace, damage to the home or injury to you and the owner. Here we will cover the more common safeties and what they do. Please keep in mind, NEVER bypass a safety and if it needs replaced you must use the exact replacement.
This is a temperature activated limit. It is normally closed and opens on temperature rise. This is used to protect the heat exchanger from temperature damage. It is designed to open at a temperature well below the thermal stress limit of the metal of the heat exchanger. If this safety does open, it will shut the gas off and turn the blower on.
The primary limit can open because of a failed blower motor, the blower running at too low of a speed, incorrect gas pressure or high static pressure in the duct system.
Flame Rollout Switch:
This safety is very important! It is designed to open if flame is sensed in an area where it shouldn't be. This safety is a normally closed temperature activated switch. One thing to remember is that this is not an automatic switch. If it opens, it stays open. It does not reset automatically. It is designed this way because if for some reason flame was detected at that point, a technician is needed to assess the situation. We do not want it to reset because it could cause further damage. Keep in mind, there can be several flame rollout switches in the system. If this safety opens, the entire unit can shut down. The gas valve will close and not reopen.
The flame rollout switch may open due to a failed heat exchanger, restricted heat exchanger, high manifold gas pressure or delayed ignition. Keep in mind, if this safety opens, it must be addressed and the cause of it opening resolved before this furnace can be put back in to operation.
The pressure switch is a normally open switch that closes when the inducer runs thus proving the inducer is operating. This switch is designed to open if there is an issue with the furnace getting air for combustion or getting rid of burnt gasses. If the pressure switch opens the furnace will shut down and retry. If it happens several times the furnace will lock out.
The pressure switch can open, or not close because of a restricted intake pipe, restricted exhaust pipe, exhaust piping holding water, the secondary heat exchanger not draining or a clogged heat exchanger or a failed inducer motor.
This is just a small list of the 3 more common safeties in modern furnaces. There are others that can be in there. Remembering what causes the safety to open will help in diagnosing the situation.
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