Tripped Rollout Switch

Anytime you have to reset a rollout switch on a furnace, warning alarms should go off in your head. The switch is not the problem, it is a symptom of a very serious problem. Just resetting the switch and going on is like turning off a fire alarm and leaving the fire burning. You need to find out why the rollout tripped. Rollout switches trip because flames are burning back where they are not supposed to be. Possible causes include a stopped up vent, a stopped up heat exchanger, low gas pressure, or a cracked heat exchanger. All of these conditions are very serious and have the potential to do great harm. Figure 1 shows a common rollout switch.

Figure 1 Rollout Switch

In the case of the plugged vent or heat exchanger. The flue gas cannot exit quickly enough, builds up and pushes the flames out of the heat exchanger into the area where normally there is only secondary combustion air. Although 80% gas furnace heat exchangers seldom become restricted, the condensing, secondary heat exchangers on 90% furnaces often become restricted. Either way, you must have a clear heat exchanger and vent to operate the furnace safely.

A cracked heat exchanger  can also lead to tripped rollout switches. A cracked heat exchanger can allow positive pressure air from the blower into the heat exchanger and reduce the draft. If the hole is big enough, the pressure in that cell of the heat exchanger can become positive, and push the flames out of the heat exchanger into the area where normally there is only secondary combustion air. Figure 2 shows an example of this. Look carefully at the burner on the left. See that there is no bright inner cone. That is because the flames are coming back out of the tube. The burner to the right of it looks normal.

Figure 2 Flames rolling out (left burner)

Low gas pressure can cause the flame to retreat from the burner port, back into the burner body, often all the way to the orifice. Figure 3 shows an example of flames burning back at the orifice due to low gas pressure.

Figure 3 Flames burning back at orifice

Whenever you find a furnace with a tripped rollout switch, you need to determine why the rollout switch has tripped. I know no professional would ever jump out a safety device, such as a rollout switch. But just to make certain – NEVER “fix” a furnace by jumping out a rollout switch. The switch is not the problem – it may be the reason nobody died.

CSST Gas Lines

If you use flexible gas connectors or CSST (corrugated stainless steel tubing) when hooking up the gas to a gas appliance, you need to make sure and do it safely. Flexible gas connectors are made of corrugated stainless steel and generally have no outer covering or protection. They are often used to connect a gas appliance to a rigid iron gas line. 

CSST, on the other hand, has an outer covering over the corrugated stainless steel, comes in large rolls, and is often used instead of black iron when piping gas lines. There are some installation practices for each of these products that need to be followed to avoid setting up a dangerous situation.

For flexible connectors, it is important that they not be used to go through walls, floors, or the unit cabinet. Iron pipe should pass through the unit cabinet to the gas valve. Contact with the metal side of the furnace cabinet can rub a hole in a flexible connector. Another reason for keeping flexible connectors out of the cabinet is the potential for loose electrical wires or connections to arc against the flexible connector and blow a hole in it. While this could also happen with black iron, there is far less likelihood of the arc blasting a hole in the iron. 

Flexible connectors can be used to make the final connection between the black iron leaving the furnace cabinet and the black iron piped into the furnace area. When using a flexible connector, the flared connectors are generally considered “unions.” Don’t forget to install a gas shutoff. Some flexible connectors are provided with a gas shutoff.

CSST is similar to flex connectors in construction with an outer layer of protective plastic. CSST can be pulled through interior walls, but metal nail protectors are required anywhere the CSST is inside the wall. CSST manufacturers make striker plates for this purpose. Protection needs to be approved by a listing agency, such as CSA or UL. Also, it is still best to use black iron to go into the furnace cabinet. The best practice is to penetrate exterior walls with black iron. If CSST is used to penetrate an exterior wall, protection is required.

One of the biggest safety concerns with both flexible connectors and CSST piping is properly grounding the gas piping system. There have been many instances where lightning strikes near a building have blown holes in CSST gas lines or connectors. The grounding is to avoid this. The most common practice is to connect a bonding ground wire to the rigid black iron pipe outside the house BEFORE the first CSST connection. This bonding ground is connected to the ground rod or run inside to the ground bus of the electrical panel.

Here are a couple of links for more information”

Grounding: http://www.csstsafety.com/CSST-solution.html

Installation: http://www.tracpipe.com/Technical/CSST_Installation_Instructions/  

Check Combustion Air

With the weather getting cooler, I thought that now would be a good time to talk about combustion air. Don’t forget to check for proper combustion air. Most codes provide detailed drawings illustrating where combustion air should come from and how much you need, but there are still many furnace installations that rely entirely on air from inside the building for combustion air. In days gone by this was often considered adequate so long as the furnace was located in a large enough space. In newer homes, combustion air should always be provided.

Most 90%  furnaces today can operate using sealed combustion. In the case of a sealed combustion furnace, the combustion air is being piped in from the outside. The combustion air is piped directly into the furnace. These are easy to spot, they have two pipes: one for combustion air and one for the vent. Also, their panels have no louvers for combustion air. 

Traditional furnaces get their combustion air from the space where they are installed. Combustion air enters through louvers in their panels.Since the furnace is drawing air from the space it is in, fresh combustion air must be supplied to the room to keep the process going. Failure to supply the correct amount of combustion air can lead to negative room pressure, vent spillage, poor combustion, and CO production. All these things together can be disastrous.

When a technician checks a furnace that does not have sealed combustion, one of the first things to look for is how the furnace receives combustion air. If the furnace is in a ventilated crawlspace or attic, the ventilation for those spaces provides the combustion air. However, even these can be a problem. A large furnace in a small crawl space may not have adequate combustion air if the crawl space vents are closed. I have also seen crawlspace vents clogged with debris, effectively reducing the combustion air.  

The most troublesome installations are furnaces located inside the house in a closet. They should have a combustion air vent near the floor and another near the ceiling. Someone asked me about a furnace installed in a closet off of a bathroom. When they turn on the bathroom vent fan, they can smell gas! Another story involved a fireplace and a furnace. When the furnace came on it sucked the smoke out of the fireplace into the room. These types of stories indicate that the furnace does not have adequate combustion air. 

What if there are no obvious combustion air vents? Sometimes the vents were never provided, other times they have been covered up. I have seen combustion air vents covered with tape or plastic. Undoubtedly, someone noticed cold air coming in the vent and “fixed” the problem – thereby creating a combustion air problem. Occasionally insulation covers the grille into the attic. Another problem is using the furnace closet for storage. This is dangerous in and of itself, but it can also cause combustion air problems if boxes are stacked in front of the combustion air grilles.  For details on combustion air requirements check your local code. Unit 53 Gas Furnace Installation in Fundamentals of HVACR, 3rd ed also has detailed drawings and specifications for the most common applications.  

Personal Gas Safety

Time for furnace tune-ups is here. Soon you will be taking the first no-heat calls of this heating season. No doubt you will be inspecting equipment for safety concerns that affect the customer. Don’t forget to pay attention to your own personal safety. Gas leaks, leaky vent systems, incorrect combustion, and lack of combustion air can all create immediate hazards to the service tech working on the system. There are pro-active steps you can take to stay safe while working on gas fired equipment.

Gas Leaks

If there are existing gas leaks it is possible that a combustible atmosphere exists in the space. Ideally, you want to check the air in the space for combustible gas before entering. You can use an electronic combustible gas detector for this. Many techs now use these to check for gas leaks, so you might already have the tool. Just turn it on and let it sample the air in the room as you enter. There are also personal detectors which you can wear to monitor the conditions wherever you are. If you do detect combustible gas in the room, do NOT turn on or off any electrical switches and leave the room. Shut off the gas outside and ventilate the room before continuing. You will have to turn the gas back on to find the leak, but you can ventilate the room and start with a safe atmosphere while you are doing it.

Carbon Monoxide

Negative room pressure, incorrect or leaky gas venting, and lack of combustion air can all contribute to a build-up of carbon monoxide in the room. You can’t smell or see carbon monoxide, so it is especially dangerous. Again, you should test the atmosphere in the room. There are electronic detectors made to check for carbon monoxide in the air, and some are made to wear to monitor the air continuously. If the monitor indicates a build-up of CO in the room, you should shut off the gas outside the room and ventilate the room. Once the room is cleared of CO you can start to look for the cause. Before turning the gas back on make sure to check for obvious things such as blocked combustion air intakes or compromised gas vents. After restarting the furnace you should check the flame color and test the flue for proper operation. Also be sure to check the flue gasses for CO.

Gas Ignition Problems

When observing a gas furnace light, you should never place your face directly in front of the furnace. If there is a delayed ignition or flame roll-out you can be inside the fire-ball. I have made that mistake. It was over before I knew what happened. For a second all I could see was yellow because my head was in the fireball that came out of the furnace. Luckily it just singed my eyebrows a bit and gave my face a slight burn similar to a sunburn. Afterwards I worked out what had happened: a leaky gas valve had allowed a build-up of gas prior to turning the furnace on. Had my face been off to the side I would not have been in the flames when they came out.

Don’t leave your personal safety to chance. 

Take steps to detect and avoid possible personal safety hazards. It is an inconvenience to the customer to have to call 911.