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/  

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.

Combustion Efficiency

Fall tune up season is here. A seasonal check on a gas or oil furnace should include a check of the combustion process. To do a good job you really need to measure the combustion efficiency. To measure combustion efficiency you need to take two temperature readings and one flue gas reading.  The temperatures are the flue gas temperature and the ambient temperature around the unit. You subtract the ambient temperature from the flue gas temperature to get what is called the net stack temperature. The flue gas reading can be either CO2 or O2.Generally oxygen is preferred. For a gas furnace, perfect stoichiometric combustion produces 12% CO2 in the flue gas. The CO2 percentage drops off if the process is either too rich (too much fuel) or too lean (too much air). At the perfect stoichiometric point the O2 will still be 0% because all of it is being used in the combustion process. As excess air is introduced into the process, the O2 begins to rise due to the oxygen content in the air that was not used in the combustion process. Excess air is introduced to insure complete combustion. Note that complete combustion is not the same as perfect combustion. In perfect (stoichiometric) combustion, ALL the fuel and ALL the oxygen are used up, producing ONLY carbon dioxide and water. In complete combustion, all the fuel is used up, but not necessarily all the oxygen. Lack of combustion air produces incomplete combustion. Incomplete combustion leaves some unburned carbon and carbon monoxide. Excess air is introduced on purpose to prevent the production of soot and CO in the flue gas. The figure below shows the relationship of CO2, CO, O2 , and excess air. Next week we will discuss some of the ore practical aspects of measuring the combustion process.

Gas Furnace Firing Rate

How do you know the rate at which your furnace is actually firing? You really can’t assume that it is firing at the manufacturer’s published rate just because the manifold pressure is at 3.5”, even if that is what is specified on the nameplate. To determine the Btuh firing rate of your furnace you must first determine the cubic feet of gas per hour being burned (CFH) and multiply that times the heat content of the gas. To determine the CFH, you need to clock the gas meter. First, you want to turn off all other gas appliances. Then clock the number of seconds it takes for the smallest dial on the gas meter to make one revolution. For more accurate results, clock the time for two or three revolutions. The CFH can be calculated by dividing the time clocked into 3600 (the number of seconds in an hour). Multiply this answer times the value of the dial. If you clocked for multiple revolutions, multiply by the number of revolutions. The formula looks something like this

(3600/time clocked) x dial value x revolutions = CFH
So if the a 1/2 ft3 dial took 90 seconds to make three revolutions
(3600/90) x 1/2 x 3 = 40 x 1/2 x 3 = 60 CFH

Next, you need to know the heat value of your gas. You get this from your local gas supplier. If you can’t get that you can get a good estimate from the EIA. The US Energy Information Administration (EIA) has a table online that shows historical values for every state. GAS HEATING VALUES.

In our example, if the furnace is in Mississippi in 2011, the gas heat content is somewhere around 1010 Btu/ft3. So our firing rate would be 60 ft3/hr x 1010 BTU/ft3 = 60,600 BTU/hr

The same results in West Virginia in 2011 would mean a firing rate of 60 ft3/hr x 1083 BTU/ft3 = 64,980 BTU/hr

If the furnace in West Virginia is rated for 60,000 Btuh, you would need to reduce the manifold pressure slightly to reduce the firing rate. Another complication is altitude. Furnaces need to be de-rated approximately 4% for every 1,000 ft of altitude. In West Virginia, it is quite possible to be 3,000 ft above sea level, necessitating a 12% reduction in firing rate. So the new firing rate should be 60,000 Btuh x 88% = 52,800 Btuh. Now you are looking at firing a furnace at 64,980 Btuh which should only be fired at 52,800 Btuh. You most likely will need to change orifices as well as adjust the manifold pressure. After making any changes to the manifold pressure or orifices, make certain to clock the meter again and recalculate the gas firing rate to insure your adjustments brought the firing rate into line. Over firing a furnace can increase the production of CO in the flue gas. In extreme examples, it can cause soot formation.  

Natural Gas Furnace Firing Rate

Do you know the heat content of the natural gas supplied to your area? Although we often use the nominal value of 1000 Btuh per cubic foot for natural gas, the heating value is really a bit higher in most places. A look at a table by the U.S. Energy Information Administration, EIA, shows that only a few states have natural gas with a heating value close to 1000.Gas Heating Values In 2011, average gas heating values ranged from 1004 in Nebraska to 1076 in West Virginia. Furnace manufacturers must choose a heating value when they select the orifices to put in the furnace. Typically, they use a value on the higher end of the range, 1075 BTUs/ft3. A furnace with the factory supplied orifices set up at the same manifold pressure in each of these states would deliver a different amount of heat. Assuming they were all installed at sea level in their respective states, they would burn the same volume of gas because they would have the same manifold pressure and the same orifice size. A furnace that burns 100,000 Btuh in West Virginia would only be firing at 93,000 Btuh in Nebraska. Small adjustments in the firing rate can be made by adjusting the manifold pressure. However, it is often necessary to change the burner orifices to get the manufacturer’s listed firing rate.

Higher altitudes can make de-rating a furnace necessary because the lower pressure, less dense air just does not have enough oxygen in it to support the full capacity of the unit. In general, furnaces are de-rated 4% for each 1,000 ft of altitude. Some manufacturers provide tables showing what orifices and manifold pressure should be used depending upon the gas heating value and altitude. The point is that setting up a furnace for the correct firing rate involves a little more than simply adjusting the manifold pressure to 3.5" wc using the manufacturer supplied orifices. You may need to change the orifices and/or adjust the manifold pressure to something other than 3.5" wc. Below is an example from one manufacturer. Note that this is an example - it does NOT apply to all furnaces.

Gas Heating Value	Sea level - 2000		2001 - 3000		3001 - 4000		4001 - 5000
	Orifice	Man press	Orifice	Man press	Orifice	Man press	Orifice	Man press
975	44	3.3” wc	44	2.8” wc	44	2.6” wc	47	3.5” wc
1000	44	3.2” wc	44	2.7” wc	44	2.5” wc	47	3.3” wc
1050	44	2.9” wc	44	2.5” wc	48	3.7” wc	48	3.4” wc
1100	46	3.3” wc	48	3.7” wc	48	3.4” wc	48	3.7” wc

Checking Gas Pressure

Heating season is upon us and it is time to get out your gas pressure measuring instruments. What do you use to check gas pressure on natural gas furnaces? A popular tool that I see a lot is probably the least accurate available. Many techs use the diaphragm gauge gas pressure test kit which comes in the blue plastic box. But these can’t read anything below 2” wc, making them useless for the low fire stage of a two stage furnace. Even 3.5” is in the first part of the scale, not in the sweet spot. If you are testing natural gas and feel that you must use one of the diaphragm pressure gauge gas test kits, at least get the model that puts 3.5” wc more towards the center of the scale. Then you can actually read a pressure of 1” wc. The reason many people don’t choose the gauge which is more appropriate for natural gas is that it does not read high enough for propane. So they buy the gauge that was really designed to read propane gas pressure and use it for both. That sounds reasonable, until you try to read gas pressure on a two stage natural gas furnace using one of those gauges and cannot get a reading.

For an inexpensive gas manometer, an actual water column manometer is a far better choice. I like the straight tube water manometers that Yellow Jacket sells. They are actually cheaper than their gauge style gas pressure test kits and more accurate. No, they are not quite as easy to use – you have to put water in them and learn to read the bubble in the center tube, but that is not too difficult. For a little more money you can buy a U-tube manometer. If you want both convenience and accuracy, a digital manometer is the thing to get. They are considerably more expensive, but also considerably more accurate and convenient. An added bonus is that you can also use most digital manometers to read duct pressures – so the tool can serve more than one function.

Why should you worry about how accurately your manometer reads the gas pressure? Because you cannot insure the furnace is operating safely at the correct firing rate and efficiency if you don’t really know what the manifold pressure is. Checking furnace operation by just seeing blue flames is similar to checking the charge of an air conditioning system by just feeling to see that the suction line is cold. You can see that it is operating, but you really don’t know it is operating as designed. Incorrect gas manifold pressure can lower system operating efficiency and increase the operating cost. Under fired furnaces will lose capacity, over-fired furnaces can be a safety risk due to increased CO output. So do yourself and your customers a favor and get a tool that will allow you to take an accurate manifold pressure, even on low fire.

Sometimes HVAC/R technicians have an opportunity to do more than make people comfortable, we can save lives. More people are sickened or killed by carbon monoxide poisoning than any other type of poison. The Center for Disease Control and Prevention (CDC) reports that each year more than 500 people in the United States accidentally die from carbon monoxide. An estimated 10,000 people in the U.S. are treated for CO poisoning in hospital emergency rooms annually. It is believed that many more people suffering CO poisoning are misdiagnosed, or never seek medical care. This is because the symptoms of CO poisoning are very similar to influenza symptoms. One big difference is that influenza causes a fever and CO poisoning does not.

Carbon monoxide is an odorless, colorless gas that is highly poisonous. It is formed by the incomplete combustion of carbon based fuels, like natural gas, oil, coal, or wood. Incomplete combustion can be caused by lack of oxygen, improper mixing of the fuel and oxygen, or too low a combustion temperature. A correctly adjusted gas or fuel oil flame should produce very low levels of CO. Ideally a correctly adjusted gas or oil flame should produce no CO, but realistically, most produce at least trace amounts. Solid fuels almost always produce large amounts of CO, that is why charcoal comes with a warning that it is not to be used inside. Even people that should know better sometimes overlook the obvious. A friend of mine was conserving heat during a cold winter power outage by using his charcoal grill inside. His daughter became very ill and had to be rushed to the hospital where they correctly diagnosed her condition. This story ended well, she recovered and is doing well. Unfortunately there are many stories about CO that do not end well.

HVAC/R technicians are in a position to help. We can make sure all combustion appliances in the home are burning correctly, insure there is enough combustion air for proper combustion and venting, and finally by making sure the vent system is adequate and working correctly. For gas and oil furnaces also remember to inspect the heat exchanger for leaks. The heat exchanger separates the combustion products from the air circulating in the home. Although a defective or cracked heat exchanger can contribute to CO poisoning, more obvious problems are frequently to blame. Stopped vents, loose or leaky vents, and lack of combustion air are common causes of CO. While every technician should learn to look for conditions that can lead to problems, testing is required to verify that a system is operating at safe levels of CO and that there is no CO in the house. Every technician should have an accurate CO tester. Household alarms are not a substitute. While every house with gas or oil appliances certainly should have CO alarms, they are not a replacement for an accurate tool for diagnosis. I highly recommend a seminar done by Bob Dwyer for COSA (Carbon Monoxide Safety Organization) Make sure and take advantage of the opportunity if you have a chance to attend one of his CO Safety Seminars.

There are many units in Fundamentals of HVAC/R to help explain how to achieve safe, efficient combustion for gas and oil furnaces, including

• Unit 37 Gas Fired Heating Systems
• Unit 38 Warm Air Furnaces
• Unit 40 Gas Furnace Installation, Startup, Checkout, and Operation
• Unit 41 Troubleshooting Gas Furnaces
• Unit 42 Oil Fired Heating Systems
• Unit 43 Oil Furnace and Boiler Service
• Unit 44 Residential Oil Heating Installation
• Unit 45 Troubleshooting Oil Heating Systems

There are many good web sites for more research on carbon monoxide poisoning.

A few are listed below.

http://www.bbc.co.uk/health/conditions/carbonmonoxide1.shtml

http://www.cosafety.org/Aboutco.htm

http://www.clima-tex.com/consumer/carbonmonaction.html

http://annhyg.oxfordjournals.org/cgi/content/abstract/18/1/79

http://www.coheadquarters.com/colimits1.htm

http://www.coheadquarters.com/CarbonMonoxideHQ.com/index.html