Toggle Switch Failure in a Crawl Space

A very common problem encountered in crawl spaces is for the toggle switch used as a furnace disconnect to die. Often you turn it off, and it will never turn on again. They can also fail on their own over time. Toggle switches are not really made for the environment found in most crawl spaces. The contacts and the switch mechanism get corroded. The contact corrosion causes voltage drop and heat, further degrading the switch. 

The picture shows what is left of the toggle switch that was serving as a furnace disconnect. The switch had failed, keeping power from reaching the furnace. It was relatively easy to diagnose: nothing happening with any thermostat setting, no transformer hum, and no LEDs glowing. My non-contact voltage detector showed power entering the switch but not leaving it. When I started to remove it so I could check it, it just fell apart.

To avoid similar problems, toggle switches in crawl spaces should be installed inside a weather proof switch box. I also recommend spending a few extra dollars to get a commercial, heavy duty toggle switch instead of the normal light duty residential toggle switch. In fact, the NEC requires the weatherproof box, but it is often overlooked. The relevant NEC sections are listed below. These are from the 2017 edition, but I do not think these particular sections have changed from previous code versions.

“404.4 Damp or Wet Locations

(A) Surface-Mounted Switch or Circuit Breaker. A surface mounted switch or circuit breaker shall be enclosed in a weatherproof enclosure or cabinet that complies with 312.2.

312.2 Damp and Wet Locations.

In damp or wet locations, surface-type enclosures within the scope of this article shall be placed or equipped so as to prevent moisture or water from entering and accumulating within the cabinet or cutout box, and shall be mounted so there is at least 6-mm (1∕4-in.) airspace between the enclosure and the wall or other supporting surface. Enclosures installed in wet locations shall be weatherproof. For enclosures in wet locations, raceways or cables entering above the level of uninsulated live parts shall use fittings listed for wet locations.

Exception: Nonmetallic enclosures shall be permitted to be installed without the airspace on a concrete, masonry, tile, or similar surface.”

CO Safety and the HVACR Tech

There is more to carbon monoxide safety than leaky furnace heat exchangers. Venting problems may actually cause more CO safety issues than leaky heat exchangers. I am not suggesting that you overlook the importance of checking furnace heat exchangers for leaks, but rather that you expand your CO safety horizons a bit. I want to talk about CO safety concerning the technician – you. A combustion appliance operating in an unsafe condition can spill vent gasses containing CO out into the room where it is operating. This can create a safety hazard for technicians going into that area to service the furnace. You should carry a CO detector that displays the CO level in the area you are working in. If the ambient CO goes above 50 ppm – you should leave. 50 ppm is the OSHA standard for the maximum allowable concentration for an 8 hour work day.

Ray Wohlfarth recently published an article in the October 2015 issue of Plumbing & Mechanical magazine in which he discusses a story of an electrician being overcome by CO fumes. The electrician was working in a mechanical room on something completely different than the boilers, but the boilers were in that room and not operating properly. When the electrician failed to report back to the school administration, the secretary was sent to see what was taking him. She found him passed out on the floor – ran and called 911 to get emergency medical help. This undoubtedly saved his life. Suppose that had been a service technician working on a furnace with nobody home? By the time someone found the tech – it would probably be too late.

Before you go to a big box store to get a home CO detector, consider that they are typically built to UL 2034 standards, which allows 15 minutes before alarming at levels above 400 ppm. This is just not fast enough in the case of high levels. The UL standard is weighted to prevent false alarms, but this also means that a UL listed device can fail to alarm at dangerous levels until it is really too late. Plus, typically these do not show what the CO level is. Kidde, a primary manufacturer of these types of CO monitors, points out that their devices are for continuous monitoring – not short term detection.

Single gas, battery operated CO detectors which display the CO level are available for $100 - $150. Alternately, you could use the CO setting on your combustion analyzer. If you don’t have a combustion analyzer, you could put that $150 you would normally spend on a single gas CO detector towards one of the lower cost $500 combustion analyzers. You will be doing both yourself and your customers a favor.

Condensing Furnace Drains

With the abundance of condensing furnaces, drain problems are no longer limited to the summer months, but are now a year round concern. Although the drain on a condensing furnace is a relatively small detail in the overall scheme of things it can shut a system down if not run properly. Normally, there are two furnace drains: one for the condensing heat exchanger and one for the vent. Some furnaces combine then inside the furnace while others require the installer to take care of that. The vent should slope towards the furnace so any water condensing inside the vent runs back to the drain. This also prevents water dripping out the vent and creating an ice dam. The drains need to be trapped, but only once. After the drain leaves the trap the pipe should never rise. Sometimes sags in PVC drain lines cause unintended secondary traps. Secondary traps will keep the water from draining out, creating a mess. Many manufacturers now provide a manufactured trap. If a furnace has a built in manufactured trap you should not add another one. Multi-poise condensing furnaces pose a special problem: you have to know which end is up (literally) to know how to position the trap and drain. Often these furnaces come configured for upflow installation but must be reconfigured for downflow or horizontal installation. Make sure the drain gets moved to the right location for whatever position the furnace is installed in.  In general, you should not run the air conditioning condensate drain and the furnace condensate drain into a common line. The positive pressure from the coil can travel through the drain to the furnace drain and cause the vent safety switch to trip. It is OK to run both into the same condensate pump basin, so long as it is open to the air and not sealed tight. If you use a condensate pump, make sure that it is rated for furnace duty. The condensate from furnaces is moderately acidic and can eat up some pumps that are not designed to handle furnace condensate. If the drain will run through unconditioned space that may drop below freezing, it will need to be wrapped with a heat tape to prevent it from freezing. If the furnace is located in an area which can be damaged by water overflow, such as an attic, it will require a secondary drain pan underneath the furnace. Finally, remember water runs down hill. The drain should slope away from the furnace until its outlet.

NOx Nox Who's There?

Cracked. Cracked who? Cracked heat exchanger. OK – not the funniest knock knock joke – especially if it is YOUR furnace. Low NOx emission inserts are causing pre-mature furnace heat exchanger failures. You can read more about this at http://ncidavid.blogspot.com/

I want to talk a bit about what NOx is, why an insert can help reduce NOX emissions, and why NOx inserts can cause trouble. NOx is a generic term for nitrogen compounds NO and NO₂ which are formed when nitrogen is present during combustion at high temperatures. These are major contributors to air pollution and acid rain. Laws requiring reduction of NOx emissions in areas of particularly high air pollution (California) have required the development of low NOx furnaces. Since the air is 78% nitrogen, any combustion process that uses air for its oxygen source has lots of nitrogen present. NOx will only form at high temperatures, but the burners in gas furnaces are definitely hot enough to form NOx. One quick and easy solution is to put something in the flame to cool it down. NOx rods and NOx inserts work by cooling the burner flame temperature, thus reducing the NOx emissions. These NOx rods or inserts are typically metal. Stick some metal in the flame, cool it down, and you have a low NOx furnace. The problem is that metal held directly in a flame eventually burns up. It does not burn quickly, but it does burn. That is why we don’t want the furnace flames actually touching the heat exchanger. The heat exchanger material cannot withstand direct flame contact, called impingement. NOx inserts stick inside a tubular heat exchanger. Unfortunately, the insert eventually degrades from being exposed to the flames. In some cases, it can twist or deform and change the flame pattern. This can cause the burner flames to touch the sides of the heat exchanger, and the heat exchanger fails prematurely. Before the heat exchanger fails, the furnace will be producing higher levels of carbon monoxide and carbon particulates because of the change in flame pattern. So you trade one type of pollution for another. If you work on furnaces with NOx inserts, be sure to check them – and watch the flames to see that they are not touching the heat exchanger. If you work in an area that does not require the NOx inserts, you can solve the problem by removing them. However, now you are increasing the amount of air pollution the furnace produces. Low NOx burner technology does exist – where NOx reduction is achieved through burner design, but I don’t know if there are any true low NOx burners for residential appliances. Keep an eye out for furnaces with low NOx inserts. It might be worth a few extra moments to wath the flame pattern and measure the CO in the flue gas. Abnormally high CO might indicate a developing problem. 

Why Excess Air Is Important

Combustion requires oxygen, which furnaces get from the air. Ventilation of the combustion products from a draft hood appliance, such as a water heater or an older natural draft furnace, requires even more air. For theoretically perfect combustion you need 10 cubic feet of air for every cubic foot of natural gas that is burned. However, the burners in even the most modern and well designed furnaces are not perfect. Combustion appliances all introduce excess air to insure there is enough oxygen for safe combustion. Too little excess air will have the burners operating in an oxygen starved condition, creating high levels of carbon monoxide (CO). Too much excess air can also be bad. Too much excess air will cool the flame, and also produce high levels of CO. Typical older natural draft appliances with atmospheric burners use around 50% excess air, turning the 10 CF of combustion air to 15 cubic feet. Nearly all residential furnaces manufactured today are induced draft appliances with atmospheric burners. In these furnaces, the excess air is more typically 20% - 40%. Excess air can safely go as low as 10% for commercial power burners that do a better job of mixing the air and gas.

In general, excess air decreases efficiency by cooling the combustion process. For any furnace, the ideal amount of excess air would produce the highest combustion efficiency without introducing an excessive level of CO in the flue gas. In most cases, as you reduce excess air you will see both the efficiency and CO increase. If the amount of excess air is excessive, reducing the excess air may actually decrease the CO produced in the flue gas. You want to keep the air-free CO below 400 ppm, the ANSI standard. Many techs try to keep the air-free below 100 ppm. Older gas furnaces had primary air adjustments, making it possible to adjust the amount of air being mixed with the gas. Newer furnaces do not have any air adjustments. You can only adjust the amount of fuel by adjusting the manifold pressure or orifice size. Increasing the gas being burned has the effect of reducing the excess air because now more air is needed. However, you should NOT overfire the furnace in an attempt to improve efficiency. When making any adjustments to manifold pressure or orifice size, always check orifice sizes and manifold pressure against the manufacturers specifications and the heat content of the gas supplied by the local gas utility. To read more on how combustion efficiency and CO production are affected by excess air, check out the Combustion Guide from Tru-Tech Tools (it is a free download HERE).

Furnace Drains

One important aspect of condensing furnace installation is sometimes overlooked – the furnace drain. A 90% condensing furnace makes a lot of water – comparable to an air conditioner evaporator. All that water needs somewhere to go. In most installations, there will also be an air conditioning evaporator and a drain for it. It is natural to just run the furnace drain into the air conditioning drain, but that can lead to problems. The positive air pressure inside the evaporator cabinet can create a slightly positive pressure on the evaporator drain. If the furnace drain is connected directly to it, this positive pressure can push back on the furnace drain. On some furnaces, this can create a positive pressure at the vent draft switch, causing it to open and shut off the furnace. One solution is to run completely separate drains for the furnace and air conditioner. If the air conditioner and furnace are connected to a common drain, you need to leave an air gap between the furnace drain and the common drain so that pressure cannot back up into the furnace drain and vent. If you are using a condensate pump, make sure it is rated to handle furnace condensate. Because furnace condensate is slightly acidic, some condensate pumps are specifically NOT approved for use with furnace condensate. In case you are concerned about the acidity, you can breathe easy. It is about the same as a carbonated beverage. However, some codes early on required that the condensate from condensing furnaces be neutralized before going into a sewer system. So manufacturers produced acid neutralizing filters which were basically large PVC containers filled with rocks (usually limestone or marble). The acid reacts with the rocks, which have a basic ph that counters the acid. I don’t believe most places require this any longer, but I am not certain. Another issue to watch is the drain outlet. If a gravity drain ices up or is blocked with snow, the draft switch can trip and shut off the furnace. Also, remember that the furnace really does make a lot of water. Try not to dump this water in a place that will cause problems – like right over a sidewalk.

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

PVC Vents for Condensing Furnaces

Since the advent of condensing furnaces, furnace manufacturers have been specifying  Schedule 40 PVC as the venting material they recommend. A good discussion of condensing furnace venting by Bob Formisano can be found at the HomeRepair site. Traditional metal vent material generally will not work because the vent gas on a 90%+ furnace is very wet and at a positive pressure. Other than stainless steel, metal vents would not last very long. Because the vent operates at a positive pressure, it must be sealed air tight. Traditional Type B vents are not air tight. With a negative pressure vent, the very small leaks in type B vents do not pose any problem. But if they were carrying pressurized vent gasses, even small leaks are a safety hazard. Since the flue gas temperature of a condensing furnace is much lower than a traditional furnace, plastic materials are feasible. So PVC seems like a good choice. It is definitely water proof and sealing it air tight is quite easy. The fact that PVC is inexpensive also makes it attractive. However, the standard referenced by many furnace manufacturers, ASTM D1785 for Schedule 40 PVC pipe, does not actually test PVC piping as a flue material. In fact, it specifically states that the standard does NOT cover its use as a vent for combustion appliances.

There have been concerns about PVC vent systems failing. Two sites I found that discuss failures and show pictures are  HeatingHelp, and Plumbing Engineering  Both sites discuss PVC venting systems on condensing water heaters and boilers where the PVC material turned brown, became brittle, and cracked or broke. In Canada, plastic vent materials must conform to ULC 636 – meaning no more “standard” PVC venting. It appears we may follow in the US. Some plastic materials made specifically for venting include UL 636 PVC, UL 636 CPVC, and a special polypropyylene pipe.

Here are a few links.
UL 636 PVC, CPVC
Polypropylene

Flue Season is Here!

No not the swine type, the furnace type! The weather is getting cold enough in many parts of the country that people are starting up their furnaces. Now is a good time to teach your students to check furnace flues during fall seasonal checks. The purpose of this article is not to discuss CO poisoning, but it deserves a mention since furnace flue problems can lead to carbon monoxide poisoning and CO poisoning shares many symptoms with influenza. One big difference is that influenza is normally accompanied by a fever and CO poisoning is not. For more information on CO poisoning check out the Carbon Monoxide Safety Organization web page.

A good place to start discussing furnace flues is to describe the four categories of vented appliances: Categories I, II, III, and IV. These categories are determined by the static pressure in the vent and the temperature of the vent gasses. For reasons of manufacturing and application limitations, Category II and III furnaces are rare. Most furnaces fall into either Category I or Category IV; 80% furnaces are category I while 90% furnaces are category IV. Category I furnaces are vented with type B gas vent. Category IV furnaces are usually vented using PVC. The combustion gas is cool enough to be safely vented through PVC and PVC is relatively easy to seal air tight.

In practical terms the vent gasses in a properly operating Category I furnace will not leak out small cracks because the vent gas pressure is less than the surrounding air. Vent gasses will generally not condense in a Category I flue because the temperature of the flue gas is considerably above dew point. Even though most 80% furnaces manufactured today have induced draft blowers, they still operate with a non-positive pressure vent because of the buoyancy of the hot combustion gas. However, the combustion gas coming from an 80% induced draft furnace is far more likely to condense in the flue than with older natural draft appliances. Oversized vents, single wall vents, masonry vents, or some combination of these can lead to condensation in the flue. Flue condensation can corrode metal vents and cause masonry vents to crack. Severe condensation can return water to the furnace and cause pre-mature heat exchanger failure. These situations most often occur when an older existing furnace is replaced with a newer, higher efficiency furnace. Even though the newer furnaces are designed for regular type B gas vent, they can not necessarily be connected to the old furnace flue. The extra heat in the combustion gas and the dilution air from the draft diverter of the older furnaces combined to make large single wall vent connectors and masonry vents work without condensation. The cooler combustion gas and lack of dilution air in the fan assisted furnaces makes their vent gas more susceptible to condensation. I have seen a single wall vent connector on an 80% induced draft furnace in a crawl space rust completely through and fall on the ground in a single year of operation. The furnace replaced a previous natural draft furnace that operated for many years without problems on the same type of vent. To prevent similar results when replacing an older furnace I recommend using only double wall vent and installing a metal flexible chimney liner when venting into a masonry chimney. An alternative to lining the masonry chimney is to vent horizontally using a power venter and not using the masonry chimney. More information on power venters is available from Field Controls

Another important step is to size the vent. The existing vent for the older natural draft furnace being replaced is often larger than is required for the 80% induced draft furnace. An oversized vent can also lead to condensation. You can download a pdf file on vent sizing from Hart & Cooley

Visual cues that furnace combustion or venting needs attention include: rust on metal vents, condensation weeping from vent joints, or carbon buildup anywhere in the vent system. Your students may run across non-condensing furnaces that were vented using a rigid plastic vent material called high temperature plastic vent, HTPV. This material has been recalled and should be replaced whenever it is found. HTPV recall If they see any of this material they should contact the furnace or vent material manufacturer or to find out what replacement vent material is recommended.

Fundamentals of HVAC/R can help your students prepare for flue season. Details on gas combustion can be found in Unit 37 Gas Fired Heating Systems. Furnace categories are discussed in Unit 38 Warm Air Furnaces. Vent sizing is discussed in Unit 40 Gas Furnace Installation, Startup, Checkout, and Operation. Gas combustion and venting problems are discussed in Unit 41 Troubleshooting Gas Furnaces.