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.
Showing posts with label vent. Show all posts
Showing posts with label vent. Show all posts
Sunday, November 1, 2015
Sunday, August 18, 2013
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
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
Sunday, October 7, 2012
Check the Draft
With the arrival of Fall weather technicians will soon be taking Fall furnace checkup calls. One of the things we should be doing is checking the vent draft pressure to insure that the vent is working properly. While many technicians are careful to check for cracks in heat exchangers, they sometimes neglect the vent. I believe that improper venting occurs far more often than cracked heat exchangers. Further, a vent that is spilling into the house has the potential to release far more vent gasses into the house than a crack in the heat exchanger. Of course it is right to check for cracks in the heat exchanger, just don’t neglect the vent operation. For all Category I furnaces, the vent should be a negative pressure. That is, it should be at a lower pressure than the room the furnace is operating in. Typically we want at least -0.02” wc. In other words, the vent pressure should be 0.02”wc less than the room pressure. This is true even with fan assisted Category I furnaces, which nearly all 80% AFUE furnaces are today. The fan helps pull the combustion gasses through the heat exchanger, it is NOT designed to PUSH vent gasses through the vent. Typical Class B vent is not air tight. If it is pressurized, it will leak out combustion gas.Even with fan assisted furnaces, the vent is supposed to operate at a negative pressure and the vent gasses leave because of their buoyancy compared to the surrounding air.
What can cause a lack of draft in a vent? One of the main culprits is lack of combustion air. The operation of the burners and the vent system can remove air from the room faster than it is being supplied, causing a negative pressure in the room. In older, leakier homes we often relied on infiltration for combustion air. Today, you really should provide outside air to the appliances. I have seen the operation of a furnace pull smoke out of a burning fireplace. There was enough air for the fireplace, but not both the fireplace and the furnace. The room went so negative that the fireplace vent was not lower than the room pressure. If the vent gasses cool off too much in the vent pressure will increase because the gasses are heavier, decreasing the draft. This can be a particular problem when replacing an older furnace with a newer one. Often, the vent is too big for the new furnace, causing the flue gas to cool off too much as it travels through. Another possibility is a plugged vent or vent cap. The flue gasses back up in the vent and then start to spill out of the appliance.
What can cause a lack of draft in a vent? One of the main culprits is lack of combustion air. The operation of the burners and the vent system can remove air from the room faster than it is being supplied, causing a negative pressure in the room. In older, leakier homes we often relied on infiltration for combustion air. Today, you really should provide outside air to the appliances. I have seen the operation of a furnace pull smoke out of a burning fireplace. There was enough air for the fireplace, but not both the fireplace and the furnace. The room went so negative that the fireplace vent was not lower than the room pressure. If the vent gasses cool off too much in the vent pressure will increase because the gasses are heavier, decreasing the draft. This can be a particular problem when replacing an older furnace with a newer one. Often, the vent is too big for the new furnace, causing the flue gas to cool off too much as it travels through. Another possibility is a plugged vent or vent cap. The flue gasses back up in the vent and then start to spill out of the appliance.
Although most Category I furnaces with draft inducer fans have draft pressure switches to shut off the burners if the draft pressure is not at the minimum setting for the switch, I have seen furnaces operating with a positive vent pressure continue to run. Draft switches are, after all, switches and can fail. So don’t assume that because the draft switch is closed, the vent pressure is OK – measure it so you know.
Monday, November 16, 2009
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.
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