CATEGORY I VENT DESIGN©
by Bill Smith, president of Elite Software
The morning newspaper proclaims, "Family of 5 Dies of Carbon Monoxide Poisoning." In your worst nightmare, you begin to read the article and recognize the victim's names. You remember changing out their old, oversized, low efficiency furnace with a new, smaller, more efficient furnace. The existing vent appeared to be in good condition and as the efficiency of the new unit was less than 83%, you were confident the old vent would work fine.
Although the above scenario rarely happens, it is by no means impossible to occur. Acidic condensate from the vent gas can easily form on the inside of a vent pipe and cause rusting and pitting. If the corrosion process continues long enough, the vent pipe will become riddled with holes and ultimately fail. When this happens, vent gas containing carbon monoxide and all the other products of combustion can leak into occupied areas of the building.
The National Fire Protection Association (NFPA) has addressed venting design with the publication,NFPA 54: National Fuel Gas Code (NFGC) 1992 Edition (800-344- 3555). The American Gas Association, Gas Appliance Manufacturer's Association, and Hydronics Institute offer their own publications for vent design based on the NFGC.
The most common type of venting problem faced by hvac contractors are for Category I type gas appliances. Category I appliances are primarily described by the NFGC as gas appliances with annual fuel utilization efficiencies (AFUE) below 83%. Unlike Categories II, III, and IV where special provisions are made for condensate, condensation is not acceptable for Category I gas appliances. The vast majority of central furnaces and hot water heaters fall into Category I.
The higher the AFUE of a furnace, the more likely it is to produce condensate. This is particularly true as the AFUE approaches 83%. Since Federal standards have raised the minimum efficiency of furnaces, there is greater possibility that some of these low end furnaces with improved efficiencies to just below 83% might produce condensate. Older design furnaces, with no provision for dealing with condensate, are especially vulnerable. Proper vent sizing and design is especially critical for those type furnaces.
There are many factors that affect the required vent size. The appliance input btuh rating, outlet diameter of the appliance, number of elbows, single or multi- story application, vent gas temperature, ambient air temperature, vent wall conductivity, vent connector type, vent thermal mass, vent pressurization (fan assisted or natural draft), vent height, and vent lateral length all affect vent design. The NFGC provides nine venting tables that take all these factors into account so that both minimum and maximum allowable vent and connector sizes can be specified for any given situation.
In designing a vent system for a Category I appliance the contractor usually does not have control over all the above mentioned factors. The most common factors that a contractor can adjust in his vent design is the selection of the vent and connector types and their sizes. The vent connector is the pipe that connects the appliance to the main vent pipe. A vent connector can be either single-wall metal pipe or Type B which is double-wall metal pipe with an insulating effect. The main vent can never be made of single-wall metal pipe. It must be either Type B pipe, tile lined masonry chimney, or a flexible metal liner within a chimney.
Selection of single-wall metal or Type B vent connectors is mainly governed by cost and applicable restrictions. Single-wall metal vent connectors are lower in cost than Type B connectors, but they operate at much higher surface temperatures than do Type B connectors. Consequently, the NFGC places many restrictions on the use of single-wall metal connectors.
For example, single-wall metal connectors cannot be used in attics due to the fire hazard, and they must have greater clearances on all structural components than Type B connectors. A good strategy for maximum safety and minimum chance of violating building code requirements is to use only Type B double-wall metal vent connectors.
If a chimney is not being used for the vent, then the NFGC specifies that only Type B pipe can be used for the vent. If a chimney is involved, it must have an appropriately sized tile liner or a flexible metal liner. Some chimney's appear to have a tile liner, but upon close inspection, the liner may only exist at the top of the chimney.
Venting combustion gases through a chimney without a proper liner often results in the collapse of the chimney as condensate can dissolve the mortar between bricks. Even if a tile liner is in place along the entire interior of the chimney, it is also possible that it may be too large for the appliances being vented through it. If there is any doubt as to the presence, size, and quality of a tile liner in a chimney, a flexible metal liner of the appropriate size should be used.
Once the vent connector and vent types are selected the contractor is ready to size the vent using the NFGC venting tables. In the simplest case with only a single appliance, the information needed to size the connector and vent includes whether the appliance venting is fan assisted or natural draft, the appliance input btuh rating, appliance outlet diameter, lateral distance from the appliance to the vent, height from the appliance to the top of the vent, and the number of 90 degree elbows. For the given data, the venting tables list the minimum and maximum size vent and connector diameters that can be used.
Although the vent tables appear fairly straight forward, one of the difficulties in using them is that there are over 30 notes and exceptions explained in the NFGC that must be considered. All too often, a size read from the table must be adjusted due to a special condition. For example, for every 90 degree elbow beyond the quantity two, the btuh capacity for a vent size must be reduced by 10%. Similarly, there are maximum horizontal connector length limits. The btuh capacity of a vent must be reduced 10% for each multiple of a specified horizontal connector length.
There are many more such qualifiers to the venting tables and making sure each of them is honored is the tough part of vent sizing. This is particularly true for multiple appliance and multi-story applications.
The table look up approach with the necessary adjustments and qualifications makes Category I gas vent sizing a perfect candidate for computerization. Using software, a designer can enter the information for his application and instantly see the minimum and maximum allowable vent sizes with all checks and adjustments automatically done.
Ideal software not only gives instant and accurate results, it also makes the data entry process easy and intuitive. Nothing provides a more visual and graphic environment for hvac design programs than does the Microsoft Windows environment. For this reason, Elite Software created a Windows based program called GASVENT especially for the hvac contractor. GASVENT is one of the very first Windows based programs ever offered to the hvac community.
As can be seen in the sample screens for GASVENT, it uses graphic detail virtually impossible under conventional DOS. Icons, radio buttons, list boxes, dialog boxes, and hypertext help are all new terms and features that hvac contractors will soon be discovering in Windows based hvac design software. Although strange sounding at first, these features genuinely improve software ease of use.
The proper design of vent systems for Category I appliances can literally be a matter of life and death. Besides carbon monoxide poisoning, there are many other consequences of a poor vent design: chimney collapse, fire hazards, corrosion of draft inducers and heat exchangers, non conformance with building codes, and major legal liabilities. Although software can't remove all the risks in vent design and installation, it can definitely help you do more accurate designs in less time than ever before.
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