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Chimney Physics

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Chimney Physics Chimney Physics Russ Dimmitt Chimney Safety Institute of America Learn what The House as a System means specifically to you as a retailer and installer of appliances that must vent to the outside atmosphere. This session will include topics like: • building envelopes, • pressure to measure pressure, • stack effect and what causes it, • house systems and exhaust fans, and • techniques to allow hearth appliances to function properly. 2020 HPBExpo Education Sponsored by: . 2/24/2020 Chimney Physics Russ Dimmittt Director of Education of the Chimney Safety Institute of America and the National Chimney Sweep Guild 1 Draft & Flow • Draft – the force or pressure • Flow – the volume of gases difference between the inside which actually pass through the and the outside of the venting system as a result of draft. system. 2 Draft Draft can be further defined as the measure of force which causes gases to move up and out of the chimney and air to be drawn into appliance. Two major factors influence draft: 1. The difference between the average flue gas temperature and the temperature of the outside air. 2. The height of the chimney. 3 1 2/24/2020 Temperature Difference • Warmer flue gases have greater buoyancy than cooler gases • Warmer less dense gases rise and are replaced by cooler, heavier gases • These rising gases create a vacuum at the appliance and combustion air is drawn into the combustion chamber 4 Temperature Difference • The greater the temperature difference between the gases inside the flue and the air outside the chimney, the greater the draft will be and air will be drawn more forcefully into the system. 5 Chimney Height Taller chimneys contain a taller column of warm, rising gas. The movement of the gases also increases the draft, however, the height is the critical factor, not the volume or movement of the gases. 6 2 2/24/2020 Measuring Draft Chimney draft is measured in inches of water column. Good draft would be represented by a negative number because the pressure in the chimney is less than the surrounding atmosphere. Lower negative numbers represent greater draft. 7 Measuring Draft ‐.08 in. WC represents greater draft than ‐.04 in. WC ‐.04 in. WC is greater draft than +.04 in. WC ‐.1 in. WC is greater draft than ‐.01 in. WC 8 Measuring Draft • Older technology utilized U‐tube manometers or analog manometers to measure draft • Technicians now use digital manometers to measure draft 9 3 2/24/2020 Flow Capacity Flow is the volume of flue gases which actually move through the venting system as a result of draft. Three factors which influence flow capacity are: 1. Draft 2. Amount of resistance to flow 3. Size of venting passageways 10 Resistance to Flow Friction always exists between moving gases and the flue walls. Variables affecting friction – or resistance to flow – include: Bends & turns in venting system Changes in size or shape Surface irregularities (mortar protrusions, etc.) Appliance air inlet settings 11 Relationship of Draft and Flow • Fireplaces require large flow capacity but not strong draft. • The dilution air sucked into the fireplace promotes rapid combustion which requires large flow capacity to vent the high volume of flue gases. Due to the large flow capacity draft may be weaker • Closed appliances such as inserts and free standing stoves require strong draft but not large flow capacity • Because combustion air is limited the combustion is slower and the reduced volume of flue gases require less flow capacity 12 4 2/24/2020 Troubleshooting • A chimney must perform three functions • Safely exhaust the products of combustion to the outside • Protect surrounding combustibles from the heat of exhaust gases • Draw oxygen into the appliance to sustain combustion 13 Troubleshooting • When troubleshooting follow a basic three step process • Gather information on the symptoms of the problem • Make a tentative diagnoses and test if possible • Specify effective solutions or refer the customer to and expert to provide solutions 14 Troubleshooting • Start gathering information by interviewing the homeowner and making observations of the installation. Some questions that may need to be answered: • Is the problem intermittent or constant? • When does the problem occur? • Is the problem worsening or does the severity vary? • Are there any consistent circumstances that are related to the problem? • What solutions have been attempted thus far? 15 5 2/24/2020 Troubleshooting • If it can be done without undue risk light the appliance and replicate the problem, make a diagnosis and attempt a trial solution • Try one solution at a time • Try cheapest and easiest solutions first • Be aware multiple problems may exist on any system 16 Troubleshooting • Draft is an important element of chimney performance • Draft is expressed in negative numbers as it is a suction • Draft must be sufficient to overcome negative pressure in the home 17 Troubleshooting • Troubleshooting appliances can be challenging as several factors may be in play that add up to a problem. Some things to investigate: • Chimney height • Vent design • Negative pressure in the home • Fuel choices • Burning habits • Wind • Remember the house will function as a system 18 6 2/24/2020 Troubleshooting • When troubleshooting open fireplace issues it is useful to understand the homeowners expectations • Performance issues with fireplaces generally are related to smoke spillage • Wind induced • Flow reversals due to pressure issues • Inadequate flow • Inadequate draft 19 Troubleshooting • Wind may cause smoke and flue gases to be forced from the chimney into the home • Wind induced issues are typically sporadic and occur under specific circumstances • The issue may be caused by high pressure at the termination or wind blowing down the flue 20 Troubleshooting • Barriers downwind from the chimney may cause wind to “pile up” and create a high pressure zone at the top of the chimney • Barriers upwind may create turbulence which blows down the chimney • Raising the chimney height may put it above the problem area • Specially designed chimney caps are available to combat wind problems 21 7 2/24/2020 Troubleshooting • Negative pressure in homes is an ever increasing issue for chimney technicians • Negative pressure has two basic causes in a home • Mechanical systems that exhaust air from the home • Stack effect 22 Troubleshooting • As homes are sealed tighter to aid in energy efficiency, it becomes more difficult for natural draft appliances to function • Exhaust devices such as clothes dryers, attic fans, kitchen range hoods and central vacuum systems all exhaust air from the home. When a home is tight often the easiest way for replacement air to enter the home is down the flue • Natural draft appliances will lose when competing with mechanical systems 23 Troubleshooting • Stack effect is caused when heated air rises and leaves the home • The home functions as a chimney and the taller the home the greater the stack effect • Pressure will change throughout levels in the home 24 8 2/24/2020 Troubleshooting • Any appliance in the negative pressure zones of the home will have to overcome the negative pressure to vent • The neutral pressure plane will follow the leaks, if a window is opened the neutral pressure plane will move to that location • The lower the location of the neutral pressure plane, the less the negative pressure at the lowest levels 25 Troubleshooting • Stack effect can not be eliminated but it’s effects can be minimized or eliminated by sealing leaks at upper levels • Other appliances operating in the home may overpower an appliance at a lower level causing spillage or back drafting • Bringing air in a the lowest level possible can minimize stack effect and allow appliances to operate 26 Interior Venting Systems • Installations completely within the structure are surrounded by warm air • Interior systems will always perform better than outdoor systems 27 9 2/24/2020 Interior Venting Systems • Promote fast, easy start‐up • Maintain better draft • Reduce condensation and creosote • Rarely have a “smoky tail‐out” • Radiates heat indoors, not outside 28 Exterior Venting Systems • Exterior systems are surrounded by cold air • Exterior systems will always perform more poorly than an interior design 29 Cold Hearth Syndrome • “Cold Hearth Syndrome” occurs when cold outside air flows down a chimney that is not currently in operation • Common causes are: • Exterior chimneys serving an appliance located below the Neutral Pressure Plane • Building envelope is higher than the chimney 30 10 2/24/2020 Exterior Venting Systems • “Cold‐Hearth Syndrome” • Makes it hard to initiate a fire • Promotes condensation of flue gases and increased creosote through cooling effect • Have a lower draft as a result 31 Higher than Heated Area • Whether located indoors or out, the chimney should terminate higher than any portion of the heated area (building envelope) Heated Area 32 Chimney Termination The 3‐2‐10 Rule All residential chimneys must terminate at least 36” above the point where they pass through the roof and 24” above any structure within 10 feet of the chimney. 33 11 2/24/2020 Troubleshooting • Leaky duct work may affect pressure levels in the home in a way unrelated to stack effect • Operating exhaust fans will exacerbate pressure issues 34 Troubleshooting • Mechanical draft inducers may create other problems in the home. Draft inducers are not a solution for pressure issues. They are to be used for sizing or construction issues • Additional air introduced into the home
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