HVAC systems not requiring Chimneys
I came across the following in the Architect's Studio Companion book.
The 3 diagrams shown below all show a chimney going from the boiler. Why does it then say to use these systems to avoid chimneys? Would really appreciate if someone can help me with this question. How do we identify HVAC systems that don’t need chimneys?
Hi. I noticed this (slight) error too. The text from page 243 is actually correct. See the bottom paragraph where it says that SOME high-efficiency fuel burning systems can be direct vented thru a wall. This is correct. The chimney would not extend up as high as shown in the diagram. And a direct vent typically needs power/fan. The power ejector (vent) is typically mounted low on an exterior wall, so if it snows a lot you need to shovel it out. It's not as good as a chimney in my opinion.
Hope this helps!
Rebekka O'Melia, R.A., NCARB, B. Arch, M. Ed, NOMA, Step UP ARE 5.0 Courses
Hey Sayali -
Nice catch. It’s not quite an error, but it’s definitely confusing. The diagrams show standard atmospherically vented boilers that use a chimney to exhaust the products of combustion from the building. Most boilers work this way. Water heaters and furnaces are also usually atmospherically vented in that they have chimneys that use natural stack pressures to get the dangerous stuff out. What’s not called out on the diagrams (but I suppose is implied) is that the air required for combustion comes not from the outside but from the house. (Crazy, eh?)
Atmospherically vented appliances are dangerous because of the potential for back drafting. For example, suppose an architect specifies a fancy commercial grade range hood (or an owner retrofits their kitchen with one later). Exhausting air from the kitchen can create a negative pressure in the conditioned space that can then cause the products of combustion from the boiler / furnace/ water heater to be drawn into the space rather than vented to the atmosphere through the chimney. With increasingly air tight homes (generally a very good thing) it doesn’t always take a whole lot of fan power to create a negative pressure strong enough to cause back drafting. This is a really big deal, and it’s the subject of a lot of discussion among building scientists and mechanical engineers as standards and codes related to air tightness, indoor air quality, and ventilation evolve.
A much safer approach is to use direct vent, sealed combustion appliances. These essentially have two ducts connecting the appliance to the exterior: one that brings in air to permit combustion and the other to exhaust the products of combustion safely outside. The whole process is sealed so it’s completely isolated from the interior environment, which means it’s not affected by any pressures in the house and it’s much safer from an indoor air quality perspective. (It’s also not expensive). This is the approach being described in the text, but it’s not the approach shown on the diagrams. Note that sealed combustion appliances still have to exhaust to the exterior! They’re just not doing it through a chimney using stack pressures.
For more information you might find this helpful from the Department of Energy’s Building America program:
I think your question gets to a level of detail that’s unlikely to be on the exams, but it’s good to know anyway:)
Thank you @Christine Williamson Cronin. Your explanation helps :)
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