Boiler room design requires an understanding of boiler room equipment, applications, local boiler code and boiler room airflow and venting requirements. It cannot be overstated that improper design can cause dangerous safety issues such as fire and excess carbon monoxide levels, or nuisance trips, or premature failure of ignition systems due to over or under drafting.
Why is proper draft important
Every boiler is a packaged system of a heat exchanger, burner, and associated onboard controls. When boilers (and water heaters) are being designed and subsequently tested, they are under very specific factory/lab conditions with a single stack. A single stack per appliance under 30’ tall is an ideal layout, and allows the boiler to achieve the best efficiency. In the real world, there are no “simple” venting systems. Especially doing retrofits in existing buildings. Deviating from the ideal stack layouts can affect the boiler in several ways.
Too much draft, and the heat is pulled through the boiler too quickly. This prevents contact time to exchange the heat from combustion into the water of the boiler. It can also lead to nuisance trips because the high draft can extinguish the pilot, or prevent main ignition of a canister type burner typically found on high-efficiency condensing boilers.
Too little draft, and the boilers can struggle to start consistently because there is back-pressure at the outlet of the boiler. The flame can also sit back too far ON the burner components, which leads to overheating and premature failure. If the back pressure is high (not enough draft), there is a chance for CO leakage if the venting system is not tight.
For proper operation and efficient fuel consumption in boilers and appliances, draft must remain constant. Draft Systems maintain consistent draft by counteracting the negative forces caused by changes in temperature and barometric pressure and outside conditions, not to mention variable boiler load, burner modulation levels, and system temperatures. There is a LOT going on when it comes to venting – it’s not static by any means.
Too Much Draft:
Relatively cool ambient air exerts pressure on the outside of the furnace or boiler, the breeching, and stack. The pressure difference between the room air and heated gas (exhaust) causes products of combustion to flow (draft) through the unit and rise through the breeching and chimney.
We usually see appliances go outside of their factory tolerances above 50’ vertical rise. If you have a system that is venting over 50’ vertical rise, you should definitely take a look at ways to control excessive draft. Or – if you have multiple boilers into a single manifold – what happens when only one boiler is running in low fire? The manifold that it’s connected to is far oversized, and thus, there will be too much draft.
One way to approach this with oil burners was to install a barometric damper. A BD would be installed in a tee section, and open to the mechanical room. This introduced heated room air to bleed into the stack system to essentially “break” draft. The damper blade is weighted to try to keep it closed when the boiler is offline. However – you are losing overall building efficiency by letting room air literally “go up the stack”. In addition, these shouldn’t be applied to condensing, or positive pressure-rated boilers (category 3 or 4). The barometric damper is not pressure-tight, and not stainless steel, which would lead to rust, so it’s not the best application.
The better way to control draft in multiple condensing boiler applications with tall stacks is to consider a US Draft damper system. These actually monitor the outlet pressure of the boiler, and modulate the stainless steel damper to maintain the draft tolerance that is outlined specifically in the boiler manual. No worries of corroding the damper with stainless steel construction. No worries about inefficient operation, or nuisance faults.
US Draft’s line of products are designed to fully control a venting system. Regardless of the weather, temperature, or operating conditions, it controls the draft precisely to what the boiler(s) or water heater(s) are designed for by the manufacturer. Boiler manufacturer’s test all of their products with individual exhaust and combustion air vents.
Figure 1 below is a common draft system scenario. US Draft offers a full line of Constant Pressure Controllers, dampers and fans.
Too Little Draft:
On the other side of the designer’s challenge are long horizontal venting runs. Or – multiple boiler vents, or too many elbows, tee sections, etc. Each boiler manufacturer will either publish the maximum lengths of exhaust duct that can be used, or, the draft tolerance of the appliance.
When you approach the lengths the manufacturer’s guidelines allow, it should give pause. When you exceed those limits, it’s time to get a venting expert involved. In addition to handling too much draft with a unique damper system, US Draft also offers a solution here with stainless steel constructed inline and termination fans.
These fan SYSTEMS are designed to measure the draft, and speed up or slow down the fan in accordance to exactly what the boiler manual calls for. These are rated to vent category 1, 2, 3, or 4 type appliances. Direct drive, no belts, and continuous temp rating up to 575 deg F (750 intermittent). Super reliable and designed to last a long time.
Reduce cost and material
For multiple boiler projects, we recommend combining the flues into one system to regulate the draft with a fan. This saves on material and installation costs compared to multiple individual stainless steel stacks, and simplifies installation, saves space, guarantees boilers will always run as efficiently as they were intended, and you have consistent and reliable ignition keeping the draft within tolerances.
Each facility has its own unique needs. From single appliances to large multi-boiler projects, US Draft has the product solutions to engineer the draft system to meet the demands of the system. Whatever the boiler manufacturer designed their boiler vent tolerances to be, we can deliver it.
Contact a venting expert at Frank I. Rounds today!!