Venting

LLL

What is the difference between induced draft, power vented and fan assisted?

billtwocase

venting

A powervent is a motorized vent that pulls the exhaust out complete with timer relays and safety controls to prevent the unit from firing should the motor not pull a negative draft. A direct vent uses a blower (burner if oil fired) to push the exhaust out complete with post purge timer relay. Draft inducer would be to a chimney vented appliance that has poor breech draft, or a long run of pipe.  Hope this helps

LLL

Venting

Thank you!

Tim McElwain

Good question, this is a subject which often the

definitions are stated incorrectly.



Let me add one and start there:



Forced Draft - this is were a blower is installed on the burner (oil or gas power conversion burner) ahead of the combustion process to premix air from within the space with the fuel. Both the air and the fuel must be properly matched to insure safe and efficient burning. Typically these are Category I venting as for gas and can vent through chimneys or metal vents. Found in NFPA 54 National Fuel  Gas Code.



Power Vented- is easily distinguished from the others because the "Power Venter" pressurizes the vent after the point of exit with a positive pressure therefore the vent material after the power venter must be sealed. Many times these are PVC or CPVC (not the best way in my opinion) or stainless AL294C sealed. This would be a Category IV vent and and is typical of 90+ equipment. These are side wall vented equipment. In some cases they are concentric vents. The venter both overcomes the resistance of the appliance heat exchanger and insures air for combustion enters the equipment to mix with the fuel being burned air for combustion is typically from outdoors. This is what is also called Mechanical Exhausting equipment and the vents must terminate 4 feet from any windows door or openings etc. Many Mod/Con appliances are what can be called Power Vented, along with premixing air with the fuel before burning much like Forced Draft. They are not forced draft however because they also pressurize the vent.



Fan Assisted- The fan in this case is used to over come the internal resistance of the heat exchanger. The air for combustion comes from within the room through conventional atmospheric burners needing primary and secondary air in order to burn safely and efficiently. The blower is positioned inside the appliance in such away as to pull the air gas mix products of combustion to a Category I vent, The vent is negative pressure and can be single wall galvanized or "B" vent and can be vented into a lined masonry chimney. Many mid efficiency appliances now days are Fan Assisted. Some of this category of appliance may be susceptible to condensing in the flue therefore they may be required to be stainless steel AL294C. In that case they would be classified as Category III venting. Some manufacturers state in the I &O manual for these appliance's that the appliance may be sidewall vented (horizontal) with say 3" vent. In that case they would be Category III. Or they can be chimney or B vented vertically with an increases say to 5" vent. An example is a Weil McLain CGa boiler there are others. This would be Category I.



I should add here that the Category of venting is on the rating plate of the equipment.It is a good idea to read the directions that come with the equipment to get these Category's correct.



Induced Draft- is typically having the inducing fan at the termination of the equipment after the combustion chamber. The purpose is typically to overcome poor venting conditions. Many times it is a device added by the contractor and is not part of the design of the equipment. Field and Tjernlund make some of these. All equipment must be interlocked when this type of venting is used.



We also have to mention what are called "Direct Vent"simply because a Direct Vent versus Mechanical Exhausting have different rules for termination Direct vent depending on the BTU rating of the appliance may be 9" or 12" from any windows doors or openings.



Sorry to be so long with the definitions but hopefully I made it clear.

billtwocase

Sorry LLL

I thought i would keep it simple. Tim has gotten into it in more depth. That should answer your question?

LLL

Venting

Thank you again, this has helped a lot, as an energy auditor I can never seem to absorb enough information...you never stop learning.  I want to leave the HVAC to the professionals but I also have to know enough to notice when something isn't safe for my customers.  Now that worst case CAZ testing is more important that ever dealing with DHW and furnaces is high priority. I'm really glad I found this forum, there is some great information to learn from.