Best Of

When Vent dampers were introduced in the 1970s It was like open season in the wild west. Everyone wanted to get in on the energy saving bandwagon. Most of those damper manufacturers have gone out of business because they left the safety of the appliance up to the installing individual. Many of the installers were electricians (or wanna bee electricians) that did not understand the safety feature that prevented the burner from operating if the damper was closed. As a result, many consumers experienced serious carbon monoxide problems as a result of improper installation or tampering with the unit when they had a “NO HEAT” call that may or may not have been related to the vent damper. Ultimately the insurance policy for those companies that made the product became too expensive (as a result of too many claims) that there was no longer a profit in it.

Effikal was the winner of the best product but was not a large enough company to stay in business and sold the rights to manufacture that best quality damper to Field Controls. They are pretty much the only vent damper that boiler and furnace manufacturers use today.

Here is the Vent Damper wiring discussion https://forum.heatinghelp.com/discussion/179453/

And my comment about it includes a wiring diagram.


Here is a possible wiring diagram that MAY help you. Post picture of the inside of the L8148E control you have, and the make & model number of YOUR boiler. It looks like a Hydro Therm but I may be wrong



The basic concept (Operational Logic) of an automatic vent damper is to close off a natural draft chimney in order to reduce the down time loss of heat from the boiler and inside the home safely. You don't need Draft Exhaust if there is no flame, because there is no dangerous exhaust when the burner is off.

This must be done Safely by not allowing the burner to operate if the damper is closed. No path for exhaust equals no burner operation.

To accomplish this Safely, the motor mechanism must have 24 V power always. This is to be able to close once the controls and safety devices determine there are no exhaust gases that require venting. The Motor is actually connected to what we call an Actuator. This is the motor and some switches, thermostats, and relays that by design, regulate when the damper may close. In the case of Field Controls and other control manufacturers like United Technologies, and Honeywell, the Molex plug terminal 1 and 4 are the terminals that have constant 24 volts for damper actuator operation. Once it is safe, 24 volts on terminal 1 and 4 will operate the motor until the damper is closed. The internal switches and relays will stop the motor from operating once the damper is in the closed position.

In this closed position the gas valve can not get power to operate the burner as long as the damper is in the closed position.

On a call for heat, IF all the other safety devices are satisfied that it is safe to operate the burner, the #2 terminal on the Molex connector will be energized with 24 volts and the damper motor will start to turn. The motor turns until the damper is in the full open position. Once fully open, the internal relays and switches stop the damper movement and the damper will stay in the full open position. Another set of switches and relays will then energize 24 volts to the #3 terminal on the Molex connector. This then sends power to the gas valve to open and allow the flame to establish. As long as there is a flame or other source of exhaust gasses (or perceived exhaust gasses... by high temperature in the exhaust pipe may be one way to tell) The damper can not close.

Most automatic vent dampers operate on this principle. Some have a spring opening and electric close, but most brands have a damper plate that rotates in the same direction making a 180° revolution every cycle or a 360° revolution every 2 complete burner cycles

Regardless, what ever brand or design of vent damper you have, the safe operation MUST be maintained by proper wiring, and free unobstructed path for the damper plate to open.

All the Professionals on this site fully understand this Sequence of operation (or Operational Logic) for vent dampers. Not all would like to deal with manually wiring of the device and prefer to just "Plug and Play" using a new control on a new damper ... where the now universally accepted "STANDARD DESIGN" makes wiring effortless and fool-proof in most cases.

I hope this helps you with your particular situation. If you feel up the the job, you now have all the information you need. If you are not comfortable with control wiring or do not fully understand the safety and importance of getting this right, then PLEASE GET A PROFESSIONAL that understands how this should work. Where do you find this technician? Go to you local Plumbing and heating or HVAC wholesaler and ask the counter man, "Which one of your customers is good at control wiring?"

They know who they are, because they are the guys that buy lots of controls and never return them because "This darn thing was bad out of the box"