In this article I would like to touch upon one of the biggest problems that I have seen with draft inducers since I started in 1967 and still continues. Many of us are familiar with the proper use and application for draft inducers , but how many of us know how to control them? The powerventer doesn’t seem to suffer as much from these control problems because most of them are sold with packaged controls. This is an older article that we updated based on a request.
Most of the draft inducer problems that I have seen are because they were not installed to Code, with an absolute minimum of controls that being an air proving switch, Figure 1. This is the most important control since if the inducer fan motor fails or there is a complete failure of the inducer due to blockage, the pressure switch will prevent the burner from operating. And answer me this. Why would you want torunaburnerwithoutpropercombustion venting?Keepinminditisatleasta Code violation and probably not a good idea. The air pressure switch shown in Figure 1 is a standard model which can normally be supplied by the manufacturer of the draft inducer, nothing special here. There is a small diaphragm inside the case which when pulled upon by the vacuum (negative pressure) created by the fan wheel again closes an electrical circuit. If the pressure is lost because of a failure of the fan motor the switch opens, and the burner stops.
Another problem has to do with making sure that the air flow (draft) is proper at both the beginning and end of the burner’s operating cycle. In Figure 2 , we have an installation which incorporates all of the elements that we could use to make sure that there is an adequate draft supply at all phases of the burner’s operating cycle while using a draft inducer. Although this picture is for oil, the rules remain the same for gas. In addition to the inducer there should be that air-proving switch and possibly a time delay and an oil (or gas) valve, found on the burner. This will ensure that there is adequate draft at burner startup and also help promote a clean shut down of the burner and become even more important if something fails. In many cases the inducer and controls may be an integral part of the appliance, Figure 3. Using Figure 3 we can find the inducer (1), the exhaust outlet (2) and the air proving switch (3) on the gas furnace shown.
The air pressure switch, Figure 1, which is normally found on or close to the draft inducer, Figure 4 , will not only verify draft conditions before operation but also during operation at all times when the burner is running and possibly create a post-purge. Another control may be a post-purge control, or relay-timer that will keep the inducer running after the burner has shut down and allows for the purging of all of the hot gases from the unit. Now, this may sound like a lot of expensive equipment, and you may ask ‘why won’t the control and burner companies make it simpler’? You see they have, and for a long time now. We’ll look at all of the most common options and one new one.
Inducer installation and wiring should always be done to the manufacturers (OEM) instructions while in compliance with all Codes. Figure 5 shows a typical installation for an inducer. Note that the sensing line can be aluminum or copper tubing. Brake line also makes good material for this use being acid resistant steel tubing.
Let’s look at a generic wiring diagram for a basic operating control for inducers, Figure 6 , with a typical hot water boiler controller . The burner can be a line voltage gas or oil burner or if a 24 Vac gas burner or control is being used, the power transformer for powering that system can be located across those terminals.
Note: this is just an example of the kind of circuits that will be found and, in all cases,refer to the OEM wiring.
Another thing to consider are the purges or ‘delays’. With many residential applications today these purges or delays are also referred to as ‘valve-on delay’ (pre-purge) and ‘motor-off delay’ (post-purge). Today you can buy many oil primary controls and gas ignition modules that will give you a guaranteed pre-purgecycleeverytimewhichcreates a nicely controlled, delayed start-up of the burner. You get constant air proving directly through that air switch and are assured of a good air flow through the appliance and venting system.
The last thing to consider is a postpurge cyclethatwillgiveyouacontrolled delayed shutdown of the burner fan and inducer. How do you get all these great features? Well, you could install a programmer ,but if you thought I had alot of expensive equipment on the table so far, waituntilyouseehowmanyhundreddollar billsyouneedtobuyaprogrammer!
Withamoderngassystem,theappliance may already have a control, Figure 7 , and wiring and be ready to accept an inducer that will work in conjunction with the ignition system module. However, with many conversions from oil to gas and the possibility of an even lower stack temperature (producing even less thermal draft) an
inducer maybe required and so the inducer controller becomesthesolutionformanyjobs.With an oilburner, there are many controls available that can work with an inducer control that can provide timed purges that will correct even the most difficult problems. Using the right control always does the trick and that brings us to our finalcontrol.
In Figure 8 is a Carlin70200 also known as a ProX with its burner wiring shown in Figure 9 . Byusingthiscontrolandthecorrectterminalsyou canhaveallthefunctionalityofapurgingoilburner primary with full control of your draft inducer. Rather than using a controller (like that shown in Figure 6) you would run two wires from your inducerprovingswitchtoyourprimary.Thetwo wires for your proving switch would go between theterminalsmarkedBV-BV, Figure 10. Thetwo wiresforyourinducermotorwouldgoacrossthe terminalsmarkedMOTORandL2inparallelwith yourburnermotor.Usingtheprogramminginthe70200 you can program your inducer and burner motor-off delays and the length of your pre-purge.
One note of caution: be sure to verify the amperage draw of your motors, it cannot exceed 10 amps total. With PSC oilburner motors your running amperage is about two amps and that can double at the start. With inducers however, residential split-phase motors still found on inducers can draw as much as five amps that can also double on start so check them! If worse comes to worse, you can always switch your motors through a contactor like they do on commercial burners.
In Figure 11 we show a switching relay or ‘RIB’ relay. Using this relay and the wiring shown in Figure 12 you can easily control and power the burner motor and inducer motor without destroying the relay from an overloaded contact.
If you use these components properly, you will have total control of the inducer and burner, but more importantly you will be able to have that necessary draft all of the time. Good draft makes for good burner operation. As I said, from going on so many jobs over the years it bothers me to have found that most of them all had the right equipment, what they were missing was the right controls. l FON
George Lanthier is the owner of Firedragon Academy. For more information, visit www. FiredragonEnt.com.