Furnace fan relays????

[troy3300]

Are the fan relays furnace specific or are they universal? I have a converted LP Lennox furnace with a/c. (24 volt)
I have just seen so many of these relays at all different prices. Ranging for $8 to $50. I have no idea which I need. Any help would be greatly appreciated.
Thanks

 

[AOD]

Most are made by Honeywell or Mars. A typical setup is a single pole, double throw relay. There is a normally closed side and a normally open side with a "tounge" in between, similar to a 3 way light switch. The NC side typically feeds power to the LOW speed of the furnace fan for heating use. the NO side is connected to the HIGH speed winding of the motor for use with air conditioning. When the relay is energized the "tounge" moves from the NC to the NO position allowing the fan to change speeds.

Confused yet?

 

[Slick]

I just looked for a long time and a couple hvac supply houses really confused me, wanted to built them etc....I didn't think it would be that hard...ended up getting one from my central boiler dealer for 29 bucks....t-stat click, relay clicks and it outputs 110 to my fan so I'm happy I think they are all pretty much the same if you only have a one speed fan to feed. Mine is a white rogers I think.

 

[mtfallsmikey]

I used Honeywell R8330D's on mine...scrounged from an old heat pump air handler. 24V. coils, can switch 2 loads, used them to switch on fan/secondary circ. RA89A/RA832 Honeywell's do the same job, bulkier tho.

 

[Labman]

Often all you need is a simple single pole, single throw SPST. Energize the coil, and it connects the 2 load terminals. A double throw one has one terminal hot until you energize the coil, and then it switches the power to another, SPDT. Double pole relays switch 2 sets of terminals the same way.

Most HVAC use 24 volt coil relays, although the newer furnaces may have 12 volt DC on their coils.

Match the coil voltages, the load voltages, and the load amps. You need to know how much the fan draws and select a relay with a higher rating. Many of the higher priced relays are lower volume things with special terminals and physical mountings. In your own design, you can make the lower priced ones work.

I am sure there is cheap, Chinese junk to avoid.