I'm planning for a project where I need a NC relay with a 24 VAC coil to disable my AC condenser fan, and I could use help selecting a correct relay to use and ensuring it's wired correctly. The condenser fan (OFM in the 1st diagram) is pretty typical with 3 wires, 1 coming from the contactor (CC) and 2 coming from the run capacitor (RC). See the attached diagrams. Any guidance on what relay to use to disable the fan and how to wire it?

 

First, I speak with a lack of knowledge on this. However, after looking at it for a short while it appears as though you have an (optional) LAC in circuit. IF you don't have this option then the two wire nuts would obviously be tied together bypassing that option. That would be a good place to put a relay. NOTE: I ASSUME ! ! ! WARNING - I'M ADVISING YOU ON A 230VAC CIRCUIT. EXTREME CAUTION IS HIGHLY WARRANTED AS I MIGHT GET YOU KILLED. I wouldn't want that.

So first, if you don't have the optional LAC then like I said it would SEEM as though this would be a good place to put an interrupting relay. One that would be wired in a normally closed (NC) configuration. When the relay is activated it would interrupt current to the fan and it should shut off. However, the relay will have to remain active to hold the fan off. On the other hand you could wire it normally open (NO) and have the OFM run when the relay is active.

Or second, if you DO have the (optional) LAC in circuit you can put the relay in series with the LAC. Again, how you want the control to work would depend on how you want to power it. Again there are two ways to wire the relay.

As for relays, you'll want one that can handle 230VAC contacts with a high enough amperage rating for the OFM. If you don't get one that is sufficiently rated (or higher) then it will burn out. Unless I'm missing it - I don't see any specs besides the voltage listed anywhere. It's apparent this is a 230 VAC AC unit. But the current draw of the OFM is not listed. I can't give you a definitive answer as to what relay would be good. Unless you don't mind overkill. Overkill would be a 24 volt control coil, likely AC type with contacts rated at 250 VAC @ 20 A. There are a host of such relays you can google and purchase.

Now for the important part - I am not an expert on this subject. Working with 230 VAC is dangerous. You must proceed with caution. MOST IMPORTANT OF ALL - TURN OFF THE BREAKERS CONTROLLING THE AC UNIT AS WELL AS THE HEATER UNIT (if so equipped). DOING SO WILL SAVE YOUR LIFE. Only then will it be safe to access the high voltage components and switching circuitry. I've been shocked with 120VAC and 65KVDC Both hurt. But believe it or not - the 230 VAC is more dangerous than either of those two voltages. 480 would be even more dangerous, but you don't likely have 480 at your home.

Please be careful.

 

Here's a doodle: (I'm replacing the LAC with a 24VAC relay. The 24VAC is referenced only to the coil voltage, not the contact ratings. If you HAVE the optional LAC then put the relay in series with the LAC. This doodle shows the relay in the NC configuration, during which time the fan will run. If you want the fan OFF unless the relay is active then wire it NOT to the NC contact but to the NO contact. That way it will only run when you command it to by activating the 24VAC relay.)

 

One final note before I shut up - - - why are you changing the operation of a refrigeration unit (AC)? It's designed to operate in a safe and reliable manor. Changing things may present unexpected consequences. If the OFM isn't running when it should be then the unit won't cool, but rather generate more heat inside the house. And not the kind of heat you might want when you want to heat the house. The heating will be insufficient for warming the interior and will be hugely wasteful and expensive to run.

 

Thanks for the response and the caution. My unit does not have a LAC installed. I was originally thinking I'd need to shutoff both of the 120v wires going to the fan, the one coming from L1 on the contactor through the LAC, and the common one coming from T3 on the contactor via the capacitor. But it makes sense that a LAC is already meant to serve as a fan shutoff, so that wire is probably all I need to put through a relay.

There may be a sticker on the bottom of the motor describing amps. I'll have a look later and make sure the relay is rated high enough.

As for why I want to do this, I appreciate the caution. I'm planning to have a water cooled AC condenser that also serves as a pool heater professionally installed alongside my normal AC condenser, which can be switched over to by a controller. When it's active, the compressed refrigerant in the air cooled condenser is sucked into the water cooled side, so the condenser fan is not needed and we can shut it off to save energy. For me, I just want to provision a few parts for the HVAC tech and make sure I understand the install.

 

Actually saw that on Ask This Old House about a year ago. Certainly last year, "the year of the Covid". I don't know when this originally aired, but here's a link to the exact thing you're talking about doing. Incidentally, with a heat pump you can push heat into the pool all summer long and then draw that heat back in the winter time. Not sure if they cover that; like I said, it's been a long time since I saw this.

 

That is exactly the system I'm installing. Since it's a bit unusual, nobody in my area is familiar with how to install it, so I'm trying to understand and plan the install as much as I can myself to help them along.

Unfortunately my unit is not a heat pump so I'm limited to summer heating, but I'm really excited just to have that.

 

Well, free heat for the pool and a more efficient running air conditioner. You DO realize that there's going to be more involved than just shutting down the OFM. Well, hopefully the short video helps you get a better idea of what's needed to accomplish the job as well as the professionals you're going to have to hire to get the job done. Regardless of cost it's a good investment. Good in that air conditioning will be more efficient and the pool will be heated (summer time), which in early spring can be helpful when you're air conditioning the home. But also consider that the pool water can get bath tub warm. That might not be as pleasant as a cool dip in the summer.

 

I have this system. What did you decide to do about the NC relay. I didn’t install the relay and my fan runs normally when FPH is active.

 

I ended up purchasing a RIB24P30, which handles up to 30 amps, which may have been overkill, but I wasn't positive what amperage I needed to support. It's pretty large and will be a squeeze to fit it into the AC, but it should fit. That said, I haven't installed it or the FPH yet, so I don't have much to report on it.