Scenario: Two gas fired water heaters with exhaust vent/fans are being supplied with fresh air from a duct with an auxillary vent/fan. The motors are plug connected to a receptacle mounted on top of each WH, so tapping into the temp controlled exhaust fan feeds to control and feed the aux. vent fan seems to be an unintrusive endeavor, and should keep the warrenty intact. All three motors are less than 2A FL each...

Here's my idea on how to use two 10A ice cubes to allow the auxillary vent fan to operate when either of the two w.h.exaust fans come on, and the aux. vent fan be off only when both W.H.s are off. All this without creating a parallel path...

My kingdom for a relay/contactor that has two independant coils that switch a single contact.



This could be easily adapted to involve a seperate feed for the aux. vent fan, and/or have said load going through a more robust contactor, with the leads marked "to aux vent fan" going to the contactor coil.

Kosher?

Is ther a simpler/cheaper way?

Thanks in advance...

-Virgil

[This message has been edited by sparky66wv (edited 05-25-2002).]

Virgil,

If you can provide a separate feed for the auxiliary fan, wouldn't it be simpler to just use relays with a SPST (normally open) contact and parallel the contacts?

The coils would each be wired independently to the outlets as per your diagram, and either relay closing would energize the aux. fan from its own supply.



[This message has been edited by pauluk (edited 05-25-2002).]

I've just looked back at your diagram again after printing it out. Two more points.

First, aren't the NO and NC markings the wrong way round on both relays?

Second, I've just realized that one of your relays is completely redundant and you could do it with one DPDT unit.

Scratch the left-hand relay and wire the hot/neutral from vent fan #1 directly to the NC contacts of the right-hand relay.

Now, with the relay in its released position the aux. fan will just go on and off with vent fan #1. When vent fan #2 kicks in the relay energizes and connects the aux. fan to that supply instead.



[This message has been edited by pauluk (edited 05-25-2002).]

Don't follow you, Paul...

I'm a little slow (and completely self-educated) when it comes to control work.

Anyway you (or someone who understands what he's telling me) can sketch, scan and email me a copy?

I'm going by memory on the half a dozen times that I've actually held any control work in my hands, so the terminals locations aren't as important as the function, and I always meter the relay to see which NO and NC contact is closed while the coil is energized and then go from there.

I know, I have no business being around control work... But I've had guys who do control work everyday ask me how to wire a 4-way switch!

66vdub-- Have you considered something like: http://www.crmagnetics.com/newprod/ProductView.asp?ProdName=CR9321 ?
[Pass a lead from each fan motor through the opening…just be sure the polarities are the same.]

For their small size I'd just let the fan motors run continuously.


[This message has been edited by Bjarney (edited 05-25-2002).]

Bjarney, thanks! That device was what I had imagined to use for another project for a aux vent fan for the dryer, but lo and behold it already had a venturi type pressure switch, which isn't as reliable as this option.

I'm glad to know that the animal exists...

No twin coil contactors?

I've done a couple of rough sketches of what I had in mind. I'll be seeing a friend with a scanner later today, so I'll send them to you this evening.

Thanks so much Paul, If there's anything I can do for you, you let me know...

-Virgil

Not very challenging, but a couple of 1N4004 diodes and a 10,000-ohm DC KRP relay is the best I could do on short notice.

The vanilla way would be two AC relays with paralleled contacts—it would probably be the most reliable too.

Not seen a 2-coil relay except for a latching type—doubt if you want that.

OK Paul, I've rceived your email, but haven't studied it yet. It dawned on me while pondering about it that indeed the relay on the left is redundant, the power running the WH fan would simply run the Aux vent, then shift to the second WH if it required the power. Elegant. And I had two engineers stumped for a while on this one... (I've got the drawings from one of them and two ice cubes waiting at the job site, but hey, if I can do it with one...).

I did come up with the above configuration on my own, I haven't seen the EE's configuration yet.

Are you control gurus laughing at me?

I'm going to look at the one Paul sent me now... BRB...

OK, good... I think I'll go with the DPDT relay idea, as long as the ampacity of the exhaust vent fans' circuits can handle the extra 1.8A load. If Joe (my partner) is concerned of voiding the warranty, or would rather have a contactor involved to put it on a seperate circuit, then we'll do so with the two SPST relays and a contator.

Thanks so much, guys!





[This message has been edited by sparky66wv (edited 05-26-2002).]

I wouldn't laugh at you. I once managed to design a TTL logic circuit (can't even remember what it was for now) and then realized I could simplify the dozen or more gates I used down to about three with the redundancy I'd designed into it. Oops!

The idea of a single relay fed from each circuit via a diode occurred to me as well, but I'm not sure whether the NEC would have anything to say about linking two circuits together in this way. And with the aux. fan & relay wired from the two vent fan outlets, you'd end up using a neutral from a different circuit for one of the relay feeds, which I'm sure would be an NEC no-no (it would here be here anyway).

One other question, Virgil: Why "ice cube" relays? Because this sort of relay is probably about the same shape and size???

That and the translucent blue package too!

(And it's what the supply house calls 'em)