At work today, I had to rewire 4 heating system boilers in a commercial building. Three of these boilers had to run off of one thermostat, I was going to use a 3 pole control relay to accomplish this. Each of these boilers has it's own 24volt control circuit, a (R) and a (W) terminal to fire the boiler. The previous installer didn't use a relay, he joined all 3 (R) terminals together and all 3 (W) terminals together, from each boiler and put them on 1 thermostat. My thinking has always been, if you join 3 separate control systems together you would short them out, possibly burning out the transformers, but these boilers have worked like this for about a year. Is it okay to do this without a relay. Thanks for any replies.

Nothing direct, but I talked to an electrician about that, this week. She told me that they must be on the same phase - something about the interconnected wiring between units. Sorry, but I didn't ask more questions. I'm a little slow before 10 cups of coffee.

It may work but it is not the way to do it IMO. Transformers do weird things when they "talk" to each other like that. It could also burn up the t-stat. I do quite a bit of heating-cooling wiring and always keep a couple of "RIB"s (relay in a box) in the truck. Constantly getting me out of a jam (about $12 each).
http://www.functionaldevices.com/ChartPilot.html

leespark, I'll ask when I see her again, but I got the impression that it was manufacturer's specs. I agree with you, and she clearly agreed with us, but the manufacturer wins. It didn't work, then, when changed, it did work.

I'll ask.

 
“Jumpering” three control systems with individual control-power transformers through a single contact is very shortsighted—likely unreliable at very least—worse, unpredictably hazardous for boiler-operational safety.

It could very well be that the boilers are not shutting down by the subject thermoset, but instead being repeatedly cycled by safety-limit controls—which may be a very serious error.




[This message has been edited by Bjarney (edited 09-25-2004).]

I feel they were very fortunate not to have had a problem on the control circuits of those boilers for such a long time. I agree with you regarding installing a three pole relay controlled by the thermostat. It looks more professional and should one boilers control circuit fail it would still allow the other ones to continue instead of dropping all three.
Ron

If I'm following you correctly you are suggesting that the 24V power supplies are interconnected. I have not seen that for some time. What Class power supply are you working with? I would check 725.41 (B) Interconnection of Power Sources. Class 2 or Class 3 power sources shall not have the output connections paralleled or otherwise interconnected unless listed for such interconnection.
In my experience parallel systems are unreliable and subject to failure also as mentioned by previous posts.

I did follow up on this. My friend's situation was two furnaces running off one stat. The manufacturer's specs were that they were to be fed from the same phase. Two interconnected furnaces, one stat, and approved that way. Sometimes we just have to be good little soldiers and, like it or not, do what we're told.