Transformer question

I killed my 240v to 24v (65va) transformer in my air handler tonight, by hooking up to the secondary side to another 24v source - I know stupid me. I know it's dead because the multimeter reads 240v across the primary terminals, but 0v across the secondary terminals. It's internally fused, but not serviceable, and had been buzzing loudly before I got to it.

So I've got another transformer on order (Mars brand, 75va rated), but have two questions:

On the Primary (240v) side, the terminals were labeled "common" and "240", but I wouldn't think it matters which way the wires are hooked up -- since both the "common" and "240" both get connected to a 120v "hot" leg. Am I right, or does it matter?

What about the secondary (24v) side? One wire ultimately goes to the "R" on the terminal board, and another to the "B". Does it matter which goes where?

Thanks,
David

Not normally...you should be okay using either. It only gets to be a problem if either side is connected to the frame, otherwise, the coil is floating, and either connection is fine. If phasing was an issue, you'd want to be careful about which lead went where. I don't think it applies here.

Worked perfectly.

Also, just to plug someone, I ordered the transformer from Patriot Supply. Turns out they were selling it on **** as "buy it now" for significantly less than their own website. It arrived in one day via UPS ground. They're product was half the price of Grainger and it's a UL listed part too.

unless it's internally protected from shorting. . .

if a fuse + holder is $3 and the xformer is $50 you should buy the fuse + holder if you judge the chance of popping the xformer for any reason over the lifetime of the furnace, at more than 3/50 (= 6%).
The $3 is the premium you pay for "xformer insurance."