1st post to this site and hope this question is not out of scope. I was on a DIY site and a question came up on an A/C unit running when microwave oven fan kicks on. Evidently in Canada, it is code so that there is fresh air introduced into the structure. Part of the blog:
"The basic issue is his ac was running when his kitchen exhaust came on. Here in canada u require the furnace fan to come on c/w a fresh air intake fan, damper and thermostat regulated heater when the exhaust is on (over 300 cfm). These exhausts have a separate pressure switch in them which most guys just wire to R and G on your furnace. The problem is that sometimes the 24v signal will backfeed from the furnace up through the thermostat and energize the Y terminal. (Y and G on some tstat subbases are interconnected. So the only real fix is to run the Y wire through a NC contact on a relay that will open Y when the exhaust is running."
My question: Wouldn't a properly-sized diode prevent the feedback and be simpler than a relay?
TIA,
Bob

 

Seems to me that the 24v is AC here in the States, but it may be different in Canada. If it's AC, no, a diode won't work.

 

I believe A/C controls are 24 v; just not sure if AC or DC. If DC, what size diode would be appropriate?
TIA,
Bob

 

If it WERE DC, you could probably use a 1N4002 through 1N4007, which is a 1 amp rectifier with respectively increasing PIV ratings of 100, 200, 400, 600, 800 and 1000V.
A 1N4001 only has a PIV of 50; a blip on the power line could cause problems for a PIV that low.