A/C Issue

Discussion in 'The Projects Forum' started by bob22, Jul 3, 2008.

  1. bob22

    Thread Starter New Member

    Jul 3, 2008
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    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
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    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.
     
  3. bob22

    Thread Starter New Member

    Jul 3, 2008
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    0
    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
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    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.
     
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