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I have an attic fan with an old (MH Rhodes 90015 /marktime) mechanical timer switch. I want to replace the timer with a thermostat (presumably a line-voltage thermostat). To my surprise, in the switch box where I expected a line and load feed as in a typical light switch, the two wires that were attached to the timer are a line and a neutral with 120 volts. Is there a thermostat that would connect to that? Or will I have to find the junction box and rewire to convert the switch feeds to typical line and load?

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    Have you considered that the thermostat might be on a switch loop? – ThreePhaseEel Nov 13 '18 at 12:36
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    @JimStewart - I think ThreePaseEel was just being tactful. If a switch actually had a hot and a neutral, it would dead short the circuit when closed, and trip the breaker immediately. So in all likelihood, the OP is mistakenly assuming the white wire is a neutral because it is white, – batsplatsterson Nov 13 '18 at 12:55
  • Are you sure you only have two wires going to the timer? For a timer in the attic I think of something like intermatic.com/en/timer-controls/mechanical-time-switches/t101 which connects to hot/neutral/ground coming in and connects to the load switched-hot/neutral/ground - i.e., the neutral is needed to power the clock. Ignoring ground (presumably it is in there properly connected to everything), that would be three wires - hot, neutral, switched-hot. – manassehkatz-Reinstate Monica Nov 13 '18 at 22:55
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There are line voltage thermostats, I use these for controlling electric heaters no battery a simple 2 pole switch with a temp dial. T522-w is the model I use 50-90 F temp range 120-240 at 22 amps 277v @ 18 amps I think it was close to 18$ at Platt but you may be able to find it cheaper online I believe when I ordered these there were some ~12$ but I needed 20 amp minimum 240V. , Added just checked online and you can find what you want using line voltage thermostat. Honeywell has one 13.95 (sold by Amazon) it's a 4 wire you only need to use 1 set, cap or tape the other set these are much heavier duty than your fan needs so they should last.

  • Do these require a neutral? – Jim Stewart Nov 14 '18 at 0:11
  • No just the 2 wires you have at the switch. They may have a few degrees more hysteresis than an electronic type but I have replaced several battery models with these in pump houses and storage containers that we need to keep warm. They have held up quite well with a heavy load. – Ed Beal Nov 14 '18 at 0:18
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As @ThreePhaseEel points out you almost certainly do not have a neutral in the switch box. Therefore you cannot use any device in the box which requires a neutral. A line thermostat might be such a device. There may be battery powered thermostats which do not require a neutral.

I haven't lived in a house with an attic fan since childhood in Dallas TX in the 1950s. All it did was pull in 95 F air through the window next to my bed, and so I think a timer would be more useful for an attic fan than a thermostat. However, maybe in a locality where the temperature drops significantly and quickly after dark, then a thermostat would be useful.

I think that you would still need a switch in line with a thermostat because an attic fan on requires windows to be be open, doesn't it? And if everyone leaves the house, you would shut the windows, right? So you don't want the attic fan to come on when no one is there and the windows are shut.

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    I think you are getting confused between an attic fan and a whole house fan. An attic fan pulls air through the attic, typically with some sort of louvers that automatically open - e.g., one on each end of the attic, to keep the air in the attic from getting too hot on sunny days. A whole house fan pulls air from the house up and out through the attic or some sort of exhaust vents - and therefore requires either a house that "breathes" a lot or actual open windows to be effective. – manassehkatz-Reinstate Monica Nov 13 '18 at 22:49
  • Yes, I was thinking of a whole house fan. Sixty-five years ago we used to call them attic fans. But why would someone ever put a mechanical timer switch on an attic exhaust fan? I thought they were all on thermostats. – Jim Stewart Nov 14 '18 at 0:04
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    All the attic fans I know of are on thermostats. But my experience with them is very limited - and people sometimes do strange things. – manassehkatz-Reinstate Monica Nov 14 '18 at 0:08

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