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When I turn the heat on on my central heat and air unit, the heat came on then continued to run getting really hot. It would not shut off unless I turned from auto to on then back. Sometimes I had to turn from auto to on and leave it for heat to work. I thought it was the thermostat, so I changed with a new digital one which worked fine for a few days. Tonight I had it set at 70°F but the temperature in the house was 80°F; it's doing the same thing again, not shutting off.

What could it be?

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Possibly a failing relay in your HVAC unit. – OrganicLawnDIY Mar 1 '14 at 17:39
if it is a failing relay which one could it be and where – gene Mar 1 '14 at 18:19
Are you sure that your wiring is correct? – Ethereal Mar 3 '14 at 13:42
yes red to r,white to w,yellow to y,green to g rewired same way as old one not 1st one ive replaced just 1st time with this problem – gene Mar 3 '14 at 18:16

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