Script keeps raising flag not meeting set condition

Script keeps raising flag not meeting set condition

The following script is to send email alerts, for some reason it keeps
bypassing the flag I set..
############################################################
## MAIN ##
############################################################
Open_serial()
while True:
line = ser.readline()
if line:
tmp = line.split()
if 'tank' in line:
tank_temp = tmp[1]
print 'Tank temperature:',tank_temp
if tank_temp >= 27:
flag = raise_flag()
if flag:
send_email('Tank temperature overheat',tank_temp)
flag = False
elif tank_temp <= 23:
flag = raise_flag()
if flag:
send_email('Tank temperature too low',tank_temp)
flag = False
if 'led' in line:
led_temp = tmp[1]
print 'Lights temperature:',led_temp
if led_temp >= 55:
flag = raise_flag()
if flag:
send_email('Lights temperature Overheat',led_temp)
flag = False
if 'White' in line:
white_pwm = tmp[1]
print 'White lights PWM value:',white_pwm
white_per = white_percent(white_pwm)
print 'White lights % value:',white_per
if 'Blue' in line:
blue_pwm = tmp[1]
print 'Blue lights PWM:',blue_pwm
blue_per = blue_percent(blue_pwm)
print 'Blue lights % value:',blue_per
And here is the flag function:
def raise_flag():
global start
interval = 900
if start > interval:
start = 0
flag = True
return flag
else:
flag = False
start = start + 1
time.sleep(1)
return flag
the script keeps sending emails when the temp is whithin range i.e. 25°C,
I dont know why it keeps sending emails? I placed the flag so when the
condition is met it doesnt send 1000 emails, only 1 every 15minutes...