A common cause is the thermostat being stuck in an open position.
Feel the temperature of the heater pipes in the engine bay, where they pass through the front bulkhead. If they are both hot then the fault is most likely the air regulator flap stuck. Look here http://tinyurl.com/67pl3d for help.
coolant level could be low not letting it circulate through the heater core, causing it to blow cold. Also, if the coolant level is low it will in turn overheat.
heater core going bad.
HEATER CORE.
sounds like your heater core went out
NO
Thermostat.....
There's a good chance that your heater core has sprung a leak.
This could either be due to the age or the materials used to make the water heater or the fact that the water heater has not been installed with vacuum breakers causing the geyser to collapse from the inside-out due to incorrect atmospheric pressure within the water heater
I had this on my mondeo and found that I had wet circuitry on the wiring loom under the bonnet - you could al;ways try a bit of WD40 or similar on the contacts.
No. It would be very uncommon for a thermostat to determine whether to turn the pool heater on based on the outdoor AIR temperature. All pool thermostats monitor the WATER temperature. In this way, they keep the water at a predetermined temperature (just like your house thermostat). When the water is cooler than the predetermined setting, the thermostat turns the heater on. When the water is cooler than the preset temperature, the heater is turned off. In this manner the pool water maintains the desired temperature regardless of changes in the air temperature. You could re-wire your thermostat to use air temperature rather than water temperature to turn the pool heater on/off so that whenever the AIR becomes colder than a preset temperature (and for as long as the air temperature stayed below that value), the heater would come on. The potential problem with this arrangement is that the heater could be continually on (as long as the air stayed cooler than the preset value) and the water would be continuously heated to a VERY high temperature! Conversely, if the air were continually warmer than the preset value, the heater might never turn on. Hope this helps ...
If it is water, it could be a plugged up AC condensation drain or if it is engine coolant, it could be a leaky heater core.