Why truck heat will not warm up? on 2000 Mazda B3000

Had a small hole in small hose running from thermostat housing to tee in lower radiator hose.Since replacing this hose the heater temp in cab is slightly warm and the temp gauge in dash reads cold.does not appear to be an air lock and have replaced the temp sender.Also what is this small hose for?

Asked by for the 2000 Mazda B3000
Agree with miles. A thermostat stuck full open will prevent the coolant from getting into regular operating temp.
1 more answer
Could be a stuck open thermostat. Might try by replaceing it.