Why would the heater on your 1988 mercury topaz not give you warm air unless it has been running for 30 minutes?

First and foremost, check your coolant levels. If you have insufficient coolant, you'll get heat but only after a long time. It could be a heater core problem, or a plugged line but low coolant is a common cause. First and foremost, check your coolant levels. If you have insufficient coolant, you may still get heat but only after a long time. It could be a heater core problem, or a plugged line but low coolant is a common cause. There should be a coolant reservoir under the hood, it will have some markings on it indicating hot and cold coolant levels. If that is empty that is a good indicator that you may have low coolant.