One thing you can do is increase your advertising for heating services in the winter, and air conditioning in the summer. These are the times when people need their HVAC systems fixed and functioning correctly, so you should be pushing this as much as possible when they are most necessary.
Lots of people will turn on their heat for the first time in November or December only to find out that it does not work anymore. So it is important to have a strong advertising presence to reach those new customers in their most significant time of need.
Another marketing strategy you should try is running specials for heating and cooling services in the months when they are not as much of a concern. Your customers may not be thinking about their heating system in July. But if they spot a good deal or promotional offer, it might end up being too good of an offer to pass up.