When you go to your home improvement store for a new air filter, it might be tempting to just find the cheapest type that fits your ventilation system and go with it. But there’s more to purchasing an air filter than cost. Like furnaces and air conditioners themselves, the key to finding the right air filter is in balancing cost and efficiency.

The effectiveness of an air filter is measured by its Minimum Efficiency Reporting Value (or “MERV”) rating. Ranging from 1 all the way to 20, a higher MERV rating equates to a more expensive air filter with better blockage of tiny particles. But higher ratings can also make it more difficult for your system to push the air through your ducts, hurting its overall efficiency.

For most home HVAC systems, a MERV rating in the intermediate range is the best balance. A filter rated between 5 and 9 will block most suspended particles while maintaining low strain on your air conditioner and furnace. But, if you’re not entirely positive about your home’s system, check the owner’s manual to find the optimal filter size and MERV rating. While it seems like a relatively small piece of the whole HVAC puzzle, the right air filter will eliminate contaminants, dampen allergies, and keep your system operating efficiently.