Q. I bought a house recently. The heat pump is right off the back porch. I would like to put a roof, flat and opened on all four sides, over it to cut down the noise. Is this a good idea? What is the rule of thumb on how much clearance I need to leav e so I will not restrict the air flow?

A. I’m glad you asked before you did anything. The simple answer is anything you would put over the top of that condenser/compressor unit to lower the noise level and placed so as to not restrict airflow would be at such a height that it would be useless for the purposes intended. I’m sure there is some formula somewhere with which you could calculate the exact level above the unit to place a cover but the simple rule of thumb we use out in the field is to keep roughly eighteen inches clear all around t he sides of the unit and open above to the sky. You don’t want to create any back pressure and even slight back pressure isn’t considered a wise thing. I’ve seen heat pump and A/C compressors tucked in all sorts of dumb locations to either keep them out of sight or to quiet them down. Under decks, behind thick bushes or imprisoned in wooden fence-like enclosures are the most common places I find them. Some communities’ homeowner covenants require that they be hidden from view from the street and altho ugh that can be easily accomplished without compromising the equipment I usually find them jammed up with intended screen. There is a major brand of heat pumps whose TV ad shows a unit all nestled among flowering plants. I’m sure most A/C service techni cians who view that TV spot just shake their heads as I do.

Some years ago an experiment was performed by researchers in Florida to see to what degree just shading the unit from the sun would accomplish relative to its efficiency. They discovered that a box roughly 50 feet in all directions would be needed to re alize any measurable results, so that’s an idea that got tossed. Compressors that serve dwellings process about 3,000 cubic feet of air per minute during operation and that’s a lot of air.

You didn’t tell me how old your unit is but if it’s approaching the age at which it will need replacement there are some things to consider. By the way, heat pumps have a history of lasting between eight and twelve years on average. Air-conditioning onl y can go seventeen plus. Heat pumps perform double duty-- both heating and air conditioning-- so that’s why they wear out faster.

Any heat pump or air conditioner manufactured after January 23 2006, just six months from now, will be required by law to be 13 SEER or better. SEER means Seasonal Energy Efficiency Rating and is a number derived from a formula that computes output again st electrical usage. The higher the SEER number the more efficient the unit will be. Obviously, increased energy efficiency comes at a price. The new units will be bigger so that more air can flow across the coil’s surfaces so the new units will use more copper, aluminum and steel plus using about 40% more refrigerant. Factor into that a required 65% reduction of the most common refrigerant, R-22 or Freon, by 2010, just 4 years away and you’ve got a quantum leap in up-front heating and cooling costs. T he manufacturers of the units are retooling now and Bryant told their wholesalers in June that it was last call for the common 10 SEER units. After January the lower SEER units can be installed until they run out of them. Expect to pay almost double for a newer system by the time it’s all done because the inside as well as the outside equipment will have to be replaced.

So while you’re planning your unit’s replacement strategy also consider having the new, bigger unit placed in another location so you can’t hear it from the deck, such as along the side of the house about ten feet from the rear corner. But I will say any of these newer units that I see installed now all have one common trait-- they are whisper quiet. Gone will be the days when a slow evening walk is serenaded by old A/C compressors whirring and grinding away all around the neighborhood.