What to consider in a central air install?

Our house was built in 2003, and roughed in (if that’s the correct term) for central air—we have ducts, wiring and piping between the side of the house and the attic. We had five contractors out to give us an estimate on installing the compressor, air handler and whatever else is needed. They all took measurements of the rooms and noted where the vents are. We’re expecting prices this week, but their quick suggestions and overview differed in a few respects.

Aside from reputation and references, what are the key things we should be looking for or preferring? For example, one suggests hanging the air handler from the attic’s rafters, others were putting it on vibration pads. Is there a significant difference? Does SEER rating make a difference aside from the cost of energy? One company wants to do a pressure test (or something like that) on the vents to “balance the load”—is that crucial? What else should we be familiar with before choosing a contractor? Any horror stories to share or lessons?


Compressors make noise when they kick on, so you may not want that in your attic. They also have large cooling fans to remove heat, that run anytime the unit is on. I would suggest a pad-mount with vibration isolators, located away from your bedroom. We have a heat pump that works in conjunction with the forced-air furnace. It is saving us a lot of money on our electric/gas bills.

If the contractor did not do a heat load calculation to determine what size unit to install, I would be careful about choosing him. A heat load calculation takes in size of house, windows, door, sun direction to house, shade, site locatiion and other information. There is a very good program out there for around $400 and the contractor should be using one.

Some contractors will just figure so many BTS per sq ft. That can be a bad guess. It is just a SWG (some wildass guess). So ask houw they came up with the load requirements.

The SEER number is in relation to the amount of energy consumed the higher the better. Especally if you live in a hot climate.

An oversized unit can cause humity problems.
An undersized unit may not cool the house properly on hot days.

I think he was suggesting putting the evaporator in the attic, not the compressor.

Balancing the load is a good idea. Duct length and turns influence the CFM reaching the various vents. Insulation of the ducts should be considered if not already done. Note that the air box/handler and compressor don’t have to be the same “size”, i.e. 2.5 ton units. A rental house I have is very efficient and the HVAC guy recommended during a replacement (35 years old) a smaller compressor with a larger air box that saved $45/month during the Alabama summer.

Thanks. To clarify, the compressor will sit outside, it’s the air handler (evaporator) that is slated for the attic.

We’re just north of New York City, so our summers can be intense, but short compared to Alabama. We probably used the window units for three months out of the year. Also, all the ducts are in place inside the walls, so there’s nothing we can do about adding insulation (or is there?).

Everyone took room measurements and noted the location of the vents. Some were more obvious about what else they were looking at (i.e., windows’ location and position), but I can’t say that others didn’t take it into account. They were all clear that they would have to go back to the office to make the calculations and determine tonnage. Asking how they came up with their numbers is a great question–if I can ask it intelligently.

From what we understand (estimates are not in yet), the company that included balancing the load is significantly more expensive than the others. Am I correct in thinking that this will influence overall comfort more than long-term cost? That is, a 20 percent increase in cost for, say, a higher SEER rating won’t (necessarily) pay for itself for 10 years, so may not be worth it (environmental concerns aside). On the other hand, if balancing the system will avoid cool or hot spots, the 20 percent difference in cost is a completely different consideration.

Oops: pre-coffee error. Read it wrong and couldn’t figure out what sort of fool would put the compressor indoors.

A fool and his conditioner are soon chilled.

Not if the compressor is in the attic. Then the fool is on the Dope wondering why his AC is cycling and his attic is 175 degrees.

Yes. The heat load calculations will ultimately (there are some intermediate considerations) tell the installer what amount of air (in CFMs) that each room should be getting.

For instance, suppose you have 5 rooms that have the following requirements:

Room 1 - 350 CFM
Room 2 - 350 CFM
Room 3 - 500 CFM
Room 4 - 200 CFM
Room 5 - 600 CFM

So you will need an air handler that will deliver 2000 CFM. But the capacity of an air handler depends on the pressure it is operating at, and that is dependant on the duct system.

The trick for this installer is that he has no control over the design of the ductwork (and the design of the ductwork is important) - it’s already installed so he has to make it work. Now, did the guy who installed the ductwork know that room 5 needed 600 CFM, or did he install a duct that is more likely to deliver 400 CFM?

The designer needs to take this into account in order to install a system that will deliver the right CFM into each room, and he may even have to modify the existing ductwork to make it work or order an air handler with an oversized fan motor.

Well worth the money to get it tested and balanced after the install, and an installer that recognizes that this will be needed up front is a guy who will probably do a good job.

Testing and balancing is standard on commercial systems, and usually performed by a third party, but the guy could do it himself on a residential job if he has the equipment.

Edited to add: The return duct system is just as important as the supply duct.

Isn’t there something called efficiency ratings? Are there tax incentives for higher efficiency ratings?

Changes from year to year. In 2010, there were tax credits for energy efficient equipment and insulation. Don’t know about 2011.

What about zones?
Prices are back and we’ve pretty much narrowed it to one contractor. Great reputation and took the time to go through the load calculations in detail. Most others wanted to put in a separate ductless system to cool the office (south-facing, multiple computers, Mrs. Devil and I are in there all day), he said he’d be able to keep it within the settings of the house during the load balancing process. Plus, he’s the only company to suggest using a hood at each supply vent to actually measure the actual CFM during the balancing process.

The odd thing is that he came up with a 3 ton condenser, while the other five’s calculations ended up at 3.5 – 4 ton machines. But other than faith and trust in his calculations, I have no way of knowing if I should push for a 3.5 ton or leave it to him to match the unit to the house. Too small = higher energy prices, correct? There’s no way to account for that over the year, we’ll be stuck with the system by that point.

His top estimate is about $13K for a Lennox 3 ton 16 SEER unit. We have the choice, for another $3K, to install a zoning package. Is zoning only a matter of energy efficiency? Or does it improve the accuracy of the thermostat settings? Working in the house and with a 1 ½ year old, it’s likely that both upper and lower floors will be cooled the same for at least five or six years.

Did any of them ask about insulation? When the house was built in 2003, it would have been prudent to have good insulation and been anal about sealing up leaks. A house properly built for NY winters might do well with a smaller A/C. How tight the house is is a factor in sizing the A/C. Too large of a system wastes energy and may cool the house down to the set point before removing enough humidity.

I don’t like evaporators in the attic, well really the ‘‘A’’ coil. If the condensate drain plugs up, you can have water in your attic. It can ruin drywall. That is more likely if it is a 1/2’’ PVC pipe with lots of short elbows. They make longer elbows meant to plug up less, but you seldom see them. An overflow pan is a good idea for attic installations. In general, I think attics are a dumb place for air handlers, hot in the summer and cold in the winter. At least make sure the air handler and the ducts in the attic are well sealed and heavily insulated.

Also, the idiots that installed our A/C put the condenser in front of the drier vent, providing the coil with warm, lint filled air.

Very true.

My dad is the King of Overkill. When he had AC installed in his house, the contractor said he needed a 2.5 ton unit. He told them to install a 4 ton unit. Big mistake. The duty cycle is very low, and the house always feels damp as a result.

I installed central AC in our previous home and in our current home. With the exception of brazing the lines and charging w/ refrigerant, I did all the work.

Our first home was a ranch on a concrete slab. I installed the air handler (blower + A coil + evaporator) in the attic. I set the air handler in a large, galvanized steel pan. The pan was there to capture any water from the air handler, should it leak. Both the air handler and the pan had drain lines. I had a local shop fabricate an output manifold and ran flexible & insulated ductwork to the ceiling of each room. Ducts were placed in front of windows. The air handler also had an electric heater which could be used a backup to the primary heating system (hot water and boiler).

Our current home had a forced-air heating system when we bought it, so all I had to do was install an A coil. BUT… I did not want the condenser unit to be located next to the house. I installed it 60 feet from the house, and dug a trench (by hand) for the electric and refrigerant lines.

A few things to consider:

  • Make sure the refrigerant lines are brazed – not soldered – to the condenser unit and A coil.

  • As others have mentioned, do not install a unit that is too big or too small.

  • Do not install a cover over the condenser unit during the winter. If leaves falling into the fan is a problem, tie a garbage can lid to the top of the unit. Do not cover the sides.

The 3 ton unit won’t really use more power. It will run longer, but draw less current. And running longer is preferable for humidity control and the life cycle of the equipment. I’d have no qualms about installing the 3 ton unit.

Zoning – for a two story building, each on its own zone? You want this also – each zone will have its own thermostat. If you don’t get this you will wish you did later. It has the potential to save money if one zone will be unoccupied, but the real value is comfort. Even if you want both zones at the same temperature, the zoning is needed due to the two levels of the home and the stratification of hot air and cold air.

Thanks. The company we’re likely to go with trumpeted all sorts of aspects of their service: “all line set connections to be brazed and properly vacuumed and filled to manufacturers specifications” being one of them. That they highlight such details makes us feel better going with them. I know others might do the same thing, but since we’re going on such faith in picking a company, it’s good to know.

They also made sure to specify that their ductwork will have an R value of 8. It’s an assumption, but that they are noting that it’s likely that they’re accounting for what’s currently installed–both duct and the general insulation they saw in the attic.

That goes right to the heart of the zones question. If it’s just money, then it’s not that wise an investment. With a home office one one floor (always occupied) and the Dudeling downstairs (always occupied for several years and then some), it will take a lot of inefficiency (and a lot of time) to balance out to three grand. Kind of like why it wouldn’t be all that economical to keep going up in SEER levels.

But if comfort ends up playing a major factor, then it sounds like zoning is better for long-term happiness with the system–especially as it has the potential to break even or save over time.

If you take part in (or read) enough HVAC discussions about two story homes, you’ll find that lack of zoning is the #1 complaint. The level without the thermostat is always too hot or too cold, and if you adjust the thermostat to compensate, then the level with the thermostat is either too hot or too cold. Each zone needs it’s own thermostat and associated duct/dampers/whatever equipment that allows for the zoning.

On a 2 story I would go with zoning or split units, one for upstairs one for downstairs. Return air on a single unit can be a problem.