Choosing the best strategy for adding central air to an old house takes a lot of research and perseverence. Here's what we found.
As we've mentioned previously, we've been busy evaluating ways to add central air for the past month as our work removing the second floor walls and ceiling came to a close. Today we made our final decision.
While there are certainly many ways to cool a house (window units and open windows work for us on good days), we made a decision back when we bought the house that we'd install central air conditioning. The basic reason was our own comfort (Chicago+August=Yuck!), but long term resale value and controlling humidity was also a consideration. Knowing this, we started our research early on, including a visit to the vendor expo at the Renovation & Restoration Expo last summer at Navy Pier. It turns out that finding reliable information is tough, but in the end we did find good answers and helpful folks out there. (Just as a tip, hvac-talk.com was one of the most helpful resources.) For us it came down to the pros and cons of two different approaches: Forced Air (or 'conventional' ) systems and High Velocity (or HiV) systems.
How They Work
These two systems work in fundamentally different ways:
|Conventional forced air systems are in the vast majority of "newer" homes. They work on the principle of diffusion. Cool air enters the room (typically at about 500-700 ft/sec) through a vent typically near the ceiling, where it naturally drops to the floor (cold air sinks, hot air rises!) before an intake vent someplace else in the room draws air back out. In this system the placement of supply and return vents in each room determine how evenly air is distributed.||High velocity systems work on the principle of aspiration. A). Air delivered to a room enters at a higher velocity (typically 2,000 ft/sec), creating air currents that circulate cooled air throughout the room. A high aspiration ratio is good, because it means that a greater quantity of air is kept in motion, with less chance of air stagnation and temperature variation within the room.|
So, the systems basically differ in the physics of how they cool a home. However, the engineering of each system also includes other important differences.
The other major differences between the systems fall in several categories:
|Duct Sizing||Conventional systems have the highest impact when installed in an old home. They use large runs of metal ductwork (6" in diameter is typical) that branches off of two main trunklines (perhaps 8"x18") for the supply of cool air and return of the warm air. The size of ductwork typically requires giving up space--typically in soffits, knee walls or--in the case of multi-story homes--a closet.||High Velocity systems use smaller 2" insulated tubing for supply lines. This smaller tubing results in a lower impact on existing space, typically fitting within walls and between floor joists.|
|Flexibility||Because of the space required for metal ductwork, conventional systems have limits on where they can go. Some areas of an old home may be inaccessable without adding soffets or bump-outs in corners. Flex tubing is available but is much less sturdy and may not meet code requirements in some locations.||High Velocity systems can place supply vents to more locations. They fit more easily between floor joists and wall studs and can make unusual turns more easily than rigid ducts can.|
|Air Flow||Conventional systems move air more slowly. The advantage of this is little noticable "breeze" when the air is on. The disadvantage is that poorly placed supply and return vents can create "dead spots" of warm air in a given room.||High Velocity systems intentionally create circulation throughout every room. Some may not like these air currents, but they do offer the benefit of a more consistent temperature throughout each room and the whole home.|
|Noise||Conventional systems have larger ducts and pass air more slowly. It stands to reason, therefore, that they are typically near-silent. (Real life experiences may vary though--more on that later.)||High Velocity systems use smaller air passages and move air at a higher velocity, so you'd expect more "wind noise" from this type of system. Manuracturers have used additional sound deadening materials in supply tubes in recent years to mitigate this issue.|
|Asthetics||Conventional systems leave a bigger visual mark on a home because they require more and larger vents. Every room needs a supply and return, which are typically rectangular.||High Velocity systems use small supply vents with cover plates the size and shape of CD-ROMs (or smaller). In addition, because of the physics behind the approach, they also only require one return vent for the entire house, instead of one per room. The result is a less noticable visual impact on the existing decor and less space lost to installing returns.|
|Mechanical Systems||Conventional systems win out here for two major reasons. First, conventional systems are less demanding on your air handler giving it a longer useful lifespan. Second, metal ductwork is a simple material that lasts indefinitely.||High Velocity systems use a more engineered tubing that is more prone to deterioriation over the long term (although manufacturers have made improvements in recent years). In addition, the higher air velocity and smaller tubing means more stress on mechanical components of the system. Finally, because the smaller tubing creates greater drag you typically need a more powerful (i.e., $$) air handler to achieve the same output as a conventional system.|
Well, we went in to this expecting the worst since retrofits of mechanical systems are certainly never easy or cheap. We weren't mistaken, unfortunately, and all the quotes from three contractors came in over our intended budget.
However, there was one major surprise: the high velocity system was consistently cheaper than the conventional option. We were stunned. Manufacturers of HiV systems typically position themselves as high end products. While the price tags we were quoted do fit that discription, we thought that a ductwork system was going to be our discount option. The good news is that this ultimately made our choice easier in the end...
We're going with a High Velocity system. The combination of the lower cost, lower visible impact and the need for a single return were our deciding factors. We're also optimistic that by choosing a quality installer we can mitigate the negative aspects of the system like noise and distracting airflows...which leads us to our final topic.
Pitfalls to Watch For
80% of all central cooling systems in the US are installed improperly. 80%!! The most prevelant errors are oversized condenser units and unacceptable leaking in ductwork. Such errors can increase energy costs and decrease the lifespan of mechanical systems, increase condensation in the attic and other spaces (which can lead to mold and/or decreased lifespan of wood and roofing material), backdraft and ventilation issues which affect air quality and safety. The solution: picking an excellent contractor for installation who is experienced and who has experienced, long-term employees who can "trouble-shoot" on the job and who love old houses.
This is one area of our house where, in the end, we decided it was essential to pay for quality. If only 20% of installers do the work right than a deal that sounds "too good to be true" is definitely a reason to be suspicious. Therefore, getting multiple bids for HVAC jobs is a must. While we've gone with non-competitive bids on other occasions when we trusted a personal referral, we didn't here.
If you do go with a HiV system, select a contractor with lots of installation experience with that type of system. Message boards we read that included discussions between contractors consistently cautioned against underestimating the differences in technique between installing these two very different systems. Some contractors were advising each other to sub-contract out the system design to the manufacturer just to make sure it is done right (again, sub-contracting eventually increases the cost). Again, system design is the fundamental factor in an effective installation.
So, there you have it. We'll be accepting the proposal from our desired contractor and talking about a schedule for installation. In the meantime, if anyone has any information to add to what we've documented here, feel free to comment. We're especially interested in anyone's "lessons learned" regarding the installation process. Given the size of the investment, we certainly want to uncover important considerations ahead of time!