Air Conditioning Systems: Conventional vs. High Velocity

Category: Hardware & Appliances

Choosing the best strategy for adding central air to an old house takes a lot of research and perseverence. Here's what we found.

Our Review

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, 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.

Major Considerations
The other major differences between the systems fall in several categories:

Duct SizingConventional 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.

FlexibilityBecause 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 FlowConventional 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.

NoiseConventional 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.

AstheticsConventional 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 SystemsConventional 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...

Our Choice
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!

Looking for More?

House in Progress Search for more on 'high velocity air conditioning' on this site. Search for 'high velocity air conditioning' on on other houseblogs like this one.
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When you finalize your plans, would you please givw us some idea of cost & also who you chose for the jpb?? Thanks ...POPS"30"

Hi Pops!

When other folks (besides our parents and friends) began to visit the site, we decided to lay down some ground rules to make everything all fair and square and respectful:

1) Don't accept ad revenue from a vendor who wants us to use their product or service.
2) Posting the cost of commodities (light fixtures, tile) is cool. Don't publish the cost of services and products created to fit the house out of respect for the vendor (I'll explain more below).
3) Be honest in publishing what we think about products--and how we use them, just in case we are using them incorrectly.
4) Only publish the contact information for vendors who have done a great job for us. Otherwise, nothing.

* Let me explain about our "no cost of special services" published rule. Since we generally have great vendors, we publish their identifying information. However, if we published their costs, that would be disrespectful. One, their competitors would have access to it through us, and that makes us feel uncomfortable. Two, every job is different and we don't want to set up anyone else for unrealistic expectations. (Cost can also depend on what area of the country you are in.)

We are happy to share the types of information that we look for to make sure that we felt we were getting a good value. What items to ask about (warranties, service, how to try to assess competence, etc.) And we can pass along information that is available online already that may estimate average costs (for example, Remodeling Online publishes a dandy cost vs. value report by city that is really helpful.)

Whew! Sorry for the ridiculously long explanation...

Air Conditioning Systems: Forced Air vs. High Velocity

I live in Chicago, in a 1920 "flat roof" two flat w/full basement. I occupy the whole building. I am deciding what type of air conditioning to put in.

A quote I had last week discouraged Space Pak because of cost. He did not mention any other brands of “high velocity.” Your experience was that “high velocity” was cheaper. What brand did you decide on?

What percentage of contractors put in high velocity?


The cost and ease of installation is definitely going to be affected by the type of building that you live in. Because our house is SO old with so few hidden spaces, installing a forced air system (a quality install) was actually MORE than high velocity. High velocity fits our locale better because of the space.

I'm sure there must be buildings where forced air would be cheaper than high velocity because there is enough rooms for ducts or ducts already exist (from forced air heat) and then they would only have to modify the system to accomodate A/C.

We've looked at both the Space Pak and the Unico systems...we like Unico. The system had everything we were looking for.

I think the thing to do is find A/C contractors who specialize in high velocity. Many forced air contractors will "dabble" in high velocity if they have to, but 80-90% of their installations will be forced air. You might want to call Unico to see if there are contractors in your area who get particularly high marks for installations.

Hi, we had Spacepak installed recently in our 1920s house in Oak Park. We are thrilled with the results thus far. It was an investment, but well worth it in our view. Good luck!

A message to op resident:

We're getting estimates for Spacepak for our 1929 Oak Park jumbo bungalow. Who did you use for your contractor?

I'm sorry...we cannot release the name of our contractor when and until they have actually done work for us...that's our policy. And we only release the names of contractors who would like to be listed and who do excellent work for us.

We did contact 4 contractors for a proposal before we selected and tried to compare "apples to apples" in order to make a selection. We would suggest checking out a few contractors, their references and possibly even going to visit another installation for a client before securing a contractor.

I was looking up airconditioning and heating system in the Journal of Light Construction Online. They had a good article about hi-velocity systems. The person profiled installs products from "Energy Saving Products LTD". I didn't know if you came across this company or how they compared to the Unico.

Todd--No, we hadn't come across Energy Savings Products until you're mention of it.

We had a unico system installed a couple of years ago and are pleased with its cooling capacity. We are adding on to our house and have the option of a traditional system or a unico and are hesitant because it's very noisy. It sounds like there's a rain storm hitting the roof of our house. Certain rooms are worse than others. Do other people who have unico systems also have loud systems or was ours badly installed? Also, does the unico system use less energy because of less heat loss in smaller tubes? Love to have some feedback to help our decision with the addition. Thanks! Elizabeth


We had also heard that these systems can be noisy. Sometimes that is due to funky "bends" in the system that were created during installation. And the newer systems encase the last 35" or so of ducting in a sound deadening material right before it reaches the outlet.

Where is the sound coming from? The air handler? The ducts? The outlets?

It would be interesting to know because others might be having the same problem...

Take care! Jm

Miss, Mister,

We are a society French of inside planning.
In the setting of our development, we search for some new products in the air-conditioning.
Could send us to you a documentation on the products that you distribute, as well as screw price.
We search for a product that doesn't require an outside evacuation mainly (as the Unico). You can join us to the:
Mobile: 00 33 6 21 74 62 66
Office: 00 33 5 62 20 95 27

We wait for your answer.

David Jousse


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