Air Flow and Diagnostics
Most high quality duct installers out there struggle with this topic. Why? Because for every 1 or 2 quality systems being installed, there are 8 or 9 systems installed without regard to real life energy performance. Do you think it’s easy for the quality HVAC contractors out there that have to compete with the cheap guys? It’s rather difficult. But now that energy costs are on the rise, this subject will eventually become a normally discussed item in the contract between the HVAC contractor and the Builder/Home Owner.
Even though building codes specify minimum standards for a duct system, the codes generally are not enforced. When they are, there generally isn’t a third party verification process in place (unless of course the building designer or the architect has required this in the scope of work). The EPA claims that as much as 30% of the conditioned air is lost through poor duct construction. Here is some advice: for anyone building a new home or considering a new HVAC system, consider spending more money on the duct system.
A duct system should last the life of the home. The furnace and air conditioner most certainly will not. Would it not make sense to invest a little more in the system (ducts) that will soon be buried in the hard-to-reach parts of the house forever? If you’re interviewing contractors and getting estimates, try this: Thanks for your prompt response, ABC Heating & Cooling. Let me ask you, how much would it cost me for a high quality duct system, one that you can verify is 90% or more air tight? You may get a variety of responses and even a blank look on occasion. Do you see what you just did? You’re asking for performance (to be documented), and you’re wondering if you’re getting their highest quality from the get-go. It may be awkward if the contractor comes back with a yes, we can build a high performance duct system. You may wonder why they didn’t offer it to begin with. I know I would be disappointed if I knew the firm I was about to hire gave me a price for their “B” system I want to have the option for the very best; at least to consider it anyway.
So enough of all that; let’s get technical. A typical AC system is designed to move 400 cubic feet of air per minute per 12,000 BTUs ( which simply means 400CFM per ton). The system will work just fine if it’s a little below 400/ton, but lets stick to practical examples for now. The system has to move a lot of air. It’s kind of like moving 1200 basketballs in one minute for a 3 ton system. That’s 72,000 balls in one hour. Now each of these balls has to carry some energy, typically 15 to 20 degrees colder than the room air in AC mode (or 40 to 60 degrees warmer for modern gas fired warm air furnaces in the heating mode). Here is some basic math for you: CFM x ∆T (change in temperature) x 1.08 = BTU.
So, to summarize, if there is a lack of air flow (improper design, restrictive ducts, high static pressure), or a leaky duct system, or dirty refrigeration coils, or an improper refrigeration charge, or lacking duct insulation in unconditioned areas, you simply won’t have the same BTU delivery to your home. The draw back is you’re still paying the electric company the same money for less cooling. If the duct system and refrigeration charge is really off the beaten path, you’ll sadly pay much more money for less cooling.
Your response may be: The house stays warm in the winter; what’s all the fuss with this measurement stuff? Well, if you don’t mind not knowing how much energy you’re wasting, then let’s drop the subject. However, with today’s bigger homes and energy prices reaching new records every year, Wilson has a hunch there will be some folks out there who will be requesting performance clauses in their agreements with the building contractor, or the HVAC installers if the owner is hiring the “subs” direct.
Please call us if you are interested in a third party verification agreement, and other consultation services we offer.
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