Why Isn't The A/C Keeping Up On The Hottest Days?!

 

It happens every year when the heat waves start to roll in... the phones blow up with clients complaining that their cooling is not working. It's 95-degrees in the shade and there is no relief in sight. Technicians are working 16-hour days, and still cannot keep up, let alone catch up. Dog AC

Call after call, techs all throughout the industry deliver the bad news, "Mrs. Smith, your A/C is working, and doing all it can do. It just can't keep up on these brutal days." Of course, the clients are "totally okay" with this diagnosis, right? WRONG! Nope, they're on the phone with the office almost before you can get all the words out; "your company put this system in", "you guys said everything was working fine when you did my maintenance", "I want a supervisor out here to check it out", "you need to put a bigger unit in"... And, oh by the way, they have no intention of paying for any of it.

So, what's really going on?

Load calculations are better and easier than ever...of course, assuming someone actually performed one and/or didn't "fudge-it" too much. That never happens, right?! Rule-of-thumbs are overkill most of the time; and by most of the time, I mean all of the time. And, it's not so far fetched that a salesman would have actually "upsized" the original system to prevent "callbacks". So, it's unlikely that the system size is the real issue.

According to the Department of Energy, 80% of residential duct systems have significant duct leakage, resulting in roughly 25-40% wasted HVAC energy. This wasted energy includes waste and/or lost system capacity; i.e. you're paying for it, but not actually getting it into the house. What does this mean to us and all these "not enough cooling" calls? Maybe, just maybe, could it be possible that all these ducts with "significant leakage" are part of the problem?

What does duct leakage mean to the system's ability to cool the house? A lot actually, much more than the average tech or salesman might realize. Unfortunately, most average techs have tunnel vision when it comes to servicing systems, focusing exclusively on the mechanical pressures, volts, amps, delta-Ts, and ignoring the "whole" system. And, a lot of salesmen think "bigger is better", either because they don't know better or maybe don't care. As a result, each associate ends up distracted by the symptoms, and never really uncovers the underlying cause.

Let's consider a 4-ton (48,000 btuh) system. Nominal system airflow should be 4-tons X 400-cfm/ton or 1600-CFM total. Even at 10% duct leakage, that's 160-cfm that is coming from or going into somewhere other than house's conditioned space. If everything was working "perfect" mechanically, but 10% of the required airflow is NOT reaching the conditioned space, the "missing" 10% cannot do its job to  help cool the space. In simplistic terms, think 90% of system airflow to/from the conditioned space translates to 90% of system capacity being delivered, turning the 48,000 btuh into 43,200 btuh. It's actually not that simple, but it makes the point that airflow equals cooling capacity. What if that 160-cfm is leaking into the return side from a 150-degree attic? Using some basic assumptions, if we run the basic sensible and latent heat formula calcs, we quickly discover that potentially over 8,000 btuh is "wasted". That would mean 17% of the cooling capacity that isn't actually going towards cooling the house.

It's worth noting, that a 10% duct leakage is often considered acceptable, especially by older mechanical and energy standards. Actual field testing has shown actual leakage rates to be much greater than 10%, meaning duct leakage is even worse than the examples above. New standards target 5% duct leakage maximum.

Think about it, DOE says that 8-out-of-every-10-calls have ducts that are leaking tons of air from all sorts of "bad" places, like brutally hot attics. That means if a service tech averages 5-calls/day, four of them potentially have duct leakage issues! So, if the system's full capacity is needed to cool the house during those heat waves, any capacity (and energy) being wasted by costly duct leaks will result in the A/C not keeping up. Airflow (CFM) is the method of delivering cooling capacity to the home; less airflow delivers less capacity.

Really, it's that simple; excessive duct leaks burn up mad cooling capacity, which means AC systems don't stand a chance of keeping up on the hottest days, unless the duct leaks are addressed. And, new system won't fix the problem. In fact, in many cases, new systems (especially variable speed blowers) can actually make it worse if the duct leaks are not resolved.

Proper duct & airflow diagnosis will result HIGHER SALES and HAPPIER CUSTOMERS, by being able to deliver (aka SELL) real solutions that address the real causes!

P.S. All ducts leaks...

 


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Billy Gouty wrote on July 14th, 2016 01:07:52pm
really like your articles, would like to get more like them.