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Over-Conditioned: The Super-Sized HVAC Problem

by Nic Darling on January 29, 2010 · 7 comments

in Building Science,HVAC,Philosophy

Technology, it seems, has given us the opportunity to defy nature’s impact on our lives in all but the most extreme conditions. This is particularly true within the confines of our houses and other buildings. In our homes, the words “climate control” no longer suggest the nefarious plan of a volcano-dwelling, Bond villain, but rather an expected level of modern comfort. Our HVAC systems are powerful enough to swing our homes from sauna to tundra regardless of the conditions. We can lay around in our favorite shorts in the middle of January and slip into a comfy sweater in August. But, what is the cost of this “comfort”? How much control do we really need?

One of the challenges of building energy efficient homes in the US is the idea that the occupant needs god-like control over their interior environment. This often means over-sizing a system to the point where it can massively overpower the influence of the most rare and extreme weather conditions. As one might expects, this sort of sizing has several major drawbacks:

Cost
This is probably the least surprising drawback of an over-sized HVAC system, but it is a particularly glaring problem in homes where serious investment has been made in insulation and air sealing. All that cash that was dumped into sealing up the house should pay back immediately by reducing the HVAC load and thus the size and cost of the HVAC system. Over-sizing your heat and air-conditioning systems simply negates that savings.

Efficiency
Huge gains have been made in HVAC efficiency over the years, but many of those gains are negated by over-sizing. Efficiency ratings on the best HVAC systems are high because when sized correctly there is little cycling loss. This means that the system isn’t constantly turning on and off which uses a lot of energy. HVAC systems are much more efficient when running for longer cycles, a condition that won’t occur if your system is over-sized. This may seem counter-intuitive but if your neighbors’ AC is running less than yours, they are probably spending more on electricity (unless of course they simply don’t mind sweating).

Humidity
Short cycle times also hurt an HVAC system’s ability to regulate humidity which can lead to condensation and mold, especially in particularly damp climates. It is much better to have your AC running constantly at an efficient level than to have it blasting for a few minutes and shutting down, particularly if it is helping regulate humidity.

Comfort
That cycling issue rears its head again when it comes to the comfort of occupants. Even a well insulated home tends to lose it’s heat at exterior walls, windows and doors. With a centrally located thermostat the quick blasts of heat that warm the center of the house may come too late or too infrequently to help the extremities. A short cycling HVAC system is like wearing a vest when the weather calls for a coat, everything outside the core is uncomfortable.

There are a variety of other reasons people might give to disparage the over-sized HVAC system including increased noise and the physical size of the thing, but I think you get the basic idea . . . over-sizing is bad. So, why is it done? If we know it is less efficient, less comfortable and more expensive, why is it such an ongoing problem?

The answer lies in the first two paragraphs of this post. People want control. Regardless of the outside conditions, there is this idea that one’s chosen house temperature is an inalienable right. If we want it to be 78 degrees in our home all the time, we expect out HVAC systems to make it so. A right-sized system might not be so compliant, particularly on those days of rare extremes.

Does this mean the occupants are destined for discomfort? Of course not. The vast majority of days temperature control will be as flexible as you could want, but on those aberrant extremes there may be some limitations imposed. One might need to settle for 68 and a long sleeve shirt on a sub-zero day in Philadelphia. In Maine, one might need to turn on a fan and wear a pair of shorts when the temp tops 90. Generally, a right-sized system expects one to behave (and dress) slightly more seasonally, but the overall effect will actually be more consistent throughout the home and thus, more comfortable.

So, what is your ideal indoor temperature? How much fluctuation would you be willing to allow the seasons to impose on you? Did I miss anything important about HVAC sizing?

Give us your thoughts, opinions and expertise in the comments.

If you enjoyed reading this post I can promise you'll love our new writing over at Postgreen Homes. Yeah, we know that's the same thing your favorite band said and their new album is nowhere near as good as their early stuff, but seriously, we are actually still getting better.

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On a Budget? These are your Top 10 Priorities When Building a Green Home « Courtland Custom Homes Blog
April 19, 2011 at 10:40 am

{ 6 comments… read them below or add one }

1 Mary @ Turkey Farm Treasures January 30, 2010 at 8:19 pm

What about ACCA manual J calculations?

2 greenbuildingindenverdotcom January 31, 2010 at 11:11 pm

Good point, but have you ever met a heat guy who actually put in a furnace or boiler that was exactly equal to the Manual J heat load?
No, they are too chicken, but who can blame them because 90% of the time they don’t get in trouble for putting in a system that is too big. That and the fact that most manufacturers still aren’t making anything small enough for efficient homes.

Because I think you guys have zeroed in on minisplit heat pumps for heating, and believe in grid-tied PV solar, here’s a brand new DC minisplit that possibly could improve efficiencies.
http://eon.businesswire.com/portal/site/eon/permalink/?ndmViewId=news_view&newsId=20100120006253&newsLang=en

Also, here’s a heat pump water heater that can be placed in a different place than the water storage tank. That might fix the awkwardness of most HPWH applications. A garage, unused basement room, or greenhouse might make sense.
http://www.northrdt.com/Geyser/index.html

3 RitaF February 1, 2010 at 10:58 am

The entire HVAC industry and process has been segmented to the point where the general approach is “throw in something that can do the job” and who cares how it interacts with everything else in the house. It is rather crazy how everyone if focused on a units SEER rating when in fact much of the benefits of an effocient unit are lost as a result of poor sizing, poor layout, unbalanced systems, etc.

4 Chad February 1, 2010 at 2:28 pm

Excellent comments all. Let me see if I can address each.

Mary – A correct Manual J calculation can help to solve this problem of over-conditioning. The problem is that most HVAC installers will cheat the Manual J by inputting incorrect figures that will output a larger than needed system. For instance, they will lower the true insulation and window specs, especially in a well insulated home. The good news is that LEED requires you to provide a correct Manual J and show all calculations that were input into it. The advice I would give, is that if you are going to have a Manual J done, ask to see all of the inputs so you can verify they are correct.

GBD – This heat pump water heater is awesome and made my morning. This could be used with a Solar Storage Tank without needing the solar to be installed up front if budget did not permit. Solar Thermal panels could be added afterwards. I have seen these DC heat pumps, but it doesn’t really help us unless we have an off-grid house. Even then I’m not sure how much this is helping the entire system, since most of the energy needed in an off-grid house needs to be converted to AC.

RitaF – Good comment. HVAC pros have a ways to go in general in the US. Proper training is needed. On top of that, Americans need to stop demanding their homes are always perfectly comfortable. This is the real driver in HVAC pro’s behavior. Nobody likes to get called and yelled at when they’ve gone through the trouble to size a system correctly and the client is still not happy…

5 Christopher Abnett June 3, 2010 at 5:04 pm

why not do the most common sense thing to do and that is install a system designed to change along with the conditions… ok for instance i designed and built a system for my 1450 sq ft house that does just this..

1] the system has 3 zones.. 3 separate areas at 3 separate temperatures…

2] the system is hybrid Gas (95% modulating furnace) and electric (3 Inverter style heat pumps)

3] the system can push out anywhere from 5000 to 51,000 BTU of cooling.. ALL inclusive….

4] the compressors AND fans vary their speed according to MANY parameters…..

here is how it works…
when a zone calls for cooling, its compressor will start slowly, and the dampers for that zone open (return and supply)..

the central blower ramps up to a set speed based on the following:
a) differential between setpoint and room temp
b) outside temp factored in
c) indoor humidity factored in

the compressor ramps up to maintain an evaporator core temperature between 40 and 50 degrees based on outside and inside room temp

as the system runs the conditions are monitored… such as:
a) how long has it been since the temperature gained toward the setpoint…
b) has the indoor humidity changed
c) has the outdoor temp changed

each degree that is gained toward the setpoint causes the system to ramp down slightly.. if it sits at a certain temperature for a while or loses ground, the system will ramp up (and remember) that it previously couldnt hold temperature so it runs itself up higher…

this system has CUT my energy bills greatly.. *AND* also allows me to wear shorts in winter and never sweat in summer…..

truly THE answer to what to size it…. with multiple small compressors that are inverter drive they are efficient in any mode EXCEPT running flat out… but RARELY do they run flat out for longer than a few minutes at a time…

just my ideas…

6 hvac westchester ny July 4, 2011 at 10:16 am

Awesome post!

This post shows that HVAC efficiency comes down to one thing: balance.

An HVAC system that is well laid out with an appropriate, proportional level of power is optimal.

A good HVAC company will try to offer maximun overall value to their customers, rather than just try to sell them the biggest, most expensive system.

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