You do not want an HVAC Unit too Big

Sometimes bigger is better. Ice cream cones and steaks and really good french fries come to mind for me.

But Bigger is not always better. Nothing really works better than fitting just right into clothes; although we all know someone trying to camouflage too much stuff in too small a pair of pants.

But, in fact, more cooling power can create more problems. Don’t let anyone, yourself included,  talk you into a larger HVAC than you have now. Particularly, when you are dealing with structural issues that leak conditioned air and waste energy. In fact, we have a lot of critical information which needs to be sorted through and understood before you invest any major amounts of money in HVAC upgrades.LINK

Briefly, the recommendation of the ACCA – Air Conditioning Contractors of America- is that before you make an upgrade to existing equipment- that the house be assessed for the size of unit the space needs. LINK That is shorthand but the days of rule of thumb are- should be long gone. And certainly, the days of bigger is better should be over. You do not want a unit  too big. For one thing, it can make your moisture issues in your home worse not better.

You really do not want more cooling power that you need. What? Who doesn’t want more cooling? What kind of silliness is this?

Well, as a matter of fact, too big can make you uncomfortable and can help make your home less healthy.

This issue is a big deal because traditionally, HVAC service companies have figured you need more cooling if you have any comfort issues and they will make sure the unit is a might extra big just to make sure you are happy. Sort of error on the big side to make sure. But it turns out that this does not work. In fact,when you look at the building science- when you look at the nature of moisture, that bigger is not better. Bigger is a problem.

Bigger  is not better for at least 2  important reasons.

  1. there are many critical reasons why a home can be too hot or too cold, or uncomfortable and none of these reasons- or the solutions to these reasons relate to the size of the air handler at all. Oh, there can be many issues with the HVAC system but none of these relate to the size of the A/C unit- unless it is too big.
  2. the second reason is an air conditioner that is too big will make you more comfortable and help make your home a less healthy place to live.

You do not want an A/C unit that is too big

You might need to take a spin by the page describing  how the cooling system in your home actually works. The heat pump system extracts the heat from you house and disperses it to the outdoors in the cooling season. The system can work in reverse in the heating system but lets skip that now as it is not germane to our issue of too big. that link is here. 

Your air handler extracts moisture from the air of your home, by running the air in your home over a cold evaporator coil. The humid air condenses on the evaporator coil drains off and escapes your home through the HVAC drainage system out of the house.

The evaporator is where you exchange heat out of your home. Too much air flow can cause short cycling and not provide proper time to dehumidify the air. You are not comfortable. The cause is too much cooling provided by too big an air handler.

The evaporator is where you exchange heat out of your home. Too much air flow can cause short cycling and not provide proper time to dehumidify the air. You are not comfortable. The cause is too much cooling provided by too big an air handler.

If your air handler is too powerful and lowers the temperature of your home too quickly, the set temperature on the thermostat can be reached too quickly and the unit shuts down before a comfortable amount of moisture in your indoor air has been condensed and removed. This is the most efficient and basic way you reduce the humidity in your home.

When the cooling cycle runs what is called a short cycle, you are left in a muggy state- you humidity level is too high and therein lies the potential for an unhealthy home. You are left uncomfortable and you might get up and turn down the thermostat thinking that you are too warm, when in fact the combination of humid and temperature is making you uncomfortable. Heaven forbid, you think, gees I need a bigger air handler, I need more cooling.

In fact, the opposite is the problem. You have too much cooling power. What you need is less cooling power, you need the air to move more slowly or less air to move the regular speed because the system is able to cool too much air too quickly and not leave enough time to extract a pleasant amount of moisture from the air.

Put this combination of too big a unit in a home which allows too much air to leak in and bring in high levels of moisture from the humid Texas summer, and you have a recipe for discomfort for you but a nice environment growing all kinds of crap you don’t want growing in your house- growing and thriving.  Make sure you see the page describing the relationship between moisture and humidity and an unhealthy house. 

And there are other problems with short cycling

Your equipment runs less efficiently because it runs and stops too often and the transient effects of getting the electric motor turning and all the parts turning is energy intensive. It is also the hardest on your equipment also. So short cycling increases your electric bill and hastens the day you will have a breakdown.

You equipment could be more noisy too and more disruptive to your environment. Or you might be distracted by air blowing on you and then not blowing on you. Most air blowing on you is not refreshing, but I leave you to assess how the air feels in your home, and how disruptive the equipment is. I just want you to know that great air handlers are basically whisper quiet, and ramp up air flow and you feel carassed if you notice air movement at all. I tell you that, because we can all become inured to stuff we really don’t want to put up with.

What can you do about this?

You need to have a good idea how well your home is performing- keeping water out and conditioned air in. Your home has it own hierarchy of needs -LINK- as relating to energy efficiency. Well, you need to figure out the actual issues with your home which are impacting your comfort, your energy bills and your peace of mind. You also need a clear idea how well the structure of you home- the air barrier boundary, and the insulation barrier  home structure and the HVAC mechanical system works together to make your home more comfy.

After you have a perspective on the structural issues affecting you comfort, then you need to assess your HVAC system and determine how effectively it delivers air around in our home. Here is a discussion of what can kind of defects in your mechanical systems can cause discomfort and even health and safety issues.

At this point, it makes sense to start evaluating the system components and decide whether new systems make sense for you. Variable speed systems match up much better to the wide range of cooling loads in North Texas, and a better matched load saves energy and makes you more comfortable.We have our discussion about HVAC equipment here, -LINK- but make sure when you undertake that information gathering expedition that the experts you choose understand home performance, see your home as a system and understand the design principles involved in fitting equipment to your home that fits well rather than just more power.

There is a whole lot of new thinking going on in the home performance industry and new home construction. But there is always a tendency for old worn out ideas to persist. For all kinds of reasons. But you know what you need to do, and you need to figure out how to influence the other stakeholders in the family to give up on the hemi-engine mentality and be prepared to push back again HVAC professionals that perpetuate the thinking they have had for the last 30 years. It is a new day. We now adhere to building science in home repair because it saves us money and reduces stress and makes us more comfortable.

Bigger is not better when the time comes to buy a new air conditioner. You need to make sure you have evaluated your home as a system, and understand what you home needs to be cozier and more comfortable, and where the sizing and design of any new air conditioner fits in here. The right fit is what we want.

Any thoughts here- feel free to add them below.




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