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How Air Conditioning Zoning Works!

How Air Conditioning Zoning Works!

We find many HVAC technicians don’t know how to explain to their customers, how to properly use their zoned heating and air conditioning systems.
They tell their customers to just set the temperature to 75 on both floors and leave it.
That’s not how zoning works!
If you wanted to have both floors or both zones at 75 degrees, why not just take the zoning out and cool the whole house at one time?
Why do we even need zoning at that point?
We will be talking about the do’s and don’ts of zoning.

Zoned HVAC in Homes

Zoned houses, or houses with two or more thermostats, are usually found in homes that have two floors or sprawling ranch style homes.

In this blog, we’re going to talk about older homes that have zoning added to their now oversized system.
We’ll also talk about what kind of lifestyle fits best with a zoned system. And finally, we’ll be discussing how to use zoning to save you money, which is really why zoning in a home is even used.
So many homes in the Red Deer area are big enough to support family sizes in the range of 4 people to 6 people and even more. Homes with a designated living area that include the kitchen, dining, family rooms, bathrooms, entryway, and other common areas.
The other part of the house consists of the master bedroom, master bathroom, larger closets, the kid’s bedrooms, bathrooms and sink areas, and the laundry room.
You could easily break this down into two “zones” with a thermostat to control each one.
Let’s just say you have a typical 1950 sq ft home of conditioned space. And that’s not including the areas of the house like the closets, pantry, and other rooms that don’t have registers supplying air to them.
Many houses like this are two stories and originally come with a big 4-ton air conditioner that would satisfy the whole house at one time.
Ten years later, however, the owner decides they want to add a thermostat to the upstairs area, so they’ll have two zones – one for upstairs, one for downstairs.
Ten years later now, the system is 20 years old, and the new owner of the house is ready to change the system out for a new one because the system doesn’t cool as it should anymore, but doesn’t understand why he’s being told the new system should be smaller. Much smaller!
Because they do not know how to use zoning!
Zoning is a great idea but keeping that big old 4-ton system there is a mistake. If the 1950 sq ft home is divided into fairly similar sizes, let’s say, 1150 downstairs and 800 sq ft upstairs, and only one zone is calling for cooling, then we have that big 4-ton system (which remember was designed to cool the whole house at one time) overwhelming the temperature change in one zone, (half of the house!) over pressurizing the ducts for that zone, sending a high velocity of air through the registers of that one zone and generally putting a big strain on the entire system. Except for the zone not being used.
This strain comes in the form of something like high blood-Pressure for the human body.
You can run on high blood pressure for a while, but if it’s not regulated, the body can suffer and fail earlier than usual.
The same goes for the compressor which is a lot like the heart of the human body. It pumps refrigerant to and from the indoor coil and outdoor coil. Too much short cycling, turning on and off quickly, makes the motor see an enormous amount of damaging heat and energy on every start-up, time after time after time.

When your system is twice the capacity that it needs to be because only one zone is needing air, it’s going to satisfy that one zone way too fast – on and off, all day long.
See, your AC wants to run for longer periods at less amperage to cool your house effectively. One to two degrees change for every 15 minutes is not unusual according to Honeywell.  But 2 to 3 degrees in five minutes is too fast.
We won’t get into this too much as we have other articles on this topic, but we want to condition the whole room, not just the human. This is how we keep proper humidity levels and prevent wide temperature swings in the room.

What Is the Right Size of HVAC Then?

If we were just conditioning that first floor at 1150 sq ft., what size would we need? Without getting too technical, we would need about a 2.5-ton system.
But what if we were trying to cool just the upstairs bedroom areas at 800 sq ft?
We would need about a 2-ton system.
Now, what if both zones happen to be calling for air at the same time?
This is where it can get tricky, but for God’s sake, we are NOT doubling the size of the system.
Our rule for our technicians is to size the system a half-ton larger than the largest zone in the house, which in this case is 2.5 tons.
So, we size the whole system at 3 tons. This is a full ton smaller than the original one installed, which surprises some prospective buyers, but it’s right.
Because of sizing issues already mentioned in this blog, we can use a 3-ton system blowing through the smaller 800 sq. ft. zone without doing major damage in the long run to the new system, especially if some bleed off dampers like the new Honeywell ARD dampers is installed.
This allows the right amount of air to get to the small zone and any extra bleed-off air into the other zone in very small amounts.
Now, what happens when both zones reach a point in the day, where they are both calling for air conditioning at the same time?
That extra half-ton will satisfy one zone or the other first. When that happens, that’s zone closes its damper and allows the other zone to continue until it’s satisfying.
We’re not talking about the laws of thermodynamics here and nothing we say is absolute.
Of course, there are variables your technician will have to take into consideration when it comes to your particular home, but an experienced installer will know what is right and what’s not when it comes to zoning your house.
So, let’s talk about certain lifestyles where it doesn’t make sense to have a two-zone air duct system.
We had a customer who has a nursery and kid’s playroom upstairs while the caregiver and other relatives occupy the downstairs portion of the house throughout most of the day.
What’s more, is their demand for cooling is considerable given they like it to be 72 degrees upstairs during the day for the kids, and would like it to be around 70 to 72 degrees downstairs at the same time of the day, for those who are downstairs.
And they expect those temps even on the hottest days of the year.
That’s not the way zoning works.
The typical home we work on is one where a parent stays home with a child or retirees that don’t have to go to work anymore. So, there is usually someone home most of the day.
We tell people in these homes to focus on running the AC downstairs where they typically are throughout the day. So, if you like it 75 degrees in your normal living areas, make it 82 degrees upstairs, in the area, you’re not using.
Run the AC primarily throughout the day downstairs at whatever temperature you’d like, until about 6 or 7 pm. Then, shut the thermostat off for downstairs and have the upstairs start cooling off so that by the time you get to bed, it’s cool enough upstairs to sleep for the whole family.
It’s already 75 degrees downstairs when it shuts off, so it’s not like it’s going to warm up super-fast and make it uncomfortable for you.
Nobody needs the AC downstairs during this time so set the downstairs to be 82 degrees. It won’t get there overnight, but at least the system doesn’t come on downstairs, so the AC can focus its efforts on cooling your second story down as quickly as possible.
You can set it up however you want on your thermostat’s schedule.
The main idea of having two thermostats is the system is not sized big enough to cool the whole house at one time. Your home’s AC system with two thermostats is designed a little smaller because it’s designed to cool one floor or one zone at a time.
You save money with smaller systems, and efficiency is huge. The smaller the system, the less you pay for the electricity it takes to run the AC. You also save money and energy when you don’t try to cool the entire house at one time.
Well, we hope this helps explain a little more about zoning, and how to use it properly.
The intent of this was to enlighten folks that running both zones at 73 degrees all day – isn’t the way zoning works.
Think of it as two separate zones that we are conditioning at two separate times of the day. If both zones happen to call for cooling at the same time, a properly sized air conditioner will manage its way through it and satisfy one or the other zone first, and then give the full system to the lagging zone.
For more HVAC tips continue to read our blog and if you need help with your furnace or air conditioner, give us a call.

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