FAQ: Temperature Zoning and Climate Control

What is a temperature zoning system?

A temperature zoning system is a system that allows to:

Support a consistent temperature in a given part of a house, regardless of external conditions, such as sun, rain, snow, clouds, and wind;
Support different temperatures in different parts of the house at different times as desired by the owner. In an essence, each zone will have its own climate.

It is possible, for example, to have some rooms be cooler or warmer than others (individual preferences), and it is also possible to keep the same rooms at different temperature depending on, for example, whether the rooms are [supposed to be] occupied or not.

What is a climate control system?

A climate control system is a temperature zoning system augmented with humidity sensors, allowing the system to control the heat instead of the temperature. The difference between them can shortly be described as latent heat (see also: enthalpy). Sometimes, the notion of indoor air quality (IAQ) is also brought in.

How do I know if I need a climate control system?

If the answer to at least some of the following questions is "yes", then you probably need a temperature zoning system:

Does your house have multiple levels?
Does your house have levels that are completely or partially underground?
Are some rooms in your house noticeably cooler (for heating) or hotter (for cooling) at certain season or time of day?
Do some rooms in your house feel "stuffy"?
Do you have rooms that are rarely used?
Do you have family members that want it hotter or colder?
Do you have a home office?
Do you have a big TV and like to gather all the family to watch movies?

But why do I need it in the first place?

Because the heat distribution in the house is affected by many static and dynamic factors, including, but not limited to:

Walls, ceilings, and windows insulation quality;
Exposure (south exposure gives a room that is generally hotter, western exposure gives a room that is hot in the evening);
Overcast (clouds in the sky make for less solar radiation absorbed by the walls and the roof);
Wind & rain (wash the heat away);
Humidity;
Ductwork quality (for forced air systems) or piping quality (for steam or hot water heating systems) - insufficient supply or return will create "starved" zones.

But the HVAC contractor says they can "balance" the house?

Sure, for the exact time of day, ambient temperature, weather and season at the moment when they do it. The house is fundamentally unbalanced - see the answer to the next question.

Why can the house be unbalanced?

Every house with a single thermostat is unbalanced by definition.

Let's consider how does the conventional single thermostat system works.

The thermostat reacts to the change of the temperature at the location it is installed. Most often, this is the living room, or a hallway somewhere in the middle of the house. Suppose we're in the cooling mode, and the temperature rises above the setpoint - this will make the thermostat do something that is called "call for cool" in HVAC jargon. The A/C will switch on, and the temperature will begin to fall. As soon as the temperature falls below the setpoint, the thermostat will command the A/C to stop (actually, it's a little bit more complicated - the temperature will rise a little bit above the setpoint, and fall little bit below setpoint to prevent constant starts and stops - it's called "hysteresis").

Now, how exactly did this process address the temperature in, say, bedroom upstairs?

It didn't.

Even though the cold air was being delivered to the bedroom upstairs, the temperature in it could have fallen just a little bit (for example, if you live in Arizona, the bedroom has a southern exposure, and it is a nice summer afternoon) before the temperature at the point where the thermostat is installed fell below the setpoint.

This effect aggravates as the ambient temperature gets close to the setpoint, because in this case the thermostat will be happy almost all the time, but the temperature in problem rooms may drift as far as 10°F apart from the setpoint. The far rooms are starving.

And the opposite effect takes place when the ambient temperature is far away from the setpoint because it takes a lot of time for the A/C to satisfy the requirements in the place where the thermostat is installed. By the time it is happy, the far rooms are saturated.

Why can't the house be balanced?

There are some measures that can be taken in order to alleviate the problem, but none of them can eliminate it as long as there's a single temperature measurement point.

"Balancing" dampers. The airflow to some rooms is restricted in order to provide more air to the rooms that don't get satisfactory conditioning. This increases the load on the A/C.
Improving insulation in the rooms that don't get satisfactory conditioning. This costs a lot of money. Moreover, this may actually aggravate the problem instead of fixing it - for the case of insufficient cooling in a room with a lot of heat producing equipment, such as kitchen, home office, or a living room with a big TV.
Running the fan constantly. This allows the air to be distributed uniformly, and the hot air influences the temperature at the point where the thermostat is installed, thus making it cycle more often. This costs money, too.
It is important to realize that none of these measures will eliminate the problem. The difference between heat loss (through conditioning) and gain (through imperfect insulation) is changing depending on time of day, direction of the wind, whether there are clouds or not, if it is raining, how many people are in the particular room, is there any heat generating equipment, and so on and so forth.

Bottomline, it is not possible to balance the house without a zoning system.

Do zoning systems shorten the A/C lifespan?

Short answer: not if they are done within [pretty wide] tolerance limits.

Long answer, from personal standpoint: you know, if my house feels like hell, I don't really care if my A/C works for 30 years or 25, or 15, or even 10. I am not going to suffer just because there is a theoretical, no, cancel that, an alleged possibility that what I do is going to shorten the A/C life.

Long answer, from technical standpoint: not necessarily. Properly designed and implemented zoning system should not cause the A/C unit to run outside of design specifications. If the statement about shortening the A/C life was even remotely true, the zoning systems manufacturers would have been out of business by now, buried under the pile of lawsuits filed against them by unhappy customers whose A/C units had died prematurely.

Very long answer, from technical standpoint: following is a list of factors that influence the lifespan of the HVAC unit, and a brief explanation of the way DZ handles them:

Most critical: compressor lifespan will be drastically shortened if a phenomenon called slugging occurs.
Using temperature sensors, the system is able to detect the trend and take corrective actions to avoid this situation. A very detailed, albeit quite technical, explanation is available here.
Restricted airflow is often pointed out to as the cause of HVAC unit premature failures. This is incorrect, in fact, the above effect (compressor slugging) is the root cause, whereas restricted airflow is just one of contributing factors. Restricted airflow can be a result of many conditions, including, but not limited to:
Dirty filter. Using pressure sensors, the system is able to detect this situation and issue a notification to the user requiring to replace or clean the filter;
Dirty indoor coil. Same as above.
Too many dampers closed with no regard to airflow. First of all, the system uses a balancing algorithm to maximize the open damper area, thus increasing the minimal damper opening. Then, again, using pressure sensors, the system is able to detect this situation and take corrective actions to avoid restricting airflow too much.
Compressor "short-cycling" is yet another behavior hurting the system longevity. Favorite metaphor to explain this is: compare highway and city driving.
the system uses a "zone resonance" algorithm to prolong a HVAC unit runtime to reduce number of cycles per time period, to reduce the energy consumption and prolong the lifespan of the unit.

In addition to these, a number of other technical solutions that positively influence the HVAC unit lifespan are used in the system, but they fall beyond the scope of this discussion.

What are the general requirements for zoning?

To zone the house properly, not only the control side of the equation has to be considered, but also the operating conditions of the unit. Some of them are obvious, some of them are not. I will try to point out the issues without really getting into details, because a complete explanation takes a lot of time and effort and is not really required for the scope of the project. If you feel the explanation is inadequate, you are welcome to fix it.

Airflow

In order to achieve zoning, airflow generated by the unit has to be directed to different zones at different times. This is achieved by installing devices called "dampers", which can control the airflow by [partially] opening and closing. The airflow is usually measured in cubic feet per minute (CFM).

Warning: If one (or few) zone is calling, all the airflow produced by air handler will be delivered to that one zoning, thereby requiring the duct to it to be able to sustain the airflow and static pressure.

Warning: The higher is the airflow, the higher is wind noise. Flex ducts are more noisy than sheetmetal because of turbulence (unless sheetmetal resonates).

Static Pressure

Static pressure is the pressure that is created in the ductwork because of its resistance to airflow. This is kind of obvious. The more dampers are closed, the higher is the static pressure. The static pressure is usually measured in inches of water column (IWC) or water gauge (IWG).

Warning: Important consideration: if the static pressure becomes too high, there's a risk of blowing up the ductwork or overloading the fan.

Consequence: static pressure has to be monitored. For details, see Excess Static Pressure Relief.

Temperature Drop

Temperature drop across the indoor coil depends on the capacity of the compressor (which we ignore for now) and the current airflow (the higher the airflow, the more heat the air carries away from the coil).

Warning: Higher temperature drop allows to remove more moisture from the air, however, there's a limit to that. If it becomes too high, there's a risk of flooding the compressor and possibly damaging it.

Consequence: temperature drop across the coil has to be monitored.

Bottomline: you must have ductwork sufficient for zoning. In the worst case, it shouldn't have leaks, or otherwise you will aggravate the situation instead of remedying it.