Duct Design 6 — Determining the Zones

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Zone dampers in a duct system

Last year I wrote five articles in a series on duct design and ever since, I keep getting requests to continue it.  Well, those of you looking for more in this area, you're getting your wish.  I've listed and linked the others in the series below so you can go back and read them first if you'd like.  I'll do a quick recap here by saying that the focus in those first articles was the physics of air flow that went into determining how big the ducts need to be.  We looked at the pressure from the blower that moves the air, the various pressure drops, and how total effective length is a measure of pressure drops.

Getting our heads out of the ducts

Now let's crank the series back up with a look at the bigger picture.  One of the main points in the earlier articles was that sizing depends on the total effective length of the most restrictive run in the duct system.  The equivalent lengths of the fittings matter more than the straight runs of well-installed duct (hardpipe or flex), so you've got to choose your fittings well to reduce the pressure drops. 

The layout of the system determines what fittings you need and thus the total effective length.  When we're working on a duct design, we consider a number of factors that affect the layout:  the client's preferences, location of the air handler, framing issues, zoning, and location of the vents.  I'll look at some of the other issues in future articles, but today I'm going to focus on zoning.

Should you zone the house?

This is a question you can address when you look at the house plans but you can't answer it fully until you've done the heating and cooling load calculation.  ACCA's Manual J (the book, not just the protocol) has a nice section called "Procedure for Evaluating Zoning Requirements" (section 11-21 in my copy) that can be helpful if you're trying to figure out if you should zone and how to do it.  The procedure includes five steps:

1.  Evaluate the architectural features.  Of course the house design is the first thing to look for.  Because warm air rises, for example, you're going to want separate zones for separate floors.  Manual J lists twelve factors that help you determine the need for zoning:

  • Number of levels
  • How much the house spreads out
  • Dwellings with limited exposures, as in apartments or condos
  • Rooms with a lot of glazing
  • Basement rooms
  • Attic rooms
  • Bonus rooms
  • Rooms on a slab-on-grade
  • Rooms over exposed floors
  • Rooms that will get used by a lot of occupants (Party!)
  • Hot tub or swimming pool rooms  (Have a dehumidification design, too!)
  • Sunrooms, solariums, or atriums

The basic idea here is to look for parts of the house that might have significantly higher or lower loads than other parts.  In addition to separate zones for separate floors, another common example is putting bedrooms and common areas in separate zones. 

Zoning a house with three zones

 The schematic above shows a house we divided into three zones.  The master suite is on the left, the common areas in the middle, and the other bedrooms on the right.

2.  Evaluate time-of-day load patterns.  The zones in that design above also factor in time-of-day patterns.  The common areas are used mainly during the day while the master suite and bedrooms are used more in the evenings, night, and mornings.  Another consideration for time of day is the amount of solar gain a room or zone gets from the windows and skylights.

3.  Evaluate air flow ratios.  Balance is a good thing.  If you're using the same ducts for heating and cooling, it's nice when you end up with ducts that deliver the right amount of heating in winter and the right amount of cooling in summer.  One way to do that is by looking at the ratio of cooling air flow to heating air flow for each room in the zone.  Here's the guidance on that from Manual J:

Ideally, the C/H ratios of all rooms in a zone are within 15 percent of the C/H ratio of the room that has the zone thermostat.

What if you have a room that's outside the 15% recommendation for any zone you could put it in?  If there's still time to make changes to the house design, you could try adjusting the insulation, window shading, or glazing area to get the C/H ratio closer.  If you can't do anything to change the ratio, it may come down to the occupants having to make seasonal adjustments to the supply vent louvers. 

4.  Evaluate room-to-room openings.  The more open the rooms are to other rooms in a zone, the more resilient the zone will be to imbalances in loads, time-of-day patterns, and air flow ratios.

5.  Finalize the zoning plan.  Take stock of all the things above to figure out the best way to divide the house into zones.  The goal is to create zones that lessen any imbalances in heating and cooling the rooms in that zone. 

The two types of zoning

The term "zoning" can be confusing because it's used two different ways.  What I've been discussing up to this point is dividing a house into separate zones, each of which will have its own thermostat.  But sometimes that term refers to setting up a single system to deliver heating and cooling to multiple zones.  It's usually clear from the context what's meant when you see the word. 

In the schematic above, the house has three zones served by a single system that's zoned.  When you're designing a duct system that matters a lot.  If the three zones were served by three systems, we'd have three air handlers, probably located near the zones they serve.  That should result in lower total effective lengths.  With a zoned system serving the three zones, you have more total effective length and it's not so much because of the greater distances as it is the increased number of fittings. To get all the way across the house, you have turn the air more.  Plus, you have the zone dampers and their pressure drops/equivalent lengths.

Zoning issues

Here are some things to keep in mind regarding zoning:

  • One zone for a whole house?  It's certainly possible to do a whole house on one zone.  You usually end up with less balance in the heating and cooling but if the house doesn't have any large loads from windows and the house isn't too big, a single zone can work fine.  The more disparities you have in the room-to-room loads and thr more spread out the house is, the more the house might benefit by zoning. 
  • Single room zoning.  Of course, the ultimate zoning would be to have each room as its own zone.  In houses without cooling systems, you get close to that sometimes with the old radiators in each room, if they're individually controlled.  With heating and cooling, the closest we can come is to use ductless mini-split heat pumps but they don't come in small enough sizes (or low enough cost) to be used in ever single room.
  • Small zones with mini-split heat pumps.  The next step up would be ducted mini-split heat pumps, which let you create small zones that include only a few rooms each.  We've done ducted mini-split designs where a house the size of the one above might have five or six zones instead of just three.  Because mini-splits come in smaller sizes, each zone in the house would have its own system.
  • Some rooms need separate equipment.  If you plan to heat or cool a garage, it must be done with a separate system.  Same goes for indoor swimming pools. 
  • Zoning a conventional forced-air system.  With conventional equipment, you divide the air up with separate trunk lines that carry the air to each zone.  A zone damper opens and closes based on what the thermostat tells it to do.  But what happens if just one of three zones is calling for air?  With a fixed-capacity, fixed-speed system, you've got to find a place to send the excess air.  One common solution has been to install a bypass duct, which is generally a bad idea.  Better would be to use a variable-capacity system with a variable-speed blower when you do zoning.  That way the air flow adjusts to what's needed (within the limits of the system chosen).

So now you have a little education on zoning a house and zoning an HVAC system.  The ACCA manuals have a lot of information on this topic if you want to go deeper, but the basic idea is that you look at the features of the house to determine if you need to create separate zones.  Then you figure out whether to heat and cool those zones with separate pieces of equipment or with one zoned system.

 

Other articles in the Duct Design series:

The Basic Principles of Duct Design, Part 1

Duct Design 2 — Available Static Pressure

Duct Design 3 — Total Effective Length

Duct Design 4 — Calculating Friction Rate

Duct Design 5 — Sizing the Ducts

 

Related Articles

Converting Heating and Cooling Loads to Air Flow - The Physics

The 2 Primary Causes of Reduced Air Flow in Ducts

Navigating the Twilight Zone: The Hidden Flaw in a Zoned Duct System

 

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Comments

One of the twelve zoning considerations you copied from Manual J: Hot tub or swimming pool rooms. This is a bit misleading. An indoor swimming pool (natatorium) should NEVER be served by the same air handler that serves other rooms.

In general, a natatorium should be conditioned by an stand-alone mechanical system operating through a dedicated duct system (heating, cooling, dehumidification).

Allison
Bailes

David, you're absolutely right.  A natatorium should definitely have its own heating, cooling, ventilation, and dehumidification system.  I didn't go into the details about what to do with the zones but I did discuss in a later section that zoning has two meanings:  It can refer to dividing up the house into areas that each have their own thermostat — and possibly their own system — or it can refer to distributing the heating and cooling from one system to different, separately contolled zones.  At the end, though, I did discuss a little about the issues with how to heat and cool different zones.

The air flow Ratio between C/H season is a new concept to me. With Large production builders could there be designs that limit the the ratios enough so that seasonal dampering/balancing would never have to happen. If so do you think the 15% rule would be it or would you need something like 10 or 5%? to get the ratio % down would you concentrate on the thermal envelope as the orientation would be harder to control in this situation. Thanks

@Robby, seasonal imbalances represented by cooling/heating airflow ratios are difficult to address via envelope tweaks. In any case, the person designing the duct system is not often in a position to dictate glazing areas, floor plan layout, etc.

I always check room-by-room C/H ratios before deciding how to zone a house. When laying out my zones I try to combine rooms with similar C/H ratios. Said differently, I avoid combining rooms with drastically different ratios. Certain rooms like foyers, hallways, stairs and utility rooms where occupants don't tend to spend much time are less critical than bedrooms, the family room or the kitchen.

Forced air systems, especially A/C and heat pumps, are far more limited than radiant when it comes to zoning. Variable capacity equipment helps but a compressor's minimum speed ('turn-down') is often way too much capacity for single-zone calls from the smallest zone.

Historically, installers were trained to use bypass to insure adequate airflow across the evaporator coil. However, bypass is and has always been a bad idea. At best, it compromises refrigeration system performance. At worst, it can damage or kill the compressor. Several years ago California Title 24 banned bypass (with a limited exception).

The best way to maintain adequate airflow across the evaporator during single-zone calls is apply all of the following three strategies in the design:
a) use variable capacity equipment
b) oversize ducts serving the smallest zones
c) define one or more 'dump' zones to receive excess air, or set all dampers to bleed a small amount of air (10% to 20%) when closed. The best zone control systems use modulating dampers to allocate excess air to non-calling zones based on which is most likely to have the next call.

Bottom line: designing a zoned system is arguably the most challenging aspect of residential HVAC work.

I have a love/hate relationship with my zoned system.

Issues:
- Bypass duct is too short. Consequently as Dr. Bails mentioned in a link to another article the bypass reduced the temp of the incoming air when the AC was running. Other then the problems mentioned by Dr. Bailes with this set up, this now cooler air reduced the amount of refrigerant which would convert into a gas so liquid refrigerant would travel back down to the compressor which over time killed it in 10 yrs of use. Solution: close off the bypass and permanently open the damper for the smallest zone. Basically we converted our 3-zone into 2-zone + 1 bypass zone. Unfortunately during winter it gets hotter than hell in the "bypass zone" during the winter because it's the 3rd floor.

- We have a single stage compressor and one speed air handler so our system is technically oversized but less so since we turned one zone into a huge by-pass duct.

I strongly suggest that you avoid zoning with dampers. This is equivalent to having a cruise control on your car that controls speed with the brake pedal rather than the gas pedal. Use multiple systems with separate thermostats whenever possible. If California wanted to do something useful, they would ban residential equipment that is over 3-tons in capacity.

@Roy, advising folks to avoid air zoning is a bit severe don't you think? I share your sentiment re: misuse of larger unitary equipment. But it's not always possible to apply multiple systems. So then what?

With homes verified to meet current energy codes, it's not uncommon to see whole-house loads under 2-tons. As you know, it's usually impossible to achieve good temperature balance with a single stat, especially in multi-level homes. In my world (beyond-code homes), load ratios greater than 1,500 ft2/ton are typical, and it's not unusual to see ratios greater than 2,000 ft2/ton. That means I can (and often do) have zone loads too small even for a mini-split.

There's no question that many air zoned systems are poorly designed, but your cruise-control analogy suggests air zoned systems are inherently bad, which is nonsense. We should be encouraging better training and design practice, not throwing the baby out with the bathwater!

David, I will stick with my analogy that using dampers to control airflow is the same as using the brakes to control car speed. The situation that drives me crazy is when people put in a 5-ton, single capacity system with undersized ductwork and higher static pressures. There will always be at least one zone that does not get enough airflow. So what do they do? Put in a zoning system which results in dampers being closed on the zones with sufficient airflow so that more air can be supplied to the under served zone.

I will agree that damper-type zoning systems can work well with multi-capacity systems if the ductwork is sized properly and the control system is compatible with the heating/cooling equipment. Adding third-party zoning systems to variable-capacity compressor systems is not likely to work well.

For many cases, I prefer the approach where you have one system with two temperature sensors, one in the living area and one in the sleeping area, especially if there is only one bedroom being used for empty nesters like me. The control logic is such that the living area thermostat controls the system during the day and the sleeping area thermostat controls it at night. The "uncontrolled" zones just float, which is OK from a comfort perspective since they are unoccupied. This type of thermostat with multiple sensors is available, but not as widely as it should be. You can argue that this type of control may waste energy by over cooling or heating unused zones, but I still claim that it is better than controlling your speed with the brake pedal.

As for two-story homes, just put a separate system on each floor, especially when you have an open stairway, and do not ever set the upstairs thermostat on cooling when the downstairs thermostat is on heating mode.

Sounds like your analogy is directed at zoned systems done poorly? I certainly can't argue with that. But a zone system designed to work within the operational range of the blower and compressor, using integrated OEM controls (as you aptly point out), should not be painted with the same brush. OTOH, retrofit zone control is usually a bad idea unless the duct system is being replaced.

I should also mention that zoned systems are often misused. The primary reason to zone is to achieve better temperature balance against dynamic loads (morning-to-afternoon, day-to-night, winter-to-summer). Of course, zoning can also be used to set different temperatures by zone. But savings can only happen to the extent that energy would otherwise be wasted overcooling or overheating part of the house to achieve comfort in another part.

It may be possible to reduce energy by turning down (or up) the stat in unoccupied rooms, but this can backfire. As we build homes tighter, wintertime humidity tends to be higher, which is a good thing to a point. But this increases condensation risk in unheated rooms.

Regarding separate systems for two-story homes: As I tried explaining in my previous comment, this option is becoming less viable as loads get smaller. And I'm not just talking about beyond-code homes.

BTW, I chuckled when I read your comment about using two sensors with one stat. I've been doing that in my own homes for 30 years (save one that had full blown zone control), ever since I beta-tested the first digital stats by Enerzone that supported remote sensors. Early on, I would install a switch to select which sensor was active. Today, different stats have different methods for selecting remote sensors.

Yea, David, I am talking about zoned systems done poorly which is most of them from my experience. I think that zoning dampers should never be added to a single-capacity system, even if it has a variable-speed blower. Bypass dampers should never be used. Dump zones are bad too.

Third-party zoning control companies claim energy savings and better comfort. I agree with you that zoning is and should be primarily done for comfort. Turning off unused zones makes sense from an energy perspective, but as you point out, it can cause other problems.

I believe that the solution for high-performance homes is a small ducted system. I would push for a 1 to 1.5 ton ducted system with one system on each floor of a two story house. The air handler and ducts would be small, easily hidden, and in the conditioned space. I am not even sure if it needs to be variable capacity. This type of system would be cheaper than current mini-splits and not require plastic boxes hanging on walls and would still allow air supply to bathrooms and large closets. There is hardware available today for doing this, and if the concept became widely used, better and more cost-effective hardware would become more widely available.

The multi-sensor thermostat is starting to become more popular today, but for the wrong reason. Electronic thermostats with big color screens and lots of bells and whistles just can't sense temperature accurately, so they have to go to a remote sensor. Adding multiple remote sensors is then an easy option, and selective zone control becomes a "free" option.

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