Duct Design 6 — Determining the Zones
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.
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.
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:
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