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To Zone or Not to Zone…and How?

Having Multiple Thermostats Can Help With Comfort, But What Are They Controlling?

Unless you have a simple, small home, you probably want more than one thermostat to control the heating and cooling.  That lets you control the conditions separately in different areas.  And that’s where zoning comes in.  A common example is a two-story house with one thermostat on each floor.  But there’s a lot more to it than just adding thermostats.

Dampers or equipment?

When you adjust a thermostat, it sends a signal that changes the conditions in the area served by that thermostat.  It sends more or less conditioned air to that part of the house, depending on which way you adjusted it.  But how that happens depends on whether your multiple thermostats are connected to one piece of heating and cooling equipment or one piece of equipment for each thermostat.

In the first case, you have a zoned system.  When one thermostat calls for conditioned air, it opens a damper to allow air to flow into the zone that’s calling.  Of course, it also has to turn on the heating and cooling system if it’s not already on.

In the other case, the one piece of equipment controlled by a thermostat has only one zone to serve.  That keeps things simpler, especially with air flow.

The downside of zoning with dampers

The problem of using dampers on a single piece of equipment is what to do about the air flow.  Let’s say you have a heating and cooling system with fixed capacity.  It doesn’t have a variable speed blower.  So when, say, only one of two zones is calling for conditioned air, what happens to the excess air the blower is trying to move?  Meet the bypass duct.

A bypass duct sends excess air from the supply plenum back to the return plenum
A bypass duct sends excess air from the supply plenum back to the return plenum

The most common way installers deal with the excess pressure created when not all zone dampers are open is the bypass duct.  It sends air from the high pressure supply side back to the return plenum.  That air re-enters the system having already been conditioned.  So it gets colder or hotter with each successive pass.  At best, that will reduce the efficiency of the system.  At worst, it can freeze the coil in cooling mode or crack a heat exchanger in heating mode.

Good HVAC companies understand that a bypass duct shouldn’t be part of a zoned system.  There are ways to do it properly and some good controls available for that purpose.  And of course, the first thing to do is use variable capacity or multi-stage equipment that can ramp up or down depending on the number of zones calling.

Zoning best practices

At Energy Vanguard, we do third party residential HVAC design all over the US (and a bit outside, too).  Nearly every house we do a design for has multiple zones.  And most of the time, we have a separate piece of equipment for each zone.  Our preference is to zone with equipment rather than dampers.

But we do some designs with zoned systems that have one piece of equipment serving multiple zones.  On those jobs, here’s what we do:

  • Use multi-stage or variable capacity equipment.
  • Never use bypass ducts!
  • Balance the air flow in the zones.
  • Allow excess pressure to bleed into dormant zones.

I explained the first two above.  Balancing the air flow in the zones means making sure you don’t have a zone that needs a lot less air than the system can ramp down to.  For example, if you have a variable capacity system whose minimum air flow rate is 500 cubic feet per minute (cfm), you shouldn’t have a zone designed for only 150 cfm.

The last tip is advice we pass on about setting the dampers.  When a damper closes, it has options for how much it closes.  You can set it to close all the way or stay open a little bit.  By leaving it open a bit, any excess pressure after the blower ramps down has an escape path…and it’s one that doesn’t have the downsides of the bypass duct.

Smaller zones and multiple thermostats are the way to go for comfort and efficiency.  And of course, as I’ve said before, five tons is never the right answer.  (Well, almost never.)

 

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia.  He has a doctorate in physics and is the author of a bestselling book on building science.  He also writes the Energy Vanguard Blog.  For more updates, you can subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn.

 

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This Post Has 20 Comments

  1. One classic zoning issue is either a 3-story townhouse or 3-story colonial. With the cooling unit in the basement the top floor is warm in the summer and cool in the winter. One solution is to install a separate unit for the 3rd floor.

    Have you had success with zoning in lieu of 2 units in the above scenarios?

    I am sure most of the houses you design the HVAC for are the more top end houses.

  2. Zoning with equipment is great when the budget allows but zoning with dampers (so long as at least two rules are never broken 1- No Bypass duct. 2-Your smallest zone meets “or at least closely meets” your systems minimum operating capacity.
    I’ve had great success installing multi zone systems, But I’ve had the displeasure of having to fix more zone systems done improperly than ones I’ve seen done properly.
    Another important part of zoning with one piece of equipment is to understand the buildings heat/gain losses throughout the day.
    If at any given time the needed capacity is 3 tons BUT the house as a whole needs a 5 tons YOU DO NOT need a 5 ton (plus the 5 never works anyways)

  3. We’ve lived with a zoned system in our townhome for over 24 years.

    It had a bypass duct
    Dampers which do not bleed air into dormant zones.
    Fixed capacity heating and cooling.
    The original evap coil did not have a TXV, so our compressor died an early death around year 10.

    We ended up replacing the AC which inclia TXV at the evap coil. Then a new 80 percent gas furnace 5 yrs after that. The bypass duct has been sealed off and the damper for the third floor is permanently opened which convertsthe floor as the dumping ground for the excess air. Unfortunately businesses will no longer touch our flex duct because of the material it was made from circa 2000. This prevents use from having the system properly balanced.

    Gotta love production builder quality.

      1. It’s just circa 1999 insulated flex duct. Something about the choice in material used for the exterior skin. That’s all I know.

  4. We’ve been zoning heat pump systems since our beginning in 2006. Allison nails it here – no bypass ducts – ever! We nearly always require at least a multistage system for zoning, though we make occasional exception with small (1.5 – 2 ton) single stage systems with only two zones – such as a typical 2 story townhouse. In those cases we try to both somewhat oversize each individual zone’s ductwork AND ‘crack’ (elevate the zero stop) both zone dampers.

    We always use variable speed air handlers and commercial grade power open / power close motor dampers, not the cheap spring loaded units. The ramping and enhanced dehumidification functions of variable air handlers both help manage excess static / noise concerns.

    Two stage systems we deploy up to 3 zones (I personally owned and lived in a 4 zone two stage system, but that’s a bridge too far, IMO) We’ll go as many as 5 zones on a true variable capacity system such as Trane XV.

    While I agree that 5 tons is rarely the right answer, we did deploy two 5 ton systems once with 10 temperature control zones…but it was an 8200 SF house with some poor envelope choices…the site super bragged how tight it was, but I had to spend an extra $1700 on a 2nd fan for our blower door rig just to get to -50 Pascals…not so tight!

  5. Zoning is also important on small houses as well. You have to take into consideration the use of the rooms not just treat the manuals as gospel. In our (1960)1750sqft house I have thought about zoning because the room used as an office gets insanely hot very quickly with two of us in there and two computers running. It will go from 74-85+ very fast. And no the door is not open and will not be open and cannot be open. I do need to add a return duct into the office which is easy since the unit is right next to it and easy to run a small return through the wall which a return would help some when the ac is running.
    Then there is the master bedroom where we would like to keep that room at 65-68* at night. Oh and the kitchen which we rarely cook in the summer because the system is sized correctly( technically undersized for actual temps here in TX)
    Zoned would improve things in our house but when I go over it all what makes sense is to just put a minisplit in the rooms that have the extra heat load and then in the future downsize the main AC unit. Yes is all sounds strange but in our use case it would be the best overall option and in TX you want backup sources of cooling. Heating is not an issue for us as heating is far easier than cooling though the office does still get too hot in the winter and I have opened the window many times to cool it off in there or closed the vent to stop heat from coming into the room.

    Unfortunately few of us actually live the way Manual J etc think we live and every ones requirements are different from a fixed formula that doesn’t take those things into consideration. Really it might be better to start using commercial formulas for some residential uses as it is closer to what many are using their houses for now.

    And lets also not forget that smaller rooms are going to require more action on behalf of the AC since they are small and the heat builds up faster than in a larger room that has the mass to steady the temps out.

    1. I would absolutely zone a 1750 SF home with the usage ‘intensity’ Robert describes – Three zones on a two stage or variable capacity system. Ideally the envelope(s) are or can be improved such that a two ton two stage system suffices. Since two stage systems come in whole ton increments, getting the load down to 2 tons would really help with duct design and minimum air flow management.

      The Man J process isn’t as user-aloof as Robert fears – coping with the home office is relatively easy – account for the occupants, computers, lighting, etc is quite simple via the ‘internal gain’ function. My default for a home office is 1 person plus 600 Btuh sensible, but that varies with the equipment / occupants. In Robert’s case, sounds like 1200-1600 Btuh sensible gain plus 2 peaople, but I’d suggest double checking using a Kill-A-Watt or similar instrument on everything routinely operated in the home office. Not only is a ducted return a great idea, twinning the supply registers is likely needed as well if the space began life as an auxiliary bedroom.

      We always look out for internal gains – Whenever I review a Man J done by staff I make a beeline for signs of internal gain – I’ve dealt with home-based businesses that include giving digital art or cooking classes, even one who trained 12 big dogs at a time and crated them in a spare bedroom (Please don’t site a dog crate right below a thermostat…)

      I do find the habit of a 65-68*F owners bedroom cringeworthy – if outdoor dewpoint runs much above that temperature (and it does in more and more places for longer and longer seasons) there is quite a risk of uncontrolled moisture in wall cavities…moisture leads to mold. Perhaps the comfort problem causing such low setpoints arises from excess humidity (wouldn’t be the FIRST time…)

      Get the humidity under better control might result in overnight comfort at 72-75*F…much safer from a building science point of view. In extreme cases, I have walked away from demands to supercool a sleeping zone…I don’t want to be the one holding the mold bag…much better to be a spectator than a participant in such building science fails!

      Industry ‘secret / fun fact – last I checked, all variable capacity two ton systems are simply factory-limited 3 ton systems, so don’t bother with a 2 ton. The minimum capacity is the same for both nominal sizes, so the two ton systems merely have less turndown. That remark applies to ‘conventional’ split heat pumps, not minisplits.

      Hope this helps.

  6. Another option is “follow me” zoning. At least one thermostat manufacturer has the ability to control from remote temperature sensors which you can schedule to be used during different parts of the day. So if you are empty nesters like us, you can have the thermostat control off the living area temperature during the day and the master bedroom at night. No dampers or multi-capacity systems needed.

    1. Follow me will provide comfort in the room you are in but unless some form of zoning is used does so by over conditioning the unused rooms.

      1. In some cases, follow-me zoning will result in over-conditioning in other rooms, but in other cases it may result in under-conditioning in the unoccupied rooms. It depends on zone loads relative to the delivered heating/cooling to each zone.

  7. Run around (return) bypass on direct expansion coils is a no no… there is a few nice multi-zone single condenser systems out there but run of the mill A/C Tech’s typically do not have the wherewithal to set them up or the customer doesn’t have the pocketbook to get it done right by someone who can. Wife caring for her mom in her 6100 SF house has 4 A/C units/zones. We live in Phoenix, her mom had under floor returns installed – absolute nightmare for cooling control. Air balance had supply air from one zone blowing air on another zones T-stat for two zones.

  8. Hello Allison,

    With much respect, I must point out how crude air based heating and cooling systems are with respect to control over heat/cool delivery. With hydronic delivery systems, because we practice heat transfer and not mass transfer, and because of the high energy density of water, it’s quite easy and natural to subdivide a single piece of equipment into several zones using low voltage zone valves and variable speed deltaP circulators. We zone spaces differently depending on how they are used and/or the extent to which they experience different thermal exposure. The heating water temperature is generally regulated by outdoor reset control according to outdoor air temp. Lowering the water temperature to be appropriate to the instantaneous load helps eliminate cycling.

    Typically, Living, Kitchen, Dining, and Great rooms are zoned together as they usually all share common space. Bedrooms and bathrooms are zoned separately from bedrooms, especially with radiant as everyone likes to “sleep cold”, and it is a feature to have warm floors in the bathroom at 2 am, ;-). Especially the a master bed and bath. Guest bedrooms are turned down most of the time as is the garage, which we often include as a part of the heated area.
    Domestic hot water is treated as another zone taken from the same equipment and often given priority over other heating zones so that the dhw load is recovered quickly or instantaneously. When using panel radiators as terminal equipment we are free to use non-electric Thermostatic Radiator Valves, (TRV) which turn each radiator into its own heating zone.
    Of course one can “zone” air handlers as well, which can. carry chilled water for cooling. However, I have learned that, to the extent to which cooling is really dehumidification, zoning is less important and can be counterproductive to comfort. As you know, vapor pressure is real pressure and moisture moves through the air from one dry zone to another which can create “clammyness” where the air is cold and wet.
    Sensible cooling with radiant panels can be zoned but such must operate with water temperatures well above the dew point.
    Thanks for listening.
    Dale

  9. I have trouble with treating domestic hot water as a separate zone…doing so requires operating a presumably complex and possibly inefficient (at part load, anyway) system 365 / 24 / 7 rather than only during winter months.

    While I agree that in-floor radiant heat is a premium solution with many advantages, the limitation posed by a cooling system unable to address latent load (i.e. dehumidify) seems a disqualifying factor throughout all but the driest desert climates. My understanding is that climate change is reducing the area / regions able to be solely sensibly cooled.

    I’m not sure I completely buy into heat transfer vs mass transfer…hydronic systems substitute pumping water through a system of pipes vs blowing air through a network of ducts. I would think that the only system able to lay claim to being purely heat transfer would be electric resistance.

  10. Hello Curt,
    Your assumptions regarding indirect domestic water heating are incorrect, especially regarding “presumably complex and possibly inefficient (at part load, anyway) system 365 / 24 / 7 rather than only during winter months”
    Condensing, modulating, closed combustion boilers achieve very high efficiencies (~96%) when operating at low temperatures under part load. Domestic water heating presents lower an energy dense load at low temperatures which allows a condensing boiler to condense through the water heating cycle. As the water in the indirect storage tank heats, the modulating burner will modulate down in output until dhw setpoint is reached. These boilers now modulate with turndown ratios as high as 14:1 so concerns over short cycling etc. are unfounded.
    Typically the boiler controls uses sensors, (transducers) to sense dhw temperature. The computer in the boiler measures the water temperature and fires the boiler and modulates the burner proportionally according to the load on the coil. None of this is that complicated, the boiler diverts it flow to the indirect storage tank hx and modulates temperature of the loops to satisfy that load.
    In the summer the boiler can fire just to make hot water without sacrificing efficiency. A good indirect storage tank will be very heavily insulated typically with urethane foam and the only good path for heat loss is through the piping. Because the capacity of the boiler is almost always larger than the dhw load* the recovery rate is very high and smaller storage tanks can be used compared with conventional storage water heater equipped with wimpy electric elements or cheap gas burners.

    Don’t confuse hydronics with radiant panel heating or cooling. Hydronics is a distribution medium and control strategy. Heat transport in an air system is strictly a matter of displacing the air in the room and moving it past the single heat exchanger at the source and delivered at a constant temperature. With hydronics we typically hold the design water flow through the terminal device at a constant and then vary the heat output to the device and room by varying the delivery water temperature to something roughly proportional to the load or the application.

    Chilled water de-humidification and/or air cooling is done with ~45°f chilled water. The hydronic system adds capacitance to the system and buffers the compressor by not directly subjecting it to the near instantaneous changes that occur in an air system operation. In a DOAS system the incoming ventilation air is subjected to the cold coil after the ERV has recovered conditioning energy in the exhaust air.
    Radiant panels, operating well above the dewpoint and close to the room temperature are very effective at removing sensible gains from an otherwise conditioned, (de-humidified) space. Often these sensible gains can make up the larger fraction of the total conditioning load. Radiant panels return warm water back to the compressor increasing it’s performance. Again, with hydronics we can vary the delivery temperature depending on the application.

    * fun facts to know and tell -it takes ~120,000 btu/hr to heat ~3.5 gpm of instantaneously water over an 80° temperature rise.

  11. Curt, about the economics.
    It is smart investing to purchase a high efficiency, high performance, reliable piece of equipment which provides energy and dollar savings over the next best alternative. This is saving and conservation over production and consumption.
    I do central system where I try to make the best use of the investment made. There are economies of scale involved in using the same investment in high efficiency equipment to accomplish several different tasks in order to maximize those energy savings, pay back the investment over time, and receive tax free cash returns on the money you didn’t spend on fuel in the future. A single, more expensive quality piece of equipment correctly applied is less likely to have maintenance issues more easily maintained when it does. Doesn’t require complete replacement.
    By way of contrast, accomplishing the same tasks with multiple pieces of equipment inevitably leads to more money spent up front on individual pieces of equipment of lesser quality. Now, the system consists of redundant controls, burners, compressors, flues, gas lines, power supplies,heat exhangers etc. are replicated across several cheap pieces of equipment more likely to fail. Failures of of even cheap components can create large service headaches and service bills, cutting into your payback.
    Consolidated systems and solid energy investments are what I do.

  12. I get what you are saying about modern ‘mod-con’ boilers as to combustion efficiency…but standby losses from anything outside a well-insulated water tank are non-trivial, and the added heat can be quite undesirable during warm months.

    That and it runs counter to the enlightened trend to remove fossil fuels from residences whenever feasible.

    I get that 45*F water flowing through a coil in an air handler is an excellent way to cool and dehumidify indoor air…with the possible added benefit of modulating that water temperature a bit either way to intelligently respond to latent load.

    I’m a huge fan of heat pump storage electric water heaters, in part because they mesh well with my warm climate operating area. The Rinnai tankless propane fired water heaters are big down here, but I dislike them for their use of fossil fuel (especially in homes off the natural gas grid – pricey propane). There is also the ‘moral hazard’ often reported by parents of teens – the capability of so-called ‘endless hot water’ returns the unanticipated and undesired outcome of teens taking virtually eternal showers!

    I have had multiple teens do that, but in the case of a tank storage water heater, the showering does end when the tank runs out – a feature I have come to value even though occasionally inconvenient.

    I repeatedly make the case for at least some sort of redundancy – an uncooled home is effectively unlivable during summer anywhere along the gulf and southeast Atlantic coasts, and gone are the days when nearly all repair components (or at least non-OEM alternatives) of even the most complex heat pump systems were continually locally stocked.

    Today’s high end variable capacity systems nearly all require proprietary controls / components often not available same day (or even same week)

  13. Curt,
    I’ve lived at fairly high latitude and elevation in the Rockies but I grew up on the coast in Corpus Christi Texas. We had an attic fan and no heating system at all when I was growing up. So I have a fair idea of the struggles and challenges that face hvac installers in the climate, especially in salty areas on the coast. Really doing all those things well, heating, cooling, dehumidifying, ventilation is much more difficult or at least multi faceted that than sensible space and water heating and ventilation, (all we need here), in many respects. This especially if you have limited fuel choices.
    Professionally I live vicariously through the experiences and anecdotes of knowledgeable people like Allison Bales.

  14. The southeast US is a tough climate. Granted, we don’t get much snow and a hard freeze is rare enough to be newsworthy…but the humidity is a bear and slowly worsening, presumably because nearby oceans are steadily warming.

    There is little I like better during dog days of summer than a road trip to North Carolina mountains – above 3000′ life is sooo much more comfy outdoors, even in summer.

    Up there I see a heckuva lot of Mitsubishi H2 ‘hyperheat’ minisplits and also Fujitsu Halcyon minisplits. I noticed the same on a recent road trip around New Brunswick and Newfoundland – testament to cold weather heat pumps.

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