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HVAC zoning and static pressure

ColbyForester | Posted in General Questions on

Hello, I am putting a HVAC system in a new two story house. I plan to use one unit for both floors so at least 2 zones will be needed. I would also like to zone the bedrooms as well to have a total of 4 zones. My concern is static pressure if only 1 zone is open. I plan to get a variable speed blower from Carrier or Trane. From their websites it seems intelligent enough to throttle back the blower or open other zones if the static pressure is to high for one zone. Is that truly the case and no other design factors should be in place to control the static pressure? This is assuming proper size ducts for the overall house as if all zones are open.

Also, is one company better than the other when it comes their technology behind zoning?

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  1. Reid Baldwin | | #1

    Zoning is generally accomplished by a zone controller. The zone controller receives input from all of the thermostats and sends signals to the zone dampers and the furnace/AC. To the furnace/AC, the zone controller looks like a single thermostat.
    The problem of excessive static pressure when only one zone calls is usually handled by having some of the zone dampers never quite close. The installer can adjust how open a zone damper is in the closed position. The installer should adjust these such that when any one zone is the only zone calling and the blower is at its highest setting for that condition, the other zones are still open enough that the static pressure isn't exceeded. The more zones you have, the more you end up sending air to non-calling zones, especially if one of your zones is substantially smaller than the others. If you go with too many zones, then you end up sending significant airflow to the whole house whenever any zone calls and just sending a little more to the zone that is calling.
    If you have a two-stage furnace/AC, some zone controllers are smart enough to select low stage when only one or two zones are calling. Other zone controllers leave staging up to the furnace/AC which goes to high stage after a fixed time, so you could be on high stage with only your small zone calling.
    Just because the blower is capable of running at a variety of speeds doesn't mean it will be able to choose one of the lower speeds when only one zone is operating. My blower has five speeds which correspond to fan only, low-stage heat, high-stage heat, air conditioning, and one unused. The zone dampers have to be set up for the air conditioning flow rate which is one of the higher blower speeds.

    1. _Stephen_ | | #5

      The Trane system ya fully communicating.

      What you are describing is the old school way of doing things. There are newer better ways now.

  2. Yupster | | #2

    Take a read through this, the design guide for the Carrier Greenspeed zoning design guide. It'll give you a more in depth understanding of how it works.
    It's a bit more complicated to setup and install than a typical installation, as well as being expensive. But it's a slick solution.

    1. ColbyForester | | #4

      That was actually the article I read before posting. Obviously you would need to balance the system as a whole but it looks like the Carrier zoning will open other zones automatically until safe static pressure is reached. Therefore static pressure per zone would not be an issue to correct as it will do it automatically by cracking open other zones if I'm reading that correctly?

      1. Yupster | | #7

        Pretty much, yup. There are still considerations to take when grouping zones to minimize overcooling of "dump zones". Not much point in microzoning, it increases cost and the equipment has a minimum capacity it has to put out anyway. So group rooms with similar loads together and make your zones as big as possible. You'll have a better, more cost effective system that way.

  3. etekberg | | #3

    I've had two add-on zoning board products in my house. 4 Zones, like you. I wouldn't do it again. They are at best a bandaid. I would do multiple mini splits now.

    But if I had to choose zoning, I would go with one of the infinitely variable (ECM) motor, built-in zoning systems, where you specify the size per zone. I'm not familiar with the anything but ground-sourced heat pumps, since that is what I have, but something like the ClimateMasters trilogy 45 zoning is what I'm talking about. I'm guessing that is the same thing as what you are referring to.

  4. _Stephen_ | | #6

    I have a Mitsubishi heat pump with an Air zone panel.

    It varies the speed of the fan depending on how many zones are calling, and how important the zones are.

    I believe the Trane system does something similar.

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