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If one uses a multi-zone hvac heat pump approach do the zones need to be closed off to one another? If not, what keeps the systems from “fighting” each other? For example, if the temp is set to 68 in one zone and 75 in another zone and the two zones are not isolated, what happens? - Thanks!

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  • To clarify, you're talking about "auto" mode where each zone has a set temperature and it will heat OR cool as required to maintain that temp ?
    – Criggie
    Apr 26, 2023 at 1:19
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    @Criggie : A friend is building an ultra-insulated/ultra-tight house and I’m trying to help him consider options for heating and cooling. For example, does he get a single zone air-handler with ducting for the entire (two story) house, or does he isolate the first and second floor and get two air-handlers with ducting, or does he get two wall-mounted air handlers, etc. His budget is limited. He will also have a Zehnder ERV.
    – Jim Luby
    Apr 26, 2023 at 2:29

3 Answers 3

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Based on your comment, you are asking what happens if, for example, you have two head units on opposite ends of a room and set one for 70 degrees and the other for 80 degrees (both heating).

This is no different than having two entirely separate units. One will stop heating when it reaches 70 and the other will continue until it reaches 80. Unless you have a lot of circulation, you’ll end up with a temperature gradient between the ends. The 80 set unit will, of course, work harder than the other.

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    And worth noting the temperature satisfied is that at the indoor head unit, specifically. The general advice is to not mind the numbers so much and adjust until you are comfortable where you are, whatever the temperature at the head unit (thus, the set temp) is from doing that.
    – Ecnerwal
    Apr 25, 2023 at 2:02
  • Or you can try what I've got in progress, which is to implement my own collection of temperature sensors, UR blasters, and programming to make the former control the latter as virtual thermostats.
    – keshlam
    Apr 25, 2023 at 16:09
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A single system typically operates only in one mode at a time, heating or cooling but not both, so it can't fight with itself. In my installation, if I try to request conflicting modes the whole system shuts down until I give it clearer instructions.

If your building is large enough that it needs multiple systems (in the product line I had installed, that means more than five indoor heads), then yes, theoretically they could be set in opposite modes and you'd waste lots of energy. The answer is the same as wasting energy by leaving windows open in the worst weather: don't do that.

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  • I know that multi-zone heat pumps must all be in the same mode (heating or cooling). I’m trying to understand what happens when, for instance, two indoor units are in heating mode but are calling for different room temperatures. - Thanks!
    – Jim Luby
    Apr 25, 2023 at 0:30
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    Just like any other multizine heating system. If a room becomes warmer than the thermostat is calling for, it stops actively heating. That doesn't mean it can't be heated indirectly. If you want to prevent that for some reason, you need to isolate and insulate. Heat can go up stairwells, for example, unless you have doors that block/limit airflow and/or some sort of active return to bring the heated air back down to ground level. Again, this is exactly like any other multi-zone heating or cooling system; basic physics, convection and conduction and so on.
    – keshlam
    Apr 25, 2023 at 0:47
  • I was thinking about two indoor units, one on the first floor and one on the second floor. More specifically was wondering if it would be wise to isolate the upper and lower floors with a door. Based on your comments it seems that I should have a door between floors. Thank you!
    – Jim Luby
    Apr 25, 2023 at 1:09
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    I happen to have two head units, one up and one down, with no door, primarily heating. If you are not trying to make upstairs cool (i.e. by heating less, not implying either unit in cooling mode) and downstairs hot, it's fine. If you are, you'll (probably) want that door. At the moment, being late spring, the upstairs unit is just in fan mode pulling air through the filters and mixing it in the room - the downstairs is carrying the heating load for "shoulder seasons" and both are used for colder weather. On the rare occasions I cool, the upstairs becomes primary.
    – Ecnerwal
    Apr 25, 2023 at 2:08
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    Depending on how far the air has to go to get upstairs, (or down, in cooling season,) relative to the head unit, the door may be more optional. At present my upstairs is a touch cooler than it would be with its head heating, but it's acceptable, and saves power to just run that head as a fan.
    – Ecnerwal
    Apr 25, 2023 at 2:12
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For example, if the temp is set to 68 in one zone and 75 in another zone and the two zones are not isolated, what happens?

Well, is your thermostat set to heat mode or cool mode?

In heat mode it's simple: "Heat zone1 up to 68 degrees and stop. Heat zone2 up to 75 and stop." The leakage of zone2's heat into zone1 is not zone1's problem.

In cool mode it's equally simple: "Cool zone1 down to 68 degrees and stop. Cool zone2 down to 75 degrees and stop." The leakage of zone1's cooling into zone2 is not zone2's problem.

In auto mode, contact the manufacturer and ask what programming logic they implemented.

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