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  1. #1
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    I'm considering twinning a dual unit HVAC system to 1 thermostat?

    I'm trying to make some improvements to a dual unit HVAC system. The building is a church, with a main auditorium, and two equally sized wings for classrooms. The building is around 10 years old has a matched pair of HVAC units. Each unit does half of the auditorium, and 1 wing of classrooms. The units have separate ducts & returns are currently controlled by separate thermostats. Both thermostats are in the auditorium, and separated by around 25-30 feet.

    The problem I'm trying to solve is temperature inconsistency in the classrooms (the west side classroom wing measures 8-9 degrees cooler than the east wing)

    The thermostats are Emerson Sensi wifi connected, allowing me to observe what's happening remotely. The thermostats are configured for a "medium" cycle rate, which allows a swing of less than 1 degree. When set to 68, I generally won't see anything other than 68. Then one or both furnaces will come on, run a while, and shut off, all while both thermostats read 68.

    Observations and experiments:

    1) With the building empty and the thermostats set to heat at 68 degrees, I observed the east side unit runs noticeably more than the west side unit. I have observed several times (remotely in the sensi app) the east side unit will come on, run a while, and shut off, without the west side unit activating at all. Though sometimes the west side unit does come on too.

    2) I changed the east side thermostat to a "slow" cycle rate, for a larger temperature swing. With this, I observed both units come on at approximately the same time (good!), but the west side unit shut off noticeably sooner than the east side unit. So the west unit's short run time issue remains.

    My theory is, there is a slight airflow difference from one side of the auditorium to the other, causing one side to be a fraction of a degree warmer than the other, resulting in one unit running more than the other. This does not cause a problem in the auditorium, as the units share the load, but this causes their classroom wings see significant temp differences.


    Undesirable options I see:

    1) Move the thermostats to the classrooms. This would even out the classrooms, but the auditorium temp is the priority. To properly control the auditorium temperature when the room fills with people, the thermostats must be in the auditorium.

    2) Move the thermostats further apart. This may help with our current problem some, but the thermostats can't move far without being put in corners. The auditorium is rectangular, with the ventilation split lengthwise - so there isn't much room to separate them further.


    So - I'm considering using an "iO-TWIN" module from iO Controls to twin the units, controlling them by a single thermostat. This will ensure equal run time between the units, which I expect to improve the temperature in the classrooms. We may still have differences east to west due to the sun, but I expect that difference to be less than what we're dealing with currently.


    Do you guys agree with my analysis and proposed solution?

  2. #2
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    Thread Starter
    So... After reading some more, I guess that's not considered twinning, since the two units are not sharing the same ducts & return. This would be two separate HVAC units controlled by a single thermostat, for the reasons described above.

    Does this sound like a reasonable solution to the situation?

  3. #3
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    Jan 2004
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    Most likely will just shorten the east side unit's run times. Giving you the same temp difference problems you have now.
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  4. #4
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    Apr 2019
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    The first of the two popular methods to control two or more HVAC units with one thermostat is to eliminate all of the low voltage transformers in all of the HVAC units, and install one large low voltage transformer in one HVAC unit. Next, interconnect all the units together with a "common" low voltage leg.
    Correct me if im wrong

  5. #5
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    Jan 2015
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    MarkBrown, Just curious, what are you saying needs to be upgraded or replaced?

  6. #6
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    Sep 2015
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    Ontari-ari-ari-o
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    If the building is only 10 years old you'd be nuts to buy new equipment to solve a simple temperature problem. I'd be looking into measuring the airflow and getting some actual readings and then seeing about balancing everything with zoned automatic dampers. The problem may not even be with the HVAC system you might have windows with some failed seals or a problem with the insulation or maybe one part of the building has more solar exposure...honestly when you are looking at a larger building there is a huge laundry list of factors to consider aside from the heating and cooling equipment.

  7. #7
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    Jan 2015
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    MarkBrown, I see these are your first posts on this website so welcome! If you have not read the rules of the site I suggest you do that. Nor do I what to jump down your throut on your first postings, but I found your comments curious. But again welcome.

    I see by your profile your are not an HVAC tech or work within the HVAC field.

    Originalinal Poster (OP) is talking about temperature mismatch from one side to another side in a larger structure, in a common area and in separaterate rooms. To me this does not constitute a major defect in the equipment and worthy of a replacement. The building and equipment are only 10 years old. The equipment should have another 10 years of service life before it needs to be replaced. I have seen, installed and worked on several setups such as this and it is just my guess this is something like; two 120,000 BTU furnaces with two 5 ton AC systems. How many thousands of dollars do you think it would cost to replace the equipment. Please keep in mind the problem may not be the AC units (proper) fault.

    When I offer recommendations I ask myself: If this were my house or property would I spend the amount of money being recommended? Does the repair match the problem?

    When you say "Company Rep" is sounds as if you are suggesting to call the manufacturer of the AC unit. To me this is an installer/architect issue. From what is written the equipment functions correctly, just that there is a temperature difference and many things other than the equipment (Air Handler, Condensing Unit and Evaporator) could be the cause. The OP needs to have a contractor (of their choice) come in and help them figure out what can be done to improve or correct the condition. After ten years there is probably no installer warranty left but maybe the original installer would be willing to make them a deal to fix the problem they caused.

    Again welcome, read the rules and happy posting

  8. #8
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    Hey Mod, where did the MarkBrown123 posts go? All three of his posts are gone, did he delete them?

  9. #9
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    Quote Originally Posted by Answer-Man View Post
    Hey Mod, where did the MarkBrown123 posts go? All three of his posts are gone, did he delete them?
    Spammers seldom delete their own post.

    But Admins and Mods get rid of them.
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  10. #10
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    Thanks for the info

  11. #11
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    Apr 2019
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    Thread Starter
    Thanks for the info guys.

    I have not specifically checked for window seal issues, but I don't think that can explain our issues. The temp within each wing was fairly consistent, while the temp from one wing to the other was very different. If one or two rooms had an issue, I could see a window seal being the cause. But I'd be surprised to have all the seals in one wing bad but none in the other.

    So another problem I'm trying to solve: I'd like to be able to set the thermostats to automatically switch between heating and cooling without the two units getting in a fight over what to do. I have observed with a 2 two degree heat/cool difference (heat to 68, cool to 70), one side can kick on the heat and warm up the other side of the auditorium enough that it is running the AC. I've been assuming that the temp issues in the wings is related to this issue (e.g. the units help each other too much in the auditorium and confuse things). So I'm thinking with each unit doing an identical load, just controlling them with one thermostat would even things up better than what we have today and keep thing from mismatching on heat/cool.


    Can you give rough guess on cost for adding zoning & dampers to an existing building? Maybe a half dozen dampers. Ceilings are suspended ceilings, so easy access (though the auditorium is high).

    Thanks again!
    Joe

  12. #12
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    Sorry, prices/pricing discussions aren't allowed here.
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  13. #13
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    ksjoe, call and have a local contractor come in and give it a look he may see something you don't. iif they charge you the cost will be low. You'll get a good second opinion. Also if you want heat/cool at the same time capability they should be able to recommend the correct tstat you'll need

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