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Room-to-room temperature differentials, Manual J, and Manual D

Aun Safe | Posted in Mechanicals on

I am in climate zone 4a.  The upstairs of my house has 3 bedrooms — two of which are in use, the third is a guest room.  So I will focus on the two bedrooms that are used.  (The house was built in 2010 and is pretty standard builders grade–R13 fiberglass insulation, an ACH50 around 3, double-pane windows.  HVAC and ducts were sized per Manual J and Manual D.).

One of them (called it “Bedroom 1”) has east and south facing exterior walls and a significant percentage of windows.  The other (“Bedroom 2”) has north and west facing walls and a smaller percentage of windows.  Because of east and south facing walls/windows, Bedroom 1 starts heating up in the morning almost as soon as the sun comes up.  Bedroom 1 can easily be 2-3 degrees warmer than Bedroom 2 before the A/C ever starts running, and Bedroom 1 has significantly more heat gain throughout the entire morning and early afternoon than Bedroom 2.

To try to keep Bedroom 1 comfortable, we have adjusted the dampers in our ductwork (at the takeoff collars at the supply plenum) to provide somewhat more air to Bedroom 1 and somewhat less air to Bedroom 2.  Static pressures remain decent (but not great — around 0.3″ on the supply side) even after damper adjustments.  The result is that, when the A/C kicks on, the two bedrooms end up being a relatively similar temperature until around bedtime.

Problem is, by the time the sun goes down (or even by the time its late afternoon), Bedroom 2 now has a heat gain load that is similar to or greater than Bedroom 1.  So over the course of a night, the temperature differential between the two rooms changes, and by 6 am, Bedroom 2 is ~3 degrees warmer than Bedroom 1.  We could solve this nighttime temperature differential problem by tweaking the duct campers at the plenum, but that would simply reverse the damper adjustments we made to address the daytime differentials.

My question is this:  Is zoning (or individual ductless heads in each bedroom) basically the only way to really solve a problem like this?  It seems to me that, when two different bedrooms have significantly different cooling loads throughout the day, the problem I describe above will inevitably occur unless you can adjust the A/C supply capacity to each room throughout the day as the load changes.

Am I correct about that, or is there a way to solve this problem apart from zoning?

I suppose the better insulated the house is and the slower the heat gain/loss, the problem could be minimized.  But ripping out the walls and spray foaming the bedrooms isn’t really something I want to do.  I would consider zoning… or else I will just live with 2-3 degree temperature differentials like a normal person who is not obsessed with temperature and air quality! 😉

Thanks for any feedback.

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Replies

  1. Expert Member
    Zephyr7 | | #1

    If a large part of the problem is solar gain, new windows with better SHGC would help. Better windows would help limit the amount of solar energy that would warm the interior of the room.

    Running the blower and not the A/C will help to equalize temperature somewhat, but isn’t particularly efficient in terms of energy usage. Another option is to use a thermostat like the ecobee that has multiple remote temperature sensors. With a sensor in each bedroom, the thermostat has the ability to run the A/C until the average temperature of the two bedrooms arrives at the setpoint. While not perfect, this at least splits the difference and will result in less of a temperature extreme in either room.

    Bill

  2. Jon R | | #2

    Open doors will help.

    Note that a commonly cited problem with zoning (low airflow) mostly doesn't occur when you use the zoning to only make small adjustments to airflow. Ie, dampers never fully (or even halfway) close.

    Sensor controlled zoning is underutilized as a comfort and efficiency improvement. I'm curious why you are trying to avoid it.

    1. Aun Safe | | #3

      Thanks for the response. I actually do have two sensors--one in each bedroom.

      Problem is, when the system is not running at all, the sensors are doing no good, but Bedroom 1 is still heating up and needing a/c before Bedroom 2 does. So suppose my set point is 73. At 8 am, temperatures in both rooms is, say 72. At 9 am Bedroom 1 may be up to 73. Bedroom 2 is atually down to 71. At 10 am Bedroom 1 is 74. Bedroom 2 is still 71. At 11 am Bedroom 1 is 75. Bedroom 2 is now 72, and since the average of the two room sensors is now 73+, A/C comes on, and perhaps by 3-4 in the afternoon the two bedrooms are about the same temperature.

      Also, for what it's worth, this is a Trane XV18 variable speed system. It modulates from about 35% capacity to 100% capacity. So when it comes on, it basically then stays on all day until midnight or so. So the blower is running 12-16 hours per day circulating air, and I leave the doors open during the day but we keep them closed at night.

      So yeah, the sensors certainly help, but even with them we still get the 2-3 degree room differentials, particularly on sunny days, and particularly before the a/c kicks on. (And then at night, when Bedroom 1 has a load that is closer to Bedroom 2, the temperature differentials reverse and Bedroom 1 gets cooler than Bedroom 2 if the A/C system runs very much during the night.)

      Anyways, thanks for the feedback.

      1. Jon R | | #4

        For your example, you need fully zoned. Either room can call for cooling with the option of delivering none to the other room.

        An even more extreme example is where rooms on the south side require cooling and the north side requires heating. For this you need a fully zoned dual system (consider hydronic).

        I agree with you that there needs to be more discussion of maximizing comfort (vs energy efficiency or cost). Lots of new technology and we still can't control temperature to within 3F?

  3. Expert Member
    Akos | | #5

    There are smart registers but anything with batteries is not a good long term solution.

    You could try putting a booster fan into the supply register of bedroom 2 connected to a plug in thermostat (typically sold for controlling window AC units). This way the booster only runs when the bedroom gets hotter. It gives you somewhat of a DIY zoned solution.

    1. Aun Safe | | #9

      Akos, your suggestion of a booster fan + thermostat had me google searching, and I found this: https://jacksonsystems.com/products/zone-twoposition-standalone-damperspfont-size25i-ut32-titan-thermostat-ordered-separatelyfonti.html.

      Appears to basically be the same concept, but instead of a booster fan, it is single damper controlled by a thermostat in the target room.

      Ever seen something like that before? It's an interesting concept but I suppose the problem is that, when the damper is closed, the static pressure on your system would probably go up significantly, potentially to undesirable levels.

      1. Expert Member
        Akos | | #11

        Aun,

        That is exatly what you want for a zoned setup, it is just a more work to install. Make sure you put it on the bedroom that is too cold when the AC is running.

        Putting a powered damper on a bedroom or two won't effect your furnace. When the zone is closed, the rest of the house will get slightly more air. Just make sure that you only control around 15% to 20% of the furnace blower's capacity.

        Those dampers morally need 24V AC, easiest to get a plug in transformer (typically sold for irrigation systems) or if your furnace is near the damper, pull a 24V power feed from it.

  4. Expert Member
    Zephyr7 | | #6

    You might try some reflective curtains in the hotter bedroom if it’s south facing. Something like an improvised radiant barrier. You might be able to drop the temp in that room a degree or two that way.

    Bill

  5. Expert Member
    Peter Yost | | #7

    Interesting that the last response from Bill brings up window coverings/attachments. This may be the most effective/least expensive approach to solving solar gain (and by the way, most energy modeling/load calcs have to make assumptions about window coverings presence/setting).

    The problem with solving solar heat gain with glass coatings or interior treatments is that once the solar gain gets to the glazing, most of the battle has already been lost. Exterior window treatments are the most effective for solar gain. To be the most effective, they also have to be adjustable, and now all of a sudden, we have gone from simple and inexpensive to potentially more complex and pricey.

    In our own home:

    1. on the most egregious solar gainers (west-facing glass first floor kitchen and second floor daughter's bedroom), we have fixed awnings that I install each late spring for the kitchen and fully adjustable exterior awnings on the second floor.

    2. On all other windows, we have double-cell top-down/bottom-up cellular shades that serve a whole slew of considerations and don't do too bad of a job at blocking solar heat gain.

    For quantitative assessment, consider the new AERCalc software from LBNL: https://windows.lbl.gov/software/aercalc.

    Peter

    1. But Why? | | #12

      Awnings are surprisingly effective and relatively low cost. Our grandparents knew this 50-70 years ago but they have gone out of style.

  6. Expert Member
    Dana Dorsett | | #8

    Having only a 2-3F room to room temperature difference would be considered a runaway success in most homes.

    Having the ducts sized by Manual-J/D/S is one thing. Having the AC or heat pump behind it sized correctly is still critical to getting the comfort out of it.

    Individual ductless heads can sometimes make the problem worse if it's on a multi-zone compressor, but if separate single zone mini-splits it could work (usually inefficiently, due to oversizing) for setting room temperatures independently.

    1. Aun Safe | | #10

      Thanks to everybody for the responses.

      And yeah, Dana, I suppose you are right that a 3*F temp difference is the norm and most people simply live with it (or more likely, don't even notice it enough to even be able to quantify it). Probably the only reason I even know about it is b/c I have a young kid who is still at a nap-taking age, so I pay attention to bedroom air quality throughout the day. Once the kid is finished taking naps, I would just set the dampers to maximize even temperatures at night, and I wouldn't even be worrying about temperature differentials during the day.

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