Trying to balance temperature between floors
I’m looking for help and ideas on how best to balance the heating between upstairs and downstairs. There is typically a 4-5 F temp differential between the floors and I’m trying to force more heat down to the first floor. We’ve started to hit 20F overnight temperatures just outside Boston, and I’m looking to see how best to push the heat pump in these temps.
Equipment 1st floor – MSZ-GL12NA-U1 2nd floor – SVZ-KP24NA Outdoor unit – MXZ-4C36NAHZ2
Manual J (Design temp of 3F in Boston, Balance Point of 13F, 2,238 sq. ft.) Total heat load downstairs – 21,293 Total heat load upstairs – 25,449
The first floor is served by a mini-split which is located in a family-room addition at the back of the house. I have the temp set to 68 (no set backs), and it is measured by a sensor in the addition which is connected to the MHK2 for that mini split. There are also ducts that run from the attic unit through bedroom closets to the living room and dining room, but little heat seems to come from those. There are also vents in the kitchen and in the addition, but the ducting for those in the attic was crushed and so no heat comes from those.
On the second floor, it is the air handler in the attic that serves the four bedrooms (each with their own vent). There is one central return at the top of the stairs. Upstairs is always warm and has no problems maintaining 68 even in the coldest Boston winter days. The air handler is in an air-sealed attic, with 5″ of closed cell insulation on the roof rafters.
I have oil baseboard heat as backup, but I’m trying to use that as little as I have to.
I tried setting the air handler in the attic to fan only (medium-speed), closing upstairs vents some of the way to see if that helped, but it just made upstairs warmer. (Also with the crushed vent in the attic, the air had little hope of making it downstairs). I have tried turning upstairs to heat mode too, but made no impact (assuming because the air handler in the attic just runs the fan anyway since the temp hits its setpoint of 68 on the MHK2 for that unit)
Is there anything else I can try? Would it be worth repairing the vent in the attic to try to serve the kitchen and addition more with the attic air handler?
Thanks for any help.
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I would repair the crushed duct and then balance your system. To balance your system, go from room to room and adjust (open/close) the dampers on the registers/diffusers/vents until each space receives an adequate amount of airflow based on the space size. You can rebalance the system multiple times until you get the heat distribution you prefer. You may need to close the dampers upstairs some and open the dampers downstairs more in order to get adequate airflow since the upstairs rooms are closer to the AHU.
Thanks, that's helpful. I do have the option of running the fan speed of the AHU in the attic at a higher speed, and if I close some of the vents upstairs, the hope is that more air will make it to the vents downstairs when I repair that duct. Appreciate the comment and suggestion.
YW. If the crushed duct is metal you can probably bend it by hand back into a round or rectangular shape. The shape doesn't really matter so long as the free area is about the same as it was previously. You may need to retape some of the joints or use mastic to seal the joints if they happen to come loose in the process. If the crushed duct is flex duct you may have to replace it but I'd think it would be hard to crush flex duct.
If I'm understanding correctly, the two floors have separate systems and separate thermostats, so they really aren't connected. Further, it sounds like the second floor is fine, the first floor isn't getting enough heat.
So it's not a question of balancing the two floors, it's a question of getting enough heat to the first floor. It sounds like your unit is big enough, you don't have distribution. I'd start with the crushed ducts for the kitchen.
Correct - the AHU in the attic is controlled by an MHK2 upstairs, and the mini-split downstairs is controlled by a separate MHK2. They both share the same outdoor multi-zone system. And yep, no problem with heating the second floor anytime during the day or night.
Will try repairing the crushed ductwork (which feeds a vent in the kitchen and a vent in the addition) and forcing more air downstairs to see if that helps balance and support the workload the mini-split is trying to do on its own right now.
It sounds like you have a ductless unit in the family room addition that serves the first floor. And a ducted unit that serves both the upstairs and first floor?
Where is the thermostat for the second floor unit?
Yes, that's correct. The AHU in the attic serves both the upstairs and downstairs (minus the run with the crushed ductwork for the kitchen and addition) We added the mini-split in the addition when we installed a heat-pump.
Thermostat for the second floor is on an interior wall at the top of the stairs, close to where the central return is.
The upstairs unit will only run to satisfy the thermostat upstairs. With the stack effect, your upstairs is gaining heat from downstairs.
I would raise the setpoint of your downstairs ductless unit, maybe to ~72 and let the house stabilize. With mini splits it’s less about setting the thermostat to a specific temperature and more about adjusting the setpoint until you are comfortable.
Around me, it is very common for multistory houses to have seasonal adjustments on the main supply trunks. You generally want more airflow delivered to the main floor than upstairs in the winter and the reverse in the summer.
Once you get the crushed ducting sorted, you might need to close/restrict some of the vents on the 2nd floor to get more flow to the main floor. Running the fan to continually mix the air is also a good idea. A ceiling fan at the top of the stairs blowing down to reduce stratification might also help.