House is Hot Upstairs and Cold Downstairs

[Kemper100]

My old (1920) house is hot upstairs in the summer and cold downstairs in the winter and I can deal better with the cold downstairs but not the heat upstairs. My 3 ton condenser can't keep the upstairs cool when the outside temps are at high 80's or low 90's and my stat and return are in the upstairs hallway.

I was thinking of installing a ductless unit to help the upstairs cool when it gets too hot out for the main condenser unit. I first thought about installing (1) wall mounted evaporator / air handler in the hallway upstairs but my thermostat and return are in the hallway so I was thinking of a multi zone system for the (3) bedrooms. My bedroom space is small only (400) sf total.

And yes I've check ductwork for leaks, adjusted dampers and have added insulation in the attic.

Thoughts?

 

[argile_tile]

I like to start with the basics since not everybody knows them:

Your supply ducts are designed to deliver air equally to all parts of the house when all vents are Open. (note: you may have levers at the air handler for shutting off whole sections, and sometimes these go loose).

Return ducts should not be blocked.

Closing vents is not suggested: your HVAC wants to circulate all the air 3 times before it shuts off. Too little air (dirty filter, or closing many vents to get one room "cooler") can even cause damage and reduce cooling everywhere including the one room.

Door of rooms need to remain partly open or the rooms will "be hotter" and have no air flow. This surprises some who think doors remain closed for privace. A hotel has 2 vents per room or a unit in each room. To leave a door closed you could only install a "through vent" above the door so it could remain closed.

The upstairs should cool. That's a problem that can be solved. And you want it to! Because cool air sinks so your main floor will "work better / stay cool longer" if there is not a stack of hot air right above it.

However - you can't do much about the basement being cool, no. You can close any vents in the basement, yes, but you may need a fan and de-humidifier after doing so. Basements are "separate zones" and there isn't anything you can do but use a separate air handling system. There just isn't another way. There's another problem: if you have natural gas furnace you need to insure any hvac vents in basement aren't "pushing air through the hot gas vents" of your hot water heater / furnace (which is illegal btw). If you have hvac in your basement there's a good chance it was "a faux design". It was sold (to someone), but it was never an approved design.

 

[argile_tile]

You said your main floor is cooling. You should have 14-20 degrees cooling (different from ambient room / thermostat).

14 is a very low value - it would be the minimum for the worst vent upstairs assuming the air is flowing - normally as discussed above. 14 might indicate your hvac needs a service person to check it for efficiency or issues such as dirty evap fins, a "normal maintenance check".

Good cooling for, say 2,000 sq/ft, would be more than 20 at vent closest to the hvac supply, 18 or 19 at a vent far from it, and 17-19 at the upstairs vent? These would be values that would cool a home on any day. (realistically, getting up high toward 100F your AC may not be designed to keep up - it may be disigned to hold steady at 96 degrees and fall a little behind at 100). You can ask for your model what the cooling should be.

You can use a milk thermometer to measure those vent temperatures.

 

[argile_tile]

> And yes I've check ductwork for leaks, adjusted dampers and have added insulation in the attic.

adjusted them which way ?