Optimizing HVAC airflow for cooling

Question

Here's the deal:

Two-story c. 2000 house in southern Missouri, lower level is a "walk-out basement" (subterranean on 3 sides) and consistently 10 degrees colder then the main level year-round. Single-zone ducted HVAC in a centrally-located basement closet. Thermostat (ecobee) upstairs in the "great room" with high vaulted ceiling, master bedroom has vaulted ceiling as well. Main level floorplan is very open... kitchen, dining, sitting room, great room, master bedroom either fully open or double-door sized archways. Each room upstairs has standard floor registers and in-wall returns. Downstairs rooms have in-ceiling registers with matching in-ceiling returns. Generally these registers and returns are not more than 12ft from each other (upstairs and down). Stairwell to go down is centrally located (almost above the HVAC closet) and fitted with a standard interior door. No zone controls or airflow controls of any kind (aside from ability to close individual registers).

I had the HVAC system replaced last fall, both the indoor part and outdoor part. I don't know much about it but it's a beefier unit than the original.

Over the winter I noticed it was sucking air inside through the wall outlets in the basement (on the walk-out side). This airflow cooled the electrical contacts so much that condensate would form inside the outlets and drip down the walls all day long. It's safe to say the house is not well insulated. I ended up plugging the outlets with baby-proofing plugs and taping over them with painters tape. That stopped the condensate problem, but air still came in around other seals and seams.

Today we had our first 88 degree day, and the HVAC ran maybe a 60% cycle trying to maintain 80 (!!) upstairs. Downstairs peaked at 71 (digital thermometer reading). It's now past sunset and still 82 out, and the HVAC has been running for 30+ minutes. Presently 81 upstairs and 70 down.

Is there ANYTHING I can do to make this system more efficient? Ideally, 75 upstairs and 72 downstairs. If I set the thermostat to 75 it will bring the downstairs into the mid-60s. But I don't care about the downstairs - it's not presently used as a living space. (If that was reversed and I didn't care about the upstairs because I was only downstairs I'd be pretty happy in the cooling season; I'd keep it at 75 down there and that would probably mean 85-88 upstairs.)

Any guidance appreciated!

(Prior to upgrading the HVAC in the fall, we had similar problems with imbalances up and downstairs but that system was not so modern and it had sections of damaged "fins" on the condenser unit so I expected some inefficiency.)

Answer 1

Cold air being sucked in through wall openings is caused by negative pressure in those rooms, which might be caused by an oversized HVAC system or by poorly designed ducts where the returns are large enough but the air supplies are too small, or perhaps too many of them have been closed by occupants.

You might need a smaller blower or you might need more upstairs ducts. A competent HVAC designer would need to figure it out.

Your first step could be a test to ensure, on a hot day, that all upstairs registers are fully open and all doors in the house are open to allow maximum airflow.

Answer 2

Improving A/C performance would either mean relocating ducts or adding fans, or ducts and fans (separate from the central system, probably running continuously, where you want to pick quiet and efficient fans) to redistribute air properly. You mention rooms with high ceilings, but don't mention having any ceiling fans (which will mix the air in those rooms.) The "floor only" ducting is a terrible arrangement for cooling. Hardly a wonder the basement cools better with ceiling-only, but ceiling and floor would improve overall air mixing, and thus balance.

As for the heating season suction,

• if the furnace is fuel-based rather than just a heat pump, it might need a combustion air intake so it's not pulling air from inside the house to burn and exhaust, thus causing the house to suck air in from outside to compensate.
• Likewise any other fuel appliance such as a water heater.
• In addition, exhaust fans in kitchen and bath areas that don't have explicit make-up air (rarely done) will add to depressurizing the house,
• and finally stack effect (warm air rising) in heating season (or poorly-balanced cooling season) will cause exhaust from leaks up high and intake from leaks down low on a poorly sealed house.

A new system on old ducts is limited by the design of the old ducts, if they were designed poorly. If the new system came with new ducts (which would be unusual given the expense, and disruption to the house that causes, but evidently might have been advisable) then taking the issue up with the contractor as being done poorly might make sense.

As a quick example of what you might do with a added ducts and fans (perhaps making use of wall cavity spaces to minimize disruption) some small fans that suck from the basement floor and dump 8 feet up in the upstairs rooms (you don't really need to cool the rest of the air above where people are that much) should help the balance. In heating season you could reverse the flow, though the ideal in heating season would be to draw from all the way up at the ceiling of the high rooms upstairs. Relocating central system supply/return ducts could achieve the same thing, it's more a matter of what you find fits your budget both monetarily and disruption to house-wise.

Answer 3

1. Add ceiling fans on the main level. The central system discharges cool air at ankle level while you feel the temperature at head level. Ceiling fans will mix the air in those rooms. You can set the fans to blow upward to circulate air while minimally feeling the movement, or you can set the fans to blow downward to circulate the air and perceive extra cooling from the movement of air across your skin. (I change my fans seasonally: blow upward during heating season and downward when cooling.)
2. Add air returns at floor level in the basement. Cold air always wants to accumulate at the basement floor, but year-round that's undesirable. Use the central HVAC to draw that cold air off the basement floor and spread it upstairs for "free" cooling in the summer, and heat that air in the winter to reduce the discomfort of cold feet in the winter (admittedly, less of an issue in your case as the basement is not presently used as living space).
3. Reduce the cooling load. This really deserves to be higher on the list but it's often not very appealing work! This is changes like exterior shading of windows (trees, awnings), window tint film, upgraded window glass units, attic ventilation, air sealing, insulation, etc.
4. Supply duct modifications. It may be the case that your system simply can't deliver enough air flow to the main floor rooms with their high ceilings (read: large air volumes). Perhaps some of these runs are 6" and should be up-sized to 7" or 8". A local HVAC pro may be better able to advise you on this than we would be. This is a somewhat intrusive undertaking, but if your equipment can't achieve the comfort you're seeking, then either it's under-sized or it is hampered by inadequate ducting.

Answer 4

This is an air balancing problem. Multi-story houses are trouble for a single unit. You may find that you need different balancing in summer versus winter.

1. Pulling in outside air (if it's only in the basement) indicates that you have too much return air there. Opening the basement door or replacing it with a louvered one will probably solve this, but that's a band-aid. I've never seen dampers in the return ducts, but it would be a solution. The next would be limiting return air at the return registers in the basement. You might also be drawing a vacuum on the entire house via duct leakage on the supply ductwork.

2. Dampers in the supply ducts are generally used to balance sections of a house (upstairs/downstairs). The best place for these dampers is right after the exit plenum of the air handler to minimize duct leakage. If you don't have duct dampers, closing the air registers is the next best thing. So if you have dampers for the downstairs, partially close them, if not, close down the registers in the there. Check for closed dampers and registers upstairs; I had a problem with a failed damper for a room in my house that had me blowing fans down the hall until I figured it out.

3. If the dampers & registers don't cut it, you may need larger supply lines to the upstairs or a booster fan. Before you do that make sure you have proper insulation in the attic and also that attic ventilation is working. Going from the old-style whirly-vents to ridge vents was a huge improvement for me. If you have power vents make sure they are working.

Answer 5

Rather than focusing on the HVAC machinery, focus on the structure. Consider air sealing and then insulation, probably guided by an energy audit.

You noticed a great amount of air infiltration during winter, around downstairs outlets and doors. A significant driver of this is "stack effect". In cold weather, hot air inside the house rises, creating a pressure different between the top and bottom of the house. If the house is not sufficiently air sealed, hot air goes out the top and draws cold air in at the bottom, just like you've seen.

The opposite happens in summer when the AC runs, cold air inside the house is more dense, sinks, flows out openings around the first floor and foundation, and draws hot air in at the top. The same drafts you've noticed in winter still exist in summer, they're just flowing in reverse.

Gaps and cracks around boards and other small parts of the structure can be tiny and seem like nothing, but all together act like a big hole. No matter how powerful or efficient the HVAC gets, if the structure is as leaky as you say then it's like trying to cool a car in summer using max AC but with the windows down. If you have a known drafty door or window you can get a demo of how strong this effect is by taping plastic over it - the plastic will balloon up when it's cold outside with quite surprising pressure, and when it's hot outside the plastic will press up against the window as all the air is sucked out.

Energy audits can help pinpoint problem areas and are often subsidized or rebated through the electric or gas company. The auditor can also put you in contact with vetted contractors and offer significant discounts towards the work. You can also do it yourself, the work is straightforward and the materials aren't expensive, but it can be difficult and time consuming.