Cold air stratification downstairs even after furnace replacement

[wwu123]

I just had a contractor replace our 18-yr old 140K, 80% Amana furnace with a new Carrier Infinity 58MVC080-14 80K, 95% with the Infinity controls after the old unit suddenly failed. The new unit is probably more fairly sized, if not still somewhat oversized, for our house after a full remodel last year with tight insulation and new windows, despite adding 50% more floor area.

I actually asked the contractor to upgrade from 95% single-stage to the top-of-the-line variable-speed and Infinity control primarily in the hopes that longer fan cycles on low-speed, low-heat would reduce stratification to the downstairs rooms, particularly an office that's the farthest run and yet another three feet lower than the rest of the house. In addition he dedicated a new 8" duct from the supply plenum to this office. I'd also asked about adding a return downstairs, but he said the air would flow up through our big open stairwell to the one return near the top of the stairs.

Whereas before I couldn't feel any air movement at the register located at the ceiling of that room, I can definitely feel warm air now at both low and high stages. The flow is even fairly strong at high stage, though the Infinity really minimizes the amount of time in high stage, and the low-stage cycles are long. However, the room is still cold as there is stlll a lot of stratificaton; the air is not mixing floor-to-ceiling, and the warmth seems like it's just following the ceilings back upstairs.

Incidentally someone in my area (CA bay area) had a very similar situation, but seems to have solved it with the opposite approach; 95% single stage to force more air downstairs, enlarged the upstairs return, even removed their 2nd return at the bottom of their stairs.

I'm going to ask the contractor to come back to see if he can do anything more, but wonder if I should suggest any of the following:
-ask him to replace the dedicated 8" flex duct he just added with rigid duct, to reduce friction and get more supply air. It is the first 25' from the supply plenum and is easily accessible (the 2nd 25' is in the ceilings and not changeable)
-ask him to enlarge the return grille, though the return itself is smaller and fixed size.
-ask him to add a low return in the downstairs bedroom across the hall from the problem room, to "pull" the cool stratified air at floor level back to the furnace. That room shares a wall with the furnace crawl space, so fairly straight shot to the return plenum. This would also increase my undersized returns (Infinity control shows 0.8 static pressure).
-ask him to add a low supply duct to the downstairs bedroom, so that warm air can "push" the cool air off the floor and promote more vertical mixing across to the problematic office as well. All the downstairs registers now have reasonable airflow, but are at ceiling level.

It's about 2 deg cooler downstairs at the ceiling, another 2 deg cooler at the floor, and then in the problematic office yet another 2-4 deg because it's lower. When I run in continuous low fan (no heat), the 70 deg upstairs air comes out that register at 68 deg, but a foot away the ceiling is only 66 deg and continues to drop.

[Jim Davis]

If you have long runs or ceiling supplies a 2-stage furnace will leave you on the chilly side.

[beenthere]

Is the register in that room a round one.
If so. They are great for cooling. but lousy for heating.
Is the door to this room left open.

[cehs]

One important thing many HVAC designers forget is that of a RETURN AIR in the basement near the floor so that problem will be GREATLY reduced.
If you pull cool air off the floor, warm air will have to replace it and the problem is reduced.
I have done this on several homes and ALL have reported an improvement.
Try it, you will be surprised.

[Shophound]

The coldest room (office); what kind of window does it have? Single pane or double pane? Is the floor of the office on a slab or is it pier and beam? If the latter, is there insulation below the floor of that office?

What? I didn't say anything about your HVAC equipment? No, I did not. Sometimes comfort problems with certain rooms can be remedied by paying closer attention to the building envelope vs. throwing more heat or cooling into a problem child area. We are engrained with relieving comfort problems by throwing more energy at them. Energy that is becoming more expensive as time goes by. Passive methods get overlooked as a result...we're also easily dazzled by whiz-bang technology.

That's not to knock your equipment choices and selection. The Infinity system can be a great performer at reduced energy cost. Digging a bit deeper in thought and focus is all I'm saying. The house and its HVAC equipment are ONE system.

[wwu123]

You're right to question the building envelope. Let me concede the envelope was built with good construction but designed with poor heat-loss - an architect's dream but an HVAC nightmare. With the furnace emergency, I've been willing to throw more $$ into HVAC upgrade remedies (though the upgrades that are not working could've already paid for a dedicated mini-split heat pump for the office). That's why I'd like to pose it as an HVAC challenge for the experts here.

So:
-new addition last year, 12x15 room, two walls are 2x6 exterior walls with R-19 insulation. I checked the insulation for gaps before the drywall went up, have measured no cold spots with an IR thermometer.
-8' ceiling faces a conditioned room upstairs. Floor is joist and beam, R-19 insulation.
-here's the tough part. 100 sq ft of glazing, so 55% of floor area. North and east facing with no direct sunlight. 3/4 of windows are well-insulated - wood-clad, double-paned, argon, but 1/4 are single-pane, field-glazed for a glass-butted corner. Curtains are not an option, concession to modern design.
-no framed doorway, instead completely open into hallway which steps up 3', so return air flows along ceiling rather than the floor. Office crawl space thus hits the concrete beam of the furnace crawl space, only way to connect would be to punch a hole through the concrete or dig under (feasible, but is that allowed?). That's why the low duct options are to the bedroom across the hall.

Two observations about the airflow:
-the supply duct rework so far has increased warmth up near the downstairs ceiling registers. Ceilings are closer to upstairs temperature, but floors are still cold.
-by simulating a ceiling fan with a Vornado fan pointed straight up, I'm seeing the ceiling/floor temps start to equalize, but I'd prefer this not be the long-term solution.

Ceiling supply register is 4x10 at far end of room from hallway, fed by 25' of 6" round flex duct and 25' of 8" new round flex duct from supply plenum. Flow is more than before, though not forceful.

[Shophound]

This past summer, with my own acquisition of a decent IR thermometer, I took some readings around my home and then sat down to perform some number crunching. What I found, if reasonably accurate, led to an eye opening experience.

I took my own home office ceiling, and for simplicity rounded it off to 12 x12, flat at eight foot height. The one exterior wall I measured 12' x 8', with the single pane window 8' x 3'. Using a simplified heat transfer formula of Area x Delta T x U factor, I discovered that, on a summer day with the outdoor temperature at 100 degrees and the attic temperature at 135, the biggest source of heat gain to the room wasn't where I thought it would be...the ceiling (floor is slab-on-grade). It wasn't the shaded, south facing wall, R11 (at best) with brick veneer on the exterior. Nope...it was the 8' x 3' single pane aluminum framed window, sitting in full shade, adding more heat to the room than the ceiling and the wall the window was in, COMBINED.

It would be similar results to crunch numbers for this same room now that the weather is cold outside. We're probably a bit chillier than the Bay Area of San Francisco right now, and I do admit that houses vary widely in design and construction quality, but my underlying thought is that it shouldn't be hard to keep rooms warm in California, period. I know this because I once lived in California, in Roseville, in a split level apartment with a 1 1/4" thick subfloor over a crawl space, no insulation. It was IMPOSSIBLE to keep that apartment warm enough for human habitation before I convinced the owners to insulate the subfloor. Even after that, had I known then what I know about single pane windows, the sliding glass doors in that apartment would've been next on my hit list. I replaced two in my own mid-century modern house early this year with new IGU equivalents...the difference in comfort is stunning.

Truth is, like my signature below says, human comfort is a four part equation. The HVAC system can take care of three of the four, to a certain extent. The fourth, mean radiant temperature, is all on the building envelope. If the skin of the building is cold, you will be cold even if you run your Infinity system full blast. Your body loses heat in more than one fashion...cold building envelopes is an overlooked factor in our quest to be comfortable in our homes.

Nevertheless, and again in accordance to my sig line, air circulation patterns and volume are also important for comfort. You're correct to look at those. It's a multiprong thing, and when you get all four aspects hitting on all cylinders, it's a wonderful thing to experience. Your budget will like it, also.

[Brian GC]

Though your building envelope is not ideal with the use of some single pane glass, it would not be enough to cause cold air stratification while the unit is running. Lack of R-value in walls or windows would cause an increase in unit run cycles; it would not prevent a unit from heating a house thoroughly if it was ducted properly. Open drafts might cause cold floors, but that doesn’t seem to be your problem.

There are many pros here and each have their own approach to your problem, but Cehs’ quote holds the most water IMO.

One important thing many HVAC designers forget is that of a RETURN AIR in the basement near the floor so that problem will be GREATLY reduced.
If you pull cool air off the floor, warm air will have to replace it and the problem is reduced.
I have done this on several homes and ALL have reported an improvement.
Try it, you will be surprised.

I have also witnessed your problem many times and have fixed it by using low returns to remove the cold air off the floor.

Cehs is correct in that the more cold air that is removed off the floor the more the warm air from above will replace it. It will also cause less warm air to escape upstairs.

Tighten the envelope and use the best registers to circulate the air, but also use low returns for heating – it can make a big difference.

Brian

[wwu123]

Thanks for the input everyone. May I seek one last piece of advice about where this low return duct should best connect into the existing returns?

It would be most convenient to connect this to an upper return plenum where there is already an unused capped off bypass opening. The upstairs shaft is 288 sq in to this plenum, and this lower return duct could be 133 sq in (if using a 12" round), but then everything constricts through a 16" round duct (200 sq in) before entering the furnace.

There is a lower plenum after the 16" round that would be possible to connect into, but is not very accessible. However, doing it this way would increase the proportion of area from downstairs (133 sq in upstairs vs 200 sq in round duct from upstairs). Would this be significantly more effective at sucking the downstairs air, as well as increasing my total return area (and reducing my somewhat high 0.8" static pressure), to be worth possibly a lot of extra cost and effort?

[cehs]

I would just come off the RA drop and use rigid or flex to a boxed in wall grill to another room in the basement.
Coming off the RA drop in the basement will also allow you to use the existing filter and will NOT need to add a filter somewhere else.
That way you will have MORE return available to the furnace for additional airflow to ALL rooms, upstairs and down.

[beenthere]

Connect it to the plenum. So you don't interfer with the air in the 16" round.

How much it will drop your static is unknown. Since we don't know if your return is more or less restrictive then your supply.

[wwu123]

Thanks everyone. I contacted the HVAC installer and he wants more additional money to install a return as it's an existing design problem. I don't dispute that, but he could have spoken up more last week that the furnace upgrades and the supply duct work weren't going to solve the problem. His work looked like solid quality, it was just mostly wasted effort with no result. He doesn't seem to fully get the solution; for example he wouldn't want to install an 8" or larger return in that bedroom when it only has a 6" supply duct, because it would cause negative pressure in that room - but the exact point is to draw in cool return air from the rest of the downstairs, otherwise I'm just heating up that room alone.

I'm going back to talk to my general contractor from my remodel last year first. I looked at the building plans, and the architect called for a floor supply duct for the office, and a "HVAC Supply to Crawl Space" rectangle in the furnace area that looks like the contractor was supposed to dig under to get to the office's lower crawl space. They saved time by running it to the ceiling instead, and I didn't realize it mattered until now.