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I got a customer I consult with on a regular basis who has a conundrum. He's got this big open floor plan house with an open central stairwell going up 2 stories from basement to 2nd fl.

When he runs the AC, the basement turns into an icebox because the spill down from the upper floors. He wants the basement to be similar temps as the rest of the house.

How can he warm up the basement, or otherwise even out the temps between floors? We've already tried improved returns, altering air flow patterns, balancing registers, and putting the Tstat in the basement (which "worked" except the upper floor was then too warm).

What to do?

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If the open area is narrow enough and tall enough, the air will stratify which exacerbates the vertical temperature differential. To counter that, you need to generate a vertical circulation loop (an 'engine' so to speak) to break the stratification layers and help distribute air temperatures more evenly between and bottom in that area.

Consider the sunlight that enters that area. If you could use sunlight to warm a vertical strip of wall surface on one side of that area, it would facilitate a vertical circulation loop.

Consider also whether the duct design (whether the conditioned air is dispersed horizontally or vertically by the ductwork of each split system) is contributing or inhibiting the vertical circulation loop. It could be either.

I'd say add a gigantic ceiling fan about mid-way from top to bottom, running in a direction that facilitates the air circulation loop but I doubt the homeowner would agree to the esthetics and noise of such a fan.

Marc

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If he had a blower fan in the basement and ductwork drawing air from the upper level down into the basement, the basement would be pressurized and the cold air would be forced up the stairwell, probably. Not any easy thing to experiment with tho.

His design is flawed as far as efficiency is concerned. How about going to mini-splits, running independently?

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Have you tried zone control dampers? Three separate zones AND continuous blower operation. Blower sucks air from return in one or more locations and pushes it ONLY to the floor in need, maybe even two stage thermostats at each level, stage one is blower only, stage two brings on a/c but only goes where it is needed.

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Supplemental heat in the basement. Window units on top floor.

Dampers and continuous blower. The basement will be colder but the humidity will be higher so the feel will be close.

Edit: on second thought, the basement dampers need to be controlled via humidistat. Only condition for humidity. Let the stratification take care of cooling.

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Have you tried zone control dampers? Three separate zones AND continuous blower operation. Blower sucks air from return in one or more locations and pushes it ONLY to the floor in need, maybe even two stage thermostats at each level, stage one is blower only, stage two brings on a/c but only goes where it is needed. . .

Be sure to always use a bypass with those dampers.

They'd make things better, but not a lot better. Even if you're only delivering AC to the upper floor, that cold air is still going to want to fall down to the lowest floor.

On the radical side, have you considered a radiant cooling system?

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Do not set the blower to always on. What happens if you do that during the cooling season is you end up dumping a lot moisture in the home from the evaporator coil.

I have a similar problem at my house and I just close the supply vents during the summer. Works great for me.

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This is a big fancy house. Simply closing registers has marginal impact.

I'm not worried about too much moisture from the coil. I suppose...maybe....but I can measure that and know. So, continuous blower helps.

I'm thinking programming and modulating temps by cycling equipment differently than we've been doing is the next experiment. Get some Nest stats and go from there.

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What happens if you do that during the cooling season is you end up dumping a lot moisture in the home from the evaporator coil...

How so? The evaporator coil causes moisture to condense on the coil so it can drain away. How does that add moisture to the house?

I think the OP's issue is quite simple. Not easily solved but the mechanics of it are quite simple.

It reminds me of an unrelated issue involving the same principles where convection currents form in a loop within a hollow exterior wall. It causes convective heat losses through the wall. Such currents in the OPs issue would help equalize the differences in temps between basement and upstairs. Same concept but beneficial in this case.

Marc

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What happens if you do that during the cooling season is you end up dumping a lot moisture in the home from the evaporator coil...

How so? The evaporator coil causes moisture to condense on the coil so it can drain away. How does that add moisture to the house?

I think the OP's issue is quite simple. Not easily solved but the mechanics of it are quite simple.

It reminds me of an unrelated issue involving the same principles where convection currents form in a loop within a hollow exterior wall. It causes convective heat losses through the wall. Such currents in the OPs issue would help equalize the differences in temps between basement and upstairs. Same concept but beneficial in this case.

Marc

You answered your own question Marc. the reason is because moisture condenses on the coil. It does not drain away immediately. The coil will stay wet and if you operate the unit with the fan in on position during the cooling months that moisture gets dumped into the home.

From the Florida Power and Light website:

Another reason to keep it on ?auto?

Setting your A/C fan to auto also helps provide better dehumidification. Have you noticed how moisture from the air condenses on the outside of a cold drink on a humid day? Your A/C unit captures moisture the same way, helping your home feel more comfortable. When the fan cycles off using the auto mode, moisture has a chance to drip from the cold cooling coils into the condensation pan and then drain outside. However, when the fan runs all the time in the ?on? setting, less moisture has a chance to drip and drain outside. Instead, some gets blown back into the air again.

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I take it it's not a variable speed system? I wouldn't trust a zoned system even with bypasses to recycle/dump the unused air to help this situation.

It sounds like a variable speed system or multiple system set up with thermostats for auto settings that can maintain a constant temperature by running either heat or cool is needed.

One system for the first and second floors and a separate system for the basement would work best. Set the basement thermostat to auto setting to the desired temperature and leave it.

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Methinks Florida Power and Light understands resid AC far less than the average AC tech.

Very little water remains on the coil at any point in time. It drains away long before it becomes a substantial amount of water.

Marc

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Methinks Florida Power and Light understands resid AC far less than the average AC tech.

Very little water remains on the coil at any point in time. It drains away long before it becomes a substantial amount of water.

Marc

Almost all modern system have a "purge" that keeps the fan running after the compressor shuts off for energy efficiency. By the time the coil has given up residual cooling, it has also given up remaining moisture on the coil.

As Marc said previously, all of the water in the condensate pan and on the coil came from the air in the house so it does not "add" moisture to the house. It might not remove as much but hardly enough of a difference to be noticeable IMHO. Far more important is the presence of proper traps to facilitate condensate drainage and reduce wasted energy from lost air.

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. . .

From the Florida Power and Light website:

Another reason to keep it on ?auto?

Setting your A/C fan to auto also helps provide better dehumidification. Have you noticed how moisture from the air condenses on the outside of a cold drink on a humid day? Your A/C unit captures moisture the same way, helping your home feel more comfortable. When the fan cycles off using the auto mode, moisture has a chance to drip from the cold cooling coils into the condensation pan and then drain outside. However, when the fan runs all the time in the ?on? setting, less moisture has a chance to drip and drain outside. Instead, some gets blown back into the air again.

FL power & light wins the award for the most stupid paragraph I've read all week, and it's been a week full of stupid paragraphs.

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