Ways to combat thermal dynamics.

[Vivid]

I always seem to start my threads off the same way, oh well here goes....

I am building a new house and I worried about the thermal dynamics. The house is 5 stories, well not really, there is a half basement, then three living levels and lastly a small access area to the roof-top deck. I had two ideas to run by you guys and get some feed back on how you think the would help with maintaining a comfortable house.

1. having a recirculating fan. Essentially one run of conduit from the top level to the basement with a bi-directional fan which would either take the hot air from the top and dump it in the basement or vice versa.

2. Have either a skylight or an extractor fan in the top level which can be turned on when the temp outside is less that the temp inside and you want a cooler home, both could be on a auto temp switch.

From my lay person point of view I think they both sound like good ideas. Each helps with a different problem and would not be used at the same time, but what do you guys think?

[dan sw fl]

I always seem to start my threads off the same way, oh well here goes....

I am building a new house and I worried about the thermal dynamics.
... , then three living levels and lastly a small access area to the roof-top deck.

1. having a recirculating fan. Essentially one run of conduit from the top level to the basement with a bi-directional fan which would either take the hot air from the top and dump it in the basement or vice versa.

Do you anticipate > 3'F between the 3 living area floors?

Use a higher end thermostat to circulate air.

[cem-bsee]

in your area, be DAMM sure the switch locks out bringing in outside air when the RH% outside is above 60F -- ie it senses enthalpy --

[Vivid]

Dan,

I'm not sure or should I say I hope not. I was thinking of the recirculating fan to mainly because hot air likes to rise and the basement will be a natural source of cooler air. I thought this system might reduce the workload on the top floors unit and maybe reduce the cooling cost because of the free cool air from the basement, and generally make the house more consistent and comfortable. Let me also say the my stair cases are stacked right on top of each other and they are floating stairs with no risers so I was thinking there could be a problem of all the heat gathering on the top level.

[Shophound]

I'd say living in Atlanta, which is likely a more humid region than my own (which isn't exactly dry), you are limited on how many days you can actually use outdoor air to effectively cool and ventilate your house. With a multi-level house you will also be dealing with significant "stack effect", which is the natural tendency for heated air to rise and cooler air to penetrate the building envelope from lower surfaces to make up for air that rises toward the top of the structure. This is aggravated by large, open areas between floors, which is a common feature in many multi-level designs now.

I think the best strategy to effectively condition a multi-level house is to build it tightly, then zone each floor...ideally so each floor has its own system and air distribution network. Minimize open area between floors, do not penetrate ceilings with unsealed can lights. Each room on every floor must have adequate return air...door undercutting is insufficient. Each room must receive proper amount of supply airflow for design conditions; adequate return air is part of this strategy.

[Vivid]

Shop,

alot of what you said is being done. I am using the sealed recessed cans, each level will have its own heatpump system with the correct ducting/sizing and I am using spray foam for the exterior walls...

But there are not that many interior walls, the stairs stacked and are open stairs with no riser. I thought the recirculating fan would kind of be like adding a ceiling fan to a room but I guess from what some of you have said its not a good idea?

[Shophound]

Item Number 2 in your OP has merit...since you have a common open area between all floors, on days when outdoor dew points are favorable, you can ventilate using clerestory or skylight type openings and open windows and screened doors on lower levels.

The key is dew point. If the common indoor design conditions of 75 degrees/50% relative humidity (for cooling purposes) is in effect in your case, that's a dew point of around 55 degrees. Whenever outdoor conditions in your area have dew points higher than 55, the degree of comfort you will experience from outdoor air ventilating the house will be diminished.

For Item Number 1, the way I see it, a large duct running from basement to top level will in itself experience a stack effect. It will continually draw air from the basement to the top level, placing the basement in a negative state. Unless you are running the fan to bring upper level warm air to the basement, the stack effect will be occurring at all other times. With the basement in a negative, air will infiltrate into the basement from any portion of basement wall penetration that is above ground level. This air will have to be cooled and dehumidified, or heated and dehumidified in winter.

[v2r]

zoning, multiple stats controlling the duct for each floor. might be worth looking into.


vaughn

[dan sw fl]

Dan,

I'm not sure or should I say I hope not.

I thought this system might reduce the workload on the top floors unit and maybe reduce the cooling cost because of the free cool air from the basement, and generally make the house more consistent and comfortable.



Use a variable speed air handler set on Continuous fan to provide uniform temperatures. The air flow is adjusted to 1/3 or 1/2 of the normal flow in this mode.

Generally, I would NOT advocate use of Continuous fan mode
in a humid climate.

Sealing the house ( including foam insulation on the underside of the attic roof ) is likely to minimize stack effect.