Ceiling fan

[sailoday28]

We have a lobby in our appartment building that has hvac. Volume and height of the lobby are relativley large. How can I determine if there will be an energy savings by installing reversible overhead ceiling fan(s)?

Thanks in advance.

 

[IRstuff]

How high a ceiling would seem to be a relevant parameter. This from one of the many HVAC websites seems relevant:

VI. My ceiling is higher, how long of a downrod do I need?
This is another one of those issues out there where there are many similar charts to answer, and yet there are a few important points to consider first. Ideally speaking, for maximum circulation, the blades should be positioned 8-10' from the floor. However having the blades too far away from the ceiling can sometimes lessen the effectiveness of heat destratification. Not to mention that there is also an aesthetic factor, having the blades 9' from the ceiling on a 15' ceiling may look a little imposing. This can also be affected by where the fan is positioned-- a fan hanging over a table will likely be positioned lower, like a chandelier, compared to one over a traffic area.
Generally speaking, the 8-10' rule works until you exceed a 2' downrod. From that point on you need to balance the above factors.
Here is a generally accepted chart for downrod length:
9' ceiling: 6-12"
10' ceiling: 12-18"
11' ceiling: 18-24"
12' ceiling: 24-30"
13' ceiling: 30-36"
14' ceiling: 36-48"
15' ceiling: 48-60"

http://www.hunterfan.com/FAQs/#a14

Note that for destratification, the fan is supposed to be much closer to the ceiling, so maybe you need some sort of telescoping downrod?

TTFN
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[EnergyProfessional]

Why do you want to reverse? All the fan does mix the air (ideally all air has exact same temp) and will do more efficiently blowing down.
Some people (falsely) think it will help cooling when reversing (since blowing down helps in winter, right? Like my car will produce gasoline when driving backwards...). but when blowing up, the air still will be mixed (just less efficiently since the fan discharges to the closer ceiling).

in summer you have 2 choices: if the fan is close to occupants, you could use it blowing down for cooling since draft makes feel people cooler (but all their paperwork will blow away). Of it the room is high, just turn it off. When cooling high rooms you WANT the air to stratify so you only cool the occupied zone.

For height you also need to consider the air the fan needs to draw in. the manufacturer will tell you. the above table may work for 4' ceiling fans. When you get 24' BAF fans it is much different since the fan wills starve of air if it only has 12" room.

 

[Drazen]

as you have system in place, you can measure temperature at height of your thermostat and at lobby top.

you can eventually make more measurements from lobby top up to thermostat height.

all excess temperature above thermostat level is straightforward waste of energy, so you can calculate it using some heat loss calculation model.

1. consider your top part of room as a separate room
2. take average of your measurements as fictive design temperature of this "room"
3. calculate heat loss of such "room"
4. calculate heat loss of the same "room" with design temperature
5. difference between 3 and 4 would be direct was of energy, should be base point to justify your destratification fans.

 

[Compositepro]

Bottom line is that a fan will consume energy and will not likely save energy unless the added comfort of the draft allows the thermostat to be set at a higher temp. If you have stairs to a second floor in your lobby you may need a fan just for comfort of any occupants.

 

[EnergyProfessional]

do you actually think you want to save heating or cooling energy? In july this may obvious, but you also asked about reversing fans. Please state what you actually want to achieve.
As stated above, the fan just running will consume energy.

 

[sailoday28]

I wish to achieve both saving cooling energy in the summer and heating energy in the winter.

 

[EnergyProfessional]

Assuming this ceiling is 15'or higher and no one is above 8' we don't care how warm or cold it is "up there"

cooling: warm air will rise and that is good. Just leave the fan off. Stratification is what you want (those are the theoretical benefits of under floor air distribution).

Heating: it depends a bit on how you heat (radiant in floor, ceiling air diffuser etc.). but to avoid the stratification a fan blowing down will help to mix the air. In this case you may want to disturb stratification. BUT don't create a draft to make people uncomfortable. Providing heat below (in-floor or radiators at floor level) probably is more effective than fans.

why you or anyone would reverse the fan is beyond me. the fan blowing against the ceiling will mix the air as blowing down, but less efficiently. It must be that someone thought "fan blowing down helps heating in high ceilings, so blowing up must help cooling". this is the same thinking as "driving a car uphill will but gasoline, so driving downhill must generate gasoline".

Maybe reversing could be similar as effective as blowing down if the fan is in the middle between floor and ceiling, but I doubt you want the fan in 8'height in a 15' room.

not only this fan uses energy, you also need to maintain it in that height.

 

[IRstuff]

"cooling: warm air will rise and that is good. Just leave the fan off. Stratification is what you want (those are the theoretical benefits of under floor air distribution)."

For cooling stratification is not the issue, is it? Isn't it more of trying to get some forced convective cooling in lieu of A/C? If just allowing the warm air to rise was sufficient, A/C wouldn't even be needed.

TTFN
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[Compositepro]

The logic of reversing the fan in winter so that it blows upward is to avoid strong drafts on people which will make them feel colder. The fan still mixes air but the draft comes down at the walls at a much lower velocity than it would coming straight down from the fan.

 

[EnergyProfessional]

IRStuff:
your theory is valid if you want to get by without AC. but the fan doesn't make it colder, or less humid. If the space is 85°F and 70%RH you can blow air all around as much as you want.... just don't expect people in the office being productive.
In order to give some cooling effect with no AC, the fan needed to be so strong that it would make all paper fly around. this likely isn't practical in an office (and still too warm and humid)

Compositepro:
IF the fan is close to the ceiling, it won't be affective when blowing against the ceiling.. the "throw" distance of a fan (or any diffuser is far. The "suck" distance of a fan (or return grill) is very short.

 

[IRstuff]

"but the fan doesn't make it colder"

It makes you FEEL cooler.

TTFN
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[willard3]

We have all heard of adiabatic cooling, haven't we?

Moving air cools you off.

 

[EnergyProfessional]

IRStuff: if the lower part is 75°F, and the fan blows down the 85°F... not sure if that is better.

and in order for moving air to cool you , it needs to be moving around you, being a draft and nuisance.