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View Full Version : How do I determine the size of ducts needed for new cooler?


StephenP
08-11-2005, 02:23 AM
It's a 6500 cfm MasterCool, the area is around 1500 sq. ft (12K cu. ft.?). I haven't installed it yet, but it will service 2 bedrooms, a kitchen, a living room, a bathroom and a hallway. How can I figure out the diameter of the ductwork I need to use for each room? The salesman said I should use at least 10" (round) flexible duct for each room, maybe smaller for the bathroom. Is this even in the ballpark?

Steve

ODDJOB
08-11-2005, 02:32 AM
6500 cfm seems like a whole lot of air to me. At one time you needed to move 450 cfm per ton of cooling. Based on that you have over 14 tons of cooling. Something dosen't sound right. I have a 3 ton unit that cools my house in VA. (outdoor design 95). Now about the duct size you can pick up a duculator at any wholesale house to size your ductwork, But you need a load calculation to know the heat load to determine your cooling needs. Who told you that you needed 6500 cfm ?

Sparks
08-11-2005, 05:58 PM
Oddjob is a gentleman and a scholar evidently, I will second his response.

StephenP
08-11-2005, 06:23 PM
Based on one sizing method which says to figure in 3 to 4 Industry Standard CFM per square foot of floor area, (http://energyoutlet.com/res/cooling/evap_coolers/). Our summers routinely average 100 F. or more, and isn't it better for me to get a cooler big enough to provide cooling for a bigger area should I add more floorspace later? Here: (http://www.swenergy.org/pubs/Evaporative_Cooling_Systems.pdf)says "The product literature from evaporative cooler manufacturers cites nominal flow rates—termed “energy standard cfm ratings”—that are routinely 30% to 50% greater than actual flow rates". Aside from that, the old house is not even close to being energy-efficient (though I'm working on it). Are these bad sources? Regardless, this is the cooler I'm going with, I was just hoping to learn something on how to size the ducts.
Thanks.

S..

Sparks
08-11-2005, 06:34 PM
An evaporative cooler, I see. I didn't realize this is what you were using, I was thinking conventional HVAC. I don't have experience with evaporative coolers, aka. swamp coolers, other than I've heard they do not provide much in terms of humidity control. What would possess you to want this in a residence, normally these are used at auto-dealerships, warehouses, etc? Would seem that they would be problematic for homeowner use. They require regular maintenance, are noisy and are prone to water leaks, at least the ones I have seen and dealt with. I for one would not be incined to purchase one, although I'm sure they have improved over the years.

StephenP
08-11-2005, 06:58 PM
What would possess you to want this in a residence, normally these are used at auto-dealerships, warehouses, etc? Would seem that they would be problematic for homeowner use. They require regular maintenance, are noisy and are prone to water leaks, at least the ones I have seen and dealt with. I for one would not be incined to purchase one, although I'm sure they have improved over the years.

Yeah, an evaporative cooler doesn't do much unless the humidity is low. They don't cool near as much as an a/c unit - but they cost maybe a fourth as much to run.
I guess they probably aren't as common in Buffalo as they are in Bakersfield. Kinda like snow-blowers - I'm 41 years old, and the only ones I've ever seen are in magazine ads...

S..

Sparks
08-11-2005, 07:35 PM
Well I'm sure those "magazine ads" are an authority on the subject. They would have no reason at all to mislead. Good luck with your swamp cooler, I'm guessing that removing humidity is not a concern. BTW, cooling a 1500 sq. ft house with a conventional high efficiency hvac system is not that expensive, but obviously, you've done your homework. After all, swamp coolers are becoming very popular and will eventually dominate the market. :roll: :D

StephenP
08-11-2005, 07:56 PM
Oops - if I came off as a smart@$$, I apologize. Not intended. All I meant was to point out differences in things we are familiar with from coast to coast. I don't believe swamp coolers are better than a/c units. I don't believe magazine ads are an authority on anything. I just meant that I'm not familiar with something that people on the east coast likely see fairly often.
Again - sorry if I left the wrong impression. (not the first time I've done that).

Steve

Sparks
08-11-2005, 08:16 PM
My apologies as well. I often misconstrue replies. Long day, lots of know-it-all customers, etc, etc. Time for some shut-eye I guess. I'm interested in your situation, but you are correct that we don't see many evaporative coolers around here other than the places I've mentioned. Hopefully a more knowledgable individual with experience with evap. coolers will offer input on your question. Didn't mean to snap at ya, my bad. :)

Ponderosa
08-12-2005, 04:32 AM
I'm from Arizona, I understand. Many, many residences use evap cooling throughout the southwest. Advantages are lower cost and fresh air. An evap can drop the temps 20-30 degrees on a dry day. To work, dewpoints need to be below 55, preferably below 50. In AZ they are not as effective in July/Aug but were used for decades before AC became "affordable". Anyway, most installations that piggy back on the HVAC system use the existing duct work - generally 10 inch flex to the major ceiling outlets. Look for a product called "up-dux" too. Vents into the attic so you can keep the windows closed.

6500 is kinda big for your house, I'd say. But oversizing a cooler is not an issue like in ac.

Sparks
08-12-2005, 06:09 AM
Good info, I guess they are more common than I thought in different areas of the country. I re-read my previous posts and couldn't believe what a jerk I can be when I'm tired. My apologies gentleman. :oops:

Ponderosa
08-12-2005, 08:05 AM
No problem. I bet there is another cooling system you rarely see east of the Mississippi - misters. Misters are common around store and mall entrances and on restaurant/bar patios in this part of the country. You can get them for your private patio areas too. They, as the name implies, spray a fine mist into the air above where people are. The mist evaporates and cools the air. Residential systems use line pressure and don't work all that great in my opinion. Commercial ones have pumps and finer spray tips. I suppose such a system would be worthless in many parts of the US. Everyone would just get soaked.

Another variation on evap cooling is used on dairy cows/feedlots. In this case a stream of water is shot into the back of a fan which makes it into small droplets/mist that flies out toward the cows, evaporates and cools. In the summer you see the cows all gathered in front of the fan, so I guess it works.

rabadger
08-12-2005, 08:10 AM
I last time I checked swamp cooler cfm is listed at 0 static pressure. The CFM capacity drops rapidly when duct systems are applied. The first thing you have to do is figure out how much cfm is needed is each room and then look at the blower performace curve to figure if you have enough air.

Do your load calculations first.

Sure the salesperson is going to tell you whatever you need to hear to make the sale. If the salesperson had any technical background he or she would have drawn something up for you.