I have read in the forum numerous times, Run from the guy who will only use a duculator instead of a manual D. Can someone tell me why using a duculator alone when determining duct sizes is wrong as opposed to using the Manual D? This is after doing a proper load calculation with the manual J.
In the equal friction method (Ductulator) you pick an arbitrary number and begin sizing according to that number. Most guys stop there and begin building their work and omit the rest of the steps as soon as they size the ducts. To complete the process the designer then needs to convert all of the ducts and fitting to equivalent feet then calculate the static pressure of the system. If the static pressure is beyond the equipment then they must pick a new number and do the process again. Whereas most guys don't do all of the steps they end up with ducts that are undersized and a system that doesn't perform as it should.
Under the Manual D you actually start off by determining all of your pressure losses (grills, filter, coil, etc) and your CFM requirement. Then you calculate your available static pressure and size the ducts from there. The end result is that you only have to go through the calculations once and when the system is built it will actually perform as intended. There are more steps involved but this is the simplified version to the Manual D.
Another big advantage of the Manual D method is that you have control over the air conditioning. In dry areas, such as where I live, we need less dehumidification. But the manufacturers rate the air conditioners for a humid climate at sea level. By installing according to Manual D and Manual J we can take an air conditioner of a given SEER rating and decrease the operating costs simply by increasing the airflow (which, in turn, decreases the amount of energy used for dehumidification). Those that use the Equal Friction method presume that we need 400 CFM/ton in our area thus the systems dry the air in the houses and cost more to operate than needed.
I have often heard guys claim that they've installed "hundreds of jobs without a problem" using the equal friction method but, intersetingly, I have never found one that operates within the manufacturers specifications for static pressure. One particular system was so far off that the furnace was riding on the limit switch... all without a problem.
[Edited by sadlier on 05-31-2005 at 09:24 AM]
Thank you for the explanation. I am having trouble finding a contractor in this area that does use the manual D. Now I have a little bit more understanding on why it is needed.
1.Furnace/air handler,has a very short return duct with one 90° turn between furnace and return grille.
ESP of equip.=.6"wc,minus .25" for pressure loss devices,leaving .35" ,Available Static Pressure for duct system.
Total Equivalent Length of duct is,300'TEL.
.35 ASP X 100 /(divided) 300' TEL=.116 Friction Rate.
Friction Rate is what the ductolator uses,it's Static9ESP) per 100' of duct,that's what the 100 is in the equation.
2. Same job ,but returns are ducted to most rooms ,adds 225' TEL to duct system.
.35 ASP X 100 / 525' TEL=.066 Friction Rate
6" round @ .066 FR ,is 87 cfms(cubic feet per minute,of air)
6" round @ .116 FR ,is 118 cfms
Easy to see what using an arbitrary FR will do to ESP and actual air flow.