Residential HVAC Questions, Please help?
Residential HVAC Questions, Please help?
(OP)
I am trying to learn more about residential HVAC design, load calculation guidelines, duct run design, etc. Is there one good resource for information?
For instance, which is better, one larger duct feeding several registers that are somewhat distant from the plenum, or three smaller diameter ducts that run all the way from the plenum to the registers? Intuition says that to minimize losses both to flow and duct wall conductivity minimize surface area by using a larger duct to feed the registers. Is this the case, or do other factors weigh more heavily?
For instance, which is better, one larger duct feeding several registers that are somewhat distant from the plenum, or three smaller diameter ducts that run all the way from the plenum to the registers? Intuition says that to minimize losses both to flow and duct wall conductivity minimize surface area by using a larger duct to feed the registers. Is this the case, or do other factors weigh more heavily?





RE: Residential HVAC Questions, Please help?
Very few residential forced air systems are designed let alone well designed. Builders are cheap and the sheet metal guys run ducts wherever for convenience. I've seen both smaller diameter ducts come off a trunk or main rectangular and also smaller diameter ducts come from a plenum near the furnace. Neither system is good insofar that the former never reduces the size of the trunk after a branch and the latter lends itself to branches going to short runs and some to excessively long runs as more often than not,the furnace is tucked away in some corner of the basement. The ideal situation is to have your furnace tucked away somewhere in the event that you might want to utilise the basement space for a well laid out rec area but to have a large rectangular duct coming from the furnace to a centrally located plenum and have the smaller diameter duct branch from there giving you reasonably equal runs to the registers and a better distribution of the air. All your branch diameters shouldn't be the same either depending on length of run as you want the velocity at each register to be pretty well equal as well as the volume. However, a properly designed and installed system would add significantly to the cost of the home and unless you are designing for custom built homes or an upscale builder rather than run of the mill tract homes, you will get very frustrated at what you are asked to put in.
RE: Residential HVAC Questions, Please help?
Go to the Sheet Metal and Air Conditioning Contractors' National Association (SMACNA)www.smacna.org for information on sheet metal duct design and fabrication.
HAGGIS is right about the contractors and residential HVAC design. They are very cost sensitive and most times the HVAC is "designed" (and I use it loosely) by the architect or by the installer or builder. Many residnetial systems are okay, but many are unsatisfactory also. Usually the homeowners think that their situation is normal.
Regards
Dave
RE: Residential HVAC Questions, Please help?
Take one person, sit in a classroom for a year or three,listen to the 'old codger' in charge and try out a few designs (supervised of course)
There are many ways to skin a cat.
First you need the load of the building. i.e. how much heating or cooling it needs.
Then you can selevct your 'furnace' or warm air heater.
The heater will be rated at a particular heat output and deliver a particular airflow (against a set system pressure drop). That is pretty much fixed but you can get bigger motors and change pulleys to vary the flow within fixed limits.
Once you have that, you can lay out your system ductwork.
You need volume control dampers to 'set up' the airflow rates to each area and good quality grilles to properly distribute the air (otherwise you will get drafts).
The ductwork can be sized on how much air you need to each room. I would size the ductwork based on the airflow rate and a typical duct velocity of about 3m/sec. Any more in a domestic situation could give you excessive noise/turbulence. Lower velocities are better but more expensive.
If you get an ASHRAE book (Fundamentals) then this has some good ductwork sizing examples and shows you how to assess your heating or cooling loads.
ASHRAE applications also might be helpful.
If you join ASHRAE, you get one free design guide a year and so in 4 years you have the full set.
It really is worth joining.
Good luck
Friar Tuck of Sherwood
RE: Residential HVAC Questions, Please help?
Cosidering a duct velocity of 600 fpm, as suggested by friartuck, if you have to size the duct for 3000cfm(say), the duct cross sectional area will be 5sq.ft. This will give you an equivalent diameter of 2.52ft and surface area will be 7.91sq.ft/linear foot.
Split it into 3 ducts of 1.45ft dia. 3 ducts to carry 1000 cfm each, and the surface area becomes 4.56*3 = 13.68sq.ft/linear foot. You have to cough up extra 5.77sq.ft of sheet metal per linear foot.
As the frictional loss is proportional to v2/d, frictional losses with one large duct will be lower(for a fixed velocity)
Further, heat losses will also be high. Increase in internal convective coefficient due to increase in Reynold's number will be offset by constant conductive and outside convective coefficients and increase in surface area.
Regards,
RE: Residential HVAC Questions, Please help?
Thanks again in advance.
RE: Residential HVAC Questions, Please help?
It's a pretty good general reference book, not just HVAC.
RE: Residential HVAC Questions, Please help?
Friar Tuck of Sherwood
RE: Residential HVAC Questions, Please help?
RE: Residential HVAC Questions, Please help?
HVAC68
RE: Residential HVAC Questions, Please help?
RE: Residential HVAC Questions, Please help?
Thanks for all the great thoughts so far.
RE: Residential HVAC Questions, Please help?
http://www.hrai.ca/site/skilltech/UStraining.html
RE: Residential HVAC Questions, Please help?
Think of ductwork systems in the same way as a motorway (Freeway in your terminology)and side roads.
In the UK our motorways are designed (and I use the word designed loosely here), for speed of 70mph with side roads coming off at lower speeds of say 50mph or so.
Ductwork works on pretty much the same principles. It really depends on the individual circumstances, design experience, knowledge blah blah blah but for general guidance, the main distribution for an office type situation would be 5 to 6m/sec and the side ducts around 3m/sec.
For residential, the noise is more critical since you don't want to be kept awake by hissing or rumbling, so I would suggest velocities of around 4 to 5m/sec for main runs and dropping down to 2.5m/sec for side duct and possibly 1.5 to 2m/sec for grille and diffusers (i think you call them registers)
This should keep the system pressure drop and noise level down.
Incidentally, you can have too little pressure as well.
I designed a system a couple of years ago using Lennox floor standing gas fired heaters. The units had pre-set fan duties so you got what you got.
I sized the system on a low velocity (it was a library and the client wanted it very quiet---plus he already had Lennox units so he wanted to use them again)
Guess what? The units overloaded because the system resistance was too low....
The fans were forward curved type which have an overloading characteristic. In the end, we addedd resistance and the system worked fine.
Good luck
Friar Tuck of Sherwood
RE: Residential HVAC Questions, Please help?
RE: Residential HVAC Questions, Please help?
RE: Residential HVAC Questions, Please help?
RE: Residential HVAC Questions, Please help?
RE: Residential HVAC Questions, Please help?
RE: Residential HVAC Questions, Please help?
http://www.venmarces.com/
http://www.lifebreath.com/
Both companies have economical HRV/ERV lines designed for houses.
RE: Residential HVAC Questions, Please help?
there are several resources for design and installation there.
John Peterson
www.wba-arch.com
RE: Residential HVAC Questions, Please help?
I would like to replace the ductwork in my house which is in poor shape and is located in a very constricted crawlspace. I am trying to roughly size a trunk and branch layout to replace my "spiderweb" layout. I would expect to replace the gas furnace at that time, but would like to retain the existing return duct in the attic and the condensing unit.
At this point, I am not concerned with heating/cooling loads and so forth, as I have existing equipment which seems to perform well enough, furnace is 106,000 input BTU, AC is 2 or 2 1/2 tons (I think) and the current diffuser layout would be retained.
The calculations are straightforward but, not being a professional in this field I do not have a feel for what are "good" numbers.
Finally! My specific questions:
1) What is a "good" value for volume flowrate, i.e. CFM flow per ton cooling?
2) What is a "good" value for system total pressure loss and what will an "average" air handler produce.
3) When balancing duct runs, how much difference in total pressure loss would be considered negligable?
4) I am also interested in further guidance on velocities in the various parts of the system, particularly "exit" velocities where the flow exits into the room space.
I have found a several good sources on this subject, but they seem to be oriented toward large commercial systems and don't contain some of the more practical aspects of small systems.
Thanks in advance for any input.