Residential HVAC Questions, Please help? 8

[elfman]

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?

 

[haggis]

Hi elfman,

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.

 

[CESSNA1]

ELFMAN: Go to the American Society of Heating, Refrigeration, and Air Conditioning (ASHRAE) for information and manuals on the design of heating and air conditioning systems and duct design.

http://www.ashrae.org

Go to the Sheet Metal and Air Conditioning Contractors' National Association (SMACNA)

http://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

 

[friartuck]

Where do I start??

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

 

[quark]

I concur with your basic idea.

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,

 

[elfman]

Thank you for all the great information. I am beginning to realize that the ACCA Manuals are the place to get the info, but for a non-member to get all the Manuals is rather pricey at this time. Is there a good book that will give me all the info and howto for doing heat load calculations and duct design?

Thanks again in advance.

 

[MintJulep]

The "Standard Handbook of Engineering Calculations" by Hicks, published by McGraw-Hill has example calculations for heating load and duct sizing (and many other things)

It's a pretty good general reference book, not just HVAC.

 

[friartuck]

try amazon.co.uk and look under ventilation. There is a book for about £17-00 for the building services handbook by frederick hall (F Hall as we call him over here...hope you get the joke)



Friar Tuck of Sherwood

 

[VEEKRISH]

Hey, try the CARRIER SYSTEM DESIGN Manual. This will give you all you need to get things right.

 

[HVAC68]

Absolutely right, Veekrish. I learnt a lot from the Carrier System Design Manual. It's one of the best avaialble resources.

HVAC68

 

[chicopee]

About one trunk line from which you would have lateral connections? a much much better solution.

 

[elfman]

With the trunk line, don't I need to reduce area as I go down, or do I just pull off the appropriate size of lateral connection and keep the trunk line the same size? Also, is it better to have round or rectangular. It will be going in the attic.

Thanks for all the great thoughts so far.

 

[samv]

HRAI has a series of short courses with many geared towards residential applications. It is Canadian, but my understanding is that its well recognized in the US, with training courses given in the US as well as Canada.

http://www.hrai.ca/site/skilltech/UStraining.html

 

[friartuck]

Hi Elfman

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

 

[chicopee]

round ducts is coming back into fashion. trunk line would be reduced after each lateral connection however economic may suggest to maintain same size eventho velocity and air flow will be reduced. Technically you would be right to reduce size after lateral connections to maintain velocity,however you would need reducers and that could become more expensive from a material and workmanship standpoint. One of the reason you would want to maintain a high velocity throughout duct work is for material transport but for residential heating velocity is not as important.

 

[sspeare]

Chapter 28 of ASHRAE Fundamentals Handbook (2001) has residential load calc procedures.

 

[lilliput1]

Don't forget return air ductwork or return air transfer openings. Note also that residential units typically have only about 0.8"wg external static pressure capacity. This has to cover losses through both the supply and return air ductwork. If returns are not ducted, transfer air openings must be sized for 250 say maximum 300 FPM to minimize required plenum negative static pressue requirement and consequent infiltration in the conditioned space. Air has to return to the HVAC unit. Residential units usually do not have an outdoor air connection because normal infiltration provides enough air for ventilation & for combustion.

 

[samv]

liliput - not sure what kind of residential construction you work with, but in Canada new houses are tight enough to require fresh air intakes. Heat Recovery Ventilators are common practice in my part of the world, in houses with all kinds of heating and cooling systems.

 

[elfman]

samv - or anyone else with some good ideas, what would you recommend in the way of an HRV or ERV? I would like to install one, but am not sure whether I should have it as it's own system or just duct it into the current forced air HVAC system? Nor am I sure what brands are good economical units. Recommendations would be appreciated.

 

[ChrisConley]

If you are in a heating climate then I recommend a HRV, in a cooling climate than an ERV is usually the way to go. Check out:

http://www.venmarces.com/

http://www.lifebreath.com/

Both companies have economical HRV/ERV lines designed for houses.