sizing return air

[hvac7]

When sizing return air for a paticular room should you only try to pull back the same amount you are supplying to that room or should you try to pull back a little more?

[phosgene]

I'll get sh*t for this but, RULE OF THUMB, shouldn't return air be 1.5* the size of supply

[ct2]

phosgene I'll get sh*t for this but, RULE OF THUMB, shouldn't return air be 1.5* the size of supply

Let me be the first : I know you know better

[davidr]

Originally posted by hvac7
When sizing return air for a paticular room should you only try to pull back the same amount you are supplying to that room or should you try to pull back a little more?

We size ours to pull back the same amount as is being supplied to the room.
This is subject to being +/- 10% of the total airflow to the room.
When sizing this way you are trying to obtain a neutral pressure in the room when the door/doors are closed.

[faith]

hvac7;

Duct sizing is best accomplished by methodologies contained within ACCA's Manual D. If you do not have a copy of this publication, obtain it immediately. Be advised that you have to calculate the actual load on the room in order to size a distribution system to meet that load.

Heating & Cooling loads are generally done by ACCA Manual J in the residential arena. That said, be warned that ACCA's Manual J, version 8 has some very serious issues that ACCA is in the process of correcting therefore please consider using ACCA's seventh edition. The HVAC-Calc Residential 4 by Don Sleeth (see top of this page) is based on this industry standard version.

But knowing the load for an individual room isn't enough; you have to know how to size the equipment for the entire structure. Please obtain ACCA's Manual S for this task; it is an excellent guide that needs more wide spread use in the industry.

BTW: Rules of thumb are what reduce the credibility of the entire residential HVAC industry. It is sad that so many good contractors are dragged down by a minority of individuals who for various reasons refuse to accept the collective wisdom of all the talented and schooled ambassadors who have gone before them.

[sandra salloum]

we are building...three zones in house...zone 1 is kitchen, dining room and half of great room...unit is located in pantry bordering walls of three above rooms...any reason that return & thermostat cannot be in dining room as opposed to great room...(less than four feet from where HVAC man wants to put it in great room)????...all rooms are very open with no doors to shut any of them off. If we insist on putting it in dining room, he wants us to sign a release saying he has no responsibility for whether it cools the area! In other words, he is saying there is no place, but his selected spot to put the return...(that would be on a $3,000 brick wall we are paying extra for)
Sandra

[Edited by sandra salloum on 06-19-2005 at 10:43 AM]

[beenthere]

Can't see the blue print from here.

you can usually relocate a stat some where that will work ok.

Ask him the exact reason it won't work where you want it, and try to find a compromise.

[dash]

Rooms near the baths and kitchen,need to be sized to return the air from those rooms as they will have no return grilles in them.


Rooms can be grouped together ,in some cases.What exactly are you trying to determine??