HVAC (forced air system) return ducts required?
NiniKins
| Posted in Energy, Heating & Insulation on
Due to remodeling (increasing the open span between the dining and living room from 4′ to 10′) I have eliminated one of two return air ducts in the central area of the house. Will this affect the efficiency of the system? If a correction needs to be made can anyone tell me the procedure for calculating supply air and the return air required?
Replies
Insufficient return air is one of the most common problems with forced-air systems.
Without going into lengthy detail, the least you should do is make sure that the original amount of RA is restored to the system, and make sure that the supplies in each room have an unrestricted path to the return grilles.
HVAC (forced air system) return ducts required?
That is as I suspected. Before I proceed with the project I would like to identify the required return air area/volume. So I can increase the return air if necessary. On the internet I have found info about duct sizes pressure loses due to bends and lengths but nothing to relate the feed with the return. Can you help me with this? Perhaps some basic formulas and how to utilize them. Thanks for any help you can provide.
Return air should be minimally as large as the supply air ... but many would say a bit larger (you want a little less static pressure on the return side, I think).
Try to provide/restore return air flow as much as practical. If you interrupted the ducts ... can't you restore their continuation? e.g. return grill above your opening and in the floor ... the room becomes your return air plenum. Just a concept ... maybe think outside the box a bit (or duct as it were).
The simple answer is that there's no simple answer.
To do this right, you would first do a room-by-room load calculation to determine the heating and cooling needs, based on ACCA's Manual J, preferably. This will result in a BTU and CFM value for each room/space. Since the equipment has already been selected and installed, you might find that its capacity does not match the loads. So, you would need to proportion the output of the equipment to the loads you calculate--if a room's load is 15% of the load, then you would send 15% of the CFM to that room, etc.
Sizing the ducts properly requires that you start with the blower's external static pressure, and then subtract the pressure losses that are created by various airside devices (cooling coil, registers, grilles, filter, etc.) to arrive at the available static pressure. You then determine the total effective length of the longest path, which includes the measured length of the supply and return, added to which are the fitting equivalent lengths; some bends can add 100 ft. or more of equivalent resistance.
Knowing the ASP and TEL allows you to calculate the friction rate: ASP x 100 divided by TEL. Knowing the FR allows you to use a duct calculator to size the ducts according to the CFM that each section of duct is supposed to deliver.
The returns should be sized according to the supplies that are paired to them, and each room with a closeable door should have an unrestricted path to the return system.
A really qualified HVAC contractor will have a software program, such as Wrightsoft, to do all this. Others will guess, based on a mix of experience, misinformation, and "magic" formulas. (like: "you need 100 CFM at each register; 1 ton of A/C for every 400 sq. ft; and size your ducts using a friction rate of 0.1 on the duct calculator.")
Oh, is that all there is to it :) Thank you for giving me an understanding of the procedure to accomplish this task. You were able to put the bits and pieces I have read into a comprehensive explanation.
do not close any inside doors
You wouldn't
run only a supply to a water fixture, would you? For every (set of) supply, there is a drain (and hopefully, a vent).
Will removing returns affect the efficiency of the system? No, but it will diminish the effectiveness of the system.
Well thoughout duct systems provide balance of supply, exhaust and return. Where good air movement is important, generally you push (or throw) the air half way, and pull it the rest.
In most houses, the approach is similar to having all the sinks overflowing onto the floor and counting on a single floor drain to take care of the "return". That is a slight exaggeration, but you get the point.
Is there a "procedure" to calculate the supply and return? Absolutely. There are many. In a real HVAC design, loads are calculated based on construction, exposure, occupancy, internal loads such as lights, computers, appliances, etc., and the supply required to meet that load is determined. OTOH, some just use a cfm/sf approach, like say 1/2 cfm per sq. ft. Better than nothing, maybe. Supply ducts in very low pressure systems, like in a house, some (like me) would design the ducts based on "equal friction" using no more than 0.08"/100 ft of duct to size the ducts based on the required air flow, and use 0.05"/100ft for the maximum friction in retun ducts. Return equals supply.
Whether by design or not, your system will only provide supply equal to the return, unless magic is involved. If the ductwork, either supply and/or return, is too restrictive, you will effectively turn a 3 ton system into a 2-1/2 ton system, or worse. In gas fired furnaces, you can potentially have problem with over heating and tripping of the high temp limit. This will be a nusiance and limit the life of the equipment.
Thanks Tim, I see there can be a lot involved in the proper sizing of the supply/return ducts.
keep it simple
As suggested in the prior post, you should install another return at least the same size that you removed. Or increase the size of the existing duct in proportion to the one removed. Basically, you want to get that cfm back that you removed. By removing that duct you increased the cfm in the return through the remaining ducts. This works because the air is moving at an increased velocity, feet per minute, fpm. this will increase the static pressure the fan has to work against and is not good for the the longevity of your blower.
Take a look at your other returns, it may be more convienent to compensate by increasing the return of a nearby room.
plate, that is a good suggestion. I believe I can increase the RA size elseware. Thanks!