Is there scientific study of how air currents move in a home ventilation system, and how to optimize vent and return placement?
My house has a lot of vents, and every bedroom has its own air return. But my master bedroom, which has a high ceiling and large bathroom, has very uneven temperatures, and some spots that seem stale. Despite three or four vents and a return, it seems like maybe the air from the vents somehow burrows an air current to the return, and instead of the air circulating through the whole room, there are just currents of air moving through. Kind of hard to test without a smoke wand like they use in the wind tunnels. And seasonal variations cause a big difference in temperature between upstairs and downstairs.
But the real question is, is there any scholarly study of this stuff, or do architects and builders just do it by the seat of their pants?
There are plenty of studies on ventilation systems and techniques, but it is exceedingly difficult to perfect a design as every building is different and minute changes in layout and installation can affect the flow of air quite dramatically. It would only really be cost effective to have a complete test appraisal carried out on a large building or installation, not generally in a domestic installation. I would doubt the architect or the builder involved in the construction of your house actually designed the ventilation system for your house - rather a manufacturer (of the system) would have designed the layout / necessary capacity / services runs etc… for the job.
Mostly (in the UK) what is aimed for under current regulations is a specific level of 'air changes per hour’ in a specific room. This can be achieved by a combination of openable windows, trickle vents and extract fans. Of course we mainly use ventilation in a domestic situation to help remove airborne condensation and prevent surface mould growth – rarely for cooling (like we ever need it :D).
I can’t speak for US methods of design and installation though, which may differ wildly from the UK.
What kind of studies into ventilation systems are you looking for? (BRE is a good place to start looking).
It isn’t so much that I want to pore over the studies themselves, I am just interested in whether there is a serious scientific interest in the topic. Wondering if anybody has done fluid dynamics studies of the stuff.
Regarding the cost-effectiveness of domestic installations, the builder has 4 designs that are quite similar and reused in its developments all over the area. My development alone has 65 houses, so I would think that the costs to do a good system distributed over hundreds or thousands of houses would be worth it.
But “worth it to who?” is the real question? If the developer can put in a system and sell the houses with a minimum of effort (as he seemingly has done) why would he care if it is super-effective, or not, at some time down the line? I do understand your point though, but the developer is a beast led only by the lure of profit. Unless he was sure an additional premium could have been gleaned from the houses sold with the specially tested ventilation system, why would he bother? Might have been better for you, not for him.
Generally, the manufacturer of the system (and his associated engineering teams) will do all the R&D of the proposed system, rigiously testing it in many different scenarios within a ‘typical’ house plan, whatever that may be. They will then extrapolate the data from their research and apply it generally to installation designs in other house types or layouts, on request. This may (or may not) be terribly effective in the amended house type. Most if not all product installations in domestic settings, from glazing, timber, slates, doors, ironmongery, heating & ventalation systems, will be tested to destruction (and statutory standards of safety) many times over prior to being cleared for sale.
But they won’t generally, except in the case of massive office block developments etc… be tested specifically in relation to that particular building. It can be done, but usually isn’t.
Bear in mind I am NOT a services or M&E engineer, nor am I American. Things may work differently that side of the pond.
The whole field of ventilation and energy efficiency is rife with debate. The building of “tight” houses has come under fire because of ventilation issues. Inadequate ventilation causes moisture buildup and subsequent mold problems. Too much ventilation results in energy losses, but a more pleasant air quality.
Our state requires contractors to meet energy efficiency standards. ASHRAE 62 gives ventilation standards, but I can’t quote any studies offhand. You might do a search under said standard and see what pops up.
I’d like to address the poorly ventilated bedroom, CookingWithGas, without getting too technical. You can change the way air moves in a room in some fairly cheap ways.
- Your master bedroom is probably the last outlet on a long duct. You can partly close the dampers on the vents to the “earlier” rooms. You can also install a booster fan inside the vent feeding the M.B.
- You can tweak the air flow in the room, too. You can use two pairs of needle nose pliers to re-aim the vanes on the metal vents. You can get vent deflectors that magnetically mount on the vents.
- A good ceiling fan will solve a lot of these problems. I’ve installed a couple of these. I’m not an electrician, and I didn’t have any major difficulties.
Aro’s comments on testing of HVAC (Heating, Ventilating, and Air Conditioning) systems might give you a false idea. I have never heard of a manufacturer doing tests in a typical house layout.
After a house is designed, the architect or engineer comes up with heating and cooling loads for the house. Based on these, they size the furnace and air conditioning systems. The catalog will tell them the flow rate of air out of this system. If it is a basic house design, they will then use rule of thumb numbers to size ducts, and specify standard vents in typical locations of the rooms. No rigorous calculations involved.
If it is a much more expensive house (or a comemrcial building), the added step will be to calculate duct sizes and specify specific vents. Here is where the research comes in. There has been a lot of reseach (theoretical fluid dynamic kind and lab testing kind) about the behavior of air jets from a vent. They calculate throw (distance the air remains in a jet), volume, etc. But this is done under strictly controlled conditions with only flat unobstructed walls. The manufacturer then puts this information into their catalogs.
The architect looks at his print, decides he (or she) needs a certain cfm (cubic feet per minute) of airflow for a certain room, and needs a certain throw to cover the desired area. He then goes to a catalog, finds a vent that matches his needs, and finds the one in the color he likes. The contractor then puts in whatever he has in his truck. The homeowner then puts a dresser in a position to block the return grill, and an amoire to block the jet from the vent.
AskNott has some good practical suggestions. Air DOES channel from the supply to return air vents. How much it channels depends on room size, volume of air flow, furniture location, etc. A ceiling fan is the best way to solve this problem. Unless you have 7’ tall ceilings.
When I studied Duct Fabrication in school we designed systems and layouts according to “Residential Duct Systems-Manual D” by ACCA (Air Conditioning Contractors of America). Given the heating and cooling loads of a house based on the results from another class “Residential Loads”, ductwork is laid out to maximum efficiency. All manufactured ductwork components are assigned a number and have an “Effective Length” for the purpose of duct sizing calculations. Efficiency, air quality issues, and noise are taken into account. In school, cost was not an object.