Everything from Building Science Corporation is excellent, but here is another good report on the same problem:
Originally Posted by bhe
This is a corporate report designed to support sales of a commercial product:
Just wanted to show another alternative that looks valid to me, a homeowner in S.Texas. But if there is already a return in the bedroom, I would first look into whether the problem could be solved by a larger duct to that return. With the advice and consent of a pro who understands airflow of course.
Hope this helps -- Pstu
Wow! Thank you so very much, all of you, in trying to solve my problem. I really, really appreciate you taking the time out to help me.
I checked last night, and all three of the bedrooms in question have air returns. Two small - 10x10, and one larger 10x20. The one with the larger return has the pressurization problem; although I haven't put my hand by the bottom of the door to actually feel the airflow, having the door actually pull itself closed when the A/C turns on seems like a confirmation of that problem. Funny, the bedrooms with the smaller returns don't have that problem, although the A/C just blows you away when it's on - it's way too strong. I have no clue about the size of the duct; I'll have to check the attic this evening.
The thermostat for the room with the larger return is in the living room, (which faces West and has a huge picture window), under one zone, and the thermostat upstairs for the two other bedrooms is in the hall, under another zone. The hall thermostat is right by the stairs, so I know that it's getting air from downstairs, which exacerbates the problem.
I feel that the location of the thermostats may be the main cause of the problem, but if we move it to the bedroom, won't that just move the problem from the bedroom to the living room? Same thing upstairs - if we move it to one of the bedrooms, the other bedroom will have a problem, especially since one bedroom faces East and the other faces west, with large windows.
So, like davo wrote, was my contractor trying to solve the problem by putting returns in every room, but ended up making it worse because of the location of the thermostats? He didn't have any recommendations about where to put them.
Pstu - thanks for the link. I had no clue that a pressurization problem could lead to mold. The Building Science info mentioned that, but it was pretty technical and I didn't quite "get it". Luckily, we have no vinyl wallpaper, but still..... The product from Tamarack Technologies looks like something we need to look into. I especially liked the baffle to eliminate sound transfer, and the notion that only a 1" undercut, combined with the jumper ducts, will make a large difference.
Does everyone else agree this is the best way, or should we move the thermostat(s)?
I just thought of something else. We had new, high-end, windows installed last year, triple-pane with argon gas and Energy Star certified. They replaced 1952 super-leaky aluminum casement windows. They're a great sound barrier and I was also expecting a lower electricity bill, but there was absolutely no difference. Wonder if the inefficiency of the HVAC system is the reason......
Thanks a lot.
If you move the stat to one of the bedrooms. You will have the reverse at night time. But then no one will be in the other area to notice it. And aslo as teh doors are open during the day, the other area may be ok.
Thermostats will sense collective air.
You are totally right if you close all the doors and are only sensing interior air. But if you just close 1 door it should not create problem. Return airs in rooms with doors closed and no room under door to allow pressure equallization will be problematic and will not heat or cool properly. Air flow is way to important. Just as you need supply air you also need return air.
Do it right the first time.
Let me reiterate, I never said returns were not required. I said it may not and probably is not her problem.
In Florida, many many zoned homes, have thermostats in the bedrooms.
I have a sensor in every single room, and even one in the master bathroom.
A door closing when the a/c turns on does not always mean the return is too small, although it could be a possibility. The room has to pressurize a little to cause displacement for the return to move air. It does not take to much to move a bedroom door.
My master bedroom has a 14" return with a 18"x 18" RHF type grill. More then large enough. My return static berfore my Infinity Air Purifier is .03 at full speed, 3 tons There is also a wall releif system I have not closed off yet.
My door closed when the zone calls and the door is left open about 2"-3".
Maybe I should add more returns? Could it be the unit is mainly drawing from all the closer returns, and a little pressure differential is needed to cause flow?
Remember, returns don't suck, air moves through them due to displacement.
When zoning, it is important to try and group areas with similar exposures and loads together to avoid this problem.
Adding returns will not hurt, but it probably will not solve the problem either.
Once the system is triggering on and off properly, air balancing becomes much easier.
Your house, your money, your comfort.
Last edited by davo; 06-10-2008 at 12:37 PM.
Reason: Added information
Davo, what is a sensor and how would it affect the thermostat / cycling on/off of the unit?
Regarding the return where the door closes when the A/C turns on: It just happens to be right next to the door, which is in a little alcove area, not in the main room. Perhaps the location is the problem.....?
Remember, none of us can "see" your problem because we are not there.
Originally Posted by bhe
You said you have a large return in this room. We don't know the duct size to the return. Depending on the size and length of the return duct, and the size of all the other return ducts on the system, the room is probably just pressurizing a very small amount before air flow starts through the return.
In my home I have a fancy system that uses remoter room sensors instead of thermostats. They basically do the same thing, sense temperature.
Thank you, BigJon3475. It's not too late - I got some other info, but this one is more detailed and easier too understand.
What I plan to do is (try to) find an A/C guy that is more up to speed on this kind of thing. Do you know if there are any certifications or anything like that I should look for? I just hate to start from scratch in the Yellow Pages. The guy that installed our system came recommended, and he was very nice and very honest, but apparently he's not too experienced with our kind of issue.
As Mr Bill said I would make sure that there is a bypass for the zone system. If not it will cause allkinds of problems as you have described. This would be a jumper duct with a damper between your supply and return by your furnace. With zoning it is critical to have the duct done right and the by-pass put in unless there is a variable speed furnace and the manufacturer has a system to tie in with the zones.
Its a good Life!