Posted: 8/5/2006 2:32:51 PM EDT
|
Hey Guys, In short, my central AC was working great until I tried to improve it.  The filter size, located in the hall ceiling under the main handler, on my 3 ton 14 seer unit is a 16 x 20. Whe the AC comes on the filter is being sucked up so hard into the plenum that it's buckling pretty good (not enough to damage it or collapse it), so I decided to add another return in a different part of the house. I added a 12 x 24 grate and filter but after installation the vent temperature coming out all the vents is quite a bit higher than before and the whole system is now unable to even maintain the thermostat setpoint. I'd guess between 5-10 degrees higher vent temp than before. To troubleshoot I covered up the entire new return. The vent temps immediately dropped to the previous quite cool levels. So, I know what is causing the problem (new return) but I do not understand why. Is there some amount of load restriction that the fan/coils need to properly cool? Do I have too much air going into the handler now? Thanks for any help. CMOS |
|
It sounds like the installing contractor set your blower speed up to max to overcome the small filter. Roughly speaking, you need at least 1 sq ft of filter per ton of a/c to keep the velocity down and allow the filter to work . When you reduced the restriction, you increased the air volume the blower moved. More cfm for the same btus equals less temperature decrease across the coil. Get your discharge temp high enough and you will cease doing meaningful humidity removal too. Sometimes people put in a super efficient filter that has so much static drop across it that they have other problems with loss of airflow and condensate from too much temperature drop. There are several things that could need to be changed to correct/improve your setup, but if your main gripe was the filter problem you might open the airhandler up and slow the blower down to reestablish the original airflow. If it's a direct drive with a multi speed motor you can select the next lower speed for the cooling cycle by switching one of the wires to the motor. The wiring diagram will tell you which wire color is what speed. If it's a belt drive system you'll need to make the motor pulley smaller. Just be sure to kill the power before putting fingers in there. Not comprehensive, but may help. |
Werd! Brother either knows his shit, or slept in a Holiday Inn Express last night. |
like he said blower motor needs to be slowed down. Its funny how these things work but its pretty basic..issue...of speed,volume, ect |
In light of what I described, do I really need to change the motor speed or just choke down the size of the new return? I could section off the new return and effectively make it a 6x12 or 12x12, etc. As I type I have a precision thermocouple in a vent that has the longest run from the handler. With the new return chocked down to a 12x12 I'm still getting a 16.3 degree drop. Next I'm going to choke the new return ALL the way down so I can try to find out where the "sweet spot" is for the filter area. Ripper, where are those wires for the motor speeds? In the attick where the main power is fed to the blower? CMOS |
Using an airbalance method is fine if you're happy with the rest of the system. You'll need to reduce the effective duct size of the old return to be able to match your old CFM using two returns though. A damper, piece of sheetmetal etc will work but I wouldn't reduce the filter box area. I've even seen people use large ty-wraps to put a crimp in the flex  I like to see a 17 to 20F drop (depends a lot on humidity) when measured at the coil, long runs of duct cause the measurments to change so I always get the number there for comparison too. Bad insulation can cause lost capacity and the people don't even know it. The wiring is inside the air handler and there are different arrangements with different units. Some blowers have several plug in terminals located on the motor itself. You select the speed by where you plug in the hot wire from the relay. Other motors have several wires coming from the motor to the electrical panel and the speed is selected by plugging one of these different colored wires into the blower relay. With this type ,the other wires are capped off unless there is heat in the unit and the heating cycle uses a different speed from the cooling. Hope this helps. |
|
Ripper, UPDATE: I have located the ONE wire that will change the blower speed. It is currently set at MEDUIM. Since we now know this, do you recommend I 1) reduce the blower speed to low amnd keep the new return at 12x24 or, 2) Leave the blower speed at MED and choke down the new return to a 12x12 or 6x12? Or a thinrd option - try both, one at a time, and see how the vent temps are to see which config is more efficient? Thanks for your hlep. CMOS |
|
I would leave the speed at medium. Fan curves are funny sometimes in that when you get near the extremes a small rpm change makes a large cfm(static actually) change. First I'd block the new one and check the temp difference. Then I'd open the new return some arbitrary amount...maybe a third of the way. Then I would throttle the old duct enough to get back to the original drop. If the entering air temp is pretty close to what you had when you measured the first time you should be GTG. Keep in mind that the extra return is going to cause a bit of a change to your flow pattern in the house and it's possible that formerly comfortable rooms may now be slightly warmer or cooler because of it. It's HOT in those attics ! I'm no physician but I'd say that a beverage of your choice and a sammich should waiting for you when you are finished slaying the beast. 
|
|
Damn, this just keeps getting worse. I just got home from running a few errands that took about 3 hours. Came home to a warm house and the AC was on the whole time I was gone. I know this because I still have a piece of plastic covering the new return - AC goes off, plastic falls to the ground. Plastic is still up there and house is 83 degrees. Vent temperature in the living room (close to the handler) was 70-71 degrees. Not good. Just checked up in the attic for some obvious leaks maybe in the new return plenum box or the original. *** I found one of the felxible ducts for the utility room vent fell completely off the air handler - I was blowing cool air directly in the attic. Had a couple spare monster tie wraps and corrected it. When I was up there I also checked the integrity of the other vent ducts. Most of them are leaking around the joint where they insert into the air handler box. The tie wraps are tight arounnd the ducting. It's that the collars themselves are loose so I guess I have some more work to do on those. Ripper - do you know if I can remove the individual ducts and tighten the collars, use duct sealant, then reattach the duct, or will I need to replace the collars? Whoever originally installed this was an asshat, that's for sure.  CMOS |
| The blown off duct was probably the whole problem to begin with. The extra air added by your modification knocked it off, most of your air would then go out the dislodged duct as air follows the path of least resistance. Quality duct tape can be used to seal your flex duct connections. |
|
Damn Dude ! No good deed and all that huh ? If your main plenum is metal you probably have "spin ins" cut into it and the flex ducts slide over them. No dampers either, usually . If by chance your plenum is made of ductboard {{shudder}}.... you will have 1" flanges on the starting collars that lay up against the plenum and you can seal them with foil tape. As an aside, tiny air leaks felt in a 130F attic make it seem like you are airconditioning the great outdoors. Don't get yourself wrapped around the axle. Of course , when I redid MY house last year I sealed every Steenkeeng draft I could find ! When you originally changed the return duct, every bit of negative static you took off the inlet of the blower was transferred to the discharge side in the form of more positive pressure. Taking that plastic off and on the new return put somewhat of a shockwave thru the ductwork and any piece that wasn't secured properly would have a tendency to slide off. Start a vacuum cleaner and put your hand over the hose a couple of times to see what I mean. It's not that exagerated but you get the idea. Plain old duct tape or foil tape works ok for the spin in /collar but you'll have to make sure the metal or backing is clean enough and dry enough to let it stick. If you have an a/c supply around there you can also use Hardcast (Irongrip) on it. Use the latex variety and put it on with a paintbrush..... thick. Sealing the air leak first is the main thing followed closely by making sure the exposed metal doesn't start condensating while the irongrip sets up. I like the belt and suspenders method , or maybe it's just the ARFCOM way, and use both. Seal the air with the tape and then goop on the latex to back it up. Once it's sealed all you have to do is butt the insulation on the flex up against the plenum and tywrap it. When a duct is off in the attic , the air in you house has to be made up from outside any way it can. Dump 200 cfm outside or in the attic and 200 cfm of unconditioned air WILL find a way in. Sure, it's only common sense but you'd be surprised at the people that don't think of it. |
Don't I know it! That's exactly what I was saying to myself when I came home to 83 degrees. As of about an hour ago whan I looked at the thermocouple meter my vent temp was 62. Much more in line wht what I'd like to see. I'll shoot for next weekend to repair those collar leaks. They have quite a bit of slop to them in the METAL air handler. I am not confident that duct tape followed by the sealant pookie will hold - not until those collars can be mechanically secured first. I'll have to take a look at them more closely when I get up there again. When I added the new collar to the intake side (for the new return) it had the metal tabs that I bent down AND a pretty decent size collar on the outside that I secured with about 8 sheetmatal screws, then duct tape, THEN pookie to seal all that (Yes, I like to be thorough...). I understand that some small leaks are okay on those vent ducts but like you Ripper, I gotta seal them all the way or I won't sleep proper! Thanks for all the help. I'll report back when I have learned more. CMOS |
Quite possible! I'll have to start the experiment again - opening the new return filter, which is currently blocked off except for a 6"x12" section. I'll be monitoring vent temps as-as, then slowly open that filter up to see if I can leave it all the way open at 12x24, or if I'll need to keep is choked off to some extent. CMOS |