Second opinion(s) on Mini-duct AC system
Well, I’m FINALLY getting around to preparing for installation of a Unico mini-duct AC system for my house. I had the company come out to do a walk-through and get data for the estimate.
I have not reason to doubt anything the rep said, but I’ve read a bit about HVAC guys not always being up-to-speed on the mini-systems, so I thought I’d post the info I have so far and see if anyone wanted to add anything.
Quick background:
The house was build in 1929 and has hot-water floor-board radiator heat. It’s a 1 1/2 story (upper floor is one room). The basement is unfinished at the moment.
The first floor is about 850 square feet, the second about 470, and the basement, eventually will have about 550 finished square feet of space.
He recommended a 2 ton unit, a main 14″ return system (on the first floor) and the appropriate number of exhausts for each room (most with one, 2 in the living room, 3 on the third floor). This all sounds good to me. The only immediate concerns I had are the following:
1) He claims the 3 vents on the second floor are enough to cool that room, provided we keep the door closed to the upstairs. My concern is that there is no return in that room. Will those three vents be enough to pump cool air into the room without a return? (I’m no expert, but it would seem to me that those three vents would have to work quite hard to pump air into an already ‘full’ room. Since the path of least resistance would be for the air to go into the main floor, as it has a return vent, little cool air would make it to the second floor).
2) He suggested that we don’t vent the basement at all, as it is relatively cool, and instewad just use 2 mini-returns to pull out the humid air. He DID say that we would still want to run a dehumidifier in the summer. Is that an ideal situation? Would it be better to run a few vents in the basment and let the AC to most of the dehumidifcation?
Replies
2 Quick follow-up questions
1) has anyone had experience with installing both Space-Pak and Unico systems? This particular contractor has experience with both and felt they were pretty evenly matched in terms of performance and ease of installation.
2) Has anyone installed these in the floor? That would be the easiest installation for us. Any reason to add a few feet to the duct length to get the vent up on the wall or celing?
Yes, you can vent through the floor, but furniture/traffic patterns can be tough to work around.
Often it's tough to come out of a wall because of the tight turn radius, which wil lead to a noisy outlet. There are toe kick boxes that have been used in walls, but they are pricey. You can come out of a soffit as well, a little more room there to make the turn. The only negative for ceilings is heating from high ceilings, but that's not a concern of yours.
One other "rule-of-thumb" that you can consider...usually, you can run about 5-6 outlets per ton of cooling. You have 2 tons, so you should have 10-12 outlets.
Ballpark. Fewer outlets could mean short-cycling of your system which may give insufficient dehumidification. Fewer outlets could also mean too much air being pushed through too few outlets. that means noise.
Mongo:
Thanks for all of the feedback. I appreciate it.
A few notes:
re: the unico plenum...I will not be using that, per yours (and others) past comments regarding the fiberglass. I was originally hoping to use spiral ductwork, but the return plenum would have to be 14" in diameter, so we'll probably go with rectangular ducting...as much as I dislike that look comparet to the cylindrical ductwork.
I should clarify the main return duct will be on the main floor, split in half so that it takes from both the hallway and the main living room. The upstairs (second floor) will have no return...one concern of mine, and the basement WILL have two mini-returns branching off of the main return. (and, now that I think about it, I may go ahead and look into running one mini-return up to the second floor to even it out a bit).
At this point, I think we have a total of 10 vents, so we have room for a handful in the basement.
I'm not too concerned about noise. I can live with noise. I can't live with extreme heat+humidity. That said, there's nothing wrong with an extra vent as it'll be easy to do the first time.
My main concern is that I get the maximum dehumidification out of the unit...which, I believe has to do mostly with a properly sized condensor and proper air-flow rate. I really don't want to be running another dehumidifier in the basement, so I'll be checking with this place to make sure we have the vents figured out properly for the basement.
A couple of ideas...
First, with the high-velocity systems, it's not a good idea to stretch the limits of cooling. Why? Because of noise.
Your designer should have donw a pretty detailed calc to get the cooling loads for each room. the amount of required cooling for a room translates into so many outlets for that room. With these systems, a "full outlet" is an unrestricted outlet with a certain length, say 15' long mini-duct, feeding that outlet. For each foot that the outlet gets lengthened, the efficiency of that outlet goes down. If the outlet is shorter, it can get noisier from turbulent airflow at the outlet.
If, for example, a room needs two "full ducts," that means that two 15' ducts will cool the room. The problem is that may be a littel noisy. While i cost more in materials, it may be better to intall three duct, each counting as two-thirds of a full outlet. You can drop the oulet from a "full" to a less efficient "two-thirds of an outlet" eaither by lengthening the run of mini-duct or by installing a baffle plate where the mini-duct departs the plenum.
What this all gets down to is that if the installer claims three outlets "should" be able to cool the room "as long as the doors remain closed"...you may want to consider running a fourth outlet to that room, and choke any or all down until you get the proper amount of airflow to cool the room. Those three, if running hard, may just be too noisy...or they may not cool the room adequately. The beauty of these systems is that there should be no operator restrictions (closed doors, etc) to get adequate cooling in your home.
Hi-v systems are mainly used for remodels or for with homes with RFH. Often times a whole floor can be cooled with only one main air return for that floor. This differs from conventional AC. However, if an area can have a return run without much problem, why not? If an area has a return, why not a supply as well? If your basement is currently unfinished, and you run a return with no supply, a question...do you have a furnace or other combustion sources in your basement that could possibly be back-drafted? It may not happen, but it's something to consider.
I like to see a return on each floor. I like to see each floor get it's own supply plenum. Even if you only have a few outlets on the second floor, sometimes it's best to run a takeoff from the main plenum up to the second floor, then run your mini-ducts off that takeoff. Shorter duct runs, and as long as access isn't a problem ot run that takeoff, it will usually ba a wash or even a savings in material.
Hi-v is great at dehumidifying. Unless there are strange things going on in your basement, the hi-v should be able to dehumidify your finished rooms down there. Still, your designer knows you house better than I do.
Space-Pak vs Unico? Overall, they're both pretty good. Unico has better marketing so they're better known. They both have good hardware, though I'd give the edge to Unico. They both have good ducting, though I'd give the edge to Space-Pak. Easy snap-together assembly.
I highly encourage you to not use the pressboard FG duct for your main plenum. It's be shedding shards into yout house for who knows how long. Use spiral of regular metal duct ad insulate it well.
Another point...SEAL ALL OF THE DUCTING. All seams. All unions. With Al foil tape. THEN insulate the ductwork, and SEAL the foil on the insulation with Al foil tape.
Off to bed...
Edit: Just wanted to add a final note. If your desinger/installer has a good reputation and can expain his design to you and have it make sense...it's likely to be a good layout. He knows you house better than any of us do. Every house, especially remodels, has a few quirks. Then again, some installers do too.<g> If you're truly concerned, call Space-Pak/Unico ans talk to a tech rep, just to see if this guy's ideas (3 outlets with door always closed, and basement return but no supply, etc) mesh with Big Brother's ideas.
Edited 4/28/2002 3:12:49 AM ET by Mongo