Central Air Q's for the HVAC Peeps--UPDATED W/Quotes

Hello All,
Planning to have central air installed this spring. We have a split-level home, 1500 sq. ft up, 1000 down. Since the basement is half underground, and usually maxes out at 75 on the hottest days, can I get by with just installing vents upstairs (via the attic)? We have all new windows, doors, siding, etc., so I'm thinking a 2.5 ton with vents in the ceilings of every upstairs room will do--and the cool air will drop to the basement. The 2.5 ton is a bit bigger, but since it will be in the attic, I think it makes sense.

Thoughts?

S

Do you already have ducts in place? If not, you might look into a ductless mini-split system. Less hassle to install, and less wasted energy to air/cooling losses from the ducts themselves.

If you are going with ducts, you need to base the size of the unit on the heat calculation done for the structure. No way to just guess and get it right. Most companies tend to install units much bigger than you need, which costs more out of pocket, and leads to frequent cycling, which wastes energy for the life of the unit.

Dave,
We do not have any ducts, since we have electric heat, but the attic is wide o pen to run ducts to all the top floor rooms. The issue I was thinking with the mini split is that it would be on only one end of the house (our wood stove is on one end and, though it does a good job of heating the back rooms, there is a few degrees difference). To put it on both would not work, since we don't have a shared wall on one end (the hall goes right into a bedroom).
As for the calculation, I did one using a calculator I found and it said between 2 and 2.5. Since the handler would be in a hot attic, I figured the higher of the two. Can I assume the HVAC people should be able to do that/that it would be part of a good service's routine?

S

My two cents:
Both living areas need to be ducted and returned - balance and humidity would be an issue
Don't put your handler in the attic - maintenance would be a nightmare
Upsizing generally is a poor idea - typically will result in more spikes in temperature and more humidity (both related to less run time of the unit)

From what I've seen, what I'm thinking of doing is fairly common- where else would one put the handler? As for both zones being ducted, that would be much more difficult and costly and result in my losing a lot of closet space. One person recommended just putting a return in the basement.

S

I retrofitted a central air (and heat pump) into a 1960 house. I agree with above that you need to run some air to the lower story for dehumidification and ventiliation, period. In my case, I ran two 6" round insulated ducts along one side wall of a bedroom closet. I walled them in, losing about 8" of 'rod length' in that closet. No bigs. We cool and heat the downstairs through those. You also likely have a plumbing stack you can drop at least on good sized line down, if it ends in the right place.

As for the attic thing, modern ducting code is R-8 insulation on rigid or flex ducting. My guy went with a rigid metal fabbed duct for the 'trunks' with a heavier insulation, with R-8 flex branches. No problem with it except the guy undersized and did not insulate or airseal the central return--but we got that fixed. And my roofers tromping through my attic stomped on everything, breaking flex and rigid seams and caving in the rigid ducts in a couple places...got that fixed too. Watch ANY non-HVAC workmen in your attic.

Make sure the air handler is located in an accessible location, you have a hatch big enough to get it in an out, and have them hang it from the roof, rather than sit it on the 'floor' (for noise). Also, remember that all systems will eventually leak condensate, no matter what they tell you about interlocks and safety systems. Mine waas mounted over a load bearing wall, so the water ended up 15' from the source, ugh. Install a water alarm yourself.

Used to install HVAC(not design). All good posts. Properly designed and balanced system is critical. I agree with the returns in the lower levels. Stay away from flex duct as much as possible, no more than 12 inches long. (Horrendous air flow restrictions) Very good point on hanging from roof, or have it on properly sized isolators. Pay attention if you hire installers. I have followed crews and "fix" the hack jobs. I have seen "fittings" made of cardboard that was duct taped together and painted silver. Don't forget, you can hide an amazing amount of screw ups when the ductwork is insulated. If they show up with duct tape anywheres on the job, be aware. Some guys use it to seal joints(connections). Horrible idea. Only use approved sealant. kind of went off on a rant, sorry. does a mini split sound better?

Am I right about needing multiple mini-splits? How far away can they be and still be effective, and will the air move around corners? I get the hanging idea, and there's a pull-down that should fit the handler through.

About the downstairs, would it be sufficient to have just one central return and no vents?

S

I also installed ac in my house. I would only recommend rigid trunk line followed by insulated R8 flex (< 5') to each register. I would also run at least one return and one supply to the lower level. They should be positioned far from each other. When installing in closets, use interior walls and install rectangular 3x10 ducts that fit inside the stud cavity. You really only need two walls to line up to get a register in the lower ceiling. This won't take up any living space. You will however need to do some sheet rock repair.

One contractor said I could run a return to the basement through a closet almost dead center of the house, but getting supplies would be harder. What about the mini split- can that be on one end of the house? It's 40 feet or so end to end.

S

Good info. Are you doing this work yourself? Remember RA (return air) duct work should be about 20% larger than SA (supply air). Also SA ducts to the exterior wall and RA on interior walls (if using more than one)

I agree on keeping flex to minimum. Code allows more than I'd recommend because of the turbulence (noise and additional flow restriction problems)

You can use soffits and wall extensions as areas to place the duct work or as pipe chases.

schlot, I was thinking about it, but from what I've heard from contractors is that the labor portion isn't all that much--specifically, that the work I wouldn't want to do (working in the attic) would only add a few hundred dollars.

I've attached a diagram of the top and bottom floors of my home--maybe it will help with suggestions. The return I've mentioned I could put in would be over the stairs (going from inside a closet to the basement).

From what I've found, a mini-split is only really good for up to 30', which is definitely not enough. The question becomes, then, how much I try to get to the basement--remember, the hottest it ever really gets down there is high 70's, but it does get humid.

S

Ductless mini-splits are not as common yet as the traditional ductwork and central system, but they are gaining popularity. Bit not many people have much experience with them. Never heard the limit to 30 feet of run. Where does that come from?

A friend here just retro'd his 1910 brick 4 floor home (3 living areas plus basement) with ductless splits. He has no complaints. There's one air handler on the top floor, two on each of the middle floors, and none in the basement.

AS for your basement, I can't imagine it would need to be actively cooled in your climate. Plenty of cool dry air will pour down from the upper floors. As for moisture, a devoted dehumidifier with drain would be much preferable IMO to keep it dry, as opposed to running the central system for that purpose, or putting in a separate air handler there, if you go ductless. Is moisture a big problem? If so, you might look into reducing the entrance of moisture into the basement, rather than trying to remove it later.

Dave,
I read about the distance issue, and thought it made sense since (if I'm not mistaken), there's only one source of air in the mini split system? I do have a dehumidifier in the basement, but it would be nice not to have to run it for sound and cost purposes. In the winter, it's off.

S

CONSIDER A HEAT PUMP

yes, I did mean to put that in caps. We had our AC replaced a few years ago. of all the companies that came to quote us only one mentioned "oh yeah, well for an extra $500 (i think) we can make it a heat pump" that was is... nothing else.

smartest $500 I ever spent! The thing is wonderful on the shoulder season. I buy my wood and if it is over 45F I let the heat pump run. The existing oil heat in the house is left there as a backup (if there is a large demand or if it goes below 20F, thermostat does all that).

Even if you use the wood stove mostly, I suspect that $500 will have a very fast return on investment! Especially when you sell your house.

Rick

^^^^^ Yeah, what mav said!

Does the heat pump thing only count for the mini split? I'm still trying to figure out if the mini split is an option.

S

my heat pump/ac is a 3 (or 3.5 ton i forget) ton outside, primary (not split) system. I would suspect that you can make any AC a heat pump depending on the manufacturer. not sure, the manufactures info should be able to identify.

Definitely go heat pump, weather traditional split system or ductless split, go heat pump. If you go heat pump, you will need to add return ducts at floor level, otherwise, you will shortcycle the airflow.

They make ductless splits that can have more than one indoor coil run off of one condensing unit (outdoor unit). IMHO, ductless splits are great if you have an open floor plan. If you dont, and you only use 1 or 2 indoor coils, you will be uncomfortable.

I like the idea of having the air handler in the attic and running drops into each room. I see air handlers in attics and behind knee walls all the time and they work fine. Biggest thing is INSULATE and AIRSEAL the ductwork. Minimize the amount of flex to no more than a five ft run off main trunk. Im assuming you are having a professional install this? Make sure he at least does the above few things. If he says he wants to flex duct your whole attic, politely ask him to leave.

2.5 Tons will be fine for your space. Your sq footage is closer to 2 ton than it is 2.5, but honestly, I like to oversize just a pinch. It gives you flexibility to add a run or two to an addition and not overwork your equipment. And your not too badly oversized to the point you will be shortcycling. If you do wind up shortcycling, you can add in a delay on make timer to keep it from straining the compressor. Certain mfg's will even give an option for adding a "anti short-cyling" or "compressor protection" kit.

You dont really need to "condition" your basement, unless it is finished. If it is unfinished, just run a dehumidfier and be done. With your slight oversize, you will always have the option of running a few drops down from the attic in a stud cavity if you ever feel the need for a/c down there.

Hope this helps.

mrfjsf--that was perfect. One piece of clarification: the basement is finished, and I've run a dehumidifier down there these last few summers. It usually needs to be emptied every other day. I think it will be easy to run a return, but maybe not a supply. Will a return help with the humidity, or should I really try to get a supply or two down there. If the latter, which rooms would I choose to put it in or do I just do the rooms I'd be able to get to through closets, etc.?

Thanks again!

S

Sorry, I just found your home layout sketch on pg 1 and realized your basement is finished. Doh!

My suggestion for the basement is; if you are happy with the comfort level in there in the winter and summer AS-IS, then leave it alone. If you only put a return in the basement, you will not actually be exchanging air in the basement for the benefit of comfort. Remember, in order to have an air exchange and maintain comfort in a given space, you must be able to pull out the same amount of air you put in. That one lonely return in the basement will draw air down your steps and straight to the return. If you want to try and take just the "staleness" out of the air, then I suggest making a supply drop at one end of the basement and a return drop at the opposite end. I suggest the supply in the downstairs living room and the return in the downstairs kitchen. It's not going to do much for the areas that are divided by walls, but it will at least allow for some airflow down there.

Once again, if you are happy with it as is, just leave it be.

As far as drawing humidity out of the basement, 1 supply and 1 return is bare minimum to accomplish any kind of dehumidification. Another thing to keep in mind (I know this is a lot), the more humidity you subject your hvac system to, the more system capacity you are using. So, if you plan to FULLY condition your basement, you will likely need to upsize to a 3 ton system. 1 or 2 supplies and a return wont hurt you at 2.5 ton, but full ductwork downstairs will. You can help offset this by running your dehumidifier on a low setting.

Hope this answers your question.

12" of flex?Might as well not use any,you`ll have no problems if the flex runs are kept to a minimum,around 6' or so.
Air handler in the attic is fine,just make sure it`s left accessible.

Think,
With regards to a return in the basement, realize the possibility of creating a negitive pressure situation. Do you have carbon producing appliances in the basement?
If you do, account for any negitive pressure that may be created by a return only, in the space. You don't want to wake up dead!
Try to equilize the pressure with a supply and return of the same size or smaller.

OK, to summarize:

1. Air handler should be suspended in attic
2. Solid ducts for the trunk, flexible for 5' or less to each of the rooms
3. All ductwork insulated at R8 or better
4. If doing anything in basement, at least BOTH a return and supply--far enough from each other
5. 2-2.5 ton is about right--to be calculated exactly by HVAC tech
6. Make sure installers don't screw things up in the attic/check for leaks, etc.

One last thing: does shape/size/placement of vents make a whole lot of difference? I've read vents are usually placed over windows, but one installer said it wouldn't really matter (I'm not sure if it's because he was looking for symmetry with my in-ceiling speakers or because it would have been an easier install, or because he's right).

S

PS My in-ceiling speakers (in every room) are round--should I get round vents?

Post up what the install guys come back to you with on a quote and scope of work. When they come to install the system, pm me and ill give you my phone number if you have any questions or if something looks fishy while they are doing the install. Im tired of seeing people get ripped off or BS'ed when in comes to hvac installs and service work because they know that the avg homeowner does know jack about the workings of an hvac system. Dont want to see that happen to a fellow member.

Cheers!