Thinking about installing central air

Hey everyone, I live in a brick ranch house 1200 sq ft above and a 900 sq feet finished basement. All we have for cooling is a 12,000 btu window unit in the dining room and a 8,000 in the one bedroom, this does seemingly nothing to keep my house cool. The main floor is mostly open, the living room and dining room kitchen and hallway is all open, no doors to seal it off. This area I would say is roughly 7-800 sq feet, Their is two bedrooms and a bathroom that have doors.

Now the real reason i'm posting is because the basement has a family room and a bedroom where we sleep because its cooler than upstairs. Lately it has been hotter downstairs, close to 78 degrees when it is usually only in the lows 70s, this is a noticeable difference. I want to cool the upstairs, which would be simple, a split unit with the ductwork in my attic, but what i'm wondering is if I would need to run some sort of ductwork to my basement. Since I don't have adequate cooling upstairs, I don't know if the cool air would sink to the basement and cool that as well. If I were to cool the basement I would have to use a mini-split system for their is no way to run ductwork into my basement.

Has anyone had any experience with this same situation? I would rather just run a 2 ton system and run ductwork in my attic.

 

20 000 BTUs total (that's just over 1.5 tons) should be plenty for a smaller house.

Your attic may be poorly ventilated or insulated.

I'm opposed to installing hvac equipment in an attic. Service access aside, attic systems cost a lot to operate (55F air moving through poorly insulated ductwork in a hot attic picks up heat. Leakage is also an issue) and can cause water damage.

Have you considered a mini-split system with 2-3 heads?

Basements require minimal cooling.

 

Yes, I was actually thinking about a mini split system, heres the thing with them though. I can buy a 36k btu unit that ha 3 zones, but that would cost over 4 grand just for the unit alone. I was wondering if I could buy multiple single zone mini splits. My plan was to use one 12k btu system in my living room, then one in the back bedroom, and one in the kitchen. I also thought about maybe running one to the family room in the basement, one should take care of any cooling needs down there. To buy 4 separate 12k btu systems, it would cost me roughly $3400, a lot cheaper than over 4k for a 3 zone system. Also with having separate systems, I could buy them and install them as I have enough money, instead of paying a large chunk of money right up front. Thoughts?

 

I would put a couple vents in the basement - I used to leave one open in the summer but went to two when I got a new dehumidifier down there. Off the top of my head, that's two open downstairs and nine open upstairs.

I have forced air heat as well and in the summer I close most (but not all) of the vents upstairs.

 

12k btus is one ton.

3 tons is way too much for a house of that size unless the walls are all glass or there's absolutely no insulation anywhere. Bigger is not better.

I can't imagine a decent 1200 sq ft house needing more than 2 tons of cooling even in a hot climate.

A load calculation should be done to determine what needs to be installed.

Given that the first floor is open with no doors, one head should do the job there. Cold air diffused in the right location spreads evenly through open floor plans.

With respect to the window units not working well, keep in mind that the cold air they put out tends to get pulled into the return, which causes short cycling and inefficient operation. Mini-split fan coils have the supply air vent at the bottom, so the cold air falls to the floor.

The extra install labour will offset any savings.

 

Okay, with that being said, which was very helpful, I'm still looking at at least 3 units. I would need one to cool the living room, dining room kitchen and hallway, and then one in the back bedroom to cool that space and the other bedroom. Then I would need one for my basement which is 900 sq ft finished, my total sq footage finished in my house is about 2100 sq ft.

I also plan on installing them myself, which will save big time, so buying 3 mini spit systems will only put me in the hole about 2500, plus any other materials i would need. I still see this being cheaper than one multi zone.

 

Just wanted to give you guys a better idea of the layout of my house. Both bedrooms have doors on them, as well as the bathroom, the rest of the doorways do not have doors.

 

If the mini-splits you purchase don't have quick-connect fittings & pre-charged lines, a tech will have to make the connections and do the startup. (includes brazing lines, pulling vacuum, checking charge, etc.)

If you insist on three heads, purchase variable capacity equipment (system varies output to match cooling load instead of turning on and off) to avoid short cycling.

Google spacepak - might be a good alternative (professional install required)

 

With good cooling on the main floor, you shouldn't need any cooling in the basement.

Let gravity do the work.

 

Well I just can't see a 12k btu system being enough for my first floor, I have a 12k and an 8k window unit and it barley takes the edge off of the heat. So I really couldn't see one 12k mini split doing much better.

The mini splits i'm looking at don't need professional installation. On top of that i'm a handy man and could do just about everything, and my wife's dad works for trane air-conditioning so he could help me if i needed.

 

By the way that spacepak would work perfectly. With the ductwork only being 2" I could run it under the floor of my house, and also add a couple of vents to the basement.

 

Try installing baffles (cardboard or anything) to keep the outlet air from your window units from immediately being sucked into the inlet. Ideally you could construct a duct that drew air from near the floor to the inlet and then a second duct directing the outlet air towards the ceiling. You might be amazed at the increased performance.

 

18-24k BTUs for the main floor with nothing in the basement should be more than enough.

The mini-splits for the bedrooms will short cycle like crazy if they aren't variable capacity. (the smallest head I believe you can get is 9000 BTUs - probably over least twice as much as you need for a bedroom)

 

As previously stated, window units are notorious for cycling off before the room cools down because they short circuit cooled air back into the return. Mini-splits don't have that problem, since the air outlet/vent isn't above the return.

 

I'm going to try the cardboard baffle for my units for now. With only one mini split I'm worried that it would be really cool in the area I have it mounted and not cool enough in the bedrooms.

 

I have a 5,000 BTU unit in my bedroom and it used to short cycle to the point of being almost worthless in keeping the room cool. I inserted a piece of bent sheet metal that directs the outgoing air towards the ceiling and it made all the difference in the world. I want to make a cardboard "duct" that will cause it to take the intake from the floor and the heat register and see how much it will go towards cooling the rest of the house with the furnace fan running.

I have a 10,000 BTU unit I had in a window of another room and I used a piece of 1/2 inch thick foam insulation board the width of the unit and extending out about 14-16 inches to keep the discharge separated from the intake and with both these units running (and the furnace fan) I was able to keep the western half of my house quite comfortable.

 

I made cardboard ducts for both my units that force the air into a large fan I have set up, I did this early yesterday and by the evening their was still no noticeable difference. The living room hit 80 like it normally does, and the back bedroom was about 76 like normal.

I did the calculation and for my home I would need at least 30k btu to cool it properly, so that is at least 2 split systems.