Advice On Multiple Ductless System
Hi folks. I'm new here. I registered because I see some very good advice offered... and I need advice.
I live south of San Antonio, TX and summers are hot and humid here. We've had summers that reach 105+ degrees every day for many months at a time. I can no longer take the heat due to health issues so cooling is, by far, my major concern. I want heat pumps for simplicity of installation and ease of operation. It seldom gets below 30 degrees here in the dead of winter, BTW.
I'm having built a small two-story 1100 ft2 home. It's well insulated with a metal roof. The windows all have awnings to keep direct sun out. I want to install several mini-split heat pump systems. I DON'T WANT ANY DUCT WORK. I really don't want any multi-zone systems BUT I'm open to suggestions. I like flexibility and overkill... if one system fails I'll still have the others working... I can shut off all but one or two systems if I choose.
AREAS TO COOL: <proposed BTU size>
living room = 3500 ft3: <18K-24K BTU?><or two smaller units?>
(above has kitchen included but I don't cook much)
first floor bath = 800 ft3: <9K BTU?>
first floor bedroom = 1000 ft3: <9K BTU?>
(above might have washer/dryer also)
second floor open space = 1500 ft3
(above also needs to partially cool a 500 ft3 closet via passive vents plus a 500 ft3 half bath -no shower or tub): <12K BTU?>
second floor bedroom = 1800 ft3: <12K BTU?>
I realize these systems are probably too large for each space but my experience here is that overkill is FAR preferable to underkill. I think these will work if I buy inverter style units. Also, I know you're aware, no ductless systems smaller than 9K Btu are available. I might even opt for 24K BTU for the living room. Another option is to install two units on opposite sides of the living room... either 2x 9K BTU or 2x 12K BTU or 1x 9K BTU plus 1x 12k BTU and swap/combine as needed. This offers more control in the largest living space... I can set one unit 2-3 degrees warmer to only start if the first unit can't quite do the job.
I'm very open to suggestions because I know nothing about HVAC systems. I only know what my experiences tell me which is buy a system that's 150-200 percent larger than most installers recommend. I need a very cool environment.
ETA: I could probably get by with just a high-volume exhaust fan in the downstairs full bath (no dedicated heat pump)... dunno.
The information you provide is not enough to give a good answer. You need to have a heat load calculation performed. It will show what the proper amount of cooling will be. Tell them how cold you want it to be and they can factor that in. There's no point in buying too much air conditioning.
A skilled contractor can help you determine what your needs are and offer ways to accomplish them.
Mini splits are a good way to do what you describe because they will adjust their capacity to the load even if you do oversize them.
I'm intentionally buying over-sized systems because I expect them to lose efficiency and cooling ability over the next ten years... and I need it very cool so I can function well enough. I might have a heat load calculation done but, frankly, I don't trust many technicians (of any trade) in this area. Finding a knowledgeable/honest one (of any trade) can be a crap-shoot.
With mini-split systems there isn't a lot of choice for the two smallest rooms except either a complete system for each or add a vent between the two and use an exhaust fan in the bathroom to pull air in from the adjacent room. A 9K BTU system is plenty for that but a separate system (might??) better mitigate moisture issues without worrying about a balancing act between the two rooms. Perhaps the best solution is to vent adjacent room A/C into the full bathroom pulling in cool air with a high-volume exhaust fan then add a small dehumidifier?? Maybe separate systems AND a dehumidifier?? Maybe I'm making this too complicated??
I like the idea of using two smaller units in the largest downstairs room because it's more efficient to operate only one if that's all that's needed. It also allows for a spare unit should one fail. I currently have a new 12K BTU window unit in that largest room and it's been keeping it "almost" cool enough with outside temps at 95 degrees with 90 percent humidity so I think I'll need 24K BTU at 105 degrees with 90+ percent humidity. That's 2x 12K BTU units. This is strictly "observation"... not proper testing.
The two areas upstairs will require slightly larger units per ft3 because, even though it has a metal roof and is fairly well insulated, it has no attic space... it's a barn-style roof. This stated, I'll be adding a PV system that (I hope) the rafters can support which will, in effect, completely shade the entire sunny side of the roof.
Bottom line is, I think I'm pretty close regarding sizing... BUT I've been wrong about many things so, if I'm being cheap and stupid, please let me know. It won't take much convincing. I'm hard-headed but I do listen.
BTW, I use A/C even in the dead of winter here because it's common to reach 80 degrees during the day even in January so temps inside can reach 90 degrees.
I don't think it ever gets 95F and 90% humidity anywhere in the world, 105F and 90% RH defintiely doesn't exist without lots of steam leaking from an industrial process.. except inside industrial facilities... and that would be considered cool is some locations where I work. It's probably more like 50-60%RH and that's still very humid.
Are you a pilot? They love redundant systems.
Have you used our AOP Contractor Locator Map? You're likely to find a knowledgeable pro there.
Mitsubishi offers some indoor units with a small duct option. These are ideal for a bedroom and bathroom.
Don't forget that the inverter systems will adjust their capacity to the load. One indoor unit should not have a problem distributing air in a room in a house that size.
In dealing with customers, I try to determine what they want and match that with what they need. If they want to spend more money beyond what they need, I make sure they understand, then I offer them a contract.
This might be a good application for using foam insulation on the bottom of the roof. It makes an enormous difference in the load.
Originally Posted by dunno_HVAC
At 105 degrees 30-60 percent humididty is far more common but it really does reach 90 percent occasionally. I'm opting for a worst-case-scenario system.
Originally Posted by motoguy128
No, I'm not a pilot. I've just always loved redundancy and flexibility.
Originally Posted by kdean1
Thanks for the contractor map... I'll have a look.
I didn't know about those Mitsubishi units... I'll look for them.
I know the variable speed scroll units are very flexible and self-adjusting... it's why I like them. I know I could install a single 24K BTU unit in the largest downstairs room but having two 12K BTU units adds even more flexibility and efficiency (small efficiency increase not important) and it has the benefit of redundancy... sort of a backup system for not much more cost.
Regarding what I want vs. what I need... yeah, I'm often out-of-balance in that regard. The things is, I'm never disappointed if I pay upfront for a bit of overkill but... I'm always disappointed when I have to pay for upgrades.
I checked the AOP Contractor Map. There are three contractors and the nearest one is approximately 90 miles (round trip) from me... might be a bit pricey??
I was wrong about 105F/90 percent humidity... more like 90/85 or 105/65 at worst. My memory isn't what it used to be.
True but too late for that. What I have is 9" fiberglass between the rafters plus a layer of good half inch foam board (aluminum layer on upper side) between the sheetrock and rafters. The felt over the decking is thick and double-layered.
Originally Posted by kdean1
Have I exposed my ignorance too thoroughly?
Redundant and oversized? There are enough safeguards in the design calcs to handle this. Oversizing will not only increase cost but also may not remove humidity as well as a right sized or even a littl undersized that will have longer run times. Remeber this is sized for the WORST CASE design conditions that are seen only hours per season
I'm intentionally buying over-sized systems because I expect them to lose efficiency and cooling ability over the next ten years..
Reconsider your assumptions
You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!
All I know is that the system sizes recommended by most technicians "here" prove to be too small for everyone I've known including me. Their systems run full-blast all the time during the peak heat of the day and they don't have the cooling they want. When we significantly step up the size we can finally get the cooling we need. This seems to happen with all the systems I've seen installed even during new construction. I don't know if this is because calculations aren't accurate for this area or some other reason... or maybe heat load calculations were improperly performed... or maybe it's just been luck-of-the-draw. At any rate, it was 100 degrees with 25 percent humidity here yesterday and the 12K BTU window unit in the 3500 ft3 room couldn't keep up while running non-stop. At 105 degrees with 40 percent humidity it wouldn't have a prayer.
One thing's for certain. I don't want any ducts nor any through-wall nor window units. That leaves only mini-split systems.
Since I live alone I can leave the door open between the two small downstairs rooms (small bedroom/office and full bath approximately 900 ft3 each) and use only one system if that works better. Those two systems will definitely be inverter type to help mitigate over-sizing issues. To be honest, I think the two 12K BTU units I'm thinking of installing in the two upstairs rooms (approximately 2000 ft3 each) will be too small because the of the barn-style roof with no attic and good (not great) insulation. If those do prove to be too small then I'll need to decide between adding additional systems or up-sizing them.
I'm not trying to be hard-headed... I'm just expressing the experiences I've seen and personally had "here" in south Texas.