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troph

HVAC design - new house

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We are considering a ducted mini split system for our new house - not a ductless one. Seems to be less of a hassle, more efficient and takes up less space. I’m not familiar with them but you can have many zones, not just a large one or two zones. Added points for us due to height restrictions so we can have more space for ceilings. Architect is moving us that way. Internet research is surprisingly positive. We are asking our AC guy (partner is a builder) who is really great at balance and attention to detail on traditional systems but I thought I’d ask y’all.

 

Here’s an example:

 

 

https://www.mitsubishicomfort.com/products/indoor-units/multi-position/gallery

 

 

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My parents have one in their ranch house. Been in for a couple years. As far as we can tell at this point, it works just as good as anything else for a hell of a lot cheaper.

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I have Mitsubishi ductless mini-splits in my house. Each room has a cassette. Mine was an existing house without AC and running a conventional ducted system throughout the house would have been an expensive pain. These are great because they are very efficient and each room is independently controlled. We keep the units off in all the rooms we're not using at a given time. Saves a lot of money in utilities. I've seen the exposed ducted versions as well and they look really cool and as mentioned do not take up a lot of ceiling space. 

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The appeal for us is we are under a significant height restriction (can’t exceed 5 feet above the road/lot slopes down dramatically so it can work) and we want two stories and 9-12 foot ceilings, so saving or that is eliminating attic space would be huge. We have plenty of basement style storage options given the excavation we will be doing regardless.

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Other plus is if the AC craps out in the summer you still have multiple rooms that work instead of being married to one or two systems

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Not an owner, but I was concerned that several people said they do not reduce humidity as well as a forced air system.....but every last house in Europe I have seen had them so they cant be that bad at it. 

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13 hours ago, beer said:

Not an owner, but I was concerned that several people said they do not reduce humidity as well as a forced air system.....but every last house in Europe I have seen had them so they cant be that bad at it. 

I don’t know why somebody would say that.

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14 hours ago, beer said:

Not an owner, but I was concerned that several people said they do not reduce humidity as well as a forced air system.....but every last house in Europe I have seen had them so they cant be that bad at it. 

Kinda hard to cool something without removing humididity. That's a big part of cooling.

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On 11/12/2019 at 9:10 AM, Zombie said:

I have Mitsubishi ductless mini-splits in my house. Each room has a cassette. Mine was an existing house without AC and running a conventional ducted system throughout the house would have been an expensive pain. These are great because they are very efficient and each room is independently controlled. We keep the units off in all the rooms we're not using at a given time. Saves a lot of money in utilities. I've seen the exposed ducted versions as well and they look really cool and as mentioned do not take up a lot of ceiling space. 

i have these in my barndominium and they work great so far. each room can be set to a different temp. I have found that the outside unit needs to be sprayed out every 6 to 8 weeks in the spring and summer. 

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3 hours ago, Onboard 2.0 said:

Kinda hard to cool something without removing humididity. That's a big part of cooling.

True and false.  Correct size ac will do this exactly.   Too big of an ac will cool the atmosphere to quickly and not dehumidify nearly as much.   Can cause some real issues

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1 minute ago, Teddyballgame said:

True and false.  Correct size ac will do this exactly.   Too big of an ac will cool the atmosphere to quickly and not dehumidify nearly as much.   Can cause some real issues

Thanks for the clarification.

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On 11/12/2019 at 10:45 AM, Updawg said:

Other plus is if the AC craps out in the summer you still have multiple rooms that work instead of being married to one or two systems

Depends on what aspect of the AC craps out.  If it's the condensing unit, you're stilll fuct like Chuck.  If it's one of the room units, true.

And I think the issue with humidity is that each room unit is cooling a relatively small volume of air and thus does it very quickly, without time for the water vapor to condense out.  From a theoretical standpoint that makes a lot of sense.  From a practical standpoint, variable speed fans in central systems are supposed to reduce humidity better because the slower fan speeds give that time to condense to the air moving past the coil.

Edited by TwiceHorn

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So how big of an issue is the humidity concern in Austin at 95-105 for 5-6 months a year and enjoying 72 inside the house?

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11 hours ago, Teddyballgame said:

True and false.  Correct size ac will do this exactly.   Too big of an ac will cool the atmosphere to quickly and not dehumidify nearly as much.   Can cause some real issues

Correctomundo!!!

I simply like the asthetics of a traditional system.  I think the best is having the AC in the attic (with some decent access to replace) and traditional ductwork. It's quietest, and does the prescribed job properly.  Sizing is a big deal, and one of the things that needs to be contemplated is the air volume of the room vs the capacity of the individual room units.  If you go from 8' ceiling to 12' you are doubling the air volume of the room.  Do you need additional ducts for those rooms to handle the volume?  

I would get the specs and do some detailed math calculations on basic air volume capacities.  You also need to (depending on your level of insulation) make some sort of determination of additional load on the unit produced by larger windows facing the sun or other energy transfer into the larger living areas. I just my traditional unit replaced with a Rheem.  Not the best of the best, but decent and it cools and heats great. I got a great deal, and would have redone the ductwork too... but that's a TBD story. 😞

If you want the tall ceilings then just make sure you have enough cooling and heating power for the volume of  those rooms.  I personally would also have ceiling fans in most every larger room, so that would help with the air flow/circulation.  I did a quick search and here is a pretty good link to how the systems work and are installed. Seems like get the right size and they should work as well as a traditional unit. What a ducted mini split system looks like during construction

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troph- thanks for putting this up. 

After looking at some of the articles the ducted versions, they might be a great option.  I am looking at some older homes to renovate that would need all new ductwork and systems and these might be an interesting option, that honestly I had never looked at before. I do NOT like the ductless look for resale. Simply reminds me of a hotel.  

 

 

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just finished asking our HVAC contractor.  he says they are great.  actually more expensive for a whole house solution but it can be better for control, certainly operational cost and the limited physical size is a big plus.  he *only* works with the Mitsubishi line that I linked above.  He said humidity and load isn't an issue if you design it right and Mitsubishi does their own design.  for forced air systems he's really good, if you tiered HVAC companies based on design and airflow in new builds he'd be tier one, and more expensive than most.  to me at this juncture, air flow is a major factor in the niceness of a home, so it's one area I intend to splurge.  he thinks the ducted mini splits are really great under these circumstances.  more expensive, but lots of savings.  also helps us capture more room for ceilings with the height maximum for the lot at 5 feet over street level.  Seems like this is a go.

Edited by troph

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3 hours ago, horn4life said:

troph- thanks for putting this up. 

After looking at some of the articles the ducted versions, they might be a great option.  I am looking at some older homes to renovate that would need all new ductwork and systems and these might be an interesting option, that honestly I had never looked at before. I do NOT like the ductless look for resale. Simply reminds me of a hotel.  

 

 

for this very reason I said no three times to the architect and then started googling one night and thought, wait a minute.  and yes, HUGE difference between ductless and ducted mini splits.  I have no interest in a ductless system.

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This is intriguing to me for some additions, and renovations I'm currently designing .  Need to look a little closer, may be a nice alternative in some instances.

 Thanks Troph.

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For reference, it appears that on the ducted systems, a compressor unit is located outside and, like a conventional unit, compresses the refrigerant.

The compressed refrigerant is then piped or ducted to individual units where it is expanded and a fan is located to circulate the air over the coil and through the room.  So the ducts are much smaller and don't carry cooled air.

So it takes up less space than conventional air ducts and localizes the cooling function.  

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Som kind of inverter thing outside too. And the units - both inside and out are smaller. Less duct work and smaller units.

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47 minutes ago, TwiceHorn said:

For reference, it appears that on the ducted systems, a compressor unit is located outside and, like a conventional unit, compresses the refrigerant.

The compressed refrigerant is then piped or ducted to individual units where it is expanded and a fan is located to circulate the air over the coil and through the room.  So the ducts are much smaller and don't carry cooled air.

So it takes up less space than conventional air ducts and localizes the cooling function.  

For older homes where space is tight, that's a great option.

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4 minutes ago, troph said:

Som kind of inverter thing outside too. And the units - both inside and out are smaller. Less duct work and smaller units.

Nope, ball bearings, it's all ball bearings now.

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Yeah, can't quite figure out the "inverter."  It's connected to the fact that most of these have variable speed compressors.  And that makes sense and doesn't.  Variable speed would mean you could compress the refrigerant less, using less electric power, when temperatures are mild, than when it's hot.  But that doesn't make a ton of sense because cooling is accomplished by expanding the compressed refrigerant through a nozzle, which gives you a certain temperature.  Usually that nozzle is designed for a certain refrigerant at a certain pressure, so varying the pressure would give odd results.  Maybe the nozzle varies somehow, too, or is somehow designed to avoid the inefficiency that using varying pressure at the nozzle inlet might entail.

But usually AC works by having a set temperature at the evaporator coil so the exit air is more or less a set temperature, which is why you run it longer when a room is hot than when its cool or to achieve a lower temp  a larger volume of air is needed to get the whole room to the desired temperature.  Lowering the temperature of the coil would seem to require a bigger volume of air to accomplish cooling.  But who knows how the optimization works out.

Maybe someone else can elaborate further.

One other thing, these rely on "heat pumps" for heating, which is going to be more expensive than a gas furnace, typically.  Maybe not by much and only while gas is dirt cheap, but probably noticeably so today in Texas.

Edited by TwiceHorn

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My current and future home don’t have gas connections only electric so the heat pump is something I would require anyway. But that was a difference I saw too that for others might make a difference.

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On 11/14/2019 at 4:51 PM, Onboard 2.0 said:

Kinda hard to cool something without removing humididity. That's a big part of cooling.

Tell that to my wine cellar

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9 hours ago, beer said:

Tell that to my wine cellar

Refrigeration removes humidity to assist in cooling. How much is determined by the system employed, and how it's sized.

A wine cellar is all about maintaining a consistent temperature, and humidity level in a fairly confined space in most cases..

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Yeah cooling air inherently will cause water vapor to condense out of the air, making it drier and producing a better cooling effect on people in a space.  

But, if air is moving too fast past a coil, the vapor won't have time to come out of the air, so it stays humid.  Same can happen if the coil isn't cold enough.  And if the coil is too cold and the air too humid or slow, it freezes. If everything's right, the water vapor becomes liquid, stays liquid, and sinks to the bottom of the coil for draining, through your hopefully clean drain tube.

It's a function of the size of the coil and duct, the coil temperature, and the air volume flow rate past it. It's a bit of a delicate balance.

Variable speed fans, in addition to being more electrically efficient, do a better job with this than a constant-speed fan.

I'm not exactly sure why these units might not be as good as conventional ducted AC at removing humidity, but the above is the general idea.  I would guess the size limitations (think how big the coil and duct at a conventional air handler is and compare to one of these) make the balance a bit more difficult.

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