Mini-Split or Conventional for HVAC?

[JFoy]

My wife and I are builiding a 1000sqft apartment to move into and then sell our older current home. Specs are:

Metal building 24x60
4in concrete slab with 1in foam board underneath and outside the footers
Climate zone 3
1in closed cell spray foam for the entire building
The apartment takes up 1050sft of the 1440sqft building
Stud walls adjacent to metal framing posts are inset 1in with foam board between wood and metal
Pink batt will be used as the main insulation
Building has 12ft sidewalls and ceiling plane is 9ft

Stud framing is almost done and the discussion about HVAC selection is getting more important.

A 2 zone system is desired as if we decide to build a full house, we can rent out approx. 600sqft and keep the other 400sqft for storage (red line in picture shows separation) plus the shop area.

The local HVAC contractors all want to use a conventional system. The issue I have with a conventional system is all the penetrations through the ceiling plane and duct work running all over the place in the attic area.

Mini-splits solve the issues with penetrations and attic space but the bedroom is only 175sqft. The local HVAC contractors say mini-splits are not as reliable and harder to service.


Any thoughts?

Thanks
JFoy

 

[shui5612]

I tried looking up zone 3 but couldn't find anything.. how hot or cold is the weather? While insulation plays the biggest role on keeping your house cool or warm. Either system will do the job perfectly fine.

The main benefit on a minisplit system is the fact that you can have multiple heads on one compressor. Aka creating many zones (including the garage) while a conventional HVAC system can't be attached to the garage (atleast in canada, due to potential co2 getting into the system and spreading it all over the house).

Depending on the weather but the apartment would need to have a minisplit in the bedroom and one in the living room for the best possible comfort, and then add one head to the study room with the ability to have one in the garage too if needed.

Problem with mini splits is they will create areas with varrying temperatures. I.e. if you only have one head in the living room, the bedroom may be too hot for comfort and may result in the need to keep the door open in order to cool it down at night.
Another issue is that minisplits dont circulate and ventilate/filter air as much as a central AC which helps maintain the house at a more comfortable level, better control over humidity, etc.

Also if you get the conventional AC in, you could add other systems to it and take advantage of the ducts, such as a furnace, humidifier, air exchanger. Etc.

Whats the price difference for the set ups?

I personally have a heatpump with furnace going through ducts at both of my places and the air quality is amazing compared to having just a mini split that will heat or cool the air only.

 

[RayfromTX]

I would use a 1 ton of AC for that space. It could be done with less but you will need it in the winter. You may need a small electric space heater for the coldest days but don't size the unit for the coldest day. A standard DX unit like they want to install will be at least 2 tons. They are oversizing your equipment. Make the space air tight. Allow for fresh air ventilation when needed. Don't depend on leakage to provide your fresh air. This should be a long discussion but I am giving you the briefest of overviews. My house is 4,000 sq feet. I am air conditioning it with 2.5 tons and yesterday we saw 103 degrees and I was getting 65% runtimes. If you oversize the unit it will short cycle all year long and especially in the low demand periods of the year. When that happens it greatly reduces the ability of the system to remove latent load(humidity) from the space. Small living spaces tend to have somewhat higher internal moisture loads per square foot. You may need supplemental dehumidification during the shoulder seasons(spring and fall). Even the smartest HVAC contractors are stuck in an obsolete mentality and suffer from a syndrome where they have never been sued for putting too much AC in a building and are afraid of not putting in enough. Distribution is very important in a poorly built building with cheap windows but less so in a well built building with good windows and good glass. I am temporarily air conditioning a 9,000 sq ft construction site built to exceptional standards with a 3 ton system that came out of my personal home. It was installed 27 years ago and sat unused for the last 13 years until we needed it for temp AC. The building ranges from comfortable to cold and the unit is set to run continuously. There are 12-15 men working inside. Every afternoon temps reach in excess of 100f. Oversizing of AC equipment is universal in the industry. I am leaving many pages unsaid. This is a big topic with many subtopics.

 

[JFoy]

Our location is central NC which means winters down to 15dF and summers up to 95dF with humidity upwards to 80%. So, humidity control is a must.

As for fresh air exchange, that definitely is a concern given the entire building is sealed with the 1in closed cell spray foam. Panasonic does have a fresh air add-on to some of their mini-split models though.

 

[shui5612]

Good points RayfromTX,
My setup is a 4.5 ton two stage central AC/heat pump that's cooling down around 6000 sqft ( on two zones) . The contractors wanted to install a 6 ton unit. We've had a heatwave that lasted a week and a half of temperatures hitting 105-115f in the day with close to 80-100% humidity and my AC kept up with no problem, with a 60% run time roughly.

How cold does it get where you're from OP? Seeing the insulation that you're doing I think a central system (heat pump) with a backup furnace could save you interior space as it would cut the need to install space heaters such as convectors or plinthes etc..

 

[kenmac]

inverter mini split system all day ! You can't beat the efficiency

 

[IXLR8]

I have a 7 year old Mitsubishi 12Kbtu mini-split, runs great, very efficient. Single unit, heating/cooling about 900 sq ft, which is fairly open allowing the single unit to do the job. Costs me about $.50/day for the cooling season and about $1.00/day for the heating season. The only issue I have had with it.. I forget to clean the filter as often as I should and output drops off. If you install a quality system, you will have few problems with it. I would put in a mini-split system, single outside unit driving 3 inside units, unless you want to 'condition' the garage as well. Mini-split systems are no more complicated/difficult to troubleshoot/repair than a high efficiency whole house heat pump system. Sounds like you need to find a different HVAC contractor.

 

[ultrarunner]

I would use a 1 ton of AC for that space. It could be done with less but you will need it in the winter. You may need a small electric space heater for the coldest days but don't size the unit for the coldest day. A standard DX unit like they want to install will be at least 2 tons. They are oversizing your equipment. Make the space air tight. Allow for fresh air ventilation when needed. Don't depend on leakage to provide your fresh air. This should be a long discussion but I am giving you the briefest of overviews. My house is 4,000 sq feet. I am air conditioning it with 2.5 tons and yesterday we saw 103 degrees and I was getting 65% runtimes. If you oversize the unit it will short cycle all year long and especially in the low demand periods of the year. When that happens it greatly reduces the ability of the system to remove latent load(humidity) from the space. Small living spaces tend to have somewhat higher internal moisture loads per square foot. You may need supplemental dehumidification during the shoulder seasons(spring and fall). Even the smartest HVAC contractors are stuck in an obsolete mentality and suffer from a syndrome where they have never been sued for putting too much AC in a building and are afraid of not putting in enough. Distribution is very important in a poorly built building with cheap windows but less so in a well built building with good windows and good glass. I am temporarily air conditioning a 9,000 sq ft construction site built to exceptional standards with a 3 ton system that came out of my personal home. It was installed 27 years ago and sat unused for the last 13 years until we needed it for temp AC. The building ranges from comfortable to cold and the unit is set to run continuously. There are 12-15 men working inside. Every afternoon temps reach in excess of 100f. Oversizing of AC equipment is universal in the industry. I am leaving many pages unsaid. This is a big topic with many subtopics.

Ran into the same issues when I installed my brother's central HVAC...

We did a lot of work on the place... new insulation, premium windows/doors, etc.

The heating contractor sized a much larger unit which would have requited a larger electrical service.

I matched the HVAC to the available 30 amp circuit and for the last 17 years it has been perfection... even on the 100+ days... home was always comfortable... 30 amp was for clothes dryer that I swapped out for a gas unit.

He said exactly what you said about contractors not getting sued going bigger.

My take away is there is more to sizing than tons of cooling... efficiency, how tight the home is, orientation and even the huge shade tree to the South that block the brunt of the summer exposure...

 

[erkme73]

We have a multi-level 1800sqft home that we built 3 years ago. We opted for mini-split all the way. Two 2-ton compressors, each driving 2 20k btu heads. Daikin (?) with a 20+ seer rating. Not only do they give you the flexibility of cooling only the areas that are occupied but we were absolutely astonished at the comfort that comes from the precision control.

The first night we slept with in the new house, was probably the best nights sleep we've ever had during the summer. We were used to the cycling of a conventional AC. Set the tempt to 70°, it turns on at 71° FULL BLAST until thermostat gets to 69°, and then it shuts off. This results in restless sleep - both from the noise of the AC cycling - and also from the too hot (uncover), too cold (cover up) with each cycle. With the mini split, you set the temp to 70° and the temp in the room stays rock solid 70°. It never shuts off - but rather slows the fan down to maintain the temp. I've watched the compressor fan outside spin so slow you can count the RPM.

The net result is the most peaceful sleep ever. I don't think either one of us moved. We agreed that if we ever move or build again, that's the first thing we'll get installed.

With a family of four - with three always home - the highest bill during the summer has been $160.

Unlike conventional A/C heat pumps which become ineffective when outside temps drop below 40°, these head units will put out 140° air when it's 0° outside - using heat exchangers (no heat strips).

We could not be happier with the decision. Other than the upfront cost - which was about twice the cost of a conventional 4-ton unit - we've had zero complaints.

A couple of other things... Since we built the house knowing we were going with mini-splits, the contractor was able to install the refrigerant lines, condensation line, power, and control lines inside the walls. This meant we have no unsightly chases running outside the brick walls. It makes for a very clean installation if you can get them to install before the sheetrock and insulation go in.

I have installed a home automation system as well, and am able to monitor circuit and whole house kW consumption. Each head unit when run separately pulls about 1300 watts. That's with the supporting compressor running. If all four are blasting away, I'm still pulling about 1/2 of what a 4-ton central AC would pull.

 

[Tractor Seabee]

If you have allergies a mini split may not be for you as they have no capability for an electronic air cleaner or bringing in outside air. For this reason I like the fan running all the time even on reduced speed when the burner or compressor is off. Continuous running fan also keeps temperatures more even in the spaces. Not sure if minis have that feature. The more rooms you want to cool start changing the economics by requiring more pieces of equipment. Mini splits do not work around corners or through doors. I have 9 rooms to cool. Too many condensing units required. I installed a new gas 98% gas furnace last year with the coil in it to add a heat pump unit in the future when I get the cash money available. Luckily here in Puget Sound we do not have many hot days. The contention on sizing is 100% right. In my younger day I was an AC&R Tech. Over-sizing was the root of a lot of our service complaints. Today with variable speed compressors a lot of that has gone away but still you do not want the compressor motor hunting for the right speed all the time. Causes the same problems as short cycling of a one speed compressor. A variable speed fan can also cause those problems and icing up of the indoor coil.

Ron

 

[JFoy]

Thanks to everyone for the advice thus far.

One thought I've considered if I go with mini-splits is to buy two dual head systems. That way, if one compressor goes out, I would have some degree of redundancy.

The comments about oversizing a conventional system are true here as well - the contractors all want to do that. They say that with the variable speed fans, an oversized unit will be fine as the fans run at low RPMs. Plus, they say that if I want that extra umph of cold air during a hot summer, the unit can do it. I suppose there is an argument there.

Still, I can't help but think of all the ceiling penetrations for the supply ducts and resulting air leakage points. And I might be overthinking it too much given the building is sealed with the 1in spray foam.

Point for the conventional unit is freshening the air better. While some mini-splits do have a fresh air exchange, we'd have to use the ceiling fans all the time to stir the air whereas a conventional unit would do a better job by default. My wife and I smoke plus have two cats so I have to wonder if a mini-split can clean the air as well. I know some of the mini-splits can be equipped with HEPA filters.

The praises of the mini-split efficiency and low operating cost wins hands down. I have to wonder about servicing the units though - is it as easy to service a mini-split as a conventional unit.

 

[SirReal63]

Thanks to everyone for the advice thus far.

One thought I've considered if I go with mini-splits is to buy two dual head systems. That way, if one compressor goes out, I would have some degree of redundancy.

The comments about oversizing a conventional system are true here as well - the contractors all want to do that. They say that with the variable speed fans, an oversized unit will be fine as the fans run at low RPMs. Plus, they say that if I want that extra umph of cold air during a hot summer, the unit can do it. I suppose there is an argument there.

Still, I can't help but think of all the ceiling penetrations for the supply ducts and resulting air leakage points. And I might be overthinking it too much given the building is sealed with the 1in spray foam.

Point for the conventional unit is freshening the air better. While some mini-splits do have a fresh air exchange, we'd have to use the ceiling fans all the time to stir the air whereas a conventional unit would do a better job by default. My wife and I smoke plus have two cats so I have to wonder if a mini-split can clean the air as well. I know some of the mini-splits can be equipped with HEPA filters.

The praises of the mini-split efficiency and low operating cost wins hands down. I have to wonder about servicing the units though - is it as easy to service a mini-split as a conventional unit.

We have two three ton Mitsu's with 8 heads to cover 3500 square feet, it has worked beautifully. It is a little more tonnage than we need but with a mini split there is no penalty for oversizing.

As far as working on them, to me it seems easier, there are no pieces of equipment in closets, all of the mechanicals are out in the open. I grew up working with my Dad who was a HVAC Tech. We have had our installer out twice since we had them installed, once because a mis-manufactured fitting slowly leaked the Puron out, it took him maybe half an hour to replace the fitting, pull the vacuum and re-charge, most of his time was with the vacuum. The second time one of the condensate level switches failed and a slow draining condensate line backed up. It took another hour to replace, half of that was yakking with me. No charge for any of it. It took two days to install them, one day to run the line sets and one day to run the head units and charge them.

Our 8 units just quietly sit here keeping this house the temp we set the thermostats for. We do use ceiling fans, one upstairs and three downstairs, to keep the air moving, I am not sure if they are really needed but I like having them. If we ever build another house, it will have the mini splits in them.

 

[LouNY]

I have two 36,000 btu Mitsubishi splits systems in my house with seven heads, it does an excellent job of cooling and a good job of heating in the early part of the season. I am glad that I had this system install.
That said it cost all most twice what a conventional systems were quoted at, the conventionals would have been using flex duct in a hot attic as no one wanted to quote running ductwork in the basement and crawl space.
The mini- splits have been much more efficient to run compared to the window units previously in use.
In your case, new construction, a compact house with future plans to split the house into two uses and being smokers with cats;
I would lean toward two small conventional systems with VFD's on the blowers mainly to get some air changes in the house and filtration of the air, good well insulated ductwork will distribute the cooling and heating in a compact dwelling with several rooms much more evenly. One unit if you don't split it in the future with powered dampeners and a few thermostats to zone the home garage areas.

 

[jaxs]

Sound's like you understand pros and cons of both systems. Since you are concurned about penitrations and the flex duct monster, I'll give you something to ask contractor about. Put an up-flow furnace/air handler in alcove behind 1/2 bath basin. Fur down hall ceiling and cut vents for each room. All outlets should have adjustable dampers in covers. There should be a considerable cost savings compared to attic ducts. Effeciency is better since duct isn't exposed to attic air temperature.

 

[Rustyiron]

I'm in a similar building, a big shop with a small living space. (about 900s/f) We added a mini split last year (outside unit with 2 "heads") and it's been one of the most satisfying money spent in a long while! Right now it's only in the mid 70's but very sticky. The de-humidfy setting is just wonderful.
Up here in the frozen north we use it for the "shoulder seasons" that's spring & fall for heating after/before we've shut down/fired up the wood boiler that heats our radiant system in both the shop & house.

 

[ovrszd]

Mini splits have their place. My wife struggles with the visual....

 

[wagne223]

For me, the best feature on mini splits is the ease for self installation.

For the most part, AC installers and sales think they can do something no one else can and they overcharge for it.

I ordered an 18k pioneer heat pump online for $800 shipped. I did the install then called a local AC company to come out to do the purge, vacuum and verify the refrigerant. They charged me $250.

Had they supplied the unit and installed, price would have been $3k

I plan to install 2 more mini splits , one in the bedroom and one in the living room.
For 2 reasons. 1. Backup in case the main central unit goes out. 2. I can run the minis on a generator when a hurricane comes through.

 

[cqaigy2]

For me, the best feature on mini splits is the ease for self installation.

For the most part, AC installers and sales think they can do something no one else can and they overcharge for it.

I ordered an 18k pioneer heat pump online for $800 shipped. I did the install then called a local AC company to come out to do the purge, vacuum and verify the refrigerant. They charged me $250.

Had they supplied the unit and installed, price would have been $3k

I plan to install 2 more mini splits , one in the bedroom and one in the living room.
For 2 reasons. 1. Backup in case the main central unit goes out. 2. I can run the minis on a generator when a hurricane comes through.

I can see where having ac might be pretty nice when it gets hot and humid and you've lost power. :thumbsup:

 

[Sigarms]

I'm a proponent of mini splits.

From what the OP has described, I'd go mini split myself.

Two single zone systems, one in the living room facing towards the kitchen, one in the bedroom. Perhaps a air exchanger in the bathroom for more ventilation from the kitchen/living area, or if that isn't possible, a 2 zone mutli zone for living area and kitchen with a dedicated horizontal ducted unit for for the bathroom and a dedicated system for the bedroom.

That said, mini splits are not the greatest thing since sliced bread. Right now I'm replacing an outdoor that is only two years old because the manufacturer does not have the compressor in stock and they're waiting for it to come off a slow boat from overseas. There a MANY issues to compare between both systems.

What's the difference between a power control board and a power filter board? For service, mini splits do require some expertise as they are nothing like a conventional system what guys are used to working on.

As much as people may claim how great multi zone systems are here, I have numerous horror stories (just like a conventional split unitary system).

First mini split I sold was Sanyo back in or around 94, before they changed names LOL

 

[ultrarunner]

For me, the best feature on mini splits is the ease for self installation.

For the most part, AC installers and sales think they can do something no one else can and they overcharge for it.

I ordered an 18k pioneer heat pump online for $800 shipped. I did the install then called a local AC company to come out to do the purge, vacuum and verify the refrigerant. They charged me $250.

Had they supplied the unit and installed, price would have been $3k

I plan to install 2 more mini splits , one in the bedroom and one in the living room.
For 2 reasons. 1. Backup in case the main central unit goes out. 2. I can run the minis on a generator when a hurricane comes through.

I have found HVAC work very expensive and more so when demand is up...

Thankfully... the only thing I have ever needed was a purge, vacuum and verify... quite reasonable.

Mom had qot quotes for a new furnace... the old one had a cracked heat exchanger and dated from the 1950's

All parts for the new furnace, sheet metal, duct sealing, permits came to $1250...

The inspector commented on how it was one of the best installs he had seen... always makes you feel good unless they tell everyone that.

I also downsized the replacement and no issues...