HVAC "Advice"

Jul 14, 2020
South of Metro Atlanta
I generally stay out of the home HVAC threads because, well, most people want to know what the cheapest solution is.

I saw in the latest thread someone asking about mini-splits. There's several versions/product lines of "mini-splits".

First off, I have been a fan of the newer technology of "mini-splits" for over 10 years. In a former life I worked as an HVAC designer at a few engineering firms (MEP) doing commercial and industrial HVAC, Piping and Plumbing design for over 20 years. I am proficient in cheap residential crap (5 tons and below), packaged DX, chilled water, VAV/PIU, water source heat pumps, with a minor in industrial cooling water, steam, process, plumbing. I have also minored in mechanical contracting doing piping, structural and some HVAC. I can fake my way around a construction site until someone smarter comes along. ;)

Anyway, I skipped on the chance to put my money where my mouth (and brain) was in 2015 when we bought a house and it needed both HVAC systems replaced. I hem-hawed at the idea of installing a Daikin 5-ton Multi-port condensing unit (VRV-S Commercial System) with two indoor 'convertible' fan coil units, connecting to the existing ductwork. More or less a typical system change-out except the equipment was going to be state of the art like no other house had.

These "mini-splits" that are multi-port condensing heat pump units can supply refrigerant to 2, 3, maybe 4 indoor units and do so with NASA-grade precision, making your home/office/etc. more comfortable than you could ever imagine while reducing your power bill to make you think you've installed solar panels. It's also quiet as a cotton ball and will last longer than your typical residential junk. They are also capable of providing heat to one area and cooling to another, but the costs climb even further than what a system is that can only heat or cool at one time.

There are residential lines of multi-port systems, but most manufacturers stop those at 3 tons. You can also opt for single-port systems, where you have one condensing unit outside for one indoor unit. Most manufacturers worth looking at make indoor units that look and act just like the indoor unit you have now.

Another advantage is there is usually no need for strip heat with these systems.

Recently I had a Daikin VRV-S system installed in my mother's house. After hours of thought and pondering, it was the only solution that made sense based on the requirements and existing infrastructure. She is 77 years old and suffers from some ailment where she is very cold all the time. She needs her house at 75 degrees or so (we've actually come down from 77-78 when she had a gas-fired furnace). She has a ~2500 sf house, all electric, with a 200-amp service. There was an existing 3-ton split system heat pump there, 15 years old (original to the house) with a zone damper system to throw some air to the upstairs when called for. They don't use the upstairs, can't climb stairs (it's a teenager suite). So I was faced with no gas, no additional electrical capacity for increased strip heat, I wanted to get rid of the zone damper crap and I needed to make darn sure the new system could supply hot air and heat the house to her demands....through the existing ductwork. Oh yeah, she had been using 1500w heaters to keep warm, December and January power bills were $415.

So I spec'd out the 4-ton Daikin VRV-S Condensing unit, a 3-ton indoor unit and 1.5-ton indoor unit. The 3-ton unit was to connect to the ductwork serving downstairs main house and the 1.5-ton unit was to connect to the ductwork serving the upstairs. This eliminated the zone damper system. Both units can run independently but use the same condensing unit. The condensing unit also only runs as much as is needed for the demand, meaning it ramps up and down all day long. You might think this is counter-intuitive to energy savings and comfort, but after watching it and observing it for 2 weeks, it is beyond the understanding of many who have only experienced the typical bang-on, slam-off residential and commercial HVAC equipment.

What this system does is 'chase' the desired setpoint all day, meaning if it is set to 72 and in heating mode, your indoor temp condition is going to show 71 but it's going to be very comfortable and you will never know it's "1 degree from setpoint" because the system is probably running at 15% or so (both the indoor fan and condensing unit) and delivering a very small amount of conditioning all the time to maintain a perfect temperature in the house. There is no "heat it up, shut off and allow the temp to fall, repeat the cycle". Comfort is unreal.

No, this was not "cheap", it wasn't even in the same ballpark compared to pricing a like-for-like system change out with typical low-efficiency residential equipment. But I am 110% satisfied and kick myself nearly everyday for not doing this at my house 5-1/2 years ago. I had this priced by 4 contractors - all commercial contractors and 3 of them were in the same ballpark price-wise. The most expensive was a friend of mine who I knew was going to be high because he runs a tight ship but I had confidence in the others or I wouldn't have called them in the first place. Before I get into the pricing- here's the scope:

1. Demolish the existing condensing unit and indoor air handling unit
2. Install new 6-space subpanel in attic, use existing 60a circuit to power this and power two new indoor air handling units with this
3. Intercept existing 40a circuit in attic, use junction box to splice in new cable to run to new condensing unit location
4. Provide Daikin 4-ton VRV-S condensing unit, refrigerant piping, y-branch splitters, dryers and other refrigerant piping accessories
5. Provide Daikin 3-ton and 1.5-ton indoor convertible air handling units, new unit stands, return and supply plenums, new additional ductwork needed to connect to existing, new power wiring, new Daikin wifi thermostats, thermostat wiring, line-hide or metal covering over new refrigerant piping up exterior wall to attic.

Daikin also wanted to include factory start-up ($$$) because they wanted to make sure everything was right.

Total cost was about $17,000. Just for reference, I had one contractor beg me to allow him to offer an alternative. That was a 16 or 18 SEER Bryant 3-ton split system heat pump for down stairs and a Daikin 1.5 ton residential line mini-split for the upstairs (a high-wall indoor unit, not ducted), this was $12,800. This was almost a slap in the face, as I have no regard for Bryant equipment and there was NO part-load capacity with the 3-ton system. Basically typical residential stuff and certainly not worth anywhere close to $4000 savings in my mind.

Anyway, a long post, but I wanted to share the experience. I know most really don't want to spend anything on HVAC, the basic theory is it shouldn't cost much, but you really get what you pay for.

I'll follow up with the February power bill and we will see what the energy savings is. I expect it to be more than $200/mo.
Thanks for posting that. I have been looking at possible options for future HVAC and have been intrigued with the mini splits. It is something that I may wind up doing in the future.
The more complex an HVAC system is the more things to fail. Do you want to be without AC in the summer. In your area, 2 complete independent systems is the way to go. That way there is always a cool, or warm space to live in.

Thanks for providing such a great info regarding your installation.

Let me asked you a question, can a mini split coil be install on top of a gas furnace and a separate cassette be on other locations.

I am trying to figure out how to heat/ cool an attic that is partially finish w/ out extending the current duct work. We tend to use a space heater and window AC when summer if needed.
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Yeah. My neighbor just installed an instantaneous water heating system, all electric, which required an entrance panel upgrade. She also installed a water conditioning system, since someone told her that hard water (which we don't have) will hurt the system.
She's 80 years old.
This was all done to "save electricity".
Her payoff will be about 38 years.
Not familiar with mini-splits living in the great white north...do these automatically entail a wall unit for internal cooling in various rooms or do they integrate with central HVAC systems?
This is awesome !! One day I aspire to build a house and condition it with mini-splits. I've read mixed reviews on the residential units (Primarily Mitsubushi which I think is assembled near Flowery Branch).

I wish some production builder in the area would get on board with them and an ERV.
Not familiar with mini-splits living in the great white north...do these automatically entail a wall unit for internal cooling in various rooms or do they integrate with central HVAC systems?
You can do either ducted or ductless (ie, wall mounted). Typically residential mini-splits aren't retrofitted into existing duct work because they're not designed to push air through the high resistant runs which is common in a traditionally designed system. Of course all homes are different.

The large ducts used on most minisplit systems are normally not well insulated, the Duct losses are significant, ducts in a hot attic notorious. There is also the issues of leaks at joints, and deteriorating flex ducts. This reduces the overall efficiency of a mini split system and is not reflected in the SEER rating.. Modern heat pumps have significant heat gain even at 0F. This makes ductless systems more efficient than the difference in the SEER might indicate.
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I run my heatpump, a standard Trane split system (it's a 15 SEER/9.7HSPF unit), without aux heat (except when it's in defrost).

As far as ducts go...none are in the attic or any other unconditioned space (that's just good design), and all joints are sealed with duct mastic (required by current code). There is also minimal usage of flex duct.
The large ducts used on most minisplit systems are normally not well insulated, the Duct losses are significant, ducts in a hot attic notorious. There is also the issues of leaks at joints, and deteriorating flex ducts. This reduces the overall efficiency of a mini split system and is not reflected in the SEER rating.. Modern heat pumps have significant heat gain even at 0F. This makes ductless systems more efficient than the difference in the SEER might indicate.

You're referring to a more common split system (central air, etc). A mini split's indoor coil is located very close/in the space it's conditioning. A common split unit moves its volume of air through ducts.

Minisplits are often referred to as ductless minisplits.
1900 sq ft modified A frame house with loft 3 bed,2 bath with crawl space. Very rural area. Pulled out 4 ton package 16 seer heatpump and all the duct. Installed 2 -2 ton gibson minis seer 21 with 5 wall units one ducted 3 head units per condenser 5 years ago.
Two systems because if one breaks parts are a few days away for shipping and still have ac or heat. The biggest pain in butt is cleaning wall units and the condensers. Condensers have split coil. Been happy with the zoning and the electric bill. Crawlspace insulated all water lines and installed crawlspace dehumidifier for controlling humidity level.
They put those mini split systems in shotgun houses in the old part of San Antonio down here. They work well as these houses never had duct work added to them, and many residents rely on space heaters, and window AC units only.

No where near 17000 dollars, maybe drop a zero and that is the final cost.

Glad to hear you took the time and your mother will be comfortable.

My heat bill is about 30 bucks a month during "winter" for a 1300 sq foot new house, and we have a 80 percent Lennox gas furnace. We typically keep it at 72 degrees.
I am starting to understand why 3 stars and below motel / hotel uses window units now. They were annoyingly loud but they are cheap to swap out and has low down time, small impact area, easy to design, and easy to service. You can literally have a bad unit and customer get a replacement within an hour, and the old unit sent out for "warranty" without an HVAC tech getting involved ($$$$$).

One thing I would want though is I wish they can be variable speed and not loud. They are really loud compare to what car AC and mini split are, and IMO that's unnecessary.
I am starting to understand why 3 stars and below motel / hotel uses window units now.

They aren't window units. They are technically PTACs, "packaged terminal air conditioners". They are all the same dimensions and fit in the same wall opening. Window units aren't nearly that standardized. A quick Google search shows that they are available in variable speed units, but they cost more.
The more complex an HVAC system is the more things to fail. Do you want to be without AC in the summer. In your area, 2 complete independent systems is the way to go. That way there is always a cool, or warm space to live in.

Yeah, I agree, with that said, the OP is clearly an expert on the subject and enjoys it. Plus he will save money.
For the typical consumer, they would be taken advantage of with something so complicated.

Me, I have 2 units in our 3000 sq ft home. Love it, each floor independent and now that the kids are off on their own, the second floor unit can be set for Max energy savings. We run our units every day of the year/meaning we dont open our windows, its either heat or AC, last night we had the AC on and this morning the heat! *L*
First floor gas heat, second floor heat pump. We use the main floor gas heat to heat the entire house, since heat rises and no one sleeps on the second floor. Never in a month does our energy bill surpass $200.
Units are getting on in years 2 15 year old contractor grade R22 Heil units, but they were the newer units at the time that can take the new gas, been dead on reliable except for a fan in one unit and some scum of the earth large local AC company that wanted to replace my units years ago when all it needed was some R22. Bought my own and filled it up been fine for years now. I dont think it was ever properly filled.

Which brings me to this, I am amazed at the repair industry and people who fall for it. People in my community been getting their units replaced for years now because anytime one of them may need a repair they get sold a new one.

Its such as scam so as far as the OP I applaud his expertise but for the typical consumer with limited financial resources I think they would lose out. Just based on a couple local companies here with big, nice new shiny trucks, fancy ad campaigns, I cringe whenever I see someone getting a new unit put in by them. They are not out to make repairs, they are out to sell, with big commissions to the techs/repair guys.
I get what the OP is saying about how the commercial stuff is better than residential and all, but when it breaks, aren't you going to be limited with respect to contractors available to fix it?
I'll address some posts-

1. Regarding complexity and break-downs/repairs with this system- I know anything mechanical and surely nowadays with everything computer-controlled, anything can and will break down and need repair. I am extremely confident in the Daikin products (I mean real "legacy" Daikin, not the Goodman line being re-vamped slowly) in that their stuff is extremely high-quality and built to perform as well as last as you would expect. Daikin is not equal to most other "mini-splits" or VRV/VRF type systems other than Mitsubishi. Most manufacturers have brought something to market in the mini-split line and it is showing that they didn't want to fund R&D or be real proud of their product line. It's sad because all the cheaper made stuff will probably give the newer technology a bad name over time. I only considered Daikin and Mitsubishi. I also have a pretty good relationship with a large number of people at Daikin, I feel that they would help me in a time of need should something major arise with a repair.

2. No, you can't install a mini-split/VRV coil over a gas furnace and cassettes/high-wall/fan coils elsewhere. Just no.

3. I wasn't concerned with payback, although my mother will probably live long enough in that house to see the payback of the additional cost over a sub-par, typical residential equipment change out with the energy savings she will see with this system. Payback with a VRV/VRF type system (not a "mini-split") might look like 15 years but when you factor in your comfort and other performance into the equation, payback is a lost thought and you realize how poor performing typical single and two-speed equipment really is.

4. The mini-split and VRV/VRF lines of equipment from most manufacturers have indoor units that look, feel and operate just like your current indoor air handling unit that is in a closet, attic or crawl space with ductwork attached. The installers can remove your existing air handling units and put in new air handling units matched with the condensing unit from the mini-split manufacturer. It's going to be a LOT quieter, tho. Strip heat is not normally needed, even in colder climates. While the condensing units do operate a defrost cycle, many manufacturers have methods to combat the introduction of cold air into the space. Daikin shuts down the evaporator (indoor) fan during defrost. The Daikin VRV-S system I had installed will produce 100°F discharge air at 0°F ambient. You're not going to see that with your Bryant, Carrier, Trane, Goodman or Lennox heat pump.

5. Nobody is getting any HVAC system that is a permanent installation (not a PTAC/Window unit) for $1700. The equipment cost of a simple 2-ton split system is $2,000. You won't get a contractor to show up for less than $2,000. The equipment cost for a 1.5 ton mini-split with a ductless high-wall unit from any reputable manufacturer is $1500+.

6. Yes, repair/etc., with this particular commercial-based system is going to be limited to certain contractors in our area. Out of the 4 contractors that I had show up (I actually sent out invitations/calls to 7, 3 did not respond), only ONE had installed this type of system previously (my friend). All perform commercial as well as residential work ( the same crews with any contractor do not cross-pollenate. There's commercial crews and residential crews. A commercial crew performed my installation.) I am not worried about repair. The contractor that I used, I have developed a relationship with and they are now more eager to try to sell these systems after this installation and actually seeing the performance themselves. With that said, I don't want just any HVAC contractor showing up at one of my properties to perform work because I'm in a tight spot. I want highly experienced and trained people.

As I stated in my OP and stressed, not many people will have much desire to spend the money necessary for a system like this, but anyone who does and can realize all the benefits will certainly say it's worth every penny when done. I'm not trying to sell anything, I'm not getting a kickback nor do I care what you do at your property. I'm simply throwing out some advice and experience to anyone wanting to listen. I have learned over the years that not many people really understand HVAC and that's an understatement. Even the people that try to sound like they do don't understand it all and don't think about many factors.

One of the biggest benefits of a system like this is the unlimited part-load capacity and the real variable speed of the compressor, condensing fan and indoor fan as well as the metering of the refrigerant, which provides minute part-load capacity. I can't stress enough how great this is. Your weather is never the same day to day and hour by hour at times, neither is the heat gain/loss of your structure.

And yes, I eat this stuff up. I'll never consider myself a nerd, but I get all nerded up in HVAC stuff. I love it.