We will go over two common designs for ADUs and go over the different options that you have for HVAC. We will start with two preapproved ADU plans from San Diego County. Since most ADUs are around 800 square feet, your ADU may be very similar to these designs. We will look at the general layout, options, and our rationale for the different options. You should be able to use our process for determining the right HVAC system for your ADU.
For most ADUs, we will be using ductless mini splits. We don’t recommend a traditional central air conditioner for an ADU because ductwork is expensive to run and it takes up more space in the attic. Most ADUs would like the floor to ceiling height to be as tall as possible to give the space the illusion of being much bigger than 800 square feet. The more attic space you take up, the more money you need to pay for materials and space that you cannot use. Most people building ADUs do not consider the extra materials and labor needed to build attic spaces for their HVAC system which is considerably more expensive than the HVAC system itself.
In the first plan, we see a two bedroom, two bath ADU with a living room and kitchen. Technically we only need to have a heating source in the living room so installing just one unit in the living room is the least expensive option. Based on the design and layout here are your options.
- One 18k BTU mini split in the living room. Either highwall unit or ceiling cassette.
- A three zone high wall mini split system (9+9+12)
- A three zone mini split system (9+9+12). For the bedrooms we can use high wall units and for the living room we can use a ceiling cassette.
The first option is the least expensive. This option should cost between 3k and 6k depending on the system that you use. You will have one larger unit in the living room and since the space is quite small, you can open up the doors in the bedrooms and still get cool air as well as heat inside the bedrooms. The one downside of this approach is that it limits privacy and is not ideal if the ADU will be used by more than one person.
Option two is the most common option. Here we would use a three zone mini split system. Typically we would use either Daikin, Fujitsu, or Mitsubishi as they make some of the most reliable products on the market. In 2021, this cost should cost between 10k and 13k total.
The third option is a combination of high wall and ceiling cassette units. Most people will only have guests in their living room and want to make that space as beautiful as possible. As a result, many of our customers choose to have a ceiling cassette in the living room and high wall units in each of the bedrooms. This option will range between 11k and 15k dollars depending upon the brand that you choose.
For the second floor plan, you will notice that although the layout is different than the first one, you still have the same three options for your mini splits. The layout does not really affect your choices much since we are mostly concerned about the size of each room and the zones for each mini split system.
For your ADU project, here are a few things to consider ahead of time.
- What are your setbacks? This determines where you can place your condenser. Every city has its own setback requirements so be sure to check them before installing your air conditioner.
- The phase of when to bring in the HVAC contractor. For new construction projects, you should involve the HVAC contractor right when framing and roofing is completed. Your major trades: mechanical, electrical, and plumbing should do their work inside the walls and ceiling before you close up the drywall. This phase is called rough-in. You should involve the HVAC contractor early since there may be lead time in acquiring the equipment and materials needed for the project. Your HVAC contractor will work in two phases. In the rough end phase, they will install the indoor units, run the copper linesets, and run the electrical and water lines. After you have completed the exterior stucco, your HVAC contractor will install the outdoor condenser unit and start up the system.
It is important to communicate with your MEP trades so that there is a smooth transition between the work and so that your project can move as fast as possible. As you can see, most two bedroom ADU projects generally have three options for mini splits.
- One 18k BTU mini split in the living room. Either highwall unit or ceiling cassette.($3000 to $6000)
- A three zone high wall mini split system (9+9+12) ($11,000 to $15,000)
- A three zone mini split system (9+9+12). For the bedrooms we can use high wall units and for the living room we can use a ceiling cassette. ($12,000 to $16,000)
As an added bonus, we will also give you the most common mini split configuration for a garage conversion.
Most garage conversions are around 400 sqft. In these cases, we would use a 12k BTU high wall mini split. The cost of these systems range from $2500 to $4000 including installation. Overall a garage conversion typically leads to the creation of a large studio. Since the space is open, you would only need one mini split unit for the entire space. If you have a panel nearby you can use a 220v system. Otherwise, a 115v system from Fujitsu or Mitsubishi would work very well for your space.