In this final installment of our 6-article deep dive into HVAC Design for High Performance Homes where we put it all together in this Case Study and illustrate all the concepts we’ve discussed in the previous 5 articles, which are: Manual J, Manual S, Manual D/T, and fresh air ventilation.
The new home in this case study is in a hot and humid southern climate and is constructed with slab-on-grade floor, ICF block exterior walls and conventional wood framed roof trusses.
The attic is a conditioned attic utilizing spray foam insulation under the roof decking. All ducts are located within the conditioned attic. Windows are double-pane, common U-Factor and SHGC specs for new construction in a southern climate zone.
Step 1 Load Calculations (Manual J)
The first step in the process is to model the house in specialized HVAC software that will perform the Manual J load calculations. In this case, Wrightsoft, which is one of the most widely used in the residential sector and what we use at American Building Performance Inc.
During this step, the entire house plan gets put into the software through a drawing interface. This models all of the separate spaces, exterior walls, windows, doors, and ceilings in the house. We will also account for other loads from occupants (people), appliances, air infiltration, and ventilation.
After the house is fully modeled and all of these items are accounted for, we are able to see both the heating and cooling loads (in Btu/h) for the entire house as well as for each space/room. Here is how those numbers come out.
We can see that the total cooling load (sensible plus latent) comes out to 18,365 Btu/h at design conditions (99°F outdoors, 75°F at 50% RH indoors). Nominally, this is a tad over 1.5-tons. On the heating side, we have a total heating load of 20,985 Btu/h at design conditions (32°F outdoor, 70°F indoors). The second image shows the breakdown of how much each component contributes to those loads (walls, windows, ceilings, infiltration, etc.)
The keen-eyed may notice that there is a portion of the loads from duct losses/gains, even though we have a conditioned attic in this example. That is because there are two ways to model the conditioned attic in the software. We can either enter it in as a normal conditioned space and level, or we can utilize the “encapsulated attic” option for the ductwork gain/loss factor. The result is the same as far as loads, since we have to account for that extra space.
Step 2 Equipment Sizing/Selection (Manual S)
Now that we have our room-by-room heating and cooling loads, we can move on to sizing a piece of equipment. This example is a simple single-zone setup, so there will be one heat pump that will serve the entire house (approximately 1,600 square feet of conditioned area).
Given the location and the loads, a heat pump is a no-brainer. Heat pumps are becoming more and more common throughout the U.S. and even Canada, as the push to all electric grows and heat-pumps have become more and more capable.
For this house, we have decided to look at a Mitsubishi P-Series multi-position air handler system (PUZ/PVA). These units are commonly referred to as “mini-splits”, but this system has a full-size ducted air handler just like a typical unitary split-system.
To determine the size of the system needed, we must consult the manufacturer’s performance data for the system we’re looking at. This information typically comes in tables that show actual capacities at various indoor and outdoor conditions. Mitsubishi also has their own software that will calculate these numbers for their systems.
Using Mitsubishi’s software, we can see that the 2-ton standard PUZ/PVA system will provide 20,842 Btu/h of total cooling capacity, with 2,906 Btu/h of that being latent capacity (moisture removal), at our design conditions (99°F outdoors, 75°F at 50% RH indoors).
This comes out to around 113% total capacity when compared to our cooling load. The sensible and latent capacity percentages also come out within the Manual S limits. So, this system passes the Manual S checks for this house.
The Mitsubishi DSB software also shows us that the heating capacity of this unit (the standard variant, not the Hyper-Heat) is 17,860 Btu/h which is only 85% of our heating load. So, the supplemental amount will be made up with electric resistance heat when needed.
During this step we also determine what the design airflow rate will be (how many CFM of air the unit will move). This information is also found in the manufacturer’s performance data. In this case, it is 875 CFM.
Remember, the 2-Ton designation is a nominal size. Always check the actual performance data as each system combination will have different total, sensible and latent capacities at your design conditions.
Step 3 Duct System Design/Sizing (Manual D & Manual T)
Now that the room-by-room loads are known, airflow amounts are known and we have the equipment chosen, we move on to laying out the registers and grilles, and the duct system.
First, the locations and sizes of supply registers and return grilles are determined based on the guidance in ACCA Manual T. Registers are placed to encourage mixing of the conditioned air within the rooms and spaces. Determining the size of the registers involves consulting the performance data for the specific register, grille or diffuser that you want to use. We want to choose a size that will have enough “throw” while also not having unacceptable noise levels.
In this example, we chose to use a central return strategy instead of ducting individual returns to each of the rooms. We like this approach in higher performance homes because it reduces the amount of ductwork (which means less resistance and friction), leading to a more efficient and cost-effective system. In this example, the central return is placed in the hallway and sized to achieve a 300-400 FPM face velocity.
With this approach, it is important that each room be provided with a return-air-pathway such as a transfer grille or jumper duct. Typical door undercuts are usually not sufficient to provide enough of a pathway on their own. In this case, we utilized a jumper duct in the Master suite and transfer grilles in the other bedrooms. Specifically, the transfer grilles are Tamarack RAPs, which help lower the amount of sound and light passthrough.
Once the registers and grilles have been laid out, we move on to laying out and sizing the ducts that will connect everything. All of this is done within the Wrightsoft software, which will perform the Manual D calculations, given the proper inputs.
In this example, we used a trunk and branch style duct system that has rigid sheet metal trunks and flexible duct branches with balancing dampers at the takeoffs. The air handler is also located in the conditioned attic in a horizontal configuration. The duct layout is shown below. The ERV and Dehumidifier are not depicted in their exact locations, but rather for better clarity on the drawing.
Step 4 Fresh Air Ventilation
Since this is an ICF house with a sealed and conditioned attic, it is safe to assume that it should be relatively air-tight (less than 3 ACH50). Because of this, mechanical fresh air ventilation will be needed. In this example, an ERV was chosen as it is efficient and provides balanced ventilation.
The ventilation rate needed is determined using the ASHRAE 62.2 formula, which is the standard for residential ventilation. This formula takes into account square footage, number of occupants and other credits for infiltration and filtration. In this example, the continuous ventilation rate is 80 CFM, and that will be a balanced rate between supply and exhaust. The ERV size for this example is a 160 CFM Broan-NuTone AI Series unit. This provides some additional “Boost” capability to be used as needed.
The most ideal way to install an ERV is with separate ducting. However, there are multiple acceptable configurations, so this is not the only way. In this example, the ERV has ducting that provides fresh air to the bedrooms and living room, and a centralized exhaust pickup. You can also choose to duct the ERV to exhaust from bathrooms. Ducts for the ERV are sized using the Manual D procedures.
Step 5 Whole-House Dehumidifier
Adding a whole-house dehumidifier allows for independent humidity control from the central air conditioning system. Whilst this is optional, even a properly sized air conditioning system simply may not need to run enough during shoulder periods (moderate temperatures with high humidity). If the air conditioner isn’t running, it isn’t removing moisture from the air. Adding a dehumidifier is highly recommended in humid climates.
In this example, we have a 98 PPD whole-home dehumidifier (Santa-Fe Ultra 98) that is ducted into the central ducts. Dehumidifier sizing is based on the latent load and the procedure described in ACCA’s Manual S for supplemental dehumidification.
Wrapping it Up
So, we’ve gone through each step of the process and put it all together in this example. A complete residential HVAC design is not just performing a Manual J load calculation. It involves all of the steps shown (Manual J, Manual S, Manual D/T), each of which plays an important role.
