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HVAC Design

As HVAC system designers we help builders, remodelers, and HVAC contractors right-size their project’s equipment and ductwork from beginning to end. When necessary, we add fresh-air mechanical ventilation. The process flow is as follows:

• Manual J, Heat Load Calculation
• Fresh-Air Mechanical Ventilation
• Manual S, Equipment Selection
• Manual D, Duct Design, incl Manual T, Airflow Distribution
• Post HVAC System Commissioning


We hold the known record for modeling the largest heating & cooling load in Wrightsoft’s Right Suite Universal Software: Right J & Right Draw. Our house was ~26,000 sqft. Because of the modeling, the homeowner is now replacing every heating & cooling system in the house—after getting the fundamentals right: air sealing & insulation.

In our early days, we like most HVAC companies and home energy consultants didn’t spend any time modeling the energy characteristics of the house. They take time. We were lucky because we could make all the recommendations for improving energy efficiency while others were responsible for results.

Member ICC
In late 2010, we decided we’d model the heating & cooling characteristics of every house. We started because Right Draw allowed us to draw pictures in a CAD-like program and it did all the math. It reduced our energy modeling time required for a HERS rating significantly. We quickly found we could use it for better energy services consulting.

We’ve gone from “We think you ought to do these things for these results” to “If you do these things, this will happen.” Now, when we speak, builders, remodelers, HVAC contractors, and homeowners listen! We’ve helped avoid disasters and correct them when they occur.

We still can’t accept legal responsibility for any installed HVAC system. Only an HVAC contractor can do that. However, we can do what most companies can’t stand doing: model the proposed HVAC system. We can build defensible load calculations and duct designs and review them with the HVAC contractor before work begins. If an HVAC contractor will work with us to model their project, the builder and homeowner will be a whole lot happier.

From ACCA Manual J, here are business issues resulting from modeling HVAC systems

Builder’s: what’s it worth to avoid call backs on the most complained about system of a house? Our homeowners of existing houses are declaring that their house are comfortable for the first time since their houses were built!

HVAC Contractors: The building code is getting stricter on your profession because communities can’t stand what has been delivered in the past. We can’t put up with it anymore because the cost of energy is going up sharply. Once you start reaping the benefits of modeling your systems, you won’t go back to using rules-of-thumb for final design decisions.

Come to think of it, what builder would start a building without a blueprint? Why are we installing heating & cooling systems without right-sized equipment and designed ductwork?

Today’s building code demands heating & cooling systems designed according to Manual J and Manual D. We can help builders and HVAC contractors make our client happy: the homeowner!

Manual J Heat Load Calculation (aka Manual J Reports)

Once the architect releases the building plans, final HVAC design can begin. For builders, the homeowner is unknown, so the building code is used in their place. Normally, the process starts by asking what the customer expects out of their heating & cooling system.

Before Home InSight starts, we need to know the specifications for several building components, particularly the insulation of walls, floors, & ceilings, as well as windows & doors. A key input is how tight the house will be. Without this information, our model is the worst house allowed by law—the building code! We ask for this information on a form, Construction Specification.

We have two choices for modeling the heat load characteristics of the house: block or room-by-room.

Block Load Model. A block load is basically a drawing the building boundary around the zone to be heated & cooled. Rooms are totally ignored.

Room Load Model. Room-by-room loads model each space of the house. After a choice is made to add supply registers or not, we know how much air is needed for each supply outlet.

A block load allows equipment selection without further design. The choice is pretty good in most simple cases. It can be used when a system is not zoned. It’s also useful when window and door placement is relatively uniformly distributed around the house. When these conditions don’t hold, a room-by-room load is needed.

To do Manual D duct design, a room-by-room heat load calculation is required.

The next step is to determine whether fresh-air mechanical ventilation is needed. If so, how much and what type.

Sample Manual J Heat Report

Fresh-Air Mechanical Ventilation

A house is considered “Tight” when the average natural air exchange rate is less than once every three hours (0.35 ACHnat). Georgia’s new energy code drives the house to this threshold. Once the house drops below this rate, fresh-air ventilation needs to be considered. Whatever method of ventilation is chosen, it negatively impacts the Manual J heat load calculation.

There are five basic types of fresh-air ventilation, two which aren’t recommended for Georgia and another that doesn’t make sense normally. The five types are . . .

Exhaust Only Supply Only Balanced Forced air Natural Air

Exhaust Only. Exhaust-only ventilation uses a fan to move air out of the house, much like a bath area fan or rangehood exhaust fan. Air is pulled through whatever air leaks there are through walls, floors, & ceilings, usually all day long every day. In summer, hot, moist, and dirty air is pulled in.

Supply Only. Supply-only ventilation uses a fan to pull outdoor air through a filter while pushing stale indoor air through holes in the building envelope. In the summer, it too pulls in hot, humid, and dirty air. The minimum difference is that the air is cleaned before it’s distributed throughout the house. Our preference is to dehumidify it first too.

Balanced. There are two types of balanced fresh-air ventilation: heat recovery ventilators (HRVs) & enthalpy recovery ventilators (ERVs). In both cases, fresh air and stale air flow past a heat exchanger to cool or heat incoming air partially, as appropriate. The ERV also removes some moisture. Normally, neither of these is appropriate for Georgia.

Forced Air. A forced air ventilation system is a type of supply-only ventilation. Outdoor air is directly drawn into the inside through a return duct connected outside somewhere. It runs when the air handler cycles air for the heating & cooling system.

When there’s no call for heating or cooling or the cycle runs in less time, then hot, humid, and dirty air is brought in at 3X the rate of a continuously running one. The fan size is 10X too big and the occupants are uncomfortable. It goes under the name, Air Cycler. This system should not be used in Georgia—though it is heavily promoted. The only advantage it has is that it’s cheap to install.

Natural Air. The final one is natural ventilation through windows, or a close cousin, whole-house fans. Except for about two weeks out of the year, this type of ventilation is useless in Georgia. During summer, this type of ventilation introduces a huge humidity load into the house that is absorbed by anything fabric or wood: furniture, bedding, drapes, towels, carpet, clothes, paper, etc. When the air conditioner is turned on the next day, the equipment spends hours removing moisture that came in overnight. Didn’t we just do this yesterday?

We understand installed cost is an issue, so we try to recommend ways that fit the builder’s budget. However, there is no getting around the fact that dehumidifying, supply-only ventilation is probably the best solution in Georgia. Everything else is a compromise. We often wonder what a homeowner would choose if they understood the range of possibilities! Is a granite countertop really more valuable than good indoor air quality?

Bottom line: the choice of fresh-air ventilation impacts the heat load that must be met by the installed HVAC system. Once it’s chosen, the method can be modeled. Then equipment can finally be chosen.

Ventilation Strategies

Manual S, Equipment Selection

After the load calculation is complete, equipment needs to be selected. Home InSight uses Manual S, Equipment Selection procedures to decide whether the chosen equipment is suitable for the house.

Don't Use AHRI Data. What amazes us is that HVAC contractors use ARI data to select equipment. Unfortunately, a cooling system isn’t going to behave in Georgia like it does on a dry dock in Arizona. Customers in Georgia have to deal with humidity and don’t keep their thermostats set at 80 degrees.

Use Expanded Performance Data. For proper equipment selection, the cooling system has to be derated by the manufacturer’s engineering performance data to decide whether the equipment will work in a particular house. It’s not uncommon for sensible heat capacity to drop by 0.5 – 1.0 tons. The higher the SEER, the worse it gets.

As a result of using Manual S procedures we’ve been able to avoid replacement of installed systems. We’ve also been able to explain why it’s better to get equipment with costlier features when it’s warranted. At the end of the day, if the homeowner isn’t comfortable, it’s a fair bet the builder and HVAC contractor will feel their pain. This can be avoided!

No equipment with ever reach its energy efficiency & effectiveness without properly designed ductwork. Without it, it’s not uncommon for really old equipment installed and functioning correctly to outperform a high SEER cooling system with leaky, wrong-size ductwork.

Limits of ARI Certification Data

Manual D Duct Design

With a room-by-room Manual J heat load calculation, it’s possible to field design ductwork. It will yield far better results than the rules-of-thumb used in the past. In our experience, this approach still may not lead to the high performance HVAC system the homeowner expects. For all but the simplest systems, Manual D, Duct Design is needed.

Manual D duct design starts with a room-by-room Manual J heat load calculation and manufacturer’s data for chosen equipment. The characteristics of the chosen filter needs to be found. The remaining choice is what type of ductwork layout is preferred. Is a radial or trunk-and-branch system going to be used? What combination of flexible and rigid ducts will be used? Is the design limited to parts availability for ducts, connectors, boots, and other fittings? Where will the supply outlets and return inlets be placed? All these decisions are collected on a Construction Specification.

At this time, Home InSight doesn’t do duct design for an HVAC system with multiple zones. We’re happy to provide our model to a company capable of designing them properly. There are several combinations of ways to build zoned systems, each with their own benefits and consequences.

Home InSight is capable of designing ductwork for single-zone heating & cooling systems. We’ll start with your preferred method of laying out ductwork. Then we’ll design a duct system capable of delivering 500-700 fpm to each supply outlet—a Manual T, Air Distribution recommendation.

The final step is installing the system and commissioning it. When do the quality assurance, we validate the system with simple HVAC diagnostic tests.

Manual T, Air Distribution

Home InSight cannot model air distribution—the way air mixes in a room. This takes the expertise of the HVAC contractor. Decisions made here will be felt by the homeowner.

When we design ductwork, we deliver air to each room at 500-700 fpm as much as possible. With this air velocity, the HVAC contractor has options for getting the air movement in the room right. Our preference is to use engineered grills and diffusers rather than the stamped variety typically available. Each cover costs ~$35. However, after properly designing the system, this makes performance awesome!

Sample Manual D Duct Design

Post HVAC System Commissioning Diagnostics

The final step in the installation of high performance heating & cooling systems is commissioning (performance tuning) the system. Racecars work at top performance when they are finely tuned. HVAC systems deliver the expected performance and efficiency when airflow, pressure, and refrigerant levels are verified.

As part of Home InSight’s quality assurance procedures, we use simple HVAC system diagnostic tests to determine the performance of the system.

Static Pressure. We start with static pressure across the air handler. If it’s too high, the airflow restriction needs to be removed. It may be as simple as replacing the air filter. Sometimes ductwork needs to be added. The most common cause of high static friction is the HVAC contractors insistence on using airflow characteristics for rigid ductwork on flexible ducts.

Capture Hood (Flow Hood). When the project calls for it, we’ll use a capture hood (aka flow hood) to verify airflow at each supply outlet and return inlet. When we can afford it, we’ll purchase our own capture hood and use it on every project.

Duct System Sizing. Without a flow hood, we can roughly estimate airflow by evaluating and comparing the plenum connection to the air handler, the sizes of and types of ductwork connected to the plenums, and the sizes of duct boots.

Duct Leakage. We test duct leakage with a Duct Blaster. With a simple modification to the set up, we can figure out what the actual airflow is through the system. Whether we do it or not depends on our ability to easily seal the return side of the system from the air handler unit.

Delta T. Typically, our final measurement is temperatures at return and supply plenums. From this, we can tell whether the fan speed is appropriate and may detect issues with refrigerant levels.

Our homeowners express bitter disappointment with their HVAC contractor when they find out how easy it is to test the performance of their system. We typically don’t use a flow hood or the Duct Blaster unless it’s necessary. Otherwise, we have the same simple tools every HVAC contractor has on their trucks.

HVAC contractors have no excuse. We do these tests for the benefit of builders because they’re the ones who get the callbacks first.

HVAC System Commissioning