Case Study 20-2: The Chen-Williams Ductwork Design and HRV Installation

Background

When Priya Chen-Williams and Marcus Williams stripped their 1963 ranch house to the studs, they stood in front of a rare opportunity: a completely open house where any mechanical system could be installed without demolition. The choices they made for ductwork and ventilation would be permanent — closing up the walls means living with those choices for decades.

Their HVAC designer, Diane, had already helped them select a hybrid cooling system (covered in Chapter 19): a 1-ton ducted mini-split for the main living area and two wall-mounted mini-split heads for the bedrooms. The ducted portion required a short duct system in the main living/kitchen area. The design and installation of this system, and the whole-house HRV that Priya and Marcus added to the scope, forms this case study.

The Duct System Design

The ducted portion of the system served the living room, dining area, kitchen, and a small home office — approximately 970 square feet of the 1,850 total.

Diane's duct design goals: 1. Minimize duct length. Every additional foot of duct adds friction, increases potential leakage, and costs money. 2. Keep ducts inside the conditioned envelope. Any leakage from ducts inside the conditioned space goes into the house, not outside — it's wasted in terms of distribution, but it's not wasted energy. 3. Design for measured performance. She specified a duct blaster test before wall close-up, with a target of 4 CFM25 per 100 sq ft of floor area or better.

The duct layout she designed: - Supply trunk: A 10×8-inch sheet metal trunk running 14 feet along the kitchen ceiling (inside the conditioned space in a dedicated soffit, not in the attic). From this trunk, five supply branches in 6-inch round sheet metal duct extended to ceiling diffusers in the living room, dining area, kitchen, and office. - Return: A single large return plenum in the hallway connecting the living area to the rest of the house, 16×10 inches, short run back to the air handler.

All sheet metal was specified with mastic-sealed joints at fabrication (the sheet metal shop pre-applied mastic to all joints before delivery). Field connections between pieces were sealed with mastic on site.

The Sheet Metal vs. Flex Duct Decision

Priya had read enough to ask Diane directly: "Should we use flex duct or sheet metal?"

Diane's answer was nuanced. "Sheet metal is better in almost every way — lower friction, no degradation, can't sag or compress. But for runs that need to be perfectly straight and don't need to flex around obstacles, sheet metal is fine. For the short runs from the trunk to the ceiling diffusers — maybe 4–6 feet each — flex duct works well, as long as it's pulled tight and supported properly."

The final design used sheet metal for the trunk and return, flex duct (fully extended, with proper supports every 4 feet) for the final branch runs to each diffuser.

One flex duct choice Diane made explicit: all flex duct would be insulated flex (R-6 rating, required for any duct in unconditioned space; also specified here for thermal consistency). Even though the duct was technically inside the conditioned envelope in a ceiling soffit, insulation on the flex duct prevented condensation on the duct exterior during air conditioning operation.

The Installation

The duct system took approximately two days to install. The HVAC crew installed the main trunk first, supported on threaded rod from the framing above — well-supported throughout its length with no sag. The sheet metal joints were mastic-sealed at each connection. Then the flex duct branches were run from sheet metal takeoff collars on the trunk to the ceiling openings, extended fully, and secured with stainless steel gear clamps and mastic at both ends.

Before any drywall went up, Diane's company returned for the duct blaster test.

Duct blaster results (pre-drywall): - Total duct leakage: 6.2 CFM25 per 100 sq ft of floor area - Duct leakage to outside: 3.8 CFM25 per 100 sq ft (well within Energy Star target of 6.0)

Diane found two locations that were contributing to the total leakage: one flex duct connection where the clamp hadn't been tightened fully, and one sheet metal joint where mastic had been applied but had a gap at a corner. Both were corrected in 20 minutes.

Post-correction results: - Duct leakage to outside: 2.1 CFM25 per 100 sq ft

This was an excellent result — significantly below Energy Star requirements and one of the tighter duct systems Diane had measured.

Adding the HRV

The final blower door test on the completed house confirmed an airtightness of 1.18 ACH50. This was excellent news for energy efficiency and a clear signal that mechanical ventilation was needed.

Priya and Marcus had planned for an HRV from the beginning — it was in Diane's mechanical scope from early in the project. The unit selected was a Panasonic Intellibalance 100 CFM HRV. This unit was chosen for three reasons: it was sized appropriately for the house (100 CFM max with variable speed control), it used a counter-flow heat exchanger with 82% heat recovery efficiency, and it had a simple control interface that Priya and Marcus felt comfortable managing themselves.

The HRV ductwork design. The HRV's two exhaust ports drew from the primary bathroom and the utility room (where the air handler was located). The two supply ports delivered fresh air to the living room and to the return duct of the air handler — using the air handler's blower to distribute fresh air through the house whenever the air handler ran.

The HRV ducts were 6-inch insulated flex duct, running through the attic (now inside the thermal envelope due to spray foam on the roof deck) to two roof penetrations for fresh air intake and exhaust. The exterior hoods were placed on opposite sides of the roof and at least 6 feet apart to prevent exhaust air from being drawn back into the intake.

Marcus noted one thing they hadn't anticipated: the HRV unit required a condensate drain. In heating mode, the warm outgoing house air loses moisture to the incoming cold air — that moisture condenses in the heat exchanger core and must drain away. They ran the drain to the primary bathroom floor drain, which required an extra 12 feet of 3/4-inch drain tubing. Diane had specified this in the design; the HVAC crew handled it during rough-in.

Commissioning and Control Settings

Diane commissioned the HRV herself, balancing the two airstreams so that exhaust flow approximately equaled fresh air flow. She set the continuous operating speed to 55 CFM — consistent with ASHRAE 62.2 requirements for a 3-bedroom, 1,850 sq ft house. She also set the "boost" function (for bathroom use) to 100 CFM for 30 minutes.

She walked Priya through the annual maintenance schedule: - Every 6 months: Remove and clean the filter cassettes (the HRV has two washable pre-filters on the intake and exhaust ports) - Annually: Remove the heat exchanger core and rinse with water to remove accumulated dust and debris. Inspect the condensate drain for blockage. - Every 2 years: Inspect the exterior hood screens for debris and bird nesting.

The whole maintenance cycle takes about 45 minutes once you know the equipment, Diane said. "Don't skip the core cleaning. A dirty core loses efficiency and eventually gets moldy. Once a year, rinse it. It's not complicated."

First Year Performance

Priya tracked the home's energy consumption carefully through the first year. Key observations:

Air quality: The difference in perceived air freshness compared to their previous house was notable. Marcus, who had mild dust allergies, continued to benefit from the MERV-11 filtration on the ducted system, and the HRV ensured continuous fresh air dilution of indoor pollutants. CO₂ spot measurements showed consistently below 900 ppm during normal occupancy — well below the 1,000 ppm threshold often cited for noticeable cognitive effects.

Energy: The HRV recovered approximately 80% of the heat it would otherwise have lost through unrecovered ventilation. Diane estimated the HRV saved approximately $280–320 per year in heating and cooling costs compared to direct fresh air introduction without heat recovery.

Comfort: Priya identified one unexpected benefit of the tight envelope + HRV combination: temperature consistency. The house maintained temperature much more uniformly room-to-room, and humidity was stable throughout the year — not too dry in winter (a common complaint in tight houses without moisture transfer), not too humid in summer. "The house is stable," Priya said. "It does what we set it to. It's boring in the best way."

The one problem: In late spring, Marcus noticed a musty smell from the HRV. He looked at the core and found it was due for cleaning — he'd missed the 6-month filter maintenance schedule. He removed the core, rinsed it thoroughly, cleaned the drain, and the smell resolved. He now has reminders set on his phone for both maintenance tasks.

What They Would Do Differently

Priya would add one thing in hindsight: a CO₂ sensor with a display, integrated with the HRV controls, so that they could see real-time indoor air quality rather than relying on periodic measurements. "We know the system is working because we measured it. But for someone who didn't measure, a sensor would give you that confirmation every day."

Diane's retrospective note: "The HRV ducting to the air handler return works well but requires the air handler fan to be running for distribution. In a very tight house, it's worth discussing a dedicated supply distribution network so fresh air distributes even when the HVAC isn't running for heating or cooling. Not essential, but better practice."

Key Questions for Discussion

1. Diane specified sheet metal trunk ducts and short flex branch runs rather than all-flex. What are the practical and performance reasons for this hybrid approach?

2. The pre-drywall duct blaster test found two leaks. Why is pre-drywall testing so valuable — what would have happened to those leaks if they hadn't been found until after the walls were closed?

3. The HRV recovered approximately 80% of ventilation heat loss, saving $280–320/year. The unit and installation cost approximately $2,400. What is the payback period, and how does it compare to Dave Kowalski's duct sealing project?

4. Marcus missed a scheduled HRV maintenance task. What were the consequences? What would have happened if maintenance had been deferred for several years?