A New Way to Duct HRVs
Instead of pulling exhaust air from bathrooms and the laundry room, consider exhausting stale air from bedrooms
UPDATED June 27, 2013 with an author's postscript
Nine years ago, I co-authored a Journal of Light Construction article with David Hansen on HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. installation. The article noted, “Stale air is exhausted from bathrooms, the laundry, and the kitchen. (An HRV is not intended to handle grease or smoke, so a range hood should be separately exhausted to the exterior.) Fresh air is supplied to the bedrooms, living room, and other living areas.” This advice is consistent with the long-standing recommendations of most HRV manufacturers.
The advice is logical: after all, it makes sense to exhaust air from the smelliest rooms in the home and to supply the fresh air to the rooms where people spend most of their indoor hours.
Although this traditional ventilation duct layout works well, I’ve begun to rethink the issue lately. It may be time to experiment with different ducting methods for HRV systems — especially for homes with single-point heating systems.
Ductless minisplits come of age
There is a strong trend among designers of superinsulated houses to use ductless minisplit systems for space heating and cooling. These systems can work well even when outdoor temperatures drop to -17°F or -20°F. Moreover, the systems are inexpensive, easy to install, and very energy efficient.
What type of ventilation system is best in such a home? Since there is no heating or cooling ductwork, a central-fan-integrated supply ventilation system is obviously out. The usual choices: either an exhaust-only system (for example, a bath exhaust fan that runs for much of the day) or a heat-recovery ventilator (HRV) with dedicated ventilation ductwork. (Throughout this article, I’ll use the term “HRV” to include energy-recovery ventilators as well as heat-recovery ventilators.)
Two problems with homes with single-point source heat and HRVs
As more HVAC engineers and builders wrestle with the details of designing a house with a single-point heat source (for example, a gas-fired space heater or a single ductless minisplit unit), they worry about two questions:
- Will the bedrooms be cold?
- Will the delivery of fresh ventilation air cause comfort problems?
Evidence on the cold-bedroom question is beginning to accumulate from a variety of projects, including R. Carter Scott’s net-zero-energy house in Townsend, Mass.; the Elakim’s Way project developed by the South Mountain Company on Martha’s Vineyard; and RDI’s Wisdom Way project in Greenfield, Mass.
It’s hard to generalize, but it seems safe to say:
- Compact superinsulated homes heated by one or two heat sources have been successfully built, and haven’t resulted in many comfort complaints.
- When bedroom doors are open, the bedroom temperature is usually within 1 or 2 degrees of the living room temperature.
- If you live in such a home, remember to keep bedroom doors open unless you need privacy.
- Monitoring studies show that the bedrooms in these homes are often several degrees cooler than the living room, but most homeowners aren’t complaining — so it’s possible that homeowners are deliberating keeping their bedrooms cool or are indifferent to lower temperatures.
Fresh ventilation air is still cool
At a presentation at the March 2011 Building Energy conference sponsored by NESEANorth East Sustainable Energy Association. A regional membership organization promoting sustainable energy solutions. NESEA is committed to advancing three core elements: sustainable solutions, proven results and cutting-edge development in the field. States included in this region stretch from Maine to Maryland. www.nesea.org, Robb Aldrich, an engineer with Steven Winter Associates, explained why there were few comfort complaints at the Wisdom Way project in Greenfield. “The winter heat loss from one of these bedrooms is in the range of 60 to 120 watts,” said Aldrich at the Boston conference. “If you’re sleeping or have one light on, you probably are generating enough heat to balance the heat loss. A sleeping person emits 60 watts. If the person is awake and has a laptop and a lamp, you might be up to 120 watts.”
Aldrich did note a problem in a house in Colrain, Mass., that is heated by ductless minisplit units. “There were comfort problem in the hallway where the fresh air was being distributed,” said Aldrich. Since the fresh air register was far from the space heater, the hallway stayed cool. Aldrich advised, “ERVs or HRVs have to dump cool air somewhere, so it’s a good idea to dump the fresh air near where the heater is.” In addition to his advice on fresh-air delivery, Aldrich suggested a novel way of moving warm, conditioned air toward remote bedrooms: why not locate exhaust grilles in the bedrooms?
Fresh air to the living room, exhaust air from the bedrooms
I think Aldrich’s suggestion is a promising avenue for experimentation. I can imagine a house with a single ductless minisplit unit on the living room wall. The HRV ductwork could direct all of the home’s fresh air to a ceiling diffuserIn a forced-air heating/cooling system, the diffuser is a register or grille attached to ducting through which heated or air conditioned air is delivered to the living space. In a tubular skylight or an electric light fixture, the diffuser is a cover plate through which scattered light is delivered. near the ductless minisplit unit, and all of the home’s exhaust air could be pulled from the upstairs bedrooms. Spot exhaust for the bathrooms and kitchen could be kept separate from the HRV.
This type of ventilation ductwork would set up a gentle air flow from the warm, conditioned living room toward the remote bedrooms. Every room gets fresh air, but by the time the fresh air reaches the bedrooms, it’s been tempered. And considering the way some unventilated bedrooms smell — especially those of some teenagers — there’s a certain logic to pulling exhaust air from bedrooms.
[Author's postscript: Dr. John Straube, a principal of the Building Science Corporation, has challenged the logic behind the novel method of HRV ducting advocated here by Robb Aldrich. In an article titled "Choosing HVAC Equipment for an Energy-Efficient Home," Straube is quoted as saying, “Ventilation air doesn’t do much to move around heat. ....Ten cfm of 72 degree air to a 65 degree bedroom won’t make any difference to the temperature in the bedroom at all. Open doors work better than HRV ducting.”]
Last week’s blog: “More Passivhaus Site Visits in Washington State.”
- Martin Holladay
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