Marc Rosenbaum discusses heat recovery ventilation considerations for zero net energy homes on the Zero Net Energy Homes course discussion board…

Exhaust-only ventilation home health concerns

Student 1: Do you know of any potential health impacts from living in a depressurized space– such as a home with exhaust-only ventilation?

Marc: You can pull in pollutants from outdoors or the ground (radon), so full time depressurization is not my preference. Occasional depressurization seems fine to me.

The effectiveness of fresh air supply from HVAC return ductworks 

Student 1: In a situation where the HRV/ERV has the fresh air supply to the house ducted into the HVAC return ductwork, wouldn’t the ventilation effectiveness be compromised when the HVAC fan is running? Any recommendations on how to approach this type of set-up?

Marc: I think that when the fan runs, the fresh air is very well distributed, and when the heating fan stops, the distribution may suffer. The product called Air Cycler turns conventional furnace fans on periodically to distribute fresh air around the house. It was designed to work with a small outdoor air duct to the return of the air handler, with a motor damper that opened when the fan turned on.

Student 1: I agree that when the air handler fan stops the fresh air distribution is probably fairly random. But when the air handler fan runs, that also seems potentially problematic because won’t the depressurization in the return HVAC side pulling on the fresh air supply of the ERV/HRV cause some kind of imbalance in the ERV/HRV?

Marc: Not enough to matter.

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Exhausting a compartmentalized bathroom: separate bath and toilet spaces

Student 2: If I’m designing a compartmentalized bath, where there is a door between a dedicated bathing space and a dedicated toilet/lavatory space, do you advise exhausting from both these spaces? And, if so, it is best to do this with separate, dedicated ducts? We’re doing more and more baths like this, and I’ve been confused as to whether or not to increase the number of exhaust points, assuming then that the CFM from each is reduced so the overall CFM for the house remains the same.

Marc: The answer to this depends on the client and the situation. I have a large Passive House I’ve been helping with where the toilet compartment gets 20 CFM and the shower/lavatory gets 40 CFM. Usually these are bigger houses anyway so one ends up with more total CFM to allocate. Depending on the layout, you might get clever and take it all out of the toilet compartment but locate a transfer grille high and adjacent to the shower.

Passive solar heat gain via stone or tile floors

Student 3: Would it be a good idea to have stone/tile floors near Southern windows to have more Solar thermal storage?

Marc: I wouldn’t spend money on the stone floors until everything that had better return was done. Prioritize securing super-insulated, airtight, triple glazed (with two low-e coating) windows and very efficient heating, cooling, and ventilation, as well as water heating and solar energy.

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About Instructor Marc Rosenbaum – Director of Engineering, South Mountain Company

Marc Rosenbaum, P.E. uses an integrated systems design approach to help people create buildings and communities which connect us to the natural world, and support both personal and planetary health. He brings this vision, experience and commitment to a collaborative design process, with the goal of profoundly understanding the interconnections between people, place, and systems that generate the best solution for each unique project. Design practiced at its highest level goes beyond efficiency and conservation to create places that regenerate and nurture the natural world and all of its inhabitants. Current areas of concentration include Passive House design, Deep Energy Retrofits, and Net Zero Energy buildings. Projects of his have won awards from the Northeast Sustainable Energy Association (NESEA – four times a winner), ASHRAE (twice a winner), and the Energy Efficient Building Association (EEBA). Three of his projects have been on the American Institute of Architects (AIA) Earth Day Top Ten list. The French Wing for the Society for the Protection of NH Forests earned a LEEDTM Gold certification, the first LEEDTM certified project in New England. In his practice of sustainable design consulting, he has worked for institutional clients such as MIT, Vermont Law School, Yale, Dartmouth College, Cambridge School of Weston, and Middlebury College; non-profit clients such as the Society for the Protection of NH Forests and the Woods Hole Research Center; commercial clients such as Stonyfield Farm, Inc., Tom’s of Maine, and the Hanover Consumer Co-op; cohousing groups such as Pioneer Valley, Pine Street, Island Cohousing, Peterborough, Cobb Hill, and Alchemy Farm; with many architects including William McDonough Architects, Maryann Thompson Architects, Kaplan Thompson Architects, Solar Design Associates, Bruner Cott Associates, Coldham & Hartman Architects, Moore Ruble Yudell, and Payette Associates.

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