Is There an Alternative to a Heat-Recovery Ventilator?
HRVs are a great idea, but they can get expensive
The tighter the house, the more it needs mechanical ventilation. That's become a rule of thumb for energy-efficient builders, and designers often turn to heat-recovery ventilators to get the job done. These relatively simple (but not necessarily cheap) devices use the temperature of outgoing air to moderate the temperature of incoming air, thus lowering the energy penalty for providing fresh air to the whole house.
Cathy O, who with her husband is building a house in Climate Zone 5 on Cape Cod, Massachusetts, is familiar with the arguments in favor of using an 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. . But the cost of the unit was a problem, and as she researched their options she was told HRVs may not even work that well.
"We want fresh air circulating in the house, to have a healthy home, and we'd also like to have enough air for the wood stoves to run," she writes in a Q&A post at GreenBuildingAdvisor. "A quote to install an HRV system was way too high for us, so my husband thought he could install the product the company recommended himself. But now when I've talked to other HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. specialists, they say the products don't even work that well and are a waste of money."
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Cathy and her husband are planning to build their house as tight as possible, sealing all the air leaks they can find and adding an inch of rigid foam insulation on the exterior.
"Does the house need an HRV?" she asks. "Would a minisplit or two do the same work of providing fresh air and some heat? Should we just open the windows from time to time (as recommended by our chimney installer)?"
An equally interesting issue is the advice that Cathy O has, or hasn't, been getting from the specialty contractors they've met with. As much as she and her husband would like to build green, it's not always easy making the right decisions when subcontractors are openly incredulous at some of their choices.
Cathy O's dilemma is the subject of this month's Q&A Spotlight.
Ventilation yes, HRV maybe
"A tight home always needs a mechanical ventilation system," GBA senior editor Martin Holladay tells Cathy. That's not to be confused with the "minisplit" she mentions. A minisplit is an air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. used for heating and cooling. It has nothing to do with ventilation.
While ventilation of some kind should be on the couple's must-have list, it doesn't necessarily have to be a heat-recovery or energy-recovery ventilator, he adds. "There is nothing wrong with an exhaust-only ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. system," Holladay writes. "Plenty of people are quite happy with the performance of such a system. Exhaust-only ventilation systems use one or more high-quality bathroom exhaust fans (usually a Panasonic) controlled by a 24-hour timer. You want the fan to run enough hours each day to satisfy ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. requirements (but no more than that)."
John Semmelhack suggests that both Panasonic and Broan make fans that can be set to a specific ventilation rate in increments of 10 cubic-feet-per-minute increments. "This should enable the fan to run continuously and would eliminate the need for a timer," Semmelback writes.
David Meiland is another fan of exhaust-only ventilation who thinks HRVs are useful only for extremely tight houses. "I would be hesitant to invest in a HRV unless I knew that the house was going to be very tight, I were in a very cold climate, and/or the house layout was such that remote rooms might not get enough circulation," Meiland says. "Our house is very tight and operates fine with exhaust-only ventilation using a WhisperGreen fan. I am more aware than most of [indoor air quality] issues and pay attention to 'operating' the house correctly, but we don't need more ventilation equipment than that. My experience of blower door testing many houses is that most people are not building tight enough to benefit from a HRV — there is already more than enough infiltration."
Now, about that wood stove
Cathy O's house won't have a central heating system, just two wood stoves. How they will work in a tight house?
"Wood stoves that rely on conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. air for combustion aren't a great idea in a very tight home," Dana Dorsett writes. "There are many units out there set up to duct in combustion air."
A second problem is finding a wood stove that won't blast them out of the house. "Also, in a house that small and well insulated, it might be tough to find wood stoves tiny enough to not turn it into a sauna if you're burning it at a high enough rate to kick a non-catalytic EPA woodstove into secondary-burn mode (necessary for it to come anywhere near meeting it's EPA emissions or efficiency ratings.)," Dorsett says.
A house as Cathy describes probably has a heat load of less than 15,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. per hour, which makes a ductless minisplit a good option, he says. They're available in smaller sizes and work well with wood stoves.
Dorsett specifically recommends a ceramic or soapstone stove with lots of mass. "An EPA rated steel or cast-iron stove will give you hot flashes getting it up the secondary burn temperature," he says.
He notes that adding a duct for combustion air shouldn't be too difficult even if the chimneys already have been installed. Although it's simpler to bring in air when the chimney is on an exterior wall, a fresh air duct can be routed from a side wall and in between floor joists.
Our expert's opinion
Here's what GBA technical director Peter Yost has to say:
I think the hardest concept to explain to the building community, and especially the general public, is the need for mechanical ventilation: You mean we just spent all that time and effort to air seal the building and the very next step after that is to spend more time and money installing a system to bring outside air into that airtight building?
The inherent problem is breaking the building’s performance down into separate issues — energy efficiency and indoor air quality — rather than understanding the performance as an integrated system issue.
The differences between air leakage and mechanical ventilation are that mechanical ventilation is:
1. Of a specified rate (cubic feet per minute);
2. Within a set schedule (air changes per day).
Some, but not all, ventilation systems are able to provide additional features, assuring that ventilation air is:
3. From a specific source;
4. Air filtered and distributed.
Since we pay so much to condition our inside air, it would be nice to keep as much of that conditioning as we can as we exchange that inside air for outside air.
Options for mechanical ventilation include an exhaust-only system, a central-fan-integrated supply (CFIS) system, an HRV, and an energy-recovery ventilator (ERV). So, what sort of mechanical ventilation options do we have and how many of the differences listed above (differences from just air leakage) does each system achieve?
Each of the listed systems can be set up to comply with the most well-known residential ventilation standard, ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. 62.2.
One final performance distinction: the first two systems are not balanced (they slightly depressurize/pressurize the building, respectively), while the last two are balanced (the same amount of air goes out as comes in).
I think the differences among these systems are more about preference than adequacy. When configured and installed correctly, each of the four systems improve the indoor air quality of an airtight home. And their performance differences pretty much follow cost: the least expensive system addresses the fewest aspects of performance; the most expensive, the most aspects.
I think the appropriate system is much more of a performance choice than a safety one, and is likely based on the philosophical and marketing approach of the builder and budget and even sensitivities of the client.
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