A Balanced Ventilation System With a Built-In Heat Pump
A Balanced Ventilation System With a Built-In Heat Pump
A father-and-son team are manufacturing a new ventilation device — a type of ‘magic box’ — called the CERV
A small manufacturing company in Illinois called Build Equinox has developed a new ventilation appliance called the Conditioning Energy Recovery Ventilator, or CERV. Build Equinox was founded by an engineer, Ty Newell, and his son Ben Newell. (Ty Newell designed and built the Equinox House, which was described in a GBA article published in 2011.)
The CERV is a balanced ventilationMechanical ventilation system in which separate, balanced fans exhaust stale indoor air and bring in fresh outdoor air in equal amounts; often includes heat recovery or heat and moisture recovery (see heat-recovery ventilator and energy-recovery ventilator). system that includes an integrated 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. — a type of appliance that has been dubbed a “magic box” by Passivhaus designers. According to Ty Newell, Build Equinox has sold about 50 CERV units.
There is just one model of the appliance; Ty Newell told me that “one size fits all.” The CERV is unlike any other appliance sold in the U.S.:
- The CERV introduces fresh outdoor air into the house while simultaneously exhausting the same volume of stale air.
- Although the CERV has the words “energy recovery ventilator” in its name, it does not include an ERV core or 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. core. Instead, the CERV uses an air-source heat pump to transfer heat from one air stream to the other.
- Like all air-source heat pumps, the heat pump in the CERV has a condenser coil and an evaporator coil. Both coils are located indoors (inside the CERV). During the winter, the exhaust air from the house passes over the evaporator coil. This allows the heat pump to scavenge some heat from the exhaust air. The condenser coil is located in the fresh-air duct; this allows the CERV to deliver heat to the incoming ventilation air. (See Image #3, below.)
- During the summer, the CERV’s heat pump removes heat and moisture from the incoming outdoor air stream and delivers heat to the exhaust air stream, thereby lowering the temperature of the incoming ventilation air. (See Image #4, below.)
- The CERV includes integrated CO2 sensors and volatile organic compound (VOCVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production.) sensors that regularly sniff the indoor air. Feedback from the sensors is used to determine when ventilation is necessary, and when the indoor air is fresh enough to allow the CERV to stop ventilating.
- The CERV can be programmed to shut off the motorized dampers connected to the outdoor grilles, and to circulate the indoor air in the house without introducing any fresh ventilation air.
- During recirculation mode, the CERV can provide limited space heating or cooling.
Operating the CERV
When the CERV is commissioned, the user has to choose the sensitivity of the CO2 and VOC sensors. Even though the human health effects of 1,000 ppm of CO2 are not really comparable to the human health effects of 1,000 ppm of VOCsVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production., the manufacturer of the CERV requires CERV users to choose a single setpoint (in parts per million) for CO2 and VOCs. While this setpoint can be raised or lowered, the CO2 setpoint and the VOC setpoint move in lock-step.
Peter Schneider, a senior project manager at the Vermont Energy Investment Corporation in Burlington, Vermont, has specified several CERV systems. “I set them at 1,000 ppm,” says Schneider.
According to the CERV owner’s manual, “In general, a lower air quality setpoint will result in better quality air and higher energy consumption, while a higher air quality setpoint will result in lower quality air and lower energy consumption. On one hand, you do not want to be overventilating your house and producing unneeded conditioning demands on your systems, while on the other hand, you should not sacrifice your air quality and health in order to save a little bit of energy.”
Air flow and heat output specifications
The CERV has an airflow rating of 250 cfm. “It is able to flush out the house in a fairly quick manner,” said Ty Newell. “If you want to ventilate at a level that will meet 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., it will work for homes up to 6,000 square feet.”
Although the manufacturer claims that the unit has a space heating capacity of 5,000 to 8,500 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. /h and a cooling capacity of 3,500 to 5,000 BTU/h, the heat output varies depending on the outdoor air temperature and the operating mode of the appliance.
According to a report prepared by the manufacturers (“CERV Performance Report: Vermont High Performance Manufactured Home Project”), “The CERV’s wintertime heating capacity varies from 500 watts to 1500 watts with ambient temperature variation from 0°F to 40°F during recirculation heating mode.”
Schneider told me, “In most applications, [the CERV] has not been used as the primary heating system. But if you couple it with their soil heat exchange loop [an option available from the manufacturer], so that all of the air that the CERV sees would be above freezing, you could probably build a very efficient small house and meet your heat demands with a CERV.” (The soil heat exchange loop is a loop of buried plastic tubing that contains glycol; it is hooked up to a small pump and a copper coil in the system's fresh air duct. The CERV's glycol loop is similar to the glycol loop offered as an option to purchasers of the Zehnder HRV.)
Gregory Whitchurch built a small Passivhaus (a house with a “treated floor area” of 1,436 square feet) in Middlesex, Vermont, for his father, using a CERV as the primary heat source and an electric-resistance space heater as the backup heat source. “When my father goes to bed, he checks the outdoor temperature,” Whitchurch told me. “If it is below about 5 or 10 degrees, he turns on the 900-watt space heater. If the forecast says that the outdoor temperature will be below -5 degrees, he switches the electric heater to the high position, which is 1500 watts. With the 1500-watt heater on, the lowest that the indoor temperature has been is 64 degrees.”
According to the manufacturer's report (“CERV Performance Report: Vermont High Performance Manufactured Home Project”), when the outdoor air temperature is 0°F, the CERV can supply 5,000 watts of heat during fresh-air-ventilation mode, but only 600 watts of heat during recirculation mode. In either mode, the appliance draws 430 watts of power. Ty Newell explained that the amount of heat that the appliance can deliver depends on the temperature difference between the refrigerant in the condenser coil and the air stream flowing past the coil. Since this temperature difference is greater during ventilation mode than during recirculation mode, the CERV delivers more heat during ventilation mode.
Whitchurch has been monitoring the energy used by his CERV. “The maximum watt draw is just under 400 watts for the CERV conditioner [the heat pump], while the fans draw 94 watts, for a maximum power draw of 494 watts,” Whitchurch told me in a phone interview. “Looking at the eMonitor, I see that it’s drawing 326 watts right now. The CERV conditioner [heat pump] used a total of 162 kWh for the month of January. The control module, fans, and flappers used 53 kWh, so that’s a total of 215 kWh for the month.”
According to Ty Newell, “If it ran flat out for every hour of the day, it would use 10 to 12 kwh a day. Maybe for 25% of the time it is providing fresh air, and maybe 75% of the time it is operating as a conditioning system.”
The energy required to run a CERV depends on many factors. According to the previously cited manufacturer's report, “An integrated analysis is required for determining the CERV’s overall energy performance as it switches through fresh air ventilation, recirculation, and defrost modes. The fraction of time spent in each mode, which depends on fresh air requirements, impacts a home’s energy requirements. … Occupancy characteristics are very important in determining a home’s fresh air ventilation needs.”
The report states, “The CERV has an electric power requirement, averaged over its recirculation, fresh air venting, and defrost cycles, of 392 watts at 0°F. The CERV supplies an average of 1,375 watts of heat to the home at 0°F relative to a ventilation air heat requirement of 1,385 watts.”
According to Schneider, “An evaluation by Ty [Newell] showed a COP of 2.5 for the whole season. I trust those numbers.”
Schneider told me, “If you put a Zehnder [HRV] in, it’s usually running continuously at 50 watts. In a very small house, it might be less — maybe 20 watts continuous — but 50 watts is typical. The CERV fans are going to run at 90 watts, but the CERV will turn off when ventilation is not needed. So if you compare the overall energy consumption to a Zehnder, it might be a wash. The CERV has higher fan wattage but it’s not continuous.”
What does it cost?
Newell told me that the CERV retails for $4,500. “Including shipping, the CERV cost us about $5,000,” Whitchurch told me. “We did the installation ourselves.”
Peter Schneider estimates that “a typical well-designed duct installation [for the CERV] will cost maybe a couple of thousand dollars.”
According to Tim Yandow, a Vermont builder who built a CERV-equipped house, “The installed cost was somewhere around $9,000.”
What about maintenance and repairs?
The CERV is a fairly sophisticated appliance that contains motorized dampers, CO2 sensors, VOC sensors, an air-source heat pump, fans, and electronic circuitry to control its multiple functions. Since it’s such an unusual piece of equipment, most builders and homeowners will be hard-pressed to find a local HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractor to service the unit.
Newell has anticipated this problem, and notes that the manufacturer can diagnose many problems remotely, as long as purchasers select the “smart appliance” option (called “CERVice” by the manufacturer) allowing a CERV to connect to the Web.
Tim Yandow reports, “Initially, there were some technical issues with the wireless monitoring system, but we worked those bugs out.”
According to Peter Schneider, “We have had dampers that stopped working. We also had evaporators that frosted up because of misuse and a bad algorithm. But the manufacturer has since modified the defrost cycle to do a longer defrost cycle at cooler temperatures.”
In spite of these problems, Schneider is a big fan of the CERV — although he notes that good duct design and commissioningProcess of testing a home after a construction or renovation project to ensure that all of the home's systems are operating correctly and at maximum efficiency. are essential. Schneider told me, “The CERV guys [manufacturer’s reps] provide you with the air flows, but you need a really good duct design to make sure you get the cfm you need. Good duct design is a weakness in our entire industry. The other issue is commissioning these systems. I’ve visited too many houses where the balancing is abysmal. It’s usually a result of poor duct design.”
Excellent indoor air quality
Schneider is impressed with CERV performance. “In a house with a CERV, the air quality is phenomenal,” he said. “But in my other homes with balanced ventilation systems — the homes with Zehnders — the air quality is phenomenal as well.”
Schneider said, “The CERV has several advantages. It doesn’t lead to overventilation. And it ensures that you can have comfort in remote spaces when you are using point-source heating.”
The CERV is one solution to the “cold bedroom” problem. In homes heated by a ductless minisplit, “it can be tricky to ensure comfort in the remote spaces,” Schneider said. “We have several homes with a wall-mounted ductless minisplit in the living room and a Zehnder HRV exhausting from the kitchen, laundry, and bath. If doors are closed, or if things aren’t operating perfectly, or if the house is not taking advantage of passive solar gains because of design issues, or if or shading strategies aren’t implemented properly, the CERV is useful — it’s a distribution system that helps mix the air in the house. The owners of our homes without CERVs are not unhappy with our high-performance houses — most of those are working extremely well. But there are often temperature variations around the house. They love having low energy bills, and they are often happy to have these temperature differentials. It is dependent on homeowners’ preference. For owners who desire even temperatures, the CERV is doing a much better job at moving more air. It only ventilates when needed. It is providing conditioned air to the living spaces. We’re monitoring and testing, and we are seeing phenomenal performance.”
High performance versus simplicity
By all accounts, the CERV solves a lot of problems without using much energy. It provides balanced ventilation. It is smart enough to stop ventilating when the indoor air is clean. It increases the ventilation rate when the CO2 level rises — for example, when there are 20 people in the house for a party — and decreases the ventilation rate when the CO2 level falls — for example, when the house is unoccupied.
The CERV is able to warm up the ventilation air during the winter and cool off the ventilation air during the summer. Ty Newell points out, “In most climates, there are 2 to 6 months — the shoulder seasons — when you don’t want energy exchange. During those months the CERV can just bring the outdoor air in without conditioning, whenever outdoor air is nicer than indoor air. It’s equivalent to opening a window.”
Finally, the CERV is able to mix the air in a home so that the temperature in a remote bedroom is close to the temperature elsewhere in the house. All of these features are desirable. It is admirable that these features are provided without much of an energy penalty compared to other types of ventilation or conditioning equipment.
There are only two downsides to the CERV. It’s fairly expensive — the installed cost is about $7,000 to $9,000 — and it is mechanically complex. Two questions arise from its mechanical complexity: (1) Will it need frequent adjustment and repair? and (2) How long will it last before it needs to be replaced?
After 10 or 20 years, we’ll have enough data to answer these two questions. In the meantime, early adopters will need to step forward to gather the needed data.
Martin Holladay’s previous blog: “NESEA Conference Highlights.”
- Image #1: Gregory Whitchurch
- Image #2: Peter Schneider
- Images #3 and #4: Build Equinox
Mar 18, 2015 12:33 PM ET
Mar 18, 2015 1:15 PM ET
Jan 17, 2017 1:05 AM ET