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Energy Use Details -

 Heat Recovery Ventilation System

The purpose of the heat recovery ventilation (HRV) system is to provide a flow of fresh air into the Residential Energy Laboratory (REL) and to exhaust a similar flow of stale air out of the REL, but to transfer a majority of the heat energy of the stale air to the fresh air before the stale air is exhausted.  In cold weather, the fresh air is heated by this process, while in hot weather, the fresh air is cooled by the stale air if the indoor temperature is cooler than the outdoor temperature. 

To put it another way, many houses use an exhaust fan to vent stale air from the home.  These pull in fresh air through unintended leaks in the house.  So, for example, if it is -15.6˚C (4˚F) outside, then the fresh air will have to be heated from that temperature to the indoor air temperature.  In the case of the HRV installed in the REL running on low speed (~ 33 l/s or ~70 CFM) with an outside air temperature of -15.6˚C (4˚F), the fresh air enters the house at 9˚C (48˚F) while the indoor thermostat is set at 19˚C (67˚F).  Use of the HRV results in a significant energy savings in heating the fresh air, with about 1120 W (3800 Btu/hr) of power being transferred from the stale air to the fresh air at this particular condition.  The power to run the HRV at this condition was 28 W, so this is hugely favorable from an energy standpoint.  The temperature transfer efficiency at this condition was measured to be 71.9%. 

As an aside, some of the inlet air heating occurs in the insulated ductwork upstream of the HRV, so the 9˚C (48˚F) temperature at the outlet of the HRV would be lower with shorter inlet ductwork.  The heat transfer in the ductwork is not included in the figure given for the heat transfer in the HRV.  The heat transfer in the ductwork is, of course, an energy loss from the house.    

Similar measurements at the high flow condition in the HRV at about 66 l/s (140 cfm) at an outside air temperature of -6˚C (22˚F) resulted in the fresh air to the house being preheated to 10˚C (50˚F), transferring 1680 W (5720 Btu/hr) of power to the inlet air stream.  The power consumed by the electric motors in the HRV is 71 W at the high flow condition, so again, this is very efficient from an energy standpoint.  The temperature transfer efficiency at this condition was measured to be 68.4%, lower than the low flow condition due to the shorter residence time in the heat exchanger. 

The HRV with the performance as described above was a Venmar AVS EKO 1.5, Model 43900.  It uses fans with brushless DC motors (BLDC) , more commonly known by their trademarked name electronically commutated motors or ECMs, to minimize energy consumption.  The fans are the only moving parts for this HRV.  The heat transfer is performed passively through a fixed heat exchanger, and the operation is easy to understand by examining Figure 1, which shows the HRV with the front cover removed.  The heat exchanger can be crudely described as like a large collection of very small soda straws arranged in alternating rows at right angles to each other, with the spaces between them filled in with plastic.  Fresh air enters the HRV from the top right in Figure 1, and is pulled through the heat exchanger by the fan partly visible on the bottom left.  The same mass flow of stale air enters the HRV on the top left, and is pulled through the heat exchanger by the fan partly visible on the bottom right.  After passing through the fans, the air flows turn vertically upward and travel to the top of the HRV unit.  In the case of the fresh air, it is ducted to the outlet side of the natural gas fired, hot-air furnace.  In the case of the stale air, it is exhausted from the house.  In the winter time, the fresh air upstream of the HRV and the stale air downstream of the HRV can be significantly below the crawl space temperature, so those two ducts are insulated to minimize cooling of the crawl space.  In the summer time, those two ducts can be significantly warmer than the crawl space, so minimizing heat transfer is still important that time of year.        

Figure 1.  HRV with Front Cover Removed Showing Flow of Fresh Air from Top Right and Stale Air from Top Left.  

There is an electrically controlled vane just above the top of the heat exchanger, not visible in Figure 1, that separates the two chambers on the top left and top right of the heat exchanger when the HRV is used to precondition the fresh air.  This vane can also be rotated such that those two chambers communicate (are connected), and the fresh air inlet to the HRV is blocked.  This allows the stale air to flow from the top left chamber to the top right chamber and then through the heat exchanger with no cross flow.  In this case, only the fan on the bottom left is operated.  This simply recirculates the stale air through the house. 

The recirculation mode is important in two cases.  In the first case, when the inlet air temperature is less than -5˚C (23˚F), the recirculation mode is alternated with the standard mode to defrost the heat exchanger by warming it during the recirculation period.  In the second case, the recirculation mode may be used to even out temperature differences in the house without bringing in fresh air.  The recirculation mode may be selected for 100% of the cycle, or it may be alternated with the standard cycle where the standard cycle runs on low speed for 20, 30, or 40 minutes per hour, with the balance of the time spent in the recirculation mode.  In the recirculation mode, the single fan on the bottom left in Figure 1 runs on high speed, while in the standard HRV mode, the two fans can be operated on low or high speed (user selectable).  The HRV is often used with some amount of recirculation at the REL to distribute the passive solar heating that occurs preferentially at the south end of the house. 

The electrical energy consumption of the Venmar HRV was low at all conditions, with the actual values measured by a Kill-A-Watt meter as shown in the table below.

Electrical Power Consumption of Venmar Eko 1.5 HRV

* The balance of the hour in recirculation mode, which uses a single fan at high speed.  . 

APPENDIX

For those interested in getting a better “feel” for the operation of an HRV, some detailed measurement results are presented below for the Venmar Eko 1.5 HRV operated at the Residential Energy Laboratory.  Temperature measurements were taken in the fresh and stale airstreams in the ducts just upstream and downstream of the HRV.  These were accomplished by drilling 3.97 mm (5/32”) holes in the ducts, and inserting an Omega thermister measuring 102 mm (4”) long and 3.175 mm (1/8 “) in diameter (Part No. TH-10-44046 -1/8-4-40) into the ducts while the HRV was operating.  The thermister output was read using a Radio Shack multimeter set on the ohm-meter scale.  The calibration table supplied by Omega was fit with a third-order polynomial to relate the ln_e(R) to the temperature in ˚C, as shown in Figure A-1 below. 

Figure A-1.  Plot of data supplied by Omega Corp. to Determine Temperature from the Measured Thermister Resistance.

Detailed measurements were taken of the HRV performance during a procedure to balance the two airflows, the fresh air into the house and the stale air exhausted from the house.  These flows are sometimes purposely unbalanced to provide a net positive or negative pressure in a building, but in this case, the object was to balance the airflows.  This is normally done by measuring pressure differentials across the heat exchanger, but since the HRV was already instrumented for temperature measurements, it was determined that more precise measurements could be performed using the temperature data. 

The objective was to balance the temperature gain in the fresh air with the temperature loss in the stale air to within about 5%.  A number of measurements were taken at both low and high speed fan operation.  Data are shown below for low speed fan operation in Table A-1 and for high speed fan operation in Table A-2.  Note that for these conditions, the heat transfer power, and, by implication, the fan speeds were balanced to within 1% for low speed fan operation, and to within 4% for high speed.  Measurements at other conditions provided similar results.  Balancing the temperature differentials for fresh and stale air required adjusting dampers in the airstreams downstream from the fans.  Note that the power transferred was more than ten times the power consumption by the fans at these conditions with a relatively large temperature differential available.     

Table A-1.  HRV Operation on Minimum Speed.

Table A-2.  HRV Operation on Maximum Speed.