I had an interesting question posed by a building science professional yesterday at an energy code workshop. She reported that an engineer she works with repeatedly claimed that an ERV used more energy than it saved. This baffled the whole crowd. Does an ERV save money? It seemed obvious to us all, at least until she asked the question. With the workshop being a bit slow for my learning pace, I set to scratching out the numbers on the back of the folder they gave us.
So I picked a unit I’m familiar with to make an evaluation. I chose the Imperial/Greentek PE 7.15. We typically advocate HRV (heat recovery ventilators) rather than ERV (energy recovery equipment) because we are trying to get rid of moisture in the winter in modern homes. But, it was a commercial code discussion and A/C loads are not insignificant so ERVs tend to make more sense in these situations.
The PE 7.15 draws 102 watts at 32F and has a total effectiveness of 74% at that temperature. It moves 150CFM at that draw as well. In our region, we are paying about $0.18/kWh for electricity. Some quick math:
0.0102kWh*0.018$/kWh=$0.0018/hr of use.
So the ERV consumes less than a penny per hour of run time. Lets now evaluate how much it saves in heat energy. This is a little more complicated. Not only do we have to calculate the energy savings, but we have to convert that to money saved in heating costs. To do this, we have to make some assumptions about the types of heating systems and fuel costs. So we will make use of our Cost per BTU Calculator on this website to evaluate the savings with a Geothermal(WSHP), Natural Gas furnace, Propane Furnace, Air Source Heat Pump (ASHP), and Electric Resistance.
For the ERV, we are assuming 32F outdoor temperature already, so let’s assume 70F indoor temperature. That is a 38F dT through the machine. The spec says it’s moving 150CFM so we can calculate the BTUs being moved with the equation:
1.08 is a conversion factor that deals with the specific heat of air as well as converting minutes to hours. Plugging in our numbers:
We will now apply the efficiency of the ERV, 74% to this BTU load
If we convert BTUh to kWh we can compare the energy balance
1BTUh = 0.293071W 4555BTUh=1335Wh
So, running the ERV at 102Wh of electricity saves 1335Wh of heat. It seems like a good deal. However, electricity is relatively expensive per “BTU” or unit energy compared to other fuels like natural gas or leveraged electric appliances like water source (geothermal) or air source heat pumps. So, just to be sure, let us compare it in dollars consumed and saved since that is was many of us really care about.
Using the local fuel prices in the area as of this post, we get a table like this.
|Air Source Heat Pump||$1.80||$0.082|
The ERV at 102W/hr consumes $0.0018/hr and Saves anywhere from 33.3X to 135X the money is consumes. So, does an ERV save money? Of course, it does. They save way more money and energy than they consume. That is why we use them. With even moderately tight houses with fiberglass batt, when properly constructed, will be tight enough to not breath out enough moisture in winter without mechanical ventilation. Never mind the VOCs, CO2 buildup, etc… we’ll talk about that later in another blog post about indoor air quality and the impacts of CO2 buildup.