Questions From an HVAC Contractor – Energy Recovery Ventilators and Make-up Air.

An HVAC contractor recently asked the following questions about Energy Recovery Ventilators and Make-Up Air:

“We are currently doing a new construction home and the General Contractor just told us they are having a 900 CFM (cubic feet per minute) kitchen range exhaust fan installed. I know an exhaust fan (typically a range hood), over 400 CFM requires make-up air (MUA). There will be a 300 CFM Energy Recovery Ventilator (ERV) installed with the Heating, ventilation, and air conditioning (HVAC) system. My question is, I know I need 900 CFM of MUA but does the 300 CFM ERV count for part of the MUA, and then I would need an additional 600 CFM of outside MUA? Or does the ERV not matter in this situation, and I need the full 900 CFM of MUA?

I was going to tie the outside MUA duct to the return air of the HVAC system with a damper in the MUA duct. Then the damper would be tied to the kitchen range exhaust fan, so the damper only opens when the range fan turns on.

Also, we plan on having the ERV run continuously along with the furnace blower in continuous operation mode, providing air filtration and circulation. Am I going to need to install any other controls when done this way? If I do need some other type of control, what do you suggest?

I have asked several people about this, and no one knows exactly what or how to do this. Any help would be appreciated.”

These are great questions.  Thanks so much for reaching out. There are many considerations to take into account in the situation you have outlined above.

1. The ERV provides balanced ventilation, where the exhaust and fresh air from outside are typically balanced or equal. It will not ‘make-up’ the air exhausted. Sometimes comments suggest setting up the ERV out of balance, i.e., with a higher fresh air supply CFM from outside and a lower exhaust CFM. This does not comply with the manufacturer’s installation instructions, but more importantly, the efficiency of the ERV will suffer from the inequality in airflow through the ERV heat exchanger. Additionally, balancing to provide MUA when the range hood operates creates an excessive positive pressure in the home when the hood is not in operation.
2. Check the specs for the kitchen range exhaust fan CFM, as manufacturers often exaggerate the actual airflow in sales brochures. Most produce a table or fan curve showing actual airflow at specific static pressures. You can expect to find a CFM considerably less than 900 on high speed with a duct attached. These numbers are often based on zero- or 0.1-inches external static, where the recommended duct size results in an actual airflow 50% lower than advertised. If the actual airflow is less than 400 CFM on high speed, MUA is not required by the code.
3. Please size the MUA duct large enough to make up the full actual exhaust CFM. Many interpret the code to only require the exhaust rate greater than 400 CFM; however, the code requires MUA approximately equal to the exhaust rate. (2015 IRC M1503.4)
4. If the home is built tighter, with closed cell spray foam, structural insulated panels (SIPs), or insulating concrete forms (ICFs), understand the home may not leak 400 CFM, and MUA may be required in most cases. 
5. When sizing a duct from outside for MUA, you’re using range hood exhaust fan power to pull in the air through the intake duct. In that case, size the intake duct for a low velocity, maybe around 400-500 FPM (feet per minute). Of course, if tied to the return air and the furnace is always running, a smaller duct is great. It sounds like you may be running the fan continuously, so a smaller duct and an interlocked damper is great.
6. Typically, a pressure switch in the range exhaust duct is interlocked to open the MUA damper whenever the range hood exhaust is in operation.
7. Broan/Nutone and others make MUA packages for this application, some with fans and re-heat, others with a damper and pressure switch. You can do a quick internet search on this.
8. If you are running the ERV and furnace fans continuously, be careful not to over ventilate as this is energy wasteful. See Table M1507.3.3(1) in the 2015 IRC for ventilation airflow sizing [JB2] . Hopefully, you can set it up to balance the ERV fans with fan speed control and dampers. This table considers occupants and floor area.
9. You can set up the ERV as a central exhaust in the middle of the home, but the ideal option is to duct exhaust from each bathroom, laundry, and maybe an inlet in the kitchen area, but not the hood. With some ERVs you can program to jump to a higher speed when switched from a bathroom or similar location. Many have a timer option where hitting the switch in the bathroom jumps to a higher speed for a selected amount of time. Occupancy and humidity sensors are also options.
10. The code requires high efficacies for all continuous ventilation products. The energy code requires at least 1.2 CFM per watt. See 2018 IRC N1103.6.1/2018 IECC R403.6.1 , adopted by the legislature for minimum efficacy (CFM/watt) requirements.

In summary, as houses become progressively tighter, whole-house ventilation has become necessary to maintain indoor air quality and to control moisture. All movement of air into or out of the home must be accurately accounted for in the design of HVAC systems.