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Controlling Two Separate ERVs Differently? Bedrooms and Living Space

gagejfoley | Posted in Mechanicals on

I am working on finalizing my HVAC systems for my home under construction. Running into questions on controlling the ERVs (and the HVAC contractor is not helpful).

The short version – with separate upstairs (sleeping) and downstairs (living) HVAC systems and ERVs, should the ERVs be programmed to run during the day/night at different rates? For example, should the upstairs ERV only run from 6PM to 9AM? If so, should it be at an increased rate?

Some more background – I work with LG and Renewair, so we will be using their products. The ERV exhaust is going to be pulled from the bathrooms continuously (with additional boost timers) and the supply ducted into the return of each separate LG air handler. I am also hoping to install air quality monitors to force boost mode on each system. Located north of Boston. Whole home is around 4,000 sq ft (including an in-law and finished basement), and requiring about 135 CFM OA total. 

Any input on the timing, CFM for each space during use times (living/sleeping), control components needed, feasibility?

Happy to share plans, OA calculations, etc. Thanks!

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  1. DennisWood | | #1

    You'll get the best efficiency from units like the Renewaire Premium L at lower CFM values. At 59CFM that unit tested at 88% efficiency, operating at 21 watts. In your situation I'd set both units up at 60-65 CFM to start and monitor VOC/CO2/Radon on both floors for a bit to see how that works. This way your baths (assuming you have a 2nd floor bath) and kitchen will be constantly ventilated, with boost mode there as needed. Your air handlers will likley be running at low speed 24/7 so air mixing will be happening regardless of which units are on/off. If your in-law suite, or any other part of the home is segregated, I think you'll want the two ERVs running at lower flow regardless.

    Then, if you want to push efficiency a bit further, you can automate the 2nd floor on occupancy so that for example if CO2 levels fall below 600ppm, turn that unit off. Even if you automate the systems, I'd consider leaving one unit running at all times at 60CFM to address VOC etc. , even if you're away. Automating for boost vs IAQ (in addition to the manual boost timers) for each unit is easy to do these days with off the shelf bits.

    Above is based on about 18 months of a "lab experiment" on our home with IAQ monitoring on each floor, live efficiency calcs, automation control of EC HRV motors, and integration with kitchen exhaust for supply air etc.

    One suprise after longer term monitoring of a commercial retrofit (logged CO2 levels for several years) is that on windy days and/or low occupancy, the air exchanger could be turned off completely and still have good IAQ inside. This was a very tight building with extensive energy retrofits.

  2. gagejfoley | | #2

    Thanks for the input Dennis. That's what Ill shoot for.

    Im having trouble coming to terms that I will need to run both air handlers at all time rather than intermittent operation. Based on Renewair controls, I dont see how to run intermittent operation, boost functions, AND occupancy timers.

    If I track this and really want to make changes later Im sure I could spend some time learning how to make my own simple controller.

    Appreciate the input.

  3. DennisWood | | #3

    The controls are usually pretty limited, although some of the newer units with EC motors allow you do more, like dial in vent rates from the wall control.

    If your air handlers are connected to heat pumps, a constant fan rate is likely required so on 24/7. On the ERV, there will be two sets of contacts, one for on/off and one set for boost. These are easy to automate using wireless Zwave or Zigbee relays from Zooz or Fibaro and give you full control. However at 21 watts, they are not using a lot of power and automating them for occupancy may not actually save much in the end.

    I’ve cranked up ventilation logic on my system to the point where the system is almost never off. This is because VOC and radon levels consistency spike with the system off. A base level of 50CFM with no one home keeps these levels in check.

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