Replies

1. 
   DennisWood | Mar 11, 2024 09:41am | #1

   I think you're seeing the results of testing at different air flows. The HVI database lists SRE at 82@36CFM, and the NRCAN numbers are [email protected] CFM for that VanEE unit. As you increase air flow over the core, efficiency will drop. During boost operation, the efficiency will drop ~10 to 15 percent further. This is why you might want to oversize your ERV a bit if you can.

   If you want the most efficient unit, figure out your target ventilation rate, then look at the HVI database for units where "Net Airflow @ Max Rated SRE (cfm)" have the best efficiency at that air flow. It's pretty easy to get good efficiency at low CFM values, but at higher air flows, the differences become more apparent. Units with large cores, or more efficienct (and larger) cross/counter flow cores like the Zehnder units will have better efficiency overall.

   You'll also see the SRE efficiency numbers drop a fair bit when testing at -25C because energy lost during defrost cycles has to be accounted for. During defrost (below -10C for most ERVs) the home's heat is used to warm the core so overall efficiency will drop.

2. 
   acrobaticnurse_Eli | Mar 12, 2024 07:34am | #2

   If targeting 110+ cfm I'd go for Vanmar/Broan's 210cfm ERV if it will fit your space. It was almost large enough to be an issue getting it through the entrance to my attic. On the spec sheet you can see the performance data at different temps/CFM. I've been happy with it so far and it's nice being able to easily adjust the CFM via the advanced controller in my bedroom. I have the ERV plugged into an Emporia Smart Plug so I can see how many watts are used at different CFM just for an idea of how much work the ECM fan is doing at different CFM.

   https://www.broan-nutone.com/en-us/product/freshairsystems/b210e75rs

   
3. 
   Greg808 | Mar 12, 2024 09:25am | #3

   Thanks for the explanation, now I can rest easy picking models from that chart.

   Sorry to digress, but now I think I need to double check my CFM, in case I'm skipping over potential models. And to get that proper 'low fan' speed you mentioned, or the proper size.

   A tech came in a while ago, we got 80cfm with what he called "the volumetric" method and 110 cfm using the 'municipal' method. But I think we missed some details, they were rough calculations.

   I think I should start form scratch.

   How should I figure out my CFM?

   I live in a three floor row house.

   The Basement is 20x40 feet, which is one open space but...you could say there is a square cut out where the garage would be on the basement and main floor. So minus 200each I think. For 600ft on the fist 2 floors.

   On the top floor there's one bedroom that overlaps the garage, but I can't tell by how much.

   And on the top floor, there are two bedrooms, one master bedroom, and 2 small bathrooms.

   The main floor is the kitchen, hall, small bathroom, but the dinning/living room is sort of open.

   Would those be all the details? Do I have to go measure every room individually?

1. 
   acrobaticnurse_Eli | Mar 12, 2024 11:15am | #4

   There is some debate as to exactly how much fresh air is needed and how to calculate it. Looking back at an older GBA article, one calculation would be 1 cfm per 100 square feet plus 7.5cfm per person, so a 2000 square foot house with 4 people would need 50 cfm of fresh air. Building Science Corporation would then increase that number if the fresh air isn't evenly distributed or balanced. 

   https://www.greenbuildingadvisor.com/article/resistance-may-not-be-futile-in-the-residential-ventilation-wars

   For higher efficiency and the ability to adjust how much fresh air I want, I went with a larger ERV even though I mostly have it run at 70-80cfm. I have an Aranet4 CO2 monitor in my main bedroom and now an Air Gradient air quality monitor in the living room that is used by most of my family when we're awake. I aim for a CFM level that keeps CO2 and other values in my desired range. I know that CO2 isn't so bad at normal levels but it's a fair indicator of fresh air levels. Some days when it's much cooler outside than in there is more natural air exchange and the ERV may be less needed, but on many other days my HVAC may not run all day and the air feels stale without the ERV or air handler running. The ERV takes only 25 watts to run continuously vs 500 for my air handler.

1. 
   Greg808 | Mar 15, 2024 07:41am | #5

   @acrobaticnurse_Eli
   Pardon the late reply.

   One last thing, you mentioned getting a larger 210 Broan . for a 110cfm target . I've gotten some differing advice on that, if you could chime in.

   From some of my notes I took when the tech guy came in, he explained (in a hypothetical house): "with a model at 150-160 cfm they may not ramp down enough to ventilate the house at the rate that it needs to, and then it 'short cycles'. "

   When I'm looking for a good slow continuous.

   Maybe some units have more specific adjustments than others, even at the continuous rate? Like is it variable? Or were you referring to it going at 70cfm on an intermittent cycle?

   My priority is the rate on a slow continuous mode, to avoid dips in the air coming in.

   I was interested though, in a larger model you could adjust because I have a heart condition and my breathing isn't great.

   I was asking myself: what if I need some wiggle room to adjust the air and go a bit higher than normal? But not too high that it dumps in too much at once and stops. Like if I still want a steady stream but at a bit higher than target.

   Is that still possible with this Broan 210cfm you recommended?

1. 
   acrobaticnurse_Eli | Mar 15, 2024 08:42am | #6

   With my current duct setup I can run the erv as low as 65cfm continuously which takes 22 watts per hour. There are intermittent and recirculate modes but I have stuck with simple continuous and boost mode so far, with boost getting activated by increased bathroom humidity. I like that I can easily adjust the cfm for what I consider low, medium, high, or boost.