How to Measure ERV Flow Rate
I have been trying to balance my ERV flow rate with my HVAC contractor.
We started by balancing the static pressure at the unit to have a slightly higher pressure supply than exhaust at the unit using a manometer.
Then we went to the intake and exhaust ports at the outside of the house and measured flow rate into and out of the house using a hood and calibrated anemometer.
All those numbers made sense.
The next step was to adjust the diffusers in each room to achieve the design supply and exhausts at the various outlets using the same hood used outside. This is where the problems start.
The flow rates are small, unboosted flow is around 20cfm to 50 cfm depending on the room.
At each exhaust he was unable to measure any flow even although there is clearly flow. The hood I don’t think makes a good seal and exhaust flow is bypassing the hood anemometer.
The supply flow rate could be measured but it surely isn’t accurate. When all supplies are added up it is about twice the flow rate that the ERV can produce under ideal conditions.
I think the diffusers shown in the picture are causing the air to accelerate and the hood and associated flow straightener are not slowing the air flow down so we are getting false results.
The other thing that happens is that the readings are extremely sensitive to how open/ closed the wall diffuser is. It will go from 0 cfm to 40cfm within a few degrees of rotation of the diffuser again leading me to believe air velocity out of the diffuser is affecting the results.
What is the best way to measure flow rate accurately through these style of diffusers?
Thanks,
Steve
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Replies
Does anybody have any tips for me on this?
The method that I used was to attach a giant garbage bag to a collar with a flat flange that will completely cover the diffuser and seal against the ceiling. Squeeze all the air out of the bag, ready your stopwatch, put the bag over the diffuser and stop the time when the bag just reaches full size. Do the math based on the bag volume and time to get your CFM. Not super accurate, but if you're consistent in your method you can get good comparative readings between diffusers.
I reduce the opening size on the hood to 2" and use a pitot tube to measure flow. This works reasonably well. The garbage bag is also a good option.
For balancing the unit, I find flow hoods to be less accurate than measuring across the core as per the installation manual ie P19:
http://vanee.edenenergy.com/literature-v2/install-manuals/90H-VECM_90H-V-install-2011-10-07.pdf
It can also be done right at the unit where you are doing the balancing adjustments. You do need a sensitive pressure gauge for it though.
How many supply outlets, and what is the outside diameter of each? What is the make and model of the ERV?
I would monitor flow at the ERV using a differential magnehelic gauge (so your overall target is maintained) combined with the the flow chart on the ERV housing. Use the bag method to compare at each supply as you adjust. If your target total is 100 CFM, then make sure you are seeing that at the ERV while adjusting supply diffusers...you'll need to go back and forth as you adjust. If you need 20 CFM from one duct, and 40 from another, you can assume that the 40 CFM supply will fill a bag in half the time of the 20.
If you keep checking flow at the ERV while you adjust, you should get your target overall, and calculate relative flow at each supply using the "time to fill bag" method.
A lot of flow hoods are not rated to give super low CFM readings. Even then, their accuracy is not perfect. Alnor, for example, has an accuracy of 3% + 5 CFM. So at 20 CFM you're potentially 6 CFM off anyways. I'd do the best you can and then get a CO2 sensor if you want to really dial things in to what they should be.
@this_page_is_left_blank
Seems like a good method for the supply vents but how do you deal with the exhaust vents?
@Arkos
Will the pitot tube measure the exhaust ports too? It seems like there was too much leakage around the HVAC guys hood. The hood was a plastic contraption with a fabric elasticated skirt supported by plastic legs to hold the skirt open in a conical shape around the supply/exhausts. I never got a picture of it.
We were able to balance the core using the RenewAire installation method and manamometer like this https://www.amazon.com/Fieldpiece-SDMN6-Pressure-Switch-Tester/dp/B00IJI0T6U/ref=sr_1_1?hvadid=247046480564&hvdev=c&hvlocphy=9029752&hvnetw=g&hvqmt=e&hvrand=16870647727440314650&hvtargid=kwd-351716122779&hydadcr=1744_10063638&keywords=sdmn6+fieldpiece&qid=1682429953&sr=8-1
@DenniseWood
We have 2 RenewAire Premium M ERV's. When the house was built the installer did not connect the ERV's in a balanced loading like had been designed. We have 4 supply outlets and 8 exhaust outlets on one ERV and 3 supply and 4 exhaust outlets on a second ERV.
One ERV system has an installed supply (unboosted) of 140 cfm and exhaust 165 cfm. The design intent was 115 supply and 115 exhaust on this ERV.
The other ERV system has an installed supply (unboosted) of 85 cfm and exhaust of 60 cfm. The design intent was to have 110 cfm supply and 110 cfm exhaust on this ERV.
I'm not sure what a magnehelic gauge is. From a quick google search it looks like a brand of manometer or pressure gauge for measuring fan and blower pressures? I think the manometer we used (linked previously) connected to the core pressure ports of the ERV would do the same right?
@stevedavis
I have had a CO2 sensor in our bedroom now for some time. This is what prompted me to get my general contractor to commission my ERV (which was never done at the install). It's an Aranet4. Overnight our Co2 PPM with 2 people in the bedroom gets up to 1570 PPM. When the room is empty the CO2 level is about 500 PPM.
I'm not sure if this is a super accurate CO2 meter but it made me worried about our sleeping CO2 levels. What CO2 levels should I be targeting?
Thanks for all your help.
Airfix
The correct CO2 level is definitely open to interpretation. However commercial spaces with demand control ventilation bring in fresh air when CO2 levels rise above 1,000 ppm. Ambient conditions outside are around 430 ppm for reference.
You can measure exhaust flow rate with a pitot tube but you need a length of pipe (~3') to do it. The pitot tube needs to point in the direction of the flow so to measure exhaust it would be installed by the hood but pointing away from it down the length of the pipe.
Balancing exhaust flows at the register isn't that important, you can do it by feel. For example if one of the bathrooms is not getting enough exhaust flow and fogging up, close down the exhaust pickup on the others ones a bit. You should be able to feel the airflow with your hands as it is adjusted.
Fresh air supply is also mostly being in the ballpark. CO2 is a good indication as it is what you care about in the end. For me at home I went by sniff test. The bedroom air smells stale in the morning, it needs some more airflow. Even pretty high CO2 is not really an issue when sleeping.
What you do care about is keeping the ERV balanced, sometimes adjusting the registers a bit too much will unbalance the ERV. The gauge your installer used should be fine. The magnehelic gauges are just lower cost.
Air, Akos gives good advice as usual :-) One thing I'm not sure of though is how accurate pitot readings will at very low CFM rates...it may be very hard to measure this way. Akos, if you reduce output to 2" for pitot measurements, are you not restricting flow (so skewing CFM) in order to increase velocity?
Air, your ERVs have separate motor controls which is great for balancing. Your asymetric balance numbers might warrant a rethink on the strategy. A differential magnehelic gauge connects to your door ports and measures pressure difference across the core (supply and return)...which give a pretty accurate measurement of CFM at the core. Most manufactures want balance numbers within 10% of each other. They do this as efficiency over the core with respect to both latent and sensible heat will be affected as you unbalance. I've measured efficiency skewing from about 55 to 90 % based on unbalancing a core. Knowing this, I'd try and balance your cores, particularly if latent efficiency (moisture transfer) is important to you. https://www.renewaire.com/wp-content/uploads/2022/03/EVPrem_Balancing_InstructionSheet_164131_001_FEB22.pdf
I've experimented a bit with a sensitive vane anemometer to estimate flow (exhaust or return) but you need to extend the port 6 feet or so using a 6" or 8" duct...kind of a DIY flow hood if you will.
The Aranet should be fine for CO2 sensing. Take it outside for 30 minutes and see if it reads around 420 ppm. If not, calibrate it as per the manual. These meters can drift over time. Once your ERVs are balanced, I would go with your CO2 targets as you have the meter in hand, as opposed to spending a lot of time measure flows at the supply ducts.
I have a few sensors now over our 2 story (with basement) and levels can fluctuate a lot depending on what the family is up to. If you can get your bedrooms down below 1000 ppm CO2 at night..perfect. I'd also look at the main living areas and bias supply to try and keep levels below 1000 ppm.
My system ramps up from 50, 65, 75 and 100 CFM based on Co2 levels, which keeps our living areas under 800 as a rule, however there are always exceptions with company over etc. On very windy days it's often dialled back to 50 CFM. With a system that is static with respect to flow, you'll want to temper your ventilation enthusiasm a bit so as not to over ventilate if you're going off spot readings with the CO2 sensor. I'm seeing some value in averaging CO2 and VOC readings over the three floors, and using average values to control vent rates.
Thanks folks. That gives me some good advice. I tend to get in the weeds a bit and need to look at big picture stuff.
Akos, with the pitot tube are you using your own Bernoulli hand calculation or is it part of a professional setup that calculates flow rates for you?
My digital manometer does the calculation based on the duct size. Trying to measure these 10 to 20 cfm flow rates is always a challenge, this is why my recommendation is not to worry too much about it if the air in the room is fine.