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Community and Q&A

Santa, Please Bring me a Balanced ERV…

Greg Labbe | Posted in PassivHaus on

We’re getting into commissioning residential heat recovery ventilation systems (balance* and optimize) and so recently invested in a Dutch-made balometer that is really accurate* for measuring low air flows. What was once done crudely with a Pitot tube and unglamorously with a garbage bag “should” now be able to be done quickly, simply and repeatably with more accuracy – perhaps too much – which may be part of the problem.

Generally, the procedure goes like this:

1- balance the adjustable dampers (if present) with the system at max speed
2- adjust flow down to Total Ventilation Capacity and label, then tweak dampers room by room to set flows as per mechanical design,
3- measure door undercuts and issue balancing report!
4- Go home for more rum and eggnog before it expires.

However, we’re having the Dickens of a time with one particular ERV. We use the balometer outside (sheltered and calm days, get a tight fit, do 3 readings in a row) to measure flows at the intake and exhaust, we can match those up to within 10% of each other (the criteria according to HRAI for a balanced system). But when we measure all supply and return grills indoors, the numbers don’t add up by a very large number.

This should be a simple reconciliation of flows on both return and supply sides. We can get the system balanced outside but flows inside are heavily lopsided or we can adjust the indoor supply return grill flows till balanced but then the readings outside are waaay off.

For example, we adjust indoor supply/return air flows to the designed rates (eg indoors we read 131cfm fresh supply and 102 return) but when we measure outdoors, we get 161 cfm stale exhaust and 90.3 cfm fresh air intake with a difference of 70cfm between intake and exhaust. For the record, the duct work in this house was meticulously air sealed.

There are two variables we haven’t accounted for yet.
1. How much air flow the ERV has by-passing though the media filter? 10% plus?
2. The ERV is “Self-balancing” and has a logic circuit in it that automatically adjusts air flows to “Balance” the ERV which might be unwittingly working against us…

As per usual, most installed HRVs and ERVs don’t lend themselves well to Pitot or flow collar insertion due to physical constraints. If you have any insights, I’d be much obliged!

*Balance: Flows into and out of home withing 10% of each other so as to not pressurize or depressurize.
Optimize: Set maximum speed on adjustable models and adjust flows room by room as per intended design by tweaking dampers.
*Nearly caused an international scene by calling the manufacturer’s claimed accuracy into question…it is pressure compensated and shouldn’t induce flow restrictions…. or so I imagine…

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  1. Marc Rosenbaum | | #1

    Can you post here (or somewhere you can point to) a diagrammatic cross section showing the relation of each of the 4 air connections, the heat exchange core, and the locations of the blower wheels relative to the core? The goal is understanding the pressure relationships across the core, to see what the natural leakage paths are, which will follow the pressure gradients. Sounds like potentially a large amount of air leaking past the heat exchange core. Is this fixed plate or rotary wheel?

    In larger buildings we've had significant leakage with some commercial rotary wheels.

    I don't have such an accurate flow device, but when we commissioned the Zehnder ComfoAir 200 in my own house about a year ago I was pleased to see that the flows we measured (two devices, Testo vane anemometer and Kimo hot wire anemometer) at the exterior louvers were very closely matched by the sum of the flows at the supply and exhaust locations inside. The ducts are the homerun Zehnder Comfotubes (hey, I don't name this stuff!) sealed with o-rings at each end - leakage not detectable. And I don't have HVI testing data on the 200 unit we used, but the 350 unit was tested to about 1% cross-leakage, which is the lowest number I have seen in HVI testing.

  2. Greg Labbe | | #2

    Marc, its a rotary wheel ERV. I'll see what I can draw up for you in the coming days.

  3. david_white | | #3

    I think a secondary issue (the main issue IMO being cross leakage) may be measurement device. You might find your device if you google this LBNL study: "Measuring Residential Ventilation System Airflows: Part 2 - Field Evaluation of Airflow Meter Devices and System Flow Verification"

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