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Thoughts on negative pressure after balancing a Venmar X24 ERV?

DAVID GOODYEAR | Posted in Mechanicals on

For those of you who have read my blog, this is a question about my HRV installation. I have a x24ERV from venmar.
Like most ventilators, the fan tables with the unit provide a way to measure air flow across the core to facilitate balancing fresh and stale air flows.
I have attempted balancing using a digital differential manometer with funny results. I can easily balance the machine within 5%.
I have an anemometer and a home built hood and have measured the air flows from the rooms and verified that they are close to the air flows measured across the core.
I have also used the anemometer to do an 50 point average accross the width and depth of the vent hood outside on the supply and exhaust and have verified that they are practically the same.

The issue is, with the machine balanced, the house is actually under negative pressure. On a still day I have opened a window and taped the gap while having one end of the anemometer hanging outside.
The pressure reads negative. If i put the machine in recirculate, the negative pressure dissappears as expected. As soon as I turn back to continious ventilation, the pressure becomes negative despite the air flows being “balanced”.
I have smoked out the mechanical room several times to determine that there are no leaks on the exhaust or supply. I thought there was a balancing issue when smoke from the chimney trickled into the space while lighting the stove. With the “balanced”
ventilator running I can feel cool air trickling into the living space through the chimney joints. Venmar says that their tables are for a closed system. I am thinking that the ventilator and house aren’t really a
closed system since the house is really like a plenum with air flowing through it. any plenum, like a house for example has leaks. There is no way to prevent air from being drawn or expelled through those holes since they are part of the
plenum and hence a part of the static pressure associated with the duct system.

Anyway, when I unbalance the system, I can balance the negative pressure of the house and I no longer feel air coming from around the chimney joints. maybe this is all that matters.

Does anybody have any thoughts as to what may be happening?

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  1. GBA Editor
    Martin Holladay | | #1

    I think that a slightly unbalanced system is unlikely to cause any IAQ problems, as long as you don't have any atmospherically vented appliances that can backdraft. After all, exhaust-only ventilation systems pull outdoor air into the house through cracks, and these systems usually (the point is arguable) don't negatively affect IAQ. Outdoor air coming into your house through cracks is a type of ventilation.

    I think you can adjust your ERV either way -- with "balanced" airflows that result in negative pressure, or with "adjusted" airflows that result in a zero difference between the indoor and outdoor air pressures. Adjust it to your preference.

  2. DAVID GOODYEAR | | #2

    I currently have it adjusted to give balanced pressure rather than air flows because of the chimney issue. It immediately took care of smoke trickling out of the chimney joints. The pressure is small ie 0.02 inH2O ie 5 Pa. This is about the same order of magnitude of a typical natural draft of a chimney so not surprising that I would see some smoke. I know that the chimney is one of the weaker parts of the envelope in terms of air-tightness and a negative pressure differential will cause air to follow the path of least resistance into the house...I guess the chimney is one of those paths.

  3. GBA Editor
    Martin Holladay | | #3

    As I'm sure you know, installing a wood-burning appliance (or including a chimney that serves a wood-burning appliance) in a tight home is tricky at best, problematic at worst. The chimney represents the biggest hole in your thermal envelope, so the chimney will be the source of much of your home's makeup air whenever the house is depressurized.

    More information in these articles:

    "Brick Chimneys With Multiple Flues"

    "All About Wood Stoves"

    "Farewell to the Chimney?"

  4. DAVID GOODYEAR | | #4

    Yes, I know now. It has been tricky to say the least. Luckily it hasn't been that problematic. I haven't really had any issues other than the issue caused by the ERV. I have been monitoring the setup and everything seems fine now that the house pressure has been balanced. Thanks for the extra info!

  5. lance_p | | #5

    David, first off great blog! Following your build has been insightful.

    Thinking through your issue, balancing the pressure across the ERV core won't be the same as balancing the pressure across the whole ERV system unless you have exactly equal restriction to flow in each of the two air paths (fresh and stale air). Since pressure changes with flow rate, a higher flow rate would be required in the less restrictive path in order to give an equal pressure drop.

    I'm guessing since you experienced a negative pressure in the house with a balanced ERV core, that you have a less restrictive exhaust (stale air) path.

    Perhaps someone with direct experience can prove or disprove my thoughts (I'm no expert), but this makes sense to me. Either way, if you've balanced the house with the outside environment you're probably good to go.

    Lance Peters

  6. DAVID GOODYEAR | | #6

    Lance, first of all thanks! Documenting the house build wasn't easy but it was certainly worth it.

    What you described here is exactly the scenario that I had scratched down on paper and I was hoping that others with more ventilation experience could confirm this. As you and Martin have pointed out, If I balance the house, I'm good to go! I have verified this. When I balance the house, I no longer feel air coming through the stove pipe joints.

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