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Advice on passive air inlets with bath fan exhaust ventilation?

Background: new construction, climate zone 6 (03846), 12" double stud walls w cellulose, heating/cooling is single Mitsubishi 12k btu heat pump, 2 story, very open floor plan downstairs (and up). Blower door test done before insulation/sheetrock and came in at 0.67 ACH at 50 Pascal.

Our ventilation strategy is Panasonic WhisperGreen bath fans (80 cfm models), one in our downstairs half bath and one in upstairs full bath, short and mostly straight ducting to exterior. Before insulation and sheetrock, we installed two Panasonic passive air inlets (they allow up to 18cfm each), one in upstairs master bedroom and one in open downstairs area. We stayed at two because we thought that might be enough and didn't want to put more holes in the envelope at that time.

I would imagine windows will be closed much of the time in winter, but open a decent amount spring through fall. Some south 2nd story windows could be cracked in winter because they are sheltered and stay warmer in sun.

Any advice especially from those that have used this system...are we not providing enough make up air for the bath fans and hampering their ability to exhaust, especially shower moisture? Maybe another air inlet in the other bedroom upstairs?

Not crazy about more holes in the wall, but I don't want to mess up the ventilation strategy. Thanks,

Brian

Asked by Brian Post
Posted Thu, 07/03/2014 - 11:27
Edited Fri, 07/04/2014 - 05:30

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5 Answers

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Brian,
Most homes have plenty of air leaks -- enough to easily meet the needs of an exhaust-only ventilation system.

Your house, however, is almost Passivhaus tight, so it never hurts to check how your fans are working. Fortunately, it's not that complicated. You need to find a home-performance contractor who has a flow hood. Ask the contractor to measure the air flow rate (in cfm) of your bath fans when they are operating.

If the airflow rate meets your ventilation goal (which is probably based on ASHRAE 62.2), you're all set.

Here's the principle behind my advice: if the cfm of the bath exhaust fan is close to what you would expect from the fan's rating and the static pressure of the duct system, and if it meets your ventilation needs, then clearly it's getting enough makeup air.

You may even find that the fans operate just fine if your passive air inlets are covered up with tape. Give it a try.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 07/03/2014 - 12:28
Edited Thu, 07/03/2014 - 12:30.

2.
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Thanks, great advice, I didn't think about about taking a measurement.

Answered by Brian Post
Posted Thu, 07/03/2014 - 13:30

3.
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This is not the site for seeking advice on exhaust-only and hybrid exhaust-only/passive-inlet ventilation. Since window, door and envelope vent seals are largely effective due to surface tension sealing, why teach the envelope to favor fugitive infiltration paths? You want the infiltration you know and control.

Your sq” of passive inlets (2 X 3” diameter) openings is likely insufficient for the volume of your house and the fresh air intake per hour needed in winter: one needs 3 to 4 times that for 1000 sq’ on a single floor. Allowing 18 cfm is not the same as supplying 18 cfm. Are your vents above or below the neutral pressure plain? Stack effect, variable external pressures, seasonal and weather, etc, will determine the true inlet performance. And are your mechanical vented? Do you have a fireplace or wood stove?

Fewer openings mean stronger drafts at those locations. Whereas more openings in the envelope means a lower velocity of air intake per vent, therefore less uncomfortable drafting, better building-wide fresh air balance and distribution, and the ability to close off/isolate sections.

Small (room sized; single point) ERV’s can do double duty: fresh air and heat recovery.

Answered by flitch plate
Posted Thu, 07/03/2014 - 23:05

4.
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The house is two stories 24x32', so conditioned space is 1320' taking into the walls. No wood burning, mechanicals and appliances are (will be) all electric.

I see what you mean about more openings giving more control. We have thought about single point HRVs down the road if the basic inlet/exhaust strategy doesn't work well for some reason.

Answered by Brian Post
Posted Fri, 07/04/2014 - 08:31

5.
Helpful? 0

More control and more comfort.

Answered by flitch plate
Posted Sat, 07/05/2014 - 00:09

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