Cen­tral fans work, but duct and fan sizes cru­cial

Winnipeg Free Press - Section G - - HOMES - ARI MARANTZ

QUES­TION: We have a 2,000-square­foot cab-over home with cen­tral ex­haust in two bath­rooms and one vent in the main liv­ing area. The cen­tral ex­haust in the large en­suite off the cab-over bed­room does not clear the mois­ture as well as it should. Re­cent- ly, we have added an­other bath­room in the base­ment. The plumber in­stalled an­other cen­tral ex­haust vent in that bath­room. We have ac­tu­ally closed off that vent as we haven’t com­pleted the bath­room so are not us­ing it yet. We have trick­les of mois­ture run­ning down the en­suite bath­room walls af­ter we shower. We would like to know how we can fix this.

I had the plumber out to in­spect the amount of air be­ing pulled from each of the vents. He said the amount is con­sis­tent through­out the house. He sug­gested we in­stall a larger ex­haust fan to move the air through the cen­tral ex­haust. We had a 195 and now he in­stalled a 250. Un­for­tu­nately, there has been no im­prove­ment with re­spect to the mois­ture on the walls. The plumber also said we should have in­stalled a six-inch pipe to the up­stairs bath­room rather than a five-inch pipe. Short of rip­ping out the walls, he has no other sug­ges­tion.

Can you think of any­thing we can do to im­prove the amount of air be­ing sucked out of the en­suite vent? We have al­most closed off the vent to the main room and we have half­closed the vent to the main-level bath­room, to no avail.

Thanks for tak­ing the time to read this email. Au­drey Kozak-Dobush, Win­nipeg

AN­SWER: Mois­ture buildup in bath­rooms due to poor or missing ex­haust fans is one of the most com­mon de­fects I see in homes, but nor­mally older ones with de­te­ri­o­rated win­dows, not newer homes with cen­tral ex­haust fans. I will of­fer a cou­ple of sug­ges­tions for sim­ple ideas that may rem­edy your sit­u­a­tion as well as a more elab­o­rate up­grade, if those are not suc­cess­ful.

The main rea­son steam or wa­ter vapour are not able to be quickly drawn from the reg­is­ter in your mas­ter bath­room is the length of duct­ing be­tween this area and the cen­tral ex­haust fan, rel­a­tive to the other bath­rooms. The main-floor bath­room and the newer base­ment bath­room likely have min­i­mal lengths of duct­ing, with only one or two el­bows, to con­nect to the cen­tral fan in the base­ment.

With each change in di­rec­tion or el­bow, and with ev­ery ad­di­tional me­tre of duct­ing, fric­tion is sig­nif­i­cantly in­creased, re­duc­ing air flow from the reg­is­ters in the bath­rooms. This will be com­pounded by the high mois­ture con­tent in the bath­room air and de­bris and dust that builds up in these metal pipes. In a nut­shell, the shorter the dis­tance be­tween the bath­room and the fan, the bet­ter the chance of ef­fec­tively re­mov­ing wa­ter vapour from your shower.

While shut­ting the dampers on the reg­is­ters on the main floor and base­ment may have min­i­mal pos­i­tive ef­fects, it will do lit­tle to elim­i­nate the prob­lem, as you have seen.

I would sug­gest open­ing them up to pre­vent mois­ture prob­lems de­vel­op­ing there, as well. Also, in­creas­ing the size of the duct­ing from the bath­room reg­is­ter may re­duce the fric­tion some­what, but would not be ef­fec­tive with­out a sig­nif­i­cant in­crease in the size of the fan, as well. A 50-CFM in­crease in air move­ment from the fan may not be nearly enough to com­pen­sate for these is­sues. Only a sig­nif­i­cant in­crease in air­flow would be enough to solve your prob­lem, from that per­spec­tive.

As most read­ers will know, I of­ten sug­gest call­ing in pro­fes­sional trades- peo­ple when prob­lems like this oc­cur but you may have gone to the wrong in­di­vid­ual in this case. While your plumber may have some knowl­edge of air move­ment and ven­ti­la­tion in homes, that is not usu­ally their forte.

Plumbers are trained about plumb­ing vents, slopes and an­gles to know the cor­rect meth­ods for in­stal­la­tion of plumb­ing drains, sup­ply pipes and fix­tures but not nec­es­sar­ily about ven­ti­la­tion sys­tems. That is the do­main of a heat­ing, ven­ti­la­tion and air con­di­tion­ing (HVAC) tech­ni­cian.

That is likely the type of trade that in­stalled the duct­ing for the cen­tral ex­haust in the first place. Un­less they made a se­ri­ous er­ror in cal­cu­la­tions, or there is some de­fect or block­age in the duct­ing to your up­per-floor bath­room, the prob­lem is not in that area. Hav­ing said that, air move­ment and air pres­sures within a mod­ern home, es­pe­cially one with a com­pli­cated floor plan like yours, can be quite com­plex.

The key to re­moval of ex­cess mois­ture from your shower is to in­crease the air­flow to the bath­room, ei­ther through the ex­haust duct­ing or the cen­tral heat­ing sys­tem.

While it may be dif­fi­cult to fix the air­flow from the cen­tral ex­haust fan, as you have noted, it may be very sim­ple to im­prove that from the heat­ing sys­tem. This can be achieved by cross­con­nect­ing the con­trols for the ex­haust fan with the fur­nace fan. In other words, when the switch is turned on in your bath­room for the cen­tral ex­haust fan, it should also en­gage the fur­nace fan con­trols, to kick the fur­nace fan on to max­i­mum.

This should be a fairly sim­ple pro­ce­dure for an elec­tri­cian or HVAC tech­ni­cian as long as the fan and fur­nace are in close prox­im­ity and the base­ment ceil­ing above both is ac­ces­si­ble. This sim­ple change may al­low enough air to be cir­cu­lated through the heat­ing sys­tem duct­ing in the en­tire home to change the air pres­sure in your bath­room and help draw con­sid­er­ably more air through the ex­haust fan.

At a min­i­mum, the ad­di­tional air move­ment will help the damp air dry more quickly or pre­vent moist air from be­ing trapped in cor­ners of the bath­room that the ex­haust fan can­not prop­erly ser­vice. If you still have prob­lems with the mois­ture af­ter fol­low­ing my sug­ges­tions, re­place­ment of your cen­tral ex­haust fan with an HRV may be the ul­ti­mate so­lu­tion.

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