HRV set­tings key to home­owner’s woes

Winnipeg Free Press - Section F - - HOMES - ARI MLRLNTZ

QUES­TION: I have a new home and have found that my fur­nace was run­ning al­most con­tin­u­ously last win­ter try­ing to keep our house tem­per­a­ture at 20 de­grees. I also found that the tem­per­a­ture vari­a­tions within our home fluc­tu­ate three to four de­grees dif­fer­ent than what the ther­mo­stat in­di­cates.

My home is a cab-over de­sign and 1,800 square feet. All the lit­er­a­ture that I have read in­di­cates that the HRV should run con­tin­u­ously, yet the HVAC com­pany has said that dur­ing cold pe­ri­ods you should shut the HRV off. What is the ac­cepted pro­ce­dure? Thanks. Al­lan Rau AN­SWER: There seems to be a lot of con­fu­sion, not only among home­own­ers but also some con­trac­tors, about in­stal­la­tion and use of Heat Re­cov­ery Ven­ti­la­tors (HRVs). I have seen sev­eral vari­a­tions of in­stal­la­tion in homes, most re­lat­ing to the con­trols for these units. I will pro­vide my in­sight into the proper use of these de­vices, which should ad­dress your is­sue.

An HRV is a me­chan­i­cal sys­tem that is used to help reg­u­late rel­a­tive hu­mid­ity (RH) in a home. These fairly sim­ple units are of­ten lo­cated near the fur­nace and have ducts which con­nect to the re­turn air for the heat­ing sys­tem, reg­is­ters in sev­eral ar­eas of the house, and in­su­lated ducts that at­tach to vent hoods on the ex­te­rior walls.

The main func­tion of an HRV is to re­move ex­ces­sive mois­ture from the air in a home and re­place it with drier, fresh air from out­side. This is done by pass­ing the in­com­ing and out­go­ing air through a “box,” which re­cov­ers some of the heat from the out­go­ing air, mak­ing the ex­change more en­ergy-ef­fi­cient. These units of­ten have ex­tra fil­ters to pre­vent clog­ging of the com­po­nents and to pre­vent ex­ces­sive de­bris from be­ing cir­cu­lated through the house.

One crit­i­cal com­po­nent of this sys­tem is a con­trol, typ­i­cally lo­cated in the cen­tral por­tion of the liv­ing space near the fur­nace ther­mo­stat, which has an in­te­gral hu­midi­s­tat. The idea is to set the hu­midi­s­tat to a de­sired RH, al­low­ing the unit to ac­ti­vate when that level is ex­ceeded in the house air.

Nor­mally, in a new home, the HRV is also con­nected to on/off switches in the bath­rooms. There should also be a switch on the main con­trol which al­lows the unit to be run con­tin­u­ously, by­pass­ing the hu­midi­s­tat set­ting. These switches al­low quick de­hu­mid­i­fi­ca­tion of iso­lated ar­eas which are not near the main con­trol, pre­vent­ing ex­ces­sive mois­ture from show­ers, baths, dish­wash­ing and cook­ing from cre­at­ing prob­lems in the home.

One of the main prob­lems that I’ve seen with HRVs is that the main con­trols aren’t func­tion­ing prop­erly. The unit op­er­ates only with the bath­room switches and on/off switch on the main con­trol, but not when the hu­midi­s­tat dial is ro­tated. This may due to an in­stal­la­tion de­fect, or sim­ply be­cause the builder has asked that the HRV run con­tin­u­ously near the end of con­struc­tion to help dry new build­ing ma­te­ri­als.

I’m not sure where you found in­struc- tions that an HRV should run con­tin­u­ously, but it was def­i­nitely not writ­ten by some­one from this cli­mate. And the ad­vice you re­ceived to turn the unit off when it is re­ally cold out­side leads me to be­lieve that even some pro­fes­sion­als are un­sure about the proper func- side air. When the HRV comes on, it will bring in much drier air that we have inside the home, re­duc­ing the RH inside the home.

In the sum­mer, es­pe­cially when we are run­ning the air con­di­tion­ing, the air out­side the house can be much tion­ing of these de­vices, or they’re tak­ing the easy way out.

To un­der­stand this bet­ter, we only have to look at the sim­ple prin­ci­ple be­hind this de­vice: We want to use the HRV pri­mar­ily to re­place hu­mid in­door air with drier out­side air to re­duce the RH in our homes, pre­vent­ing con­den­sa­tion and mould growth.

In the win­ter, this works well, as the out­side air is much colder and can hold much less mois­ture that the warm in- warmer and hold much more mois­ture than the air in­doors. Run­ning the HRV will bring this el­e­vated hu­mid­ity into the home while ex­haust­ing the drier air-con­di­tioned air. This is con­trary to the pur­pose of the HRV and will make the air con­di­tioner work much harder and in­crease elec­tric­ity con­sump­tion.

There­fore, the HRV should be shut off in warm weather, not cold, con­trary to what your HVAC source has told you.

Fi­nally, it may not re­quire very much fresh-air in­take at -20C to re­duce the in­door air to a rea­son­able RH level. Run­ning the HRV all the time may bring in far too much cold air, caus­ing your fur­nace to work harder than nec­es­sary. This is likely what is oc­cur­ring in your home and why the HVAC com­pany told you to shut it off. That’s bad ad­vice, but it’s true that it will stop your fur­nace from run­ning con­stantly.

The proper pro­ce­dure is to set the hu­midi­s­tat con­trol some­where be­tween 25 and 40 per cent in the heat­ing sea­son, which will en­gage the HRV only when the RH in the home ex­ceeds that level. As the weather gets colder out­side, you should lower the set­ting on the con­trol to pre­vent con­den­sa­tion on colder win­dows and other sur­faces that may lead to mould growth.

It’s pos­si­ble that there is some­thing wrong with the op­er­a­tion of your fur­nace or ther­mo­stat, or it’s un­der­sized for your home, but the likely cause of your is­sue is the HRV con­trol. If the HRV is run­ning con­stantly, year­round, it’s bring­ing in too much warm, moist air in the sum­mer and too much cold air in the dead of win­ter, caus­ing the fur­nace to run con­stantly. Proper use of the hu­midi­s­tat set­tings dur­ing the heat­ing sea­son and avoid­ing use of the main con­trol switch in the sum­mer should elim­i­nate your prob­lems. Ari Marantz is the owner of Trained Eye Home In­spec­tion Ltd. and the pres­i­dent of the Cana­dian As­so­ci­a­tion of Home & Prop­erty In­spec­tors — Man­i­toba (www. Ques­tions can be e-mailed to the ad­dress be­low. Ari can be reached at (204) 291-5358 or check out his web

site at www.trained­

This di­a­gram il­lus­trates the in­ner work­ings of an HRV unit, a me­chan­i­cal sys­tem

that is used to help reg­u­late rel­a­tive hu­mid­ity.

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