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2-story home, "out of the box" question from a homeowner

Hi Guys - I'd like to hear your opinions on this.

I went to look at a job yesterday - two story home with small 2nd floor and approximately half the 1st floor's ceiling vaulted all the way up to the roof. The heating system is a 15 year old Bard high efficiency gas counterflow furnace. Slab floor with no access to any ductwork. Main return on the first floor, in the hall ceiling. Another main return on the second floor whose ductwork IS accessible. Supply ducts for the second floor are all individual branch runs (5 total) up the walls from the main trunk in the floor of the first floor. All 2nd floor branches are unaccessible without ripping walls apart. With me so far?

Homeowner wants to add cooling. The second floor is always uncomfortably warm in the summer time - much warmer than the first floor. His concern is that just adding central air will not resolve the issue of a hot second floor, and I agree with him. Adding a zone control system is not feasible due to lack of duct accessibility. We talked about the benefits of variable speed furnaces using 'constant fan' to provide constant air circulation. Then we talked about ductless mini-splits - maybe a dual zone unit with wall units upstairs and downstairs (my personal preference).

His question: Is there a way to install an auxiliary fan in the return duct serving the second floor? In his mind, he sees this forcing more cold air upstairs when the blower is running. I'm trying to wrap my mind around this idea, along with the resulting air flow patterns and subsequent comfort conditions. I'm not having much luck. Would this fan improve the comfort by "sucking" more air into the upstairs return? What would be going on inside the duct system and what other effects on air distribution would this have?

I would appreciate any feedback you may have!!!




  • Techman
    Techman Member Posts: 2,144
    edited August 2010
    Several possibilities

    The second floor is bedrooms?Are the doors closed most of the time? What is the size of the return grills?What is the size of the 2nd floor return duct?Does most of the return air go into the 1st floor return grill?I think by using an auxillary return fan on the 2nd floor ,it would have to be the proper "type"of fan.Then it might force air out of the 1st floor grill,depending.Then it might force more air into the return plenum over working the regular blower motor causing it to trip on overload,depending.Then the extra air might blow the condensate off of the evap coil,depending.What is the tonnage of the unit &how is it divided up between the two floors? How much total air gets to the 2nd floor?More supply air thru properly sized  supply ducts is noisier .Ductless splits are looking pretty good about now!A load estimate would be useful in giving the best advise to your customer. 5lbs of _ _ _ _ in a 4lb bag doesn't quiet make it!!!!!!
  • Jack
    Jack Member Posts: 1,046
    You might want to check out

    the Airshare by Tjernlund. It is a room to room/floor to floor fan. It's a problem solver. I represent Tjernlund so bias noted, but it has helped me in installations of mini-splits in Capes for instance. Two small upstairs bedrooms would typically require an evap in each room, but this is just expensive and oversized. By installing an Airshare between rooms it allows, in some circumsatnces, the elimination of one evaporator and actually brings the equipment sizing more in line.
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