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zacman 06-16-2010 11:36 AM

Whole house fan using old soffit duct work
2000 sq ft ranch style older house - hallway runs down the center of house with lower ceiling in the hallway which served as the duct for furnace - with wall vents into each room. I installed new furnace with new attic duct runs to each room thru ceilings. Now I want to utilize the old - abandoned - hallway-soffit duct run for a whole house fan. Plan is to mount a WH Fan in the attic above the hallway duct - build a box there and then use the existing ductwork which has louvered vents into each room already. My thoughts are that it will be less noisy as not mounted directly through the ceiling and should be somewhat efficient as each room has its own "intake" vent with adjustable louvers to fine tune the air flow. With windows open in rooms - or selected rooms - should get the cooler air into the rooms and out the attic. See any problems with this? What WH Fan is recommended? Want at least a two speed system and probably a remote control as well. . . Looking for any help I can get before I start this, and need help with selection of the unit. Thanks. . .

xxPaulCPxx 06-16-2010 12:58 PM

Fist, make sure it's actually abandoned! When I had new duct installed, I made sure each room had a supply and a return. Make sure the old stuff isn't set up as the return already to the furnace.

Otherwise, sure this could work! Just make sure you plan on adding extra exaust ventalation to wherever you mount the fan, otherwise you will pressurize the attic and blow air through your outlets!

zacman 06-16-2010 03:54 PM

Thanks for response. It is clearly abandoned - did that when I put the new furnace in. The return air for the new furnace is near the floor and below the furnace closet. . I had that size increased when I put the furnace in. All that is left of the old system is the duct- actually the soffit - (not lined with anything by the way) and one outlet to a swamp cooler thru the attic an to the roof - I will take out the swamp cooler (which used the same duct run to put moist air in). I will then increase the size of the swamp cooler box-duct access to accommodate the new WH Fan, and wire it there. I will calculate size of WH Fan and then figure out the attic venting requirements- ie the outflow. since we are also reroofing, this is a good time to do this and I understand the roof will have continuous ridge vents but not sure of the toal sq inches of venting that provides. I will also need to make sure the old duct vents will have enough size to provide sufficient air for the WH Fan. . .that is, that the openings are enough to let enough air pull thru. . . the sq inch for the ducts total about 450 sq inches but I will add one for a room that did not have a duct to the old system - so I guess I will have about 600 sq inches, and I could add one in the hallway directly into the duct run.

When you say to make sure each room has a supply and return, are you referring to having the windows open for the supply? I may not understand that part. Thanks again.

Wildie 06-17-2010 01:44 PM

Pardon my ignorance, but what is a ' swamp cooler' ? Never heard of that, before.

operagost 06-17-2010 02:29 PM

A swamp cooler cools the air using water instead of a heat pump. Naturally, it works best in a dry climate.

zacman 06-17-2010 04:36 PM

Opergost is correct. . . it is very dry here and many houses use swamp coolers, especially in the foothills where a/c is not essential. They do cool you off by adding moisture to the air and are very energy efficient, but I have never liked the feel of the effect at times. Interestingly, some homes have both a/c and swamp coolers which would fight each other if on at the same time as one adds humidity and the other takes it out as I understand it.

By the way, just spoke to a WH Fan dealer and they recommend against using the existing duct work as I planned. . . stating that the different duct openings in the rooms would be fighting for the air. . not sure I understand that as I thought each room would have a neat way of drawing air in from the windows in that room while taking air out of the room and out thru the attic. . . I will have to investigate and ponder this a bit more. . .

xxPaulCPxx 06-17-2010 04:59 PM

The supply to a room blows in the air (warm or cold), the return pulls air out of the room and brings it back to the blower to be sent through the heater or cooler.

I like your idea, and I think you are doing it right.

One thing - I would create a specific exhaust path for the fan, with louvers to keep out the hot/cold outside air. That way you keep your attic air flow properly balanced and remove worries of accidentally pushing attic air back down into the house.

Wildie 06-17-2010 06:00 PM


Originally Posted by operagost (Post 457657)
A swamp cooler cools the air using water instead of a heat pump. Naturally, it works best in a dry climate.

Thanks! In my area, I have seen this principle used in large commercial buildings. If I remember correctly, these were called 'cooling towers'.
Swamp cooler sounds 'cooler' to me! :laughing:

High Gear 06-19-2010 12:51 AM

I installed a whole house fan in my previous house years ago.

Here's the company I purchased from

Look for belt drive , ball bearing , 2 speed and timer .

That's what I bought and it was totally reliable.

Most times we'd turn it on high for an hour before bed , then low at bedtime ( fairly quiet) and set the timer as some times it would get to cool.

I used a 30" in a 1800sq ft home .

These fans move a lot of air ( A LOT ! ) so your duct size may be an issue here.

Another issue can be adequate ventilation in your attic , here's where a ridge vent can save the day.

Do you have enough volume in the attic so as to not create a wind tunnel.

If you have blown in insulation you don't want it blowing out your vents or plugging them.

Another thing to think about is how you plan on insulating them in the heating climate.

Gas water heater and possible back drafting , don't starve it for air.

Don't mean to throw a wet blanket just some things to be aware of .

We enjoyed the one we had .

Gary in WA 06-19-2010 06:36 AM

The negative pressure was a valid point. Would the fan ducting inter-connect the sleeping areas which would be bad for a fire? Ceiling and floor fire-stopping is required in most habitable spaces. Ducting connected to a furnace is acceptable, but is WHF ducting safe? Ask your local Building Official about the fire-stopping and smoke spread. Hopefully there are no issues and it will work!

Be safe, Gary

zacman 06-19-2010 11:09 AM

Thanks to all. . great points to consider. I had not considered the fire path issue at all.

In winter - heating cycle time, we would close the louvers on the intended WH Fan vents (ie, the old abandoned duct vents), and would, of course, still have louvers below the wh fan in the attic as in the typical installation, tho I don't know if that would be enough. Now those vents have a path to the swamp cooler which will be taken out when or if we put in a WH Fan - whether we use the existing abandoned ducts or just cut a hole in the hallway ceiling.

Gas water heater gets air from crawl space and from attic, but it is in the same utility closet (small) as the furnace -a/c.

I would likely build a box above the wh fan in the attic so the air would blow up toward the top of the attic and then be dispersed there to avoid the insullation blowing around. . .would need to do that in any event.

Not sure about volume in the attic issue. . . in last house 25 years ago we put in a wh fan and loved it but had a very large attic. . . this one is big enough to enter and crawl around in, tho cannot stand up . . .will have to think about the volume issue. We were thinking about a 30 inch unit.

You have given me a lot to think about. . thanks again. . .

High Gear 06-19-2010 12:57 PM

I had blown in cellulose with a max attic height probably around 3 1/2 feet.

I had some issues with the fan stirring up the ins dust

You should be able to build some baffles to control this.

Blown glass may not give you any trouble

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