Dryer Booster fans ?

[Agu]

After some use I've learned a couple of things.

When cleaning the lint filter turn on the dryer for a few seconds so the auxillary fan is running. That way loose lint gets sucked into the screen instead of falling on clean laundy,the dryer, or on the floor when the lint trap is opened.

The aux fan is on a 10 minute timer. To shut it off when the dryer cycle is done I've installed a switchable power strip. Although the fan doesn't use much power, sucking out heated or air conditioned air is a waste of energy. Don't know if there's a code for this but if it's a code violation please ignore my comment .

[Chris in Dallas]

Hello, this is my first post here. Regarding boosting the dryer exhaust: what about boosting the dryer intake? The fan could be a a cheapy (not a $200 exhaust booster, made to work with lint), and the air source could be from outside the house. Obviously, the intake would require some fabrication, and large leaks in the cabinet would have to be sealed. Could providing some pressure to the make-up air improve exhaust venting? Would it affect dryer performance?

[jadnashua]

Hello, this is my first post here. Regarding boosting the dryer exhaust: what about boosting the dryer intake? The fan could be a a cheapy (not a $200 exhaust booster, made to work with lint), and the air source could be from outside the house. Obviously, the intake would require some fabrication, and large leaks in the cabinet would have to be sealed. Could providing some pressure to the make-up air improve exhaust venting? Would it affect dryer performance?

Unless there is an obstruction, a dryer pushes as much air as it can. The problem is the outlet is too restricted, so new air into the space, while important, is rarely a big issue. Now, for max economy, a heat recovery system for that air pulled into the room might be a good idea, but just ensuring more air won't cut it...a booster fan in the duct does.

[Chris in Dallas]

I don't think I was clear enough in my post. If the dryer intake is sealed, so that the make-up air is pressurized in the dryer, how would that affect performance?

[Chris in Dallas]

For heat recovery, I'm using a coaxial counterflow setup, with make-up air for the gas dryer coming from outside of the house. 4" duct for centered in 6" supply duct. Will have a 4" dryer booster right after a supplemental lint screen after about 6' of run with two 90 degree bends and one 45 bend. They join via a 6" tee right above the dryer (stacked dryer, vents through ceiling in closet in middle of house, then goes through attic, which is sealed and semi-conditioned), and will separate right at the gable, so that each can have its own spring-loaded damper. The 6" duct supplies air to the dryer, via a plenum that covers almost all of the intake louvers on the dryer. Shortly after the booster, the duct will slope downhill to let any condensate flow out. There shouldn't be any condensation prior to the booster fan.

[jadnashua]

Pressurizing the makeup air would likely mess up the air/fuel mixture, so I wouldn't do it. Now, if that path was too restrictive, then you'd need to ensure it was big enough. A 6" duct probably doesn't meet the manufacturer's requirements for free air intake. Often, they want one square in per K-BTU - check to be sure. For a typical gas dryer, it would use about 45K-BTU, and need 45 sqin of inlet. An 8" duct without too many elbows would just barely qualify...a 6" is too small, especially if it is coaxial, and part of that volume is overtaken by the exhaust. Code-wise, I think you might have problems with what you've done...performance-wise as well.

[Chris in Dallas]

Well, a 6" inline duct booster will provide a maximum of 250 cfm when it is boosting existing airflow. I figure the dryer will draw 150-200 cfm for both combustion and drying. The fan, working from a restricted source (the 1" gap around a 4" pipe, over a distance of about 12', plus bends), but assisted by the draw of the dryer fan, should not actually be able to really pressurize the dryer cabinet. Even if that were possible, a rheostat on the fan could solve that problem. My house gets tighter every year, as I continue to insulate and seal, and I plan to install an ERV eventually.

If a dryer requires the equivalent of an 8" unobstructed duct for adequate makeup air, then a house would really whistle from air infiltration when the dryer was running. Regarding code, there is nothing in the 2006 IRC section G2439 that prohibits anything I've described. Makeup air is only to be provided by "approved methods."

[jadnashua]

I just looked at the manual for my gas dryer. It says 72 sqin minimum of unobstructed air intake when installed in something like a closet. By closing off your air intake you are functionally doing the same thing as putting it in a closet. I don't think that what you are doing would meet the manufacturer's minimum requirements and would impact operation and safety. If it doesn't get enough air, it could shut down, and using forced air could disrupt the fuel/air ratio and risk soot buildup and a fire. Codes usually have a provision that also states "installed per the manufacturer's instructions"...you are NOT doing that.

[Chris in Dallas]

I have the 4" exhaust run finished, with a Broan Ecovent damper. The run has two 90's and a 45, and about 17' straight. The manual allows 45' of straight pipe with three 90's, so I'm well inside that requirement. I changed the make-up air supply a bit. The last ten feet of exhaust run (except the gable penetration) runs through a rectangular duct, made of 1.5" fiberglass ductboard, with the foil scrim side facing in. The return air simply flows around the 4" pipe down that duct, exchanging heat from it. The duct interior dimensions are 6" high x 11" wide (66 sq in). Subtracting the 4" duct from that leaves 53.4 sq in.

I separated the supply duct at the end of the ductboard run, with a 6" round takeoff in the top of the rectangular duct. That turns an immediate 90 degrees, and runs 5.5' along the gable wall, then exits, creating a 5.5' lateral separation between intake and exhaust. The intake is through a standard hood, over which I have fitted window screen for dust and bugs. Just before the 6" duct turns its final 90 degrees to exit the wall, I've put a gravity operated foam-sealed damper ( http://stores.hvacexpress.com/files/...mper_Specs.pdf ) to keep outside air out until there is a draw.

At the other end of the ductboard, the return air comes back out via another 6" takeoff, turns down through the ceiling, and into the dryer plenum (the box I built over the dryer intake grille). So the return air goes through three 90 degree 6" round duct fittings, one damper, 7' of straight 6" duct, and ten feet of ductboard.

I'm debating the need for the booster fan on the supply side. It is a 4" inline fan made by CFM, which is operated by a current sensor switch from the dryer. At 0" SP, it provides 152 cfm, and draws 85 watts. What do you think?

[Chris in Dallas]

Regarding what the dryer manual says about 72 sq in unobstructed air intake when installed in a closet: what are the assumptions there? If I install to that requirement in a house with 1 air change per hour at 50 pascals, do I satisfy the manufacturer's requirements? Don't know, do we? Surely, the mfr anticipates some ACH value in their recommendations for intake area in the closet, but they don't tell us that. How much depressurization can the dryer handle and still perform well? We don't know.

Without knowing the necessary assumptions behind their guidelines, it's impossible to know what intake system is necessary under what are obviously very different assumptions. So my design is a guess, and can't be rationally evaluated by comparing it to the manufacturer's standard installation instructions.

I'll fire up the system soon, and check the exhaust. I'm not equipped to do a professional airflow test, so I'll have to do that by "feel." Then I'll just have to put in a load of damp clothes and see how it goes.

[jadnashua]

A dryer is designed for a specific exhaust path (which you're within the guidelines of), and assumes a freestanding access to air. You are not providing that, and thus, in the strict interpretation of the installation instructions, are NOT following them, and thus, could void your warranty any any safety designed into the system. I'm not qualified to evaluate whether it will work, but my gut feeling is that your forced induction of air will impact the overall operation of the system. Forcing more air than the device wants into the combustion chamber will force it to run lean, potentially overheat, and maybe not even fire up properly. Time will tell. But, your safety and the device longevity isn't worth the effort, IMHO. What would work, and is tried and true, would be an energy recovery ventilation system to the room where the dryer is, then it would operate as it was designed with minimal losses, maintain the warranty and safety of the unit, and preserve your peace of mind.

[Chris in Dallas]

I didn't make it clear enough that I'm going to skip the makeup air blower, and just provide the air via the 6" round duct and the larger rectangular duct. As that provides more than twice the cross sectional area for the return air as is provided for the exhaust, over only a slightly longer length, I think it should work fine. Now, if a mfr specified that the dryer could not operate properly with any depressurization at all (that is, it had to effectively be outdoors) then I'd anticipate a problem. But my bet is that a 6" and larger duct to the outdoors will be easier for the dryer than running the dryer in the middle of a large room in a reasonably well weatherstripped house.

As for the warranty, my experience is that they're never worth pursuing anyway, and they can always find a reason to deny it. Better just to repair it myself - it's not rocket surgery.

Also, an HRV or ERV does not provide makeup air. It only balances intake vs exhaust, so they won't work with a dryer unless you run the dryer exhaust through the HRV itself, which I'd guess would create more resistance to flow than the system I've built.

[jadnashua]

The manufacturers DO list the minimum ventillation required for a gas appliance when in an enclosed place like a closet (which, if you've closed off any other air to the device, you've effectively created a closed space), AND, it is far more than that from a 6" duct. You've set yourself up for problems. Good luck. Make sure your CO detector works!

[Chris in Dallas]

The manufacturers DO list the minimum ventillation required for a gas appliance when in an enclosed place like a closet (which, if you've closed off any other air to the device, you've effectively created a closed space), AND, it is far more than that from a 6" duct. You've set yourself up for problems. Good luck. Make sure your CO detector works!

Again, though, the "minimum ventillation required for a gas appliance when in an enclosed place like a closet" is with the assumption that outside the closet, the air is drawn through accidental discontinuities in the conditioned space envelope, and those should be minimal. So the dryer must be designed ultimately to pull air through restrictive orifices, and my contention is that a 6" and larger smooth duct to the exterior causes significantly less resistance than the dryer intake is designed to overcome.

But, beyond all this hypothesizing, I've now run the dryer. The exhaust velocity is quite high - higher than I've ever felt from a residential dryer. The ball in the Broan Ecovent is slammed against the top of its housing. I'm sure that this is partially attributable to the powerful exhaust fans found in most modern dryers like this one (Samsung ANW330). The CO monitor, less than 3' from the dryer shows zero ppm. The 6" intake is also very quiet - no apparent turbulence caused by high velocity. That is so much the case that I'm wondering how effectively I actually sealed up the dryer cabinet outside of the intake plenum.

I don't know how much the counterflow heat exchanger is actually doing yet. I'll have to measure the incoming air temp close to the dryer vs the exterior air temp to determine that. With the 78 degree current outside temp, the exhaust temp is still high. That would certainly change on a day below 40 degrees, which I'm sure we'll have again before long.

[Agu]

I just came online to search for this thread and it's at the top .

It's been just over a year and the dryer is again taking two cycles to dry clothes and getting very warm to the touch. The dryer blower is working but when I get only the Fantech Fan running I can't feel any suction in the auxiliary lint screen housing. It starts, makes the usual noise it makes when running and when I look up the vent with a mirror the fan motor is spinning. However I can't see the fan blades without disassembling the whole thing. Any suggestions ?

BTW Chris, I understand the concept of introducing make up air. Now that you've done it I hope it works for you. I wouldn't do it because I doubt the payoff using humid air from outside in Florida would offset the air conditioning savings. Besides which, even tight homes need some air exchange to keep interior air fresh.