Dryer Booster fans ?

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.

 

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."

 

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.

 

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/product-specs/HVAC_Express_Butterfly_Damper_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?

 

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.

 

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.

 

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.

 

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.

 

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.

 

If the fan motor is turning, the blades are turning. It may be that the booster is working against pressure, and is not able to reach a rate of airflow that you can feel. Additionally, the lint screen box may be leaky.

Regarding tight homes needing air exchanges, that is definitely true. But it is ideal to control the rate of those changes, and to filter and condition the outside air as it enters the house. An HRV in a tightly sealed house allows that. The humid air entering from outside will still be dried out by your AC system before it goes to the dryer, so, one way or another, changing the RH of that air to absorb moisture from the clothing is accomplished.

 

I like the pipe in a pipe idea to scavenge the waste heat and bring in make up air. I do it with a Polaris water heater. But you might have a serious condensation issue at certain times. Better use aluminum pipe.

 

If I'd stuck with the coaxial pipe idea, I'd have switched to an 8" outside pipe to be sure to have very little resistance on the intake side. But because I would have to insulate the whole thing anyway, I just went with 1.5" duct board, and cut it to fit very tightly between my ceiling joists (it extends beyond the top of the joists). Here in Dallas, condensation should only occur during a few times a year, so I just sloped the exhaust pipe down from the beginning of the duct board heat exchanger to the vent outside, and put the seam up.

 

Solved the problem with the malfunctioning FanTech. Turns out the increased air velocity moved old lint in the ductwork down the pipe until the duct clogged. Pushed a drain cleaner snake up the outlet from outside about ten feet and knocked loose a huge ball of lint.

Showed the Mrs the ball of lint and she responded, "That's the biggest chunk of belly button lint I've ever seen." I just threw it in the trash and shut up .....

 

Update on my modified dryer venting: Finished the system, installing a gravity-operated damper on the intake, insulated all the ducts, and still working just great. Lots of velocity at the exit, full loads dry in 40-50 minutes. So, I can tentatively recommend this type of system, though it adds expense, and is detail-intensive. I've not needed any boosting for either the intake or the exhaust, and that is probably at least partially because this modern dryer seems to have a high-capacity exhaust fan. The manual lists acceptable equivalent duct runs that far exceed the typical recommended run of 25'.

I'm a bit taken aback still by what I consider unwarranted negativity regarding the design, and I think that negativity comes from a failure to understand the mechanism and assumptions, as well as the common reactionary aversion to innovation and breach of convention. I've dealt with that a lot, particularly from engineers approaching retirement. Maybe I'll be like that one day, too.

 

Industry standards for free air makeup on a gas appliance are fairly well estabilished...you don't have that. Regardless, it's your house, and if it works, fine for you. there may be situations where it doesn't, as the standards are set to cover many contingencies, which you may never experience, or have yet to experience. What works in one situation, may not work for all. Providing unconditioned combustion air to a gas appliance is a good way to save money, but it has to be designed properly.

 

Code dictates that when a combustion appliance draws air through a duct directly from the outside, an opening of 2 sq in per 4 kbtu/h is required. A dryer produces less than 40 kbtu/hr, so only 20 sq. in are required. My 6" duct provides 28 square inches. BUT, more importantly, the combustion air for a dryer is not supplied passively, which is what that particular code is written for. More relevantly, for forced air combustion appliances, code dictates that all of 1 cfm of outdoor air is required for every 2400 btu/hr. So for 40 kbtu/hr, that comes out to all of 17 cfm. A dryer PULLS 150 cfm or so of air through the cabinet.

Incomplete combustion is almost completely impossible.