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I recently had a Panasonic Whisper Series bath fan installed in my bathroom by a plumbing/HVAC company and wired up by a separate electrician. The fan starts up extremely slowly: it sounds like nothing is happening until 5-10 seconds after the switch is thrown, and it takes about 45 seconds to reach full speed.

Here is a video I made to demonstrate: https://youtu.be/X2nrfbXdfRU (note: if I'd had the cover off, the video would show the fan won't move at all for 1-2 seconds after throwing the switch).

Unfortunately, I don't know the exact model of the fan, but I believe it to be a FV-0510VS1. The plumbing/HVAC people didn't tell me and didn't leave any manuals with me. It seems to have a FV-0510VSB1 motor assembly, as that model number is referenced on a the Spanish part warning sticker I can barely make out.

The electrician that hooked it up said that "all Panasonic fans are like that." Is that true? I'm skeptical because I can't imagine anyone would want a fan that works that way -- I thought it was broken the first time I tried it.

What's going on with my bath fan? Is there any way I can fix it to start up more quickly? Is there some kind of setting or could I hire another electrician to wire it up differently?

  • Does it meet the quietness standard that it promises? That may be a factor. – Harper Feb 9 at 22:53
  • I don't think meeting a "quietness standards" has anything to do with this. After 45 seconds the noise level is stable (and fairly quiet). I would prefer it be at that noise level the moment it turns on. However, I'd be willing to settle for it to make at least some audible noise the moment its switched on. Right now, it basically does nothing for some time after you throw the switch. – Kaypro II Feb 10 at 1:19
  • Check the mtg spec sheet, this could be a design feature and how are you measuring the speed? Sound is not a reliable way to do this. Way back in high school an instructor proved that with a simple tone generator speaker and power monitor. Out of 30 students in the class only 1 got the power vs sound correct. – Ed Beal Feb 10 at 11:33
  • Are these DC motors? – isherwood Apr 8 at 16:41
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To make these fans quieter, the fans are designed to not create as much turbulence, which means they "slip" more when the air is not moving already, i.e. when you first turn them on. In addition the Panasonic fans have a built-in damper that prevents the cold air from flowing back into the room when the fan is off, so before it can move air it must get that damper to open, and when it is just starting there is not enough air pressure yet to open that damper. All of that can give you the impression that they are not yet working, but they are. This is likely just something you need to learn to live with, i.e. turn the fan on as soon as you enter and maybe use a timer to have it run on a while after you leave.

That said, there are a couple of things to check;

  1. Assuming this was a retrofit? If so, did the old fan system have a damper somewhere else in the air duct, like at the vent hood at the very end where it comes out of the outside wall? If so, then your poor little fan has to open TWO dampers. In that case, get rid of one of them.
  2. Is the vent pipe too long or has too many bends in it? If so, is there a way to shorten / straighten it? If not then you again may have to just live with it.
  3. Is there a proper vent pipe at all? Some unscrupulous types will just dump the air into the attic area and there may be an obstruction or insulation blocking the air flow. if there is no vent pipe, it may be a code violation and your HVAC contractor should correct it.
  4. Do you have proper "make up" air flow coming into the bathroom? A common mistake is that people have no grill on the door and then have thick carpet coming right up to the door, so that air cannot get through the crack in the bottom. When a vent fan pulls air OUT of the room, air (from the rest of the house) must be able to get back IN to make up for it, those little fans can't pull a vacuum. So if there is no way for the air to get into that bathroom, or even (in extreme cases) the house is sealed up so tight that NO air gets in ANYWHERE, the fan has to struggle to get anything moving.
  • I tried starting the fan with the cover off, and the fan mechanism doesn't move at all for a couple of seconds after the power is applied. After a few seconds it starts to spin slowly and ramp up. – Kaypro II Feb 21 at 20:24
  • To answer your other questions, this was a retrofit, and when I looked in the attic, the flexible vent pipe seemed too long. There's a couple feet laying on the attic insulation and at least one bend before it goes up to the vent. I don't know if there's another damper on the exterior vent hood (since it's on the roof), but the original ones on the side of my house seem to have dampers. There's plenty of make up air, since it does this when the door is totally open. – Kaypro II Feb 21 at 20:28
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I have this exact same problem with my Panasonic WhisperCeiling fan. I think it’s just how this model is. 10 second delay before it turns on, then slowly ramps up to speed. I bought the WhisperSense version for my second bathroom and it doesn’t have the delay. The slow ramp up is normal for the whisper series.

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The Panasonic fans are designed to move a specific CFM of air, no matter what the resistance of the air path (within reason). To accomplish this, they run at higher speeds if there is more resistance, and lower speeds (and thus more quietly) if there is less resistance. That means they end up running as quietly as they can given the air resistance.

Problem: they don't have an air flow measuring device, as it would add to the cost of the unit; they just have a variable speed fan, with (I believe) a brushless DC motor. So, to measure the current CFM, they measure the resistance at the fan by how much energy it takes to drive the motor.

Consequent problem: it takes a while to determine the actual CFM at a given speed, so they can't quickly adjust the speed to adapt to the resistance. But, it doesn't really matter if it takes, say, 45 seconds to get up to speed, so they slow down the response time to allow for careful measurement of the resistance so they can get the CFM right.

Try this at home: let it get to its final speed, and then block the air path (e.g. throw a paper towel across the inlet). Then listen to the fan speed; it'll slowly speed up until it gets to its maximum speed (which still won't be delivering its design CFM, but such is life). Then unblock the path; the fan will slowly slow down to its previous rate.

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I emailed Panasonic support about this and they replied:

It is normal for fan to start slowly. It does not use a rush of current to start up saving energy.

  • The 'rush of current' they describe is referred to as inrush current. You can sometimes observe this phenomenon by starting a vacuum cleaner and watching the lights dim for a quarter second. – calcium3000 Apr 8 at 17:22

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