Hi all.. I've got a 12VDC furnace in our RV (the furnace is gas powered but uses a 12VDC fan to help with combustion & emit the heat out the front).. However, when the thermostat decides it time to power up the fan & furnace proper it just hits the full 12V on the fan and it starts lettin loose really fast and can easily wake people up from a dead sleep.. I was hoping I could use something like a motor controller with soft start to bring the fan up to speed at a reduced rate -- perhaps within a few seconds of powerup. -- still ahead of starting the burner which is usually >5 seconds after the fan kicks in.

One item that I found on a quick google search was this one.. I'm not sure the soft start delay is long enough but it's a good start.. Is something like this or similar devices a way to achieve my goal of slowly bringing the fan speed from 0 RPM to whatever the max is at 12VDC within several seconds of powerup? Thx!

 

Should work good assuming your fan draws less than 30 amps.

 

The slow startup time for that device is 1-1.5s. Will that be slow enough for you?

 

Cool.. Thanks guys.. Now if I can only find one that has a slow-start delay in the 5 second range that would be perfect.. I think in this case the 1.5s delay is a bit too short. If you know of some kit boards or ready-to-go items that fit the bill that would be great. The fan in this case is WAY below 30A current draw -- I'd guess somewhere closer to 1A or perhaps 2A max.

 

Please excuse my curiosity but...I fix these for a living and I've never seen such a problem. My first instinct is, "fix the fan". (Is the cat stuck in it?) There is no reason a fan should be that noisy, even at 10 or 20 amps.

My second instinct is, use a stack switch. It refuses to let the fan come on until the heater gets to (adjustable) maybe 130 F degrees, but you probably don't have room to install one if the furnace didn't come with one originally. There are also fixed temperature switches called, "Klixon". You would need one called, "F-120" to, "F-150" or something like that. Attach that to a slow start module.

You are allowed to ignore me, too.

 

No worries.. The furnace in question is for an RV and I gather they don't have all the bells-n-whistles that newer home equipment might have.. I should also note that this particular unit is 27 years old and I believe has NO smarts of any sort (from 1985).. So, its all discrete electronics -- mostly I suspect thermistors/thermocouples,etc.. Anyway, once the wall-mounted thermostat decides it's time to turn on the furnace, the power is applied to the fan motor first and then later the burner is lit (backwards from current home units I believe) -- perhaps 5-10 seconds later. However, when the power kicks on the fan it's enough to startle you quite a bit.. I've still got to find (or build?) a slow-start module -- not too many of them out there apparently. I've pinged the company that designed the kit linked to above to see if the 1-1.5 second delay can be extended whether via a resistor or some other means.. If it's programmed into the IC then it's probably out..

 

There isn't enough room on that board for a microprocessor. You can probably find the delay capacitor and increase its capacitance value.

 

It claims to have a PIC controller on it (8 pin?) -- perhaps right behind one of the POTS?

 

Oops. My bad.

 

I don't know about your RV furnace, but mine runs the fan first to purge the fire box, then waits for proof of air flow before allowing ignition. What will the soft start do to your built in safety devices. Hate to see you blow your RV up (especially if your in it)

 

That certainly argues against a significant delay! (Are we getting back to, "fix the noisy fan"?)
It also raises the question: Why would the fire box have a flammable charge in it when the furnace has been off? Just naturally leaky gas valves?
Then there is, "help with combustion & emit heat out the front". One fan for both jobs?
(Apparently I've missed out on learning about a lot of really cheap equipment by not working on mobile homes.)

 

Osx, the unit probably has an airflow sensor as a safety device. That is why the fan stats first. The burner will not fire until the fan has sufficient airflow to keep the unit from over heating. Hence full power at start. A one to one and one half second start delay should be significant to reducing the noise. You have to make sure that you don't slow the startup to the point that its own circuits think the motor did not start.

 

Thanks guys.. In this case the furnace is close to 30 years old and I'd be really surprised if it has much along the ways of 'smarts' in it.. As it is now, the fan comes on and something close to 10 seconds passes before the burner even considers being enabled. Regardless, the noisy fan needs to be replaced soon anyway so I'll be able to find out what other electronics are hiding beneath.. This may be one of those cases where I just leave it as-is (no enhancements) -- particularly if it's going to interfere with safety features.

 

(Sound of applause)
Yay. Good sense prevails

"Enhancing" furnaces is strictly forbidden by law. It causes house fires and dead people!
Put it back the way you found it! (but with new parts)

 

...the furnace is close to 30 years old and I'd be really surprised if it has much along the ways of 'smarts' in it..

Don't be so sure. Folks 30 years ago weren't keen on dying of carbon monoxide poisoning either. And changing a working design has risks - these things tend to move slowly.