# Converting my furnace fan to DC



## SunnyJim (Oct 28, 2008)

Had a power outage here yesterday for about 4 hours that got the old noodle working.

I have a heating oil furnace that uses an AC fan to blow the hot air through the house, so obviously it doesn't work when the power is off. There must also be an interlock that prevents the furnace from firing without the fan.

So my question is, is it possible to replace the fan motor with a DC motor so that it could be connected to battery power during utility power outages?

Additionally, what kind of battery needs would that create?

Thanks for any help, I am just kind of thinking out loud here. If there is any aspect of this plan that makes it unworkable, please say so!


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## Jim-mi (May 15, 2002)

Dog gone now yer gonna make me ponder on that.
Yes there is limit switches for the fan.
I hate the energy hog fan that I have in my furnace.

So the question of the hour is; Is there a appropriate sized DC squirrel cage fan available that will fit up . . . .with out big modifications..........................??????
Hmmm


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## Explorer (Dec 2, 2003)

Jim-mi do you know how many running watts yours draws?


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## artificer (Feb 26, 2007)

I don't know what you have for a burner, but don't they tend to have pump and igniter as well as the fan? You would have to convert them as well. It would probably be easier to just get an inverter to run the furnace.

Michael


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## Jim-mi (May 15, 2002)

Ross, what happened to my reply . . ??
And your post ?

Seems as tho a bunch of stuff on HT went poof . . . . . . . . . .(this afternoon)

OK . . I just read MD's thing about the outage.................


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## mightybooboo (Feb 10, 2004)

artificer said:


> I don't know what you have for a burner, but don't they tend to have pump and igniter as well as the fan? You would have to convert them as well. It would probably be easier to just get an inverter to run the furnace.
> 
> Michael


Agree with Art,thats a VERY GOOD answer.


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## Jim-mi (May 15, 2002)

OK....I'll try again.
Ross had a good post on this . . .but it too got zapped.

I just watched the 'running' watts on my propane furnace at 480
In rush was well over 600.
I'd leave the existing igniter and 24vac thermostat stuff alone but add a relay to switch the DC for the fan. 
But . . thats all providing that I could obtain (reasonably) a squirrel cage that would fit.........with out a bunch of sheet metal mods.

Yup . . . at what point does the $$$$ to convert kill the deal. .??


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## Explorer (Dec 2, 2003)

Thanks, Jim-mi. That is a little more than I expected and sounds like we have a similar setup. I have been thinking about running a tap to my solar AC source, but don't have that much to spare at the current time.


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## artificer (Feb 26, 2007)

I was wondering yesterday what happened to the posts as well.

Anyway... I can't remember who posted about the efficiency of a DC/variable speed fan. The comment was that the variable speed fan only used 10% of the energy as a regular fan motor.

I would like some further clarification on that, or just post my thoughts about this "efficiency."

You have to be careful when you use that word "efficiency". A typical AC induction motor will be around 75% efficient. A permanent magnet DC motor is better at around 80%?. So if you run the fan at the same speed, you'll use slightly less energy with the permanent magnet motor. The energy saving comes in when you slow the speed of the blower down. Less air flow = less power. You could get the same savings by going to a different pulley ratio on the blower/motor, if its belt drive. Power savings due to less air flow means... less air flow. Are you still pumping enough air to let the system work properly? Will the furnace overheat at the lower air flow?

Jim-mi: I would ask why you want to add a DC powered squirrel cage fan to your system? If your system is functioning properly now, you can replace the motor with a variable speed one to use less energy, or tweak the fan's rpms for less power usage. I don't think you'll see much power savings by just using the DC fan/motor. One way to decrease the energy usage on DC power is to get a two speed blower motor. You might already have one. Wire the slow speed up to a relay for DC power operation. 

If you post the make/model of the furnace and the blower motors faceplate data, I might be able to provide a couple of suggestions.

Michael


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## Jim-mi (May 15, 2002)

"Change"-ing to a dc motor just for power savings.
Induction motors are notorious for their inrush requirements.

Yes your right . . .It ain't broke . .so why fix it . . .
I have put in my posts that "If I could do it for reasonable bucks" 

wishful thinking
Michael see your pm


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## artificer (Feb 26, 2007)

Jim-mi: answered your PM... 

As for inrush current, you can add a soft start kit, I believe. Should decrease the inrush current to a more reasonable number, and allow an inverter to power it.

Inrush of 600 with a 480w load isn't that much. Most inverters have double the running wattage rating for short periods of time. If you had a 750w inverter, it should handle the furnace. But then you add lights, the fridge... 

Michael


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## Jim-mi (May 15, 2002)

The control panel for my inverter is kinda slow reading watts . . . so I can't get any where near real readings for the inrush currant . . that 600 was all I saw . . .I'm sure it is far more than that.
My 4500 watt inverter handles all those things . .all at the same time just fine thank you ..............allways thinking about reducing the load . . .lol


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## Ross (May 9, 2002)

I'd replace my post better if I wasn't beat, but I am. Essentially it'd be far easier to use a portable genny to power up an oil furnace. They make the parts for DC operation, .....they're pricey and then you have to run it on DC. Not all furnaces are built the same. Some use very efficient PSC motors some use the older ones with start windings. Some use transformers to generate the spark (and consume around 400 watts) some use ignitors that use 40. You can put on PSC burner motors and ignitors for fairly reasonable $ but you still have the fan. Modern direct drive fans are much better than the old belt drives but you're still needing some power to put heat into the house!! Virtually all oil furnaces will power up on a small 2500 watt generator. A good one will save the electronics and motors a cheap one will likely fry something. Personally I heat with an outdoor boiler, and oil furnace and a wood stove in that order. I prefer redundant back ups to one dedicated system. If my boiler blows up I can crank up my furnace. If it isn't too cold the wood stove will do. If they all fail I'll have an air handler cut in some where running off my DWH ASAP! A single heat source with multiple power sources is great but one failed part and your investment leaves you cold. The same money into a wood stove or pellet stove or propane heater is an option you might want to consider.


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