Adding a Furnace - Page 2 - Fiberglass RV


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Old 03-22-2010, 06:39 PM   #15
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The RV store finally got the Grill in!! I decided to add a cold air return on the bottom door as an experiment. It seems to do a good job pulling all that cold air out of the floor pan. I still have some touch up work to do but It did come out pretty good! Looks like it might be time to repaint
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Old 03-24-2010, 01:02 PM   #16
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Our furnace noise drives us batty, too. Perhaps one day I'll design a circuit that drops the voltage supplied to the furnace fan after it starts up, shunting the current through one or two resistors so I can drop the voltage supplied to the fan to 10.5 volts. That'll reduce the fan noise and cut power consumption in our trailer.

The trick will be to make sure the fan gets at least 10.5 volts power, meaning the circuit will need to adapt to changing charge levels as the battery discharges and its output voltage drops.
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Old 03-24-2010, 02:42 PM   #17
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Our furnace noise drives us batty, too. Perhaps one day I'll design a circuit that drops the voltage supplied to the furnace fan after it starts up, shunting the current through one or two resistors so I can drop the voltage supplied to the fan to 10.5 volts. That'll reduce the fan noise and cut power consumption in our trailer.

The trick will be to make sure the fan gets at least 10.5 volts power, meaning the circuit will need to adapt to changing charge levels as the battery discharges and its output voltage drops.
That is a good Idea. You could try using transistor based transformers. You can order them to a specific voltage. Just parallel them in the circuit until you have enough current rating. You could put a relay circuit on a timer. Have it automatically cut over to your 10.5 Volt converter after about 30 seconds or so. Where did you come up with 10.5? having it kick down would be good so that you guarantee a good start on the fan, and you wont put start load on your digital converter. You can order a timer circuit kit from DigiKey as well as the other parts. As I recall they have a pulse timer that comes as a small board with the IC and potentiometer that you adjust for the timing. I just briefly worked up a diagram of how I think it would work I will PM it to you. If you can make it work I would be interested in modi'n mine as well.

Oh note on the transistors: They can usually take any kind of power you through at them and keep and even constant voltage. As long as you have enough in the bank to cover the amperage you need. So if your fan is 2Amps and each transistor is 500mH you would want 4 transistors. I would add one more for reliability. They will also need a heatsink. You could mount it in the cold air return... Keep it relay cool!


!

BTW will you be at Silver Falls?
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Old 03-24-2010, 06:29 PM   #18
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Old 03-24-2010, 11:39 PM   #19
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Using several 78xx in parallel would not be my first choice. Yes, each unit is rated at up to one amp of current and three of them could theoretically handle the load, but in practice there are slight variations in the 78xx output voltages, and since electricity prefers to follow the path of least resistance that means a circuit like the one you suggested would shunt more current through the more permissive regulator and overheat it.

My thinking was to use three switching transistors with matching resistors to shunt power around one or two big (10w) ceramic resistors, passing all current through both resistors at 12.6 volts and above, around one low-value resistor at 11.5 to 12.6 volts, around one high-value resistor at 11.0-11.5 volts. Below 11.0 volts the third transistor would pass pass power directly to the fan.

By doing the voltage reduction in a pair of robust, high-watt ceramic resistors I'd be passing the current handling and regulation tasks to components designed to handle a high-current connection as well as high-resistance heat loss. This part of the approach is very similar to what I've done to our Fantastic Fan, which has a 10-Ohm, 10-watt resistor circuit added to it to create a very slow & quiet fan speed setting.

As for a timer that would allow the fan to come up to speed, a fourth transistor/capacitor pair could shunt power directly to the fan until the capacitor reached full charge and its voltage rose to match that of the battery.
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Old 03-25-2010, 02:27 AM   #20
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I see you have a furnace. That should work fine. 12K BTU is probably required because windows have to be left open to combat the moisture from breathing.

You started out talking about catalytic heaters. Here is a good discussion of them:

http://www.rverscorner.com/catalytic.html

They do put a lot of moisture into the air.

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Old 03-25-2010, 10:24 AM   #21
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Using several 78xx in parallel would not be my first choice. Yes, each unit is rated at up to one amp of current and three of them could theoretically handle the load, but in practice there are slight variations in the 78xx output voltages, and since electricity prefers to follow the path of least resistance that means a circuit like the one you suggested would shunt more current through the more permissive regulator and overheat it.

My thinking was to use three switching transistors with matching resistors to shunt power around one or two big (10w) ceramic resistors, passing all current through both resistors at 12.6 volts and above, around one low-value resistor at 11.5 to 12.6 volts, around one high-value resistor at 11.0-11.5 volts. Below 11.0 volts the third transistor would pass pass power directly to the fan.

By doing the voltage reduction in a pair of robust, high-watt ceramic resistors I'd be passing the current handling and regulation tasks to components designed to handle a high-current connection as well as high-resistance heat loss. This part of the approach is very similar to what I've done to our Fantastic Fan, which has a 10-Ohm, 10-watt resistor circuit added to it to create a very slow & quiet fan speed setting.

As for a timer that would allow the fan to come up to speed, a fourth transistor/capacitor pair could shunt power directly to the fan until the capacitor reached full charge and its voltage rose to match that of the battery.
Well that is why I usually toss my ideas out to the smart people to build them. I have built power supplies for small electronic and computer applications. Usually not over 1 amp. I used timer circuits like that to delay start remote radio equipment so if I had a dirty shut down stuff would come up in order. I think the highest power draw was 700mH on some of the Cisco units. Now that you mention it you are completely right about the amperage shunting. That could explain some of the issues I have had with some of my power supplies. I am always thinking about digital control, in this application it is just a fan. A simple capacitor would work fine. If you ever get around to making it I wouldn't mind seeing a diagram. I think this would be a great modification to these noisy furnaces.

Thank you again!
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Old 03-25-2010, 10:36 AM   #22
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I see you have a furnace. That should work fine. 12K BTU is probably required because windows have to be left open to combat the moisture from breathing.

You started out talking about catalytic heaters. Here is a good discussion of them:

http://www.rverscorner.com/catalytic.html

They do put a lot of moisture into the air.
I removed my catalytic heater for several reasons.

Condensation

Very uneven heating

Danger of open burner with Kids around

The fumes. Even though the Buddy heater I have has safety features. We can still smell it.

Took forever to heat up the trailer.

Catalytic's work well, just not well for me.


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