Amps Draw and Voltage Drop Question 2

The question on how much power (Amps) the furnace will draw is actually rather complex.. But in general I suspect the higher the voltage the greater the amps but it's not a 1:1 relationship also the faster the blower will blow which might result in greater comfort. But you cited a 0.2 volt drop.. NOT WORTH the cost of the wire to upgrade it for 0.2 volts. for 2 volts yes but 0.2 no.

Gdetrailer wrote:
Will draw slightly MORE current due to less voltage drop, results in motor spinning a bit faster.

RV furnace motor is a DC motor, DC for practical intents can be considered like a resistor since you are not dealing with impedance/inductance like you do with AC devices.

If you want the furnace motor to draw less current, you would need to REDUCE the voltage going to the motor. However, that results in less RPMs for the fan which results in less air flow which results in possibility of overheating the furnace and/or creating a fire hazard..

Lights dim with lower voltage and also draw fewer amps when dimmed, so that fits.

Slower fan speed in the furnace with lower voltage would get to the point the sail switch would open, so that is the safety there.

My 8531 older furnace says the 12v to it should not be above 13.5v. That is in the manual's section about the flame sensor's microamps, but I can't find a minimum voltage specified for when the blower fan gets too slow.

The furnace does work ok when battery charging is happening and the battery voltage is above 13.5v. I have not measured for voltage at the furnace then. Doug R and the other techs here would know about all that.
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Part 2--

Not sure it is the same thing for how amps work--I am now looking at the DC-DC charger

With the input of that thing, you must use fatter wire to the battery to keep the amps down and the voltage up.

Could relate to the point made above re wasted amps making heat with voltage drop? Fatter wire means the alternator does not have to do extra amps to make that heat?

Yes the LFP does seem to move a bit more air at 13.4 volts. Amp draw is significantly lower with steady 13+ volts vs. 11.x with the old battery running the kitchen.

time2roll wrote:
Yes the LFP does seem to move a bit more air at 13.4 volts. Amp draw is significantly lower with steady 13+ volts vs. 11.x with the old battery running the kitchen.

You mean the inverter's amp draw is less without so much voltage drop? It does not have do so much to maintain the watts? That is different from the furnace draw, but might be like the DC-DC input draw, not sure.

BFL13 wrote:


Lights dim with lower voltage and also draw fewer amps when dimmed, so that fits.

Slower fan speed in the furnace with lower voltage would get to the point the sail switch would open, so that is the safety there.

My 8531 older furnace says the 12v to it should not be above 13.5v. That is in the manual's section about the flame sensor's microamps, but I can't find a minimum voltage specified for when the blower fan gets too slow.

The furnace does work ok when battery charging is happening and the battery voltage is above 13.5v. I have not measured for voltage at the furnace then. Doug R and the other techs here would know about all that.
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Part 2--

Not sure it is the same thing for how amps work--I am now looking at the DC-DC charger

With the input of that thing, you must use fatter wire to the battery to keep the amps down and the voltage up.

Could relate to the point made above re wasted amps making heat with voltage drop? Fatter wire means the alternator does not have to do extra amps to make that heat?

Flame sensor could be an issue with a too low of a battery voltage if the control board doesn't have good voltage regulation built in. Flame sensor works on the idea that the flame conducts electricity. So control board places a voltage on the sensor rod. No flame, no current flows. When there is a good flame, the flame will conduct a small amount of electricity which creates a small measurable one way current flow. Controller will monitor the current flow, if not correct current will shut down the furnace..

Potentially you could reduce the voltage to the fan motor to reduce the current drawn but I believe it is in your best interest overall to leave well enough alone as slowing down the fan affects not only your inside air flow but the burner side airflow.

As far as DC-DC setups goes, they work in the same principle as a autoformer does for AC but with DC and perhaps a bit more efficiency.

Lower input voltage will require more current to get a higher voltage at a certain amperage. So say you have a 20A DC to DC power supply and you are taking 13V and changing it to 14V you would draw say 20 amp (260W) on the input but get 18.5A (260W) on the output.. There is no free lunch.. Wattage is the same, but amperage drops..

Using a DC to DC charger, you are simply trying to boost the voltage high enough to get over the voltage lost due to higher resistance of small wiring.

You are also trying to get past the vehicle alternator's regulator which typically tapers down the voltage from the alternator from initial charging voltage after starting of around 14.4V down to 13.8V which basically means your RV battery only gets a tiny trickle charge after a minute or two of engine running..

Keeping in mind, in my example that is assuming 100% efficiency of the DC to DC charger.. Nothing is 100% efficiency in this world and DC to DC chargers are no exception and will run 80%-94% efficient.

OK,do you have an issue with your furnace? A little background-- yes the motor will slow down when voltage drops, all is normal.You are protected by the sail switch. Manufacturers build them to be in a RV, so they anticipate lower voltages.
They are rated to operate fine and safely down to 10.5 volts. That is why they have 10 volt motors in them.I have seen older Hydroflames run until voltage dropped below 10.0. If you suspect an issue, you must measure the voltage AT the furnace, not on the RV's panel,which reads the battery itself.

MT BOB wrote:
OK,do you have an issue with your furnace? A little background-- yes the motor will slow down when voltage drops, all is normal.You are protected by the sail switch. Manufacturers build them to be in a RV, so they anticipate lower voltages.
They are rated to operate fine and safely down to 10.5 volts. That is why they have 10 volt motors in them.I have seen older Hydroflames run until voltage dropped below 10.0. If you suspect an issue, you must measure the voltage AT the furnace, not on the RV's panel,which reads the battery itself.

I think the OP is more concerned about when the furnace is running plus the inverter is heavily loaded the combined load is enough to send the inverter into low voltage alarm or shut down when the batteries have been discharged to 50%..

Op may have though reducing the voltage drop at the furnace would reduce the current drawn by furnace, in the case of DC motors, nope..

OP could try much bigger wires to the inverter, even reducing the voltage drop by .1V at the inverter terminals can make a big difference.. Reducing the voltage drop between inverter and batteries can have a pretty positive effect, as the DC input voltage goes up to the inverter, the lower the DC current will be required to maintain the 120V load.. Watts is Watts..

Just pondering all this (to no great effect)

The current to the furnace fan rises with more supply voltage but the current to the input of the DC-DC falls with more supply voltage.

The reason is the furnace fan is not a fixed demand like the DC-DC or inverter's output-- it will take more if you give it more.

The 8531 furnace is rated at 8.2 amps at 12v. So that is the expected "load", but it is not the "Demand". The fan runs faster with more amps if the "Supply" voltage is higher, so the demand is a variable depending on the supply.

The inverter load side is fixed by the load's needed input watts so the "demand" is not a variable. The DC supply tries to meet that demand. As the battery voltage falls the amps drawn go up to maintain the demand watts.

The DC-DC charger has an output set at eg, 20 amps at 14.6 volts, so that is the fixed demand, not a variable. The input supply tries to meet that demand.

However, the input is itself a demand on the engine battery and alternator, which has to meet the input demand for the output demand.

Using fatter wire in the DC-DC input to the battery keeps the amps demand lower with the input voltage higher, so the alternator doesn't need to supply so many amps

The engine battery voltage is somewhat fixed as maintained by the regulator. The supply variable is the alternator amps to meet the demand.

The Dc-Dc charger "size" (output setting demand) has to be chosen, limited to match the alternator "size", where using fatter wire will let you have somewhat more Dc-Dc size for the same alternator.

Unless I’m missing something, most of responses are missing the point of the issue (as the OP might have with how he’s thinking about this).

I believe the 0.2V drop is not IR from the 8A flowing through wiring to the furnace, but instead is internal voltage drop in the batteries (assuming normal battery connection wires). OP is not measuring 0.2V drop across the wire to the furnace, but sees that drop in battery voltage. Doesn’t sound like a big battery bank, or strong (low internal resistance) batteries like Lithium or Lifeline AGMs. There is a reason that SOC is measured with resting batteries and not under load, any battery will have some voltage drop when sourcing significant current.

To answer the OP question - no, a larger wire to the furnace will not help the situation. Any small IR change by beefing up this wire would have 2nd or 3rd order trivial change in motor draw, fan speed, etc. Better batteries and/or more capacity is the issue you’re dancing around.

MNRon wrote:
Unless I’m missing something, most of responses are missing the point of the issue (as the OP might have with how he’s thinking about this).

I believe the 0.2V drop is not IR from the 8A flowing through wiring to the furnace, but instead is internal voltage drop in the batteries (assuming normal battery connection wires). OP is not measuring 0.2V drop across the wire to the furnace, but sees that drop in battery voltage. Doesn’t sound like a big battery bank, or strong (low internal resistance) batteries like Lithium or Lifeline AGMs. There is a reason that SOC is measured with resting batteries and not under load, any battery will have some voltage drop when sourcing significant current.

To answer the OP question - no, a larger wire to the furnace will not help the situation. Any small IR change by beefing up this wire would have 2nd or 3rd order trivial change in motor draw, fan speed, etc. Better batteries and/or more capacity is the issue you’re dancing around.

OPs voltage measurement is highly dependent as to exactly where the voltage meter is connected.

If not directly connected to the battery post the measurement is taking into account the IR of the wires to the connection point of the voltage meter.

I would however concur, part of the voltage drop is most likely from the internal resistance of the battery bank which goes up as the batteries are depleted. That can't be changed other than adding extra battery capacity.

However, what one can do is minimize as much as possible the voltage drop to the high amperage loads like an big inverter (OP is using the inverter to run a toaster if I remember correctly, typical two slice toasters are 850W). That IS done by using as large as possible wire ga that will fit the inverter terminals and keep both the pos and neg wire runs as short as possible. With 12V items, even saving .1V drop under a heavy load can make a difference when dealing with depleted batteries.