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Old 05-16-2010, 09:34 PM   #1
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Suburban NT-40 Furnace Question

I was wondering if anyone had any experience using the NT 40 furnace on 12 volts. The Furnace utilizes roughly 9 amps of power (12 volt). I am looking for actual results of using the unit on 12 volts. I have a 392QBBS (Wildwood Lodge) that came with the P-40 (same exact furnace but this model is 120volt only). I am wrestling with converting to the NT 40 so I can run the unit solely on the 12 volt battery (without the generator) at night. I have the top of the line Blue top Optima (D31, Reserve Capacity 155). Theoretically it should run about 24 hours if the furnace would run 10 minutes per hour. However, there are many variables, ambient temp, condition of battery, lights in coach etc..Thus my question has anyone actually tried this, and, in a coach this big (40 ft, 7" ceilings, double insulated).

Feedback would be much appreciated....
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Old 05-16-2010, 10:48 PM   #2
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Using your math, furnace running 10 minutes per hour equals 16% duty cycle. (Better be above 40 outside) and you can run 24 hours, until what? Battery completely discharged? Won't happen. When the battery hits 10 volts or so, the fan speed will be too low to make the sail switch, and the furnace will not light. With any single 12 volt battery, regardless of brand, most have found they can't depend on the furnace one night, and that is in a smaller camper. You also haven't figured water pump. lights, radio, etc., draw on your battery. My personal opinion is you should have forgot the optima, they are way overrated, and got 4 Trojan 6 volt batteries for your application, and a lot of solar panels. Just my opinion only. I also think 10 minutes per hour of furnace is unrealistic for a camper, regardless of insulation.
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Old 05-16-2010, 11:17 PM   #3
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Originally Posted by windrider View Post
Using your math, furnace running 10 minutes per hour equals 16% duty cycle. (Better be above 40 outside) and you can run 24 hours, until what? Battery completely discharged? Won't happen. When the battery hits 10 volts or so, the fan speed will be too low to make the sail switch, and the furnace will not light. With any single 12 volt battery, regardless of brand, most have found they can't depend on the furnace one night, and that is in a smaller camper. You also haven't figured water pump. lights, radio, etc., draw on your battery. My personal opinion is you should have forgot the optima, they are way overrated, and got 4 Trojan 6 volt batteries for your application, and a lot of solar panels. Just my opinion only. I also think 10 minutes per hour of furnace is unrealistic for a camper, regardless of insulation.
As stated-very "questionable" Mathematics and, quite a few variables. I called the Optima tech line and had their tech go through the "what if's "as well. Agree with you though-many variables (ambient temp outside is a big one). I will say though, in our much smaller camper, we could go 2 days/nights with runing the entire coach on the 12 volt battery (same Optima). What I was hoping to do is to run the furnace on 12 volt at night (shouldn't need the water pump, lights much), then , fire up the generator in the morning to recharge. Appreciate the info though-same page for sure. I am hoping to hear from a few folks that have run this large of furnace on 12 volt
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Old 05-16-2010, 11:39 PM   #4
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Typical RV furnaces (Suburban SF35 for instance) draw 7amps per hour. Their start up and shut down sequences can total nearly 5 minutes and that isn't producing any heat. In our experience with a 37' motorhome and temps outside in the 40's your furnace will run at a MINIMUM 30 minutes for every hour. If your math is correct and you are drawing 9 amps with the larger furnace I would give your battery about one night. Remember the optima blue top is a marine starting/deep cycle batter, not a true deep cycle battery. Deep cycle batteries should never be discharged lower than 50% and typically no lower than necessary without being recharged. I really don't think you will be happy with this type of setup. Time to start thinking of a larger battery bank and doing some research on solar if it's feasible to keep from running the generator all the time.
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Old 05-17-2010, 10:16 PM   #5
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Thanks Guys.

I am begining to think I should just leave the P40 in. Last weekend we stayed in the coach two nights. I just kept the Honda 2000i Companion running all night. It was 47 degrees outside, we set the therm at 68. It had no trouble running the furnace, used well under 1 gallon of gas in 10 hours, makes next to no noise.

The Optima did run my other coach fairly well in the winter-but again, it was MUCH smaller, and the furnace was too. The Optima D31M actually is listed as a deep cycle-blue top , gray body (unless I am missing something ). Not sure if there is a better battery you are referring to NWJ?
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Old 05-17-2010, 11:11 PM   #6
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Since you are parked permanently, why not look in to one of the big bulldozer batteries? You wouldn't have to mount it to the trailer, but could be put it in a storage box under the tongue. They are very large and heavy, but should give you what you need for a night, maybe two. Can't remember the numbers on these batteries, but should be able to find something on the net. One bulldozer battery has to be cheaper than a new furnace plus install. Might have enough left to look in to some solar back up. Just look up a battery for Caterpillar D10 dozer.
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Old 05-18-2010, 07:33 AM   #7
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Sams Club carries large semi truck battery.
Many houseboaters use them.
They cost a lot less than 4 6volt (yike$)
Just a thought....
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Old 05-18-2010, 11:13 PM   #8
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Thanks folks!
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Old 05-20-2010, 05:37 PM   #9
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Originally Posted by es335 View Post
Thanks Guys.

I am begining to think I should just leave the P40 in. Last weekend we stayed in the coach two nights. I just kept the Honda 2000i Companion running all night. It was 47 degrees outside, we set the therm at 68. It had no trouble running the furnace, used well under 1 gallon of gas in 10 hours, makes next to no noise.

The Optima did run my other coach fairly well in the winter-but again, it was MUCH smaller, and the furnace was too. The Optima D31M actually is listed as a deep cycle-blue top , gray body (unless I am missing something ). Not sure if there is a better battery you are referring to NWJ?
Trojan (or similar) 6V golf cart batteries are the preffered choice for setting up dry camping systems on RVs The plates in a true deep cycle are much thicker than starting/deep cycle batteries. Off grid homes tend to use the same type of basic idea but they have the huge L16 or similar type batteries. Optimas being AGM (absorbed glass mat) batteries are a bit different and have different charging requirements related to voltage and amperage than led acid batteries. The charger you use to charge them should be capable of charging them correctly to get the full potential out of them and RV conveter/chargers just aren't set up for them. Their cost/amp hour is also higher in most cases by quite a bit over the golf cart batteries.

Don't get me wrong, Optimas are awesome batteries and I use them in my boat and Jeep. They can take the abuse of lots of pounding and being mounted in awkward postitions that led acids can't.

Bottom line is you need to sit down with a pad of paper and really run the numbers on how many amps you use total over a days time from your batteries. Look at lighting, pumps, fans, inverter (if you have one), furnace etc. You need to size the bank of batteries to handle that and keep the run time on the generator down. I know for us we tend to be very power hungry so I have 6 batteries and 260W of solar charging (I think I need more).
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Old 05-22-2010, 11:50 PM   #10
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Originally Posted by NWJeeper View Post
Trojan (or similar) 6V golf cart batteries are the preffered choice for setting up dry camping systems on RVs The plates in a true deep cycle are much thicker than starting/deep cycle batteries. Off grid homes tend to use the same type of basic idea but they have the huge L16 or similar type batteries. Optimas being AGM (absorbed glass mat) batteries are a bit different and have different charging requirements related to voltage and amperage than led acid batteries. The charger you use to charge them should be capable of charging them correctly to get the full potential out of them and RV conveter/chargers just aren't set up for them. Their cost/amp hour is also higher in most cases by quite a bit over the golf cart batteries.

Don't get me wrong, Optimas are awesome batteries and I use them in my boat and Jeep. They can take the abuse of lots of pounding and being mounted in awkward postitions that led acids can't.

Bottom line is you need to sit down with a pad of paper and really run the numbers on how many amps you use total over a days time from your batteries. Look at lighting, pumps, fans, inverter (if you have one), furnace etc. You need to size the bank of batteries to handle that and keep the run time on the generator down. I know for us we tend to be very power hungry so I have 6 batteries and 260W of solar charging (I think I need more).
Ok that makes sense-got it. I do need to run the numbers and get some more exact figures. The other item I left out, I do not leave the battery "on site" due to potential theft. I bring it home and plug it into the charger after every use. The more I have been using the 2000I, the more I am inclined to stick with it. The Honda Companion actually weighs less than the D31 M Optima.
It does not look like it would be too difficult to swap the P40 for the NT40, I just need to decide if it makes sense to do so. Appreciate the advice.
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