forced air furnace and power

[nico]

So I am learning the nuances of my new grandby flatbed. I spent two nights in the desert recently with low temps and my heater was real finicky. I did not realize how much power the heater fan draws and even though my batteries were at 12.5 ish volts, i could not get the heater to work and my kids were freezing. I had to trouble shoot everything before realizing it was a power issue and had to run the truck intermittenly for the rest of the trip which was kinda annoying. SO moving forward i am trying to determine what this system needs (at least in the winter time) to have enough power that I dont have to run my truck constantly to charge up the batteries. I am currently running the dual lead acid batteries with a 160 watt solar panel. I am open to adding another panel but im not sure this will do the trick in the winter time. I imigine the 160 watt panel will be adequate in the summer (albeit we probably wont need the heater in the summer/fall). i had another camper owner tell my to jsut get a small heater such as the mr buddy heaters but that seems kinda dangerous to run in the camper if it fell over or something. maybe that could be an emergency measure but again I can always run the truck for 20 minutes or so. Open to suggestions.

 

[SigSanDiego]

My 2021 Grandby with single AGM did same thing.

With 130 frig, basic light use and overnight heater use you’re lucky to get 12-20 hours parked, no engine idle. Got up in middle of night to start truck to get warm. Only took one of these episodes to upgrade to:

- dual 100 Lithium batteries
- upgraded alternator truck wiring gauge
- 190 watt roof solar

We had “battery anxiety” until the above upgrade. No way to live.

We don’t really need the solar in hindsight since our style of camping is to break camp (one full day maybe 1.5 days) before battery depletion. Truck charging is amazing with very little drive time. Wish we did portable panels instead of roof mount solar ( would not have down roof rails either for roof integrity insurance.

Get battery and wire upgrade for sure. Solar is camp style dependent in my opinion.

 

[rando]

So I am learning the nuances of my new grandby flatbed. I spent two nights in the desert recently with low temps and my heater was real finicky. I did not realize how much power the heater fan draws and even though my batteries were at 12.5 ish volts, i could not get the heater to work and my kids were freezing. I had to trouble shoot everything before realizing it was a power issue and had to run the truck intermittenly for the rest of the trip which was kinda annoying. SO moving forward i am trying to determine what this system needs (at least in the winter time) to have enough power that I dont have to run my truck constantly to charge up the batteries. I am currently running the dual lead acid batteries with a 160 watt solar panel. I am open to adding another panel but im not sure this will do the trick in the winter time. I imigine the 160 watt panel will be adequate in the summer (albeit we probably wont need the heater in the summer/fall). i had another camper owner tell my to jsut get a small heater such as the mr buddy heaters but that seems kinda dangerous to run in the camper if it fell over or something. maybe that could be an emergency measure but again I can always run the truck for 20 minutes or so. Open to suggestions.

Something doesn't sound quite right. 2 AGMs and 160W of solar should be plenty to run the furnace for a couple of days in the desert. If you battery is at 12.5V, that is around 80% - 90% charged and should be able to run the furnace just fine. I wonder if there is an issue with either the wiring to your furnace causing a voltage drop, or an issue with the sail switch, which can make the furnace hyper sensitive to battery voltage. What year is your camper, and which furnace does it have?

For comparison we ran one 75Ah AGM battery and 160W solar panel for a year with no power issues, and now run a 150Ah LiFePO4 with the same 160W panel (which is about the same as what you have) and can camp for days with no sun and be fine.

 

[Vic Harder]

+ to what rando said. I wonder if that 12.5v is with no load on it, or with a load? In either case, I would suggest getting a battery monitor that can tell you the SOC and that uses a shunt... something like the Victron Smart Shunt. Do that before you spend a lot of coin on other upgrades so you can be sure you are spending your $$ in the right places

 

[nico]

It’s a 2021 grandby I’m not sure of the heater model I’ll have to find the manuals. The power draws down so fast once we start running the heat. I was very surprised

 

[rando]

I think the current Dometic furnace draws about 3-4A when running - so a pair of fully charged AGMs (~150Ah) should easily be able to run the furnace for 30hrs or so continuously With the thermostat cycling on and off, it should last for at least 3 nights without taxing the batteries. It sounds like you aren't seeing even close to this, so either there is something up with your batteries, or something up with the furnace.

I would follow Vic's suggestion and get a battery monitor, it will make it much easier to track down the issue.

 

[Jon R]

The 12k BTU/hr furnace in my 2021 slide in Grandby draws about 3 amps.

Do you have a three way fridge, and if so are you running it off the battery?

 

[Adventurebound]

This may sound crazy and i might get flack for this but I don't think it's a power issue. Remove the duct work from your heater to the grate. That 100% solved all my heater problems. Do the opposite of what I did, start with the easy stuff and move to the complicated (expensive) stuff.

 

[Jon R]

This may sound crazy and i might get flack for this but I don't think it's a power issue. Remove the duct work from your heater to the grate. That 100% solved all my heater problems. Do the opposite of what I did, start with the easy stuff and move to the complicated (expensive) stuff.

One of the functions of this forum is brainstorming with multiple brains. Nobody should give any flack to someone making an earnest suggestion, especially one that says “here’s how I solved my similar issue.”

 

[Vic Harder]

This may sound crazy and i might get flack for this but I don't think it's a power issue. Remove the duct work from your heater to the grate. That 100% solved all my heater problems. Do the opposite of what I did, start with the easy stuff and move to the complicated (expensive) stuff.

+1 good idea. Start with the easy/physical stuff first

 

[nico]

The fridge runs off battery only.

 

[Jon R]

So a two way compressor fridge. I asked because the three way fridges (propane/120vac/12vdc) use a lot of power when operated off the battery.

 

[Jkron]

So I am learning the nuances of my new grandby flatbed. I spent two nights in the desert recently with low temps and my heater was real finicky. I did not realize how much power the heater fan draws and even though my batteries were at 12.5 ish volts, i could not get the heater to work and my kids were freezing. I had to trouble shoot everything before realizing it was a power issue and had to run the truck intermittenly for the rest of the trip which was kinda annoying. SO moving forward i am trying to determine what this system needs (at least in the winter time) to have enough power that I dont have to run my truck constantly to charge up the batteries. I am currently running the dual lead acid batteries with a 160 watt solar panel. I am open to adding another panel but im not sure this will do the trick in the winter time. I imigine the 160 watt panel will be adequate in the summer (albeit we probably wont need the heater in the summer/fall). i had another camper owner tell my to jsut get a small heater such as the mr buddy heaters but that seems kinda dangerous to run in the camper if it fell over or something. maybe that could be an emergency measure but again I can always run the truck for 20 minutes or so. Open to suggestions.

I am willing to bet it is the foil ducting on the front of the furnace causing your issue. I will assume the issue you were experiencing is the furnace would turn on, but would fail to ignite? And after you started your truck, it was able to perform and ignition cycle?

The furnace blower motor draws the most power for the appliance, and it sends air through the combustion chamber and through the heat exchanger and out the exhaust. In order for the furnace to move into the ignition cycle, it needs to prove draft, (adequate venting) and it uses a sail switch to close a contact to prove draft. With that foil ducting on, its just restrictive enough to limit air flow from the blower motor to close the sail switch. When you started the truck, the voltage increased, allowing the blower motor to ramp up its RPMs and close the sail switch.

I would try removing the silver foil duct. Its 4 screws on the face, and just pop it off. I had to do this on both of my flatbeds. I have a 2021 Hawk UTE with lithium, and when I got into some elevation, I still had a problem even with the batteries at 13.4V. Once I removed the ductwork, it went away. I think it is an issue with how the furnace is mounted on the newer flatbeds, and its lack of "make up air" since it is pretty closed off, and the blower needs to work hard to pull in make up air for combustion and to blow across the heat exchanger.

 

[Jon R]

Jkron - Are you saying the furnace gets its combustion air from inside the camper rather than from outside?

 

[Jkron]

Jkron - Are you saying the furnace gets its combustion air from inside the camper rather than from outside?

Sort of. It pulls air from essentially a concentric vent around the exhaust vent. But I have found that having ventilation inside the furnace cabinet can help as well. On some of the later FWC's they added a small louvered vent on the side of the furnace cabinet to help, but it depends on the floor plan. The flatbed floor plan kind of closes off the furnace cabinet area, it is not as open as some other floor plans.

I would remove the interior ducting, as this is what I have done. Most of the issues with the furnace are related to air flow. That little bit of ducting restricts just enough to give issues sometimes, especially as battery power depletes. You can also remove the exterior cover, which can help in a bind. I had to do that at over 10k feet, since the air was thin.

 

[rando]

These furnaces are so cheaply built that they use the same motor for the blower fan and the induction fan. If you choke the blower fan side with small vents, you slow the motor a little and that also slows the induction fan? Coupled with lower voltage and you may not activate the sail switch. Some (@Charlie maybe?) one also figured out that the problem is exacerbated by the orientation of the furnace - mounted in portrait orientation the sail switch is also fighting gravity. I think there was a fix where you increased the area of the sail slightly - but find the original discussion of this before trying this.

Next time you have this problem, take a look at the LED on the furnace circuit board, if it is flashing once then it is a sail switch problem.

 

[Jkron]

yea, they do share the same blower motor assembly to handle intake and exhaust. its a crappy furnace, no doubt about it. You can tweak the sail switch slightly, but its only a little but before you run into issues with it contacting. Crappy furnace, and poor installation considerations just equal it not working. The small bit of ductwork should not need to be removed in reality, but the blower motor is just barley capable of running the system through its proving stages, so here we are with removing the ductwork to get it to work.

I am working on a diesel heater conversion, and gathering a parts list along with some instructions, since the only way to solve the furnace issues is with replacement at this point.

 

[Colorado Mark]

I'm with a couple others. I don't think its a power problem. I just went camping a while back and ran 4 days in below freezing temps with 2) 12V AGM & 160W panel. 12V, 85L compressor fridge. It was even overcast & snowing day 2 and barely got any charging from the panel.

Saw battery voltage drop to 11.9 at worst.

Zero issues other than running out of LP in one tank in the middle of the night on day 3. Should have topped it off. Was only about 1/2 full.

I did have some problems earlier (about a year ago) and did like others suggested. Remove the foil duct and just use the entire front grille.

So far, so good.

 

[Dphillip]

I suffered several years with intermittent heater problems, failure to fire until I removed the foil duct hose and cut out about 6 inches of excessive hose. The air from the hose now has a more direct path through the vent rather than making a slight turn. This simple task completely eliminated my problem. No more sleepless nights wondering if the heater is going to work.

 

[wicked1]

I noticed the furnace drains my battery surprisingly fast, too.
I have 3 solar panels (two on top, and a portable), and am almost always at 100% charge by sun down. Even on the shortest day of the year under clouds! I am a very light sleeper, and get the fridge very cold during the day, and turn it down at night.. In the cool weather, it doesn't run at all, all night. So, furnace is the only thing drawing power.
I noticed that running the furnace just a few times over the night.. when I wake and am freezing I turn it on for a while.. Then turn it off and go back to sleep. That happened maybe 3 times one night, and my battery was down 20% by morning.

It's fine.. My 3 panels will fill my 200ah battery the next day. But it's relatively a lot of power usage! (I haven't noticed how much power it pulls, since I have only run it at night, but I'll do a test later.. 2015 fleet flatbed)