Furnase whirring and suction on stove after battery solar controler upgrade

[kip22]

Hey all


Hoping someone has an idea on this one for me. I just changed out my 12v Lead acid for a new lifepo4 and renogy charge controller and went camping this weekend. I'm too bagged today to tear it apart and start troubleshooting but here is what I know so far.



When the suburban NT16 seq furnace kicks on the fan whirs up and down. It did not do this last week. The furnace is not even a year old and has maybe 4 hours use on it. Further to that when the furnace is running and I use the gas burners there is suction draft downwards into the cavity below. Again did not do this last week. Obviously the furnace is creating the suction but why now? I thought maybe the voltage was too high but I checked with a meter while camping and it was down to 12.4 volts on the output of the controller this morning...




Thoughts?

[Shocksll]

Did you install the battery and solar controller yourself?

[CharlesinGA]

Polarity reversed?

[Marv]

That voltage reading is really low for a lithium battery. If the controller output is that low in the sun it is not charging. But I have no idea how low voltage could affect the gas burner flame.

[kip22]

LOL yes I did install it myself. (I'm a DC electrician so it should be good. I usually work with -54 volts DC so +12 is no prob).


I think I may have figured it out. I'm thinking the voltage really is higher than with the old batteries. I was using 3 recycled 12 volt 8 ah batteries in parallel. They were originally just to power my LED lights and charge a phone here and there so it was fine. When the furnace kicked on the draw would drop the voltage below 12 volts regardless of charge state. That was the catalyst for the lithium upgrade. Plus I got my eye on a compressor fridge now.



When I get home from work I'm going to swap the battery back to the lead acid and see what happens. With the fan whirring and the obvious extra suction in the cabinet cavity causing negative pressure at my burners this makes the most sense to me. I was reading another post on here from a ways back someone installed a DC-DC converter between power and their suburban furnace to quiet it down. He dropped it to 10.5 volts and said it halved the noise. That may be the best solution for me too. 16,000 BTU heats up my boler in about 4 minutes so I'm really not concerned with tamping it down a little. Will report back later today.

[Marv]

Hey Dave,
A couple things: most RV furnaces recirculate and heat the air inside the RV and do not draw in outside air. So it should not cause a change in pressure unless the combustion passage to the outside is blocked or open to the inside. Also, if you slow the fan, the fire is still generating 16K BTU. Except now the fan might not be moving enough air to cool the combustion chamber. Be careful.

[kip22]

You are correct. For the fire box there is a fresh air supply and exhaust on the outside of the RV. On the back/sides of the furnace Inside the cabinet beside the stove is the fresh air/return air for the blower. This is the negative pressure I'm talking about inside the cabinet. Its also just as possible something rattled loose fell in and is partially blocking the blower air causing the noise and odd suction. This model has a low voltage shut off around 10.5 volts. The mfg has spec'd it as safe down to that voltage. I always keep two Carbon monoxide detectors in the boler when camping. That would be a **** way to go.


Thanks,

[kip22]

Okay I kind of got er figured. I tried with the old lead acid battery. Furnace worked fine no whirring noise 12.2 volts on load output while running. There was some slight downward suction on the burners. I swapped back to the new lithium battery charged up. 13.2 volts load output running. No whirring noise so that's gone now. I have a feeling it will come back just not sure how or when....


There was significant down suction on the burners. Foil duct tape fixed that. I pulled the drawer below the stove and taped the openings to the cabinet beside with the furnace to stop the negative pressure coming through and all good now.



I did measure draw on the furnace with both batteries connected. With the lead acid at the lower voltage I was pulling 2.5 amps and with the lithium 2.9 amps. That's quite a big difference in draw with the higher voltage. There is not really a noticeable difference in heating performance but the fan is a little louder. I think I may still install the DC-DC converter to drop the voltage a little. Some of them claim to be 98% efficient so given my measurements and the lower volume its probably worth dropping the voltage to around 12 volts.

[LarryB.]

Originally Posted by Marv

... ...A couple things: most RV furnaces recirculate and heat the air inside the RV and do not draw in outside air. So it should not cause a change in pressure unless the combustion passage to the outside is blocked or open to the inside. Also, if you slow the fan, the fire is still generating 16K BTU. Except now the fan might not be moving enough air to cool the combustion chamber. Be careful.

Forced air heating systems have two separate air flow systems: combustion and heating. The objective is to insure that they are totally isolated from each other.

The combustion system draws outside air into the furnace to mix with the heating fuel and burn inside the (sealed) heat exchanger. Once the air and fuel burn the exhaust is forced outside. The heating flow takes in inside air and reheats it by blowing it over the sealed heat exchanger for (re)circulation back into the living area.

Taping up the holes inside the cabinet to minimize the downdraft on your stove burners seems to tell me that the problem might be in the hot air recirculating flow system.

On my Casita, there is a (large) vent on the dinette side of the cabinet that houses the furnace and the stove. That vent serves as the cold-air return for the heating air flow. The furnace fan sucks air thru that vent, (re)heats it as it flows over the heat exchanger and blows it out the front of the furnace into the living area.

[kip22]

Hi Larry


Yes you are correct that was the cause. Even with the cabinet doors open it was still happening due to the proximity to the cooktop. The furnace recirc air intake is right beside and the plywood divider inside the cabinet and had a 1 inch gap at the top between them.

[kip22]

So I figured out why the furnace was whirring. It started doing it again while I was home doing some rewiring setup etc. With my volt meter on the battery terminals the voltage was going from 12.75 ish to 13.6 cycling back and forth every 3 seconds. Didn't matter if there was load or not. I disconnected and reconnected everything one at a time and isolated it to the solar controller. With load direct to battery no problem. With just the battery connected to the charge controller and nothing else connected sometimes the voltage goes up and down. I'm over 30 days so I can't return it but it was only 30 bucks for the charge controller so I guess I'm gonna get a new one. With the voltages I'm getting I don't think its going to hurt anything but it is annoying when the furnace fan spins up and down.



You get what you pay for I guess but a 10 amp PWM charge controller should only be about 30 bucks so......

[Perryb67]

It would be helpful if you post a wiring schematic of you Boler. It's amazing what can be found when others look at your wiring scheme. In our case Escape wired the controller ground to the frame ground (not directly to the battery) causing us to lose .2 volts to the battery.

Food for thought,

Perry