Freaked-out furnace.

Topic: Freaked-out furnace. (Read 1450 times) previous topic - next topic

Re: Freaked-out furnace.

Reply #25 – February 12, 2016, 01:24:19 pm

FINAL UPDATE (I hope) New card worked for a few days and the furnace choked again. VOM showed 11 volts from battery, but zero when I put a load on it. Hmm...Pulled & cleaned the fuse in the rear 12 volt, still no fire-up. Finally opened the gen bay below , and, after tracing the hot from the furnace, found this wire (the hot) coming from gen set power. As you can see, IT HAD GOTTEN CRISPY ! Put in a new connector w/fuse (never even found a fuse in the existing mess), and the furnace was back in the biz! I'm wondering if there were previous electrical problems and the yellow "shunt" wire was plugged into the gen to get power for the furnace? Saw no paperwork from Brett, so he's off the hook.

Re: Freaked-out furnace.

Reply #26 – February 12, 2016, 01:31:49 pm

Mike,

That certainly explains the lack of power to the furnace.

And because low voltage (voltage drop in long, small-gauge wire from back fuse box to furnace) when dry camping is a problem for furnaces (low voltage= slower fan= sail switch does not stay closed= furnace stops working), using heavy gauge wire from batteries (that very ample hot for generator in the next bay) is a common "better source" for the furnace power. And fused as you verified.

Now, the question is what caused that fuse and fuse holder to melt??? Nothing else in that area show signs of overheating. So, check the short run from fuse to furnace for any place where the positive wire could have chafed through to ground. Even then, it is bazaar for the fuse holder to melt before the fuse blows.

Re: Freaked-out furnace.

Reply #27 – February 12, 2016, 01:40:24 pm

I have seen this happen when the fuse gets a bad connection and the resistance builds heat on the power side of the fuse holder so the fuse does not blow.

Re: Freaked-out furnace.

Reply #28 – February 12, 2016, 02:39:51 pm

Quote from: wolfe10 – February 12, 2016, 01:31:49 pm

Now, the question is what caused that fuse and fuse holder to melt??? Nothing else in that area show signs of overheating. So, check the short run from fuse to furnace for any place where the positive wire could have chafed through to ground. Even then, it is bazaar for the fuse holder to melt before the fuse blows.

Could be an anomaly, I doubt it. My thought, after searching thru the coach wiring and noticing the ATS has been replaced, as well as the shore power connections (male & female) in the wet bay, I'm thinking the coach took a huge spike at one time, possibly with both ACs on and the ATS tried to fire the gen set, trashing the furnace fuse. Now, whether it was off the pedestal or someone overloading the 50 amp system, no clue. If the ATS took a hit, it should have blown the furnace fuse, which it mite of, but maybe fused the connections together for working/not working, depending on the load. That is "Leary's Theory", at this point in time. $:-\$

Re: Freaked-out furnace.

Reply #29 – February 12, 2016, 02:47:48 pm

Chuck,

Mike may be able to find the paper work on it, but as I recall, that is about a 3' run of 10 gauge wire. 20 amp fuse if memory serves.

I think Craneman may have hit on it-- corrosion in fuse holder caused resistance, caused heat..... Kind of hard to tell after the fact, and no sign of water intrusion, corrosion, etc on any other near-by generator parts.

Anyway, glad you found and fixed it, Mike.

Re: Freaked-out furnace.

Reply #30 – February 12, 2016, 03:05:46 pm

Quote from: wolfe10 – February 12, 2016, 02:47:48 pm

20 amp fuse if memory serves.

I recently picked up several of these wired fuse blocks, all of which had 30 amp fuses. Of course, they can be pulled and any size could be inserted. How does one know what size fuse is needed for a given circuit?

Thanks,

Trent

Re: Freaked-out furnace.

Reply #31 – February 12, 2016, 03:14:42 pm

Trent,

Two basic considerations:

1. The fuse is there to protect the wire-- fuse should be speced to blow before overheating the wire.
2. Adequate wire size and fuse size so no significant voltage drop between source and appliance. I generally use a 3% voltage drop table: Marine Wire Size and Ampacity | West Marine

3' of 10 gauge wire can handle a lot more than 20 amps, but 20 amps is plenty to insure no voltage drop in fuse or wiring to the furnace. Said another way, unless the fan motor seizes up or some other major issue, 20 amps is well-more than the furnace will draw. And 3' of 10 gauge wire is certainly not a limiting factor.

Re: Freaked-out furnace.

Reply #32 – February 12, 2016, 03:24:35 pm

30 amps is a lot! Not being able to find the fuse on my blown furnace connection, I threw in a 20 amp.

Re: Freaked-out furnace.

Reply #33 – February 12, 2016, 04:12:13 pm

Mike,

20 is a perfect size. Again, if the it blows, there is either a dead short (nick in hot wire touching bare metal, for example) or a seized fan motor. Have not looked at that furnace's electrical diagram, but very likely it has its own fuse to protect in case of motor seizing.

Re: Freaked-out furnace.

Reply #34 – February 12, 2016, 04:20:18 pm

If the fuse or fuse holder is melted:

In my experience of over 30 years as an electrician, if the fuse is sized properly for the fuse holder and the wire..... It is almost always a bad or poor connection between the fuse and fuse clips or the fuse holder terminal and wire.

To use Brett's term: said another way, melted because of heat, heat created when current flows across a resistor ( resistance ) the fuse link will melt at its set current, but the heat generated at the clips or "stabs" may melt a plastic holder before the internal link gets hot enough to "open" the circuit.
Circuit doesn't have to have an "over current" situation for this to happen.

Re: Freaked-out furnace.

Reply #35 – February 12, 2016, 05:35:58 pm

I have bought many of those fuse holder on the last boat I owned. I went thru so may of them I always carried a spare and had quick connects on them so I could change them out. They say they are rated for 30 amps and have @ 10 gauge wire going to them but I am thinking the connector between the wire and the fuse is not a good connection. I had tried everything to make them live thinking the connection between the fuse and holder becomes corroded but after going thru so many of them, I just think they are cheap.

Someone asked wire and fuse sizes. Here is good rule of thumb. There are exceptions
#14 gauge wire 15 amps
#12 gauge 20 amps
#10 gauge 30 amps.

The above is the max the wire can carry before bad things start to happen. In reality you want to fuse no more then above but aprox. 25% more then what the device / circuit needs. That is why you may see a 5 or 10 amp fuse on a 14 or 12 gauge wire.

Last point I want to make is as the voltage goes down the current (amps) goes up. So if a fuse blows it could have been caused by low voltage.

Re: Freaked-out furnace.

Reply #36 – February 12, 2016, 06:11:17 pm

Thanks all! My brother-in-law (Fluke engineer) gave me this for a wedding present, made me lotsa $$$. Now, I gota remember how to use it!

Re: Freaked-out furnace.

Reply #37 – February 12, 2016, 06:11:56 pm

Mike as you well know when a problem arises and gets posted on this forum more help pours in than you can get at any rv dealer. When I read the various issues that come up everyday its amazing the knowledge thats in this forum. Well as you always say time for a cold one.

Re: Freaked-out furnace.

Reply #38 – February 12, 2016, 06:32:42 pm

Quote from: joeszeidel – February 12, 2016, 06:11:56 pm

Well as you always say time for a cold one.

Yup, roger that! One of the great things about this forum, is something one had not thought of, is posted!

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Re: Freaked-out furnace.

Reply #25 – February 12, 2016, 01:24:19 pm

Re: Freaked-out furnace.

Reply #26 – February 12, 2016, 01:31:49 pm

Re: Freaked-out furnace.

Reply #27 – February 12, 2016, 01:40:24 pm

Re: Freaked-out furnace.

Reply #28 – February 12, 2016, 02:39:51 pm

Re: Freaked-out furnace.

Reply #29 – February 12, 2016, 02:47:48 pm

Re: Freaked-out furnace.

Reply #30 – February 12, 2016, 03:05:46 pm

Re: Freaked-out furnace.

Reply #31 – February 12, 2016, 03:14:42 pm

Re: Freaked-out furnace.

Reply #32 – February 12, 2016, 03:24:35 pm

Re: Freaked-out furnace.

Reply #33 – February 12, 2016, 04:12:13 pm

Re: Freaked-out furnace.

Reply #34 – February 12, 2016, 04:20:18 pm

Re: Freaked-out furnace.

Reply #35 – February 12, 2016, 05:35:58 pm

Re: Freaked-out furnace.

Reply #36 – February 12, 2016, 06:11:17 pm

Re: Freaked-out furnace.

Reply #37 – February 12, 2016, 06:11:56 pm

Re: Freaked-out furnace.

Reply #38 – February 12, 2016, 06:32:42 pm

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