Suburban NT34-SP works Sometimes, mostly just blows cold air.

[Pogrom88]

I'm new to the RV thing, I've owned my 1988 Itasca for around 6 months. It's cold here in Washington state, so I rely on the furnace a lot. At first...it would always work fine...it would light and shut itself off according to the thermostat...but then I noticed that throughout the night...the second lighting would only blow cold air. No problem...as long as I manually controlled it...it worked fine...until a week ago..

Now...it only blows cold air....however...on some random occasions...it will blow hot air...but I cannot find any consistent way to make this happen...

I watched videos on this...and most describe a "sail switch" as being the problem...but I don't believe it's my problem...and here is why..

The thermostat does what it's suppose to...it enacts a blower..then..about 10 seconds later..I hear a faint click...this is the sail switch being engaged...and then shortly after the click...I Can see through a smokey window...the ignitor attempting to ignite...but it fails...it runs this cycle 3 times...and then it quits attempting to ignite..it simply keeps blowing cold air...because it does not reach the temperature required to shut off...it remains blowing cold air until I turn it off at the thermostat.

What could this problem be...I'm ready to buy parts...but I only want to buy necessary replacement parts.

 

[Rene T]

For starters Do you have propane? Does your stove work OK?

 

[Pogrom88]

For starters Do you have propane? Does your stove work OK?

I forgot to mention...yes..I'm full on propane...my stove works fine...although my hot water heater has only functioned twice since owning the motorhome...same thing...I can see it attempt to ignite...but it appears that gas is my issue here as well. Now...it's a 1988...so I'm assuming that the solenoid is out in both cases...or the solenoid is not being commanded to open.

 

[John From Detroit]

One cause of intermittent ignition is the spark gap on the ignitor... NOT by any means the only cause.

 

[Pogrom88]

I'm able to see the spark...and it's a factory set up...the gap (as far as I know of), has not been altered...but I can see eventual deterioration of the ignitor gradually adjusting the gap, and creating an intermittent problem...I want to place all my chips in on it being the solenoid..being at a point that it's not truly out of commission but it no longer functions as the day it was made...but I am also curious...could this be a control board issue? I'm hoping not...but is there any condition where the control board stops sending the solenoid the command to allow gas to flow? If so...is this discovered by resting the solenoid?

 

[Henry J Fate]

Yes. Possible to be a bad control board. The first and cheapest thing to do is replace the igniter/sensor. The igniter/ sensor has 2 functions. It will spark to ignite the gas then it senses a flame and reports to the board. If either one of those functions fail, the gas valve will close resulting in no heat. If the fan continues to run to no end, that could be a bad board or bad connection.

When replacing the igniter, check all your connectors and board receivers for continuity and that all is clean.

 

[Kirk]

First I suggest that you download a copy of the service manual for your furnace.

.I Can see through a smokey window...the ignitor attempting to ignite...but it fails...it runs this cycle 3 times...and then it quits attempting to ignite..it simply keeps blowing cold air..

The 3 attempts and lockout is how things work with RV furnaces and until recently the blower would continue to run indefinitely once lockout occurred. In recent years some models turn the blower back off after a couple of minutes. Are you absolutely sure that the propane fails to ignite? One fairly common problem is that the heat of the burning propane is not detected and so it shuts the valve and goes into lockout because it didn't detect that the propane was burning. The same probe that supplies the spark also has a thermal couple built into it that sends a very small voltage back to the control board to tell it that the propane is burning. If that isn't received but the circuit board it assumes that the propane didn't ignite and closes the propane supply valve but leaves the blower running to make sure that all propane is exhausted from the heat exchanger for safety. Much more recently they have made it such that the blower shuts down after 1 minute but that didn't happen until around 2005 or later and some models still run as yours does.

...although my hot water heater has only functioned twice since owning the motorhome...same thing...I can see it attempt to ignite...but it appears that gas is my issue here as well. Now...it's a 1988...so I'm assuming that the solenoid is out in both cases...or the solenoid is not being commanded to open.

Much more likely since you have two appliances with the same problem is that your propane pressure regulator needs to be replaced and is not supplying the proper gas pressure. Consider this, is it more likely that two appliances with fail in the exact same way, or that 1 thing that could cause both problems has happened? You need to have your propane pressure checked or replace the pressure regulator if you want to just replace parts and hope to make it work. Any qualified RV tech should be able to check the pressure for you. It is probable that a new regulator would be properly set when purchased, if you choose to go that way. Propane pressure should be measured with a manometer and it should be 11" of water column at the entry of each appliance.

It this were only 1 appliance, I would agree that it is likely the ignition probe that has failed, but not so probable with 2 appliances the same failure.

 

[Lou Schneider]

Make sure you check the pressure with the furnace running as it may be a quantity problem where the pressure decreases under load. Don't rely on watching a stove burner to determine the propane pressure is OK. Many stoves have a secondary regulator to smooth out pressure variations before gas reaches the burners and this can mask supply problems. The furnace and water heater don't have this.

If it was only the furnace I'd suspect the gas valve. Over time the mercaptin oil added to propane so you can smell a leak accumulates in the valve and can make it stick closed when cold. The same thing can happen in the propane regulator.

I had a water heater when it was cold I had to tap on the gas valve while the circuit board was trying to light the burner to get gas to flow. Once the tank was hot it kept the valve warm and it worked fine. When I disassembled the valve I found a buildup that resembled heavy grease fouling it.

 

[Pogrom88]

So I have found yet another great quirk to this...if I run my generator...and plugged into it is a trickle battery charger...and once the voltage of my battery reaches 13 v...and as long as I fire it while my generator is running...then I get 100% results...but if I attempt to run the furnace off of DC voltage...it is random if it fires or not...I am believing it's an amperage problem...funny thing...the hot water heater works under the exact same conditions...so I know I'm having amperage shortage...my battery is not sufficient...and that saddens me...it's a brand new 8D battery that replaced the previous 4D battery.

 

[Lou Schneider]

So I have found yet another great quirk to this...if I run my generator...and plugged into it is a trickle battery charger...and once the voltage of my battery reaches 13 v...and as long as I fire it while my generator is running...then I get 100% results...but if I attempt to run the furnace off of DC voltage...it is random if it fires or not...I am believing it's an amperage problem...funny thing...the hot water heater works under the exact same conditions...so I know I'm having amperage shortage...my battery is not sufficient...and that saddens me...it's a brand new 8D battery that replaced the previous 4D battery.

So what is the battery voltage when the furnace blower is running? Is that 8D a deep cycle battery or a starting battery that isn't intended for deep cycle use?

 

[Gary RV_Wizard]

.I am believing it's an amperage problem..

More likely a voltage problem - everything works at charging voltage (probably 13.3v-14.3v) but not a battery voltage (around 12.6 if fully charged). I would suspect a poor quality connection reducing voltage to the furnace and such.

 

[Pogrom88]

So what is the battery voltage when the furnace blower is running? Is that 8D a deep cycle battery or a starting battery that isn't intended for deep cycle use?

It's a deep cycle battery...a direct replacement...and the blower is capable of blowing at less then 11 V. But it will not ignite any gas if the voltage is less then 13.0 volts...and it must be AC power...any attempts to replicate with DC current fails.