Furnace starts out great & then later not?

[Blanco1]

Two trips in a row now are furnace starts out great!
Lots of heat & by morning when we wake it blowing cold.

Can anyone help me fix this?

Suppose to another cold night.

[Blanco1]

I've been thinking about adding a vent into the bathroom shouldn't be too hard?
& would save me the trouble of placing an electric heater in there to keep the toilet from freezing.

[time2roll]

Second added vent went in today. Not super cold to test but the burner ran continuous for over 20 minutes while temperature rose from 60 to 75. Amazing the volume of hot air that is now coming into the RV.

[time2roll]

I added an additional duct last summer and it was too warm to really test. This morning I made it over to check for improvements. Here are the results.

Was burner on about 2 minutes, burner off about 1 minute and repeat.

Today the burner ran 18 minutes before it cycled off for 75 seconds. Then was about 5 minutes on, 1 minute off. I am very pleased. Still debating if I should install a second additional duct as it will encroach on the limited storage compartment space below the bunk.

[oldbeek]

smkettner wrote:

Blanco1 wrote:
To be honest these last two post are much more technical than I understand?

Yet appreciate it, in time I 'll start understand your lingo. 🙂

RV furnaces have a high temperature limit switch to prevent the box around the furnace propane burner from overheating. Generally the normal airflow should keep the burner box cool enough to operate continuously.

Due to general poor engineering and workmanship it seems to be fairly common for RV to not have enough airflow through the furnace. This causes the box around the burner to get too hot. There is a "high limit switch" that turns off the propane burner when this happens. The fan continues to run and cools the burner box. Once the temperature drops to a safe level the burner will come back on.

My furnace does this. Fan runs continuous but the burner flame cycles on for about 2 minutes, off 1 minute, on 2 minutes etc. until the RV gets warm and the thermostat says to quit.

My ducting in the floor was collapsed when built. I discovered this with a mirror looking down into the ducting. Unfortunately mine was long out of warranty when discovered.

You may have a similar issue and eventually the propane burner does not relight so it blows cold air until morning.

What you need to do is sit and focus on the sounds the furnace is making when it fires up. You should first hear the fan and then the burner a few seconds later. Then you need to continue to focus for a least 10 minutes and listen for the burner to cycle off and back on. I find it difficult to notice the burner going off with the fan noise etc. but when it relights the burner the sound is much more obvious.

If the burner does run continuously you just eliminate this issue and we move to the next....

Good luck.

As I stated before My heater high limit switch opens with every duct opening removed and the basement wide open with wind at 50f blowing through it. The opening area is twice of what Suburban says is needed. Every gas cloths dryer made uses the same snap switch to control dryer temperature. As every rv I have ever worked on cycles the hi-temp switch, I wonder if Suburban even knows how their heater is designed. Dinasour says that the 6 time out is designed into the original boards. I will agree most rv,s could use better ducting. -*

[time2roll]

Blanco1 wrote:
To be honest these last two post are much more technical than I understand?

Yet appreciate it, in time I 'll start understand your lingo. 🙂

RV furnaces have a high temperature limit switch to prevent the box around the furnace propane burner from overheating. Generally the normal airflow should keep the burner box cool enough to operate continuously.

Due to general poor engineering and workmanship it seems to be fairly common for RV to not have enough airflow through the furnace. This causes the box around the burner to get too hot. There is a "high limit switch" that turns off the propane burner when this happens. The fan continues to run and cools the burner box. Once the temperature drops to a safe level the burner will come back on.

My furnace does this. Fan runs continuous but the burner flame cycles on for about 2 minutes, off 1 minute, on 2 minutes etc. until the RV gets warm and the thermostat says to quit.

My ducting in the floor was collapsed when built. I discovered this with a mirror looking down into the ducting. Unfortunately mine was long out of warranty when discovered.

You may have a similar issue and eventually the propane burner does not relight so it blows cold air until morning.

What you need to do is sit and focus on the sounds the furnace is making when it fires up. You should first hear the fan and then the burner a few seconds later. Then you need to continue to focus for a least 10 minutes and listen for the burner to cycle off and back on. I find it difficult to notice the burner going off with the fan noise etc. but when it relights the burner the sound is much more obvious.

If the burner does run continuously you just eliminate this issue and we move to the next....

Good luck.

[steveh27]

When the same thing happened to me, my sail switch needed cleaned. The rv shop charged $140, ouch.

[JaxDad]

Blanco, the translation is;

First, the furnace has a sensor which turns off the heat, but leaves the blower going, if the furnace gets too hot.

Secondly, they're asking if the burner keeps trying to start when the blower is running. The difference is there's a micro swith in the furnace called the "sail switch". Basically it's got a little tab connected to the switch, the tab sticks out into the airflow of the blower. If there's enough air flow (which pushes the switch shut) to safely allow the burner to run, it does. If not the burner will never light. If your batteries are low the blower motor turns too slowly and doesn't make enough airflow to close the switch.

The simplest, read very low-tech, test is when it's the coldest light every burner you have, put them on highest setting and see if a) they even light, b) get to full output, c) stay lit and at full output. NB: you may need to crack a window and / or put on the exhaust fan.

If you get less than perfect results from the above you'll know straight away it's a gas problem.

[Blanco1]

smkettner wrote:
oldbeek, I have to agree with Suburban that you have restricted ducting causing the high limit switch to open. Mine does the same but I don't think mine cuts out at six cycles. Mine runs a long time with about 2 minutes burner on and 1 minute burner off. I have also added an additional duct but have not tested it yet. I have an OEM board not a Dinosaur.

To the OP, can you tell if the burner is cycling while the blower continues to run?

To be honest these last two post are much more technical than I understand?

Yet appreciate it, in time I 'll start understand your lingo. 🙂

[time2roll]

oldbeek, I have to agree with Suburban that you have restricted ducting causing the high limit switch to open. Mine does the same but I don't think mine cuts out at six cycles. Mine runs a long time with about 2 minutes burner on and 1 minute burner off. I have also added an additional duct but have not tested it yet. I have an OEM board not a Dinosaur.

To the OP, can you tell if the burner is cycling while the blower continues to run?

[oldbeek]

Blanco1 wrote:
Ok, camping right now & I've learned, that it's not a battery issue being the battery seems to stay above 12 volts or more & still stops blowing hot air after a while?


Yet turning it off & turning it back on seems to get the heat going again ?

So what you guys think?

Ok, I know what is going on. My heater did the same thing. I have test equipment to test the motor speed. OK. NASA checked my hi temp snap switch in their lab.OK. 11.6 in gas pressure tested with a manometer.OK. Used drills to check the gas jet size.OK. Removed all the ducts and bench tested the unit on a 50f day with the wind blowing across the heater. ( NO duct restriction), Contacted Dinosaur and Suburban tech support. Dinosaur says there is a safety built into the board that allows your heater to overheat (hi-temp snap switch opens) 6 times the heater can cycle on and off (if it still hasn't met the thermostat set temp)the board times the heater out for 1 hour. In real cold weather sit down outside next to your heater when you first start it. You will hear the roar of the burner then it will go quiet for a while then start to roar again. this is one cycle. I confirmed this with temp recorder installed inside the fire box. Fire box hits 410, snap switch opens, 390 snap switch closes as the blower continues to run. LOOONG POST.. What to do? the quick fix is to pull the heater 12volt fuse and reinstall it after the sixth shut down. This reboots the board. Suburban says the snap switch should not open. I have restricted ducts.(BS) The can the heater is installed in had all the duct openings removed. I did install solid and flex 4 inch ducting that produced 30% more heat to the registers inside the rig. I installed 2 sizes smaller main jet. These 2 things lengthened the 6 cycles but in 15f weather the heater still cycles 6 times before I can get up to t-stat temp if set at 70f. When first bringing up the temp I set t-stat to 55. when that is attained I reset the t-stat to 70. This winter I installed an extra catylic heater. No 12v needed. I only have about 40 hours of time invested in all this testing. Glad to share it. Most techs don't even know how these things work. P.S. They work perfect on a warm day at the rv shop.

[jwmII]

A battery that reads only 12 volts is technically dead. 12.6-12.7 is fully charged. Older batteries reading low will not have the oomph to continually maintain the needed blower RPM to keep the sail switch closed allowing the furnace to blow cold air and freeze you out. A battery reading 10-11 volts will run the blower motor and it will sound as if it is running ok, But you need that RPM that only a fully charged battery can provide.

[Harvard]

If related to battery operation then most likely a low voltage sail switch issue.

If not, could be an oil trap in the copper line to the furnace IF it works when warm (lower viscosity) BUT not when cold (higher viscosity). This type of issue is probably more common with using propane bottles then with frame mounted propane tanks..

[Blanco1]

Ok, yes I got our propane in Los Angeles.
1st night in New Mexico, it was about 8 degrees & we had no electrical hook up & ran the Geni for a while then the furnace ran strictly on battery.

& did not stay hot all night, very cold night for us.

Next night we had electrical hook ups & same thing, temps were about 10 degrees.

Then back to Arizona & it was about 16 degrees it seemed to keep warm through the night.

Then last night here in Laughlin we just just ran the electric heaters, so don't know if it would kept warm, our last night will be boon docking in the desert so we'll see how it works that night.

[JaxDad]

Blanco, have you bought more propane since this problem started?

What is sold as "propane" differs quite a bit based on when & where you buy it. If you filled up near LA in warmer weather you well could have gotten nearly straight, if not straight, butane.

Butane boils at 30 to 34 deg. F. so if it's that cold or colder you will have little to no gas vapour. In other words you could run out of gas with a full tank of butane.

By comparison propane boils at anything above about -44 deg. F. so you'd still have lots of gas vapour down to well below most people would be camping in.