1991 beaver Contessa LP furnace question

[Jason Worman]

Hi everyone

Another question about our Suburban gas furnace. We have to turn the unit on, let it run several minutes and then shut off, turn on and then we get heat, is this normal. Also our LP tank has liquid and vapor on/off valves. What runs on vapor and what runs on liquid. Thanks

[Joel Ashley]

I’m not sure about your Suburban, as our old ‘84 Pace Arrow had a different brand of furnace.  But operation should be similar, and I don’t recall the routine you mention. 

When the thermostat calls for heat, the blower motor should start.  When air speed is adequate, the sailswitch closes allowing 12v to open the furnace’s gas flow.  As long as the burner box overlimit switch stays closed (box isn’t dangerously hot) the unit should continue operating.  There’s more to it than that... see this:   http://www.bdub.net/manuals/Suburban_Service_Manual.pdf
The unit requires occasional service by experienced techs... 12v valve and switches check, nozzle replacement, burner box cleaning, intake and exhaust port checks (wasps build nests), etc.

As to the propane situation, I don’t know of a use for the liquid form other than as a means of storage and transfer.  Pressure in the tank keeps it liquid, but vapor upon slight depressurization (call for fuel by an onboard device) occupies the at least 20% unfilled tank space, and expands through the opened line.  Interestingly, firemen at an RV fire will ask how full the tank is because 10% full is more of an explosion hazard than an 80% full one.  It’s the vapor that combines with air to combust.  Our coach has (and our Pace Arrow had) a fill valve for taking on liquid propane, and a valve for releasing gas (vapor) to the coach through the regulator.  Perhaps that’s what you’re seeing?

Joel

[Keith Phillips]

Jason, the motor in my suburban furnace got real noisy last winter. I found a u tube video on how to change the motor. I was surprised how easy the unit is to remove from the coach and work on. It was very dirty inside, a complete cleaning and a new blower motor for about $120.00 and it works great and real quiet.

[Jason Worman]

Thanks for the responses. Our local service tech changed out the hot water heater yesterday. I will have him service the furnace on his next visit. I read on the unit itself about having to start/stop the furnace, it just seems odd.

[Joel Ashley]

I reviewed instructions for several Suburban units and didn’t see any reference to a two-start process.  Find your model number off a tag or plate on the unit, then see if it’s specific operation (and/or service) manual is available from Suburban, or reference these options: 
http://docs.renegaderv.com/Suburban/Furnaces/FURNACE_Service%20and%20Training%20Manual%2011-05-2015.pdf

https://www.manualslib.com/brand/suburban/furnace.html

One note, to save costs I once (~28 years ago) dismantled mine for nozzle and burner box service and remember there was a service warning of particular concern.  The firebox was ceramic or something and from heat, age, and vibration could develop microcracks.  Heat, smoke, and flame could conceivably exit the box via such disintegrities.  The service instructions insisted due to that risk that only qualified techs work on them.  I saw infinitesimal hairlines in the box and was reticent to go any further. 

If the unit is very old and your tech agrees given any parts and labor costs, consider a new modern furnace.

Joel

[Gerald Farris]

Jason,
Pertaining to you propane question, usually the only thing that uses liquid propane is a propane generator, and everything else uses propane vapor, including the refrigerator, water heater, furnace, and cooktop.

Gerald

[Jason Worman]

Thank you everybody for all the help. After the tech replaced hot water heater, our furnace is not doing anything; just sitting dead. Checked wiring diagram and I think the 12v dc got disconnected in the process. Having him return.

[Jason Worman]

To continue, after I thought about the furnace not doing anything, we had hooked up temporarily to a house outlet and when a space heater was turned on it tripped a breaker. Breaker was reset and since the furnace sits idle. I  am wondering does the furnace have a fuse or something. There appears to be power to the thermostat as the fan side operates just fine. Any ideas would helpful as tech is gone for holiday weekend. As always thank you to everyone that had helped us newbies along.

[Joel Ashley]

If you mean the fan in the furnace works fine, but there’s no ignition, it implies that the sailswitch may not be closing to allow the gas valve to open.  This can be because the fan isn’t blowing strongly enough to push the sail, and hints that your batteries aren’t fully charged. 

Memory is very dim, but isn’t there a gas shutoff in the line going into the valve just outside the furnace box?  Check that the tech didn’t shut it for some reason.  If you can hear the ignitor “snapping” but no flame, that’s another valve-off clue.  It could also be the ignition points are fouled and not sparking across (so there’s no snapping sound), and need cleaning/alignment.  Or as previously noted, a fouled nozzle may not be letting gas through to ignition, or wasps may have built a nest in the outside intake or exhaust restricting adequate burner air flow, again perhaps handcuffing the gas valve solenoid.

Also, are you being patient enough... my wife used to gripe when the furnace was first turned on cuz on some models it can take up to several minutes of blowing cold air before the various delay switches allow voltage through to the gas valve solenoid.

If you Google or YouTube “suburban furnace wiring diagram“ you’ll find more than you really want, but including perhaps insightful videos.

Joel

[Jason Worman]

The furnace does nothing, no fan, no blower, not anything.  When the tech replaced water heater he could not find switch for 12V for the water heater. He had to wire the heater with 12v on all the time. Under the kitchen counter is a switch that was 12 v for water heater and furnace. I'm thinking this switch to furnace got disconnected.

[Joel Ashley]

Methinks you are right.  If he snipped the original wire at its source thinking it only went to an unlocated water heater switch, that wire needs to be spliced back in.

Joel