HVAC system acting odd

Good afternoon. I am a new member and this is my 1st post. I believe this is going to be a winner. I have a 2013 Jayco bp toyhauler. The furnce is a Atwood and the a/c is dometic. First thing, my furnace wouldn't ignite, but the fan would come on. Next time to visit my RV the furnace fan wouldn't come on when I engaged the heater using the thermostat. This weekend I opened the door to find the furnace fan running with all systems in the off position. The only way to turn off the fan was the switch on top of the furnace. Then I went to turn on the a/c and it didn't do a thing. So I went to the switch on top of the furnace and turned it back on. The furnace fan and the a/c restarted. ??????? That is my question. The situation keeps changing. Could it be the thermostat? I can normally troubleshoot pretty well, but this one has got me scratching my head. Any info or suggestions would be greatly appreciated. Thanks for your time

I hate it when I type a reply and "Connection interupped"

First the T-Stat and A/C
Is that a digital T-Stat?  If it is not disregard this part.
Digital T-Stats have an internal battery. Often rechargable (And even replaceable, Rechargables only last a few years) and to re-charge them they often take power from the FURNACE side of the operation.

Thus turning off the Circuit Breaker atop the blower housing... (yes it's a breaker)  Cut power to the T-Stat.

(Check for an internal battery. Try to find the manua for it or google make, model Manual .. Note this ine appies to the next part as well)

Blows but no neat
First,The sequence
T-Stat feels a bit chilly so it calls for HEAT
Control board starts blower and starts watching for SAIL SWITCH COSED
A sail switch is a spring loaded switch with a "Sail" attached, the force of wind (Created by the blower) Pulls the switch closed... If the batteries are low.. not enough wind  (Also defective switch or poor connections)

You can find out where to probe with a meter (Multi meter on volts) by use of the Install/Service manual.

Then once it's up to speed the control board calls for Propane and fires up the spark generator for a few seconds. Then it checks to see "Flame on". If so operation continuesk if not restart 2 times if still no success lock out.

Mine will keep blowing in lock out (3rd party board) OEM did not... but OEM did not sense the flame so I had to replace it.

New (Dinosaur board) much higher quality.

That's all I have for the first round. More info in the forum libraries and archives if you need it.

First: Not all digital t-stats are battery powered, but some certainly are.  Please identify the make & model of t-stat in your rig. Probably one of the Dometics, but there are several models.  Also, does this rig have two or more heat/cool zones? If so, there is a zone controller in the mix between the t-stat and both the furnace & the a/c unit(s).  That could mess u both if it is quirky.

As Jon says, low battery, or at least low 12v power, could explain some of these problems. Might make the whole HVAC system flaky.

The furnace receives its power directly from the RV's 12v system and the switch in the furnace shuts that off.  The furnace responds to an external "heat needed" request but uses its own power source. "Heat needed" is simply +12v detected on a wire from the thermostat, but if there is a zone controller in between, the wire actually comes from the zone board, not direct from the thermostat. 

Another known problem with some Dometic multi-zone thermostats is called zone mismatch, e.g. one zone set to cool and another set to heat.  See this articleto see if it applies to you:
http://www.sundancecustomrv.com/DuoThermTstat.pdf

Thank you for the great insight. My thermostat is manual with a slide bar for temp and another for mode. It also has a 3rd bar for (auto, on, off). The thermostat operates the a/c fine but the a/c fan only will not start. This all happened rapid fire. Also always shore power. I was thinking/hoping thermostat. Unfortunately I'm unable to return to my preferred home for a couple weeks to further any troubleshooting. Right now, here in Texas it's warming up. My problem is can't turn on a.c. without turning furnace fan on. Can't remove furnace without loosing ac.

The thermostat is a Coleman 12V Heat-Cool Thermostat 7330F3852. The part of this that I really don't get is the switch on the furnace controling the a.c. Can I bypass that switch? Or could the thermostat really be the culprit to my problem? Thanks in advance for any help understanding this.

Gary; It's a single unit, 30amp. Thanks

Here's a link to the RVP Service manual for RV air conditioning/heating systems.  It has detailed information about what signals you should be seeing on the thermostat wires and several troubleshooting flowcharts that can help isolate the problem:

http://www.cleggind.com/postal_tech/pdfs/AC2/airxcel_1976376%5B1%5D.pdf

Thanks Lou! I was looking at that chart last night. I'm really keeping my fingers crossed its in the thermostat

Most a/c systems use 12v to power the circuit board even though the a/c itself is a 120vac appliance. The thermostat also requires 12v power.  It's possible your rig was wired such that the furnace 12v supply also provides 12v to thermostat and maybe the a/c board.  If the thermostat doesn't get 12v, it can't trigger the a/c or the furnace, so neither will start.  It's common to wire the thermostat power from the terminal strip at the furnace, but not so common to use the furnace breaker/switch to control power to that strip, but anything is possible in an RV.

Thanks Gary. I might be more confused  now . The ac works as intended from the thermostat as long as the switch is on on the furnace. The furnace fan blows when the switch is on regardless of what I do to the thermostat. Where should the circuit board be located? If it wasn't for everything being fed htru the furnace switch I wouldn't be so confused or even as much of a hurry. I could remove the furnace and take it for repair but I need the ac!

Knowing the situation, what is the 1st thing you would do if it was your rig? Thank you for your time and assist, much appreciated

I'd look at your furnace and find out why it's fan runs as soon as you turn on it's switch.  The only thing the thermostat does is tell the furnace to turn on or off, all of the other steps needed to do so are controlled from within the furnace.  If the thermostat was commanding the furnace to start, it would go through all of the steps needed to start and would begin blowing heat.  Just the fan running points to a problem in the furnace itself.

As to why the thermostat and a/c won't work unless the furnace switch is on, that's where the thermostat and the a/c's relay logic gets it's 12 volts.  It's more convenient than running separate 12 volt circuits to the thermostat and control box.

What model furnace do you have?  Do a Google search on it and you'll find lots of troubleshooting help.

Thanks Lou! The furnace is an Atwood Hydro-flame (30551 by looking at a picture of it). Its only 3 years old and hadn't been used nearly as much as the ac. I just have a feeling its something stupid. Three consecutive visits, three diferent issues. Next time I go down to the rv I'll cap the gas supply and pull the furnace. Do you know if its possible to by pass the switch so I can run the ac while the furnace is out of the rv? Summer's almost here (TX gulf coast) and no ac might cause family issues!  8)

Thanks for all of the input from you all!

Chuck Chretien said:

Do you know if its possible to by pass the switch so I can run the ac while the furnace is out of the rv? Summer's almost here (TX gulf coast) and no ac might cause family issues!  8)

Click to expand...

All you have to do to make the a/c operational while the furnace is out is supply +12 volts and ground to the thermostat, since the furnace to supplied these.  Usually the red wire is +12 volts, but check your thermostat's manual to be sure.

That's what I'll do next visit. I'm at the beach and I'd rather fish but... 1st things 1st. Thanks again , Lou

Most Coleman Mach thermostats act this way if the fan setting is in the wrong position. Try using the Auto setting. If it is in the auto position then try high. Can't remember which setting it is but one of those makes the AC fan run even when it is cold and you want heat.

Thanks for the info, Keymaster. I have already worked the t-stat settings over a few times. I'm going down to my rv sunday and see if pulling the fuse on rhe furnace might allow me to run the ac without the furnace fan blowing. I'm keeping my fingers crossed. Don't need heat anymore for probably the next 8 months. Thanks again for your input.