Furnace not working

[Peteyboy]

We do have 12 volts at the furnace when we call for heat.  Power goes all the way to the board we replaced but does not come out so it appears that the thermostat is doing it's job.  We tested the on off switch that is on the furnace itself and it is working properly. What you said earlier that it is still not receiving a signal somewhere is the most probable issue.  But what signal ?

 

[Gary RV_Wizard]

We do have 12 volts at the furnace when we call for heat.  Power goes all the way to the board we replaced but does not come out so it appears that the thermostat is doing it's job

The control board has two "12v out" functions: (1) start the fan in response to the thermostat, and (2) open the gas solenoid once the fan is up to speed. Are you saying the fan doesn't come on?  Nothing  else even tries to happen until the fan comes on and gets up to operating speed (determined by the sail switch). Then there is a short delay for purging the burner before the gas valve opens and ignition is attempted.

If the fan doesn't run, you have either a bad fan or bad wiring path to it.

Your original post said the shop determined that the furnace runs "directly" if the thermostat is bypassed. Now you are replacing circuit boards and saying no 12v out of the board? Doesn't make any sense! Both of those things cannot be true at the same time.

 

[Peteyboy]

Things now are pointing to the heat isolation relay that is supposed to be in the return air plennm.  Any ideas where that may be ?

 

[Peteyboy]

The control board has two "12v out" functions: (1) start the fan in response to the thermostat, and (2) open the gas solenoid once the fan is up to speed. Are you saying the fan doesn't come on?  Nothing  else even tries to happen until the fan comes on and gets up to operating speed (determined by the sail switch). Then there is a short delay for purging the burner before the gas valve opens and ignition is attempted.

If the fan doesn't run, you have either a bad fan or bad wiring path to it.

Your original post said the shop determined that the furnace runs "directly" if the thermostat is bypassed. Now you are replacing circuit boards and saying no 12v out of the board? Doesn't make any sense! Both of those things cannot be true at the same time.

Gary,  originally, the shop said that the furnace worked when they powered it up "directly".  What that meant I am not sure.  I would bet, knowing what I know now is that they got the fan to run.  Today we got the furnace to run and throw heat by jumping wires at the furnace itself.  We also determined that the thermostat is sending the proper signals to the furnace.  Three of the wires do not go to the furnace and supposedly go to a heat isolation relay somewhere in the return air plenum. The 3 remaining wires at the plug,at the furnace, other than the white wire do not come from the thermostat.  Whew,,,, I think I have that right.

 

[Peteyboy]

The new thermostat we had replaced in the spring was wired incorrectly from the factory.

 

[QZ]

Thanks for the follow up post and I hope your winter is comfortable. It's great learning material. I'd try to get schematics of the  thermostat, furnace, ac unit and control box and plenum and anything else all put together in one place. I have copied and increased the size of  pages of schematics for various pieces of mobile equipment and taped them together. It made a nice fold out map when trying to troubleshoot. 

The good thing is that now you are at square one. It may be best if you learned  the system and keep others fingers out of it. Find out where all the sensors etc are. While looking at your issue it did seem like a good number of posts were about resetting the thermostat. You need  to be able to program and reset before assuming any repairs are needed although I do understand how it is when we dont put our hands on some of these gadgets regularly.

 

[Peteyboy]

Luckily I knew a local HVAC tech who was willing to look at the furnace and did not give up until he found the problem.  The best thing was he came to my house 5 times to work on the rig as we progressed through the issue.  A couple of false leads were followed but he and I both learned a ton through the process. You are certainly correct when you say it is important to know as much as you can about these machines, as you never know when it may come in handy,,,, Thanks !!

 

[Peteyboy]

BTW,,, it's a piece of cake getting the furnace out of a 37F.

 

[John Canfield]

... I'd try to get schematics of the  thermostat, furnace, ac unit and control box and plenum and anything else all put together in one place. I have copied and increased the size of  pages of schematics for various pieces of mobile equipment and taped them together. It made a nice fold out map when trying to troubleshoot. ....

When our coach was new, I printed out all of our plumbing and wiring diagrams and put in a drawer. Rarely do I need to refer to them thank goodness.

 

[Peteyboy]

Winnebagos schematics leave a lot to be desired as even when I talked to a tech at Winnebago, he had a very hard time telling me where some relays were hidden.  He expressed that he wished they had the old schematics to help locating various items. 

 

[Baileyracing99]

We have a 98 Itasca sunflyer.  I cannot even get the blower to turn on when in heat mode on thermostat. If I switch to cool I hear it come on.  We are at out wits end. It's getting cold now

 

[afchap]

Baileyracing99 ... Welcome to the forum.

You will get much better responses if you start a new thread when you start a new topic rather than doing it one that already exists.

If I understand you correctly you are trying to turn on the blower from the thermostat when in heat mode. That blower is for the AC / heat pump. The thermostat has electric heat mode, and gas heat mode. When in the electric heat mode the blower comes on only when the heat pump calls for it after the compressors start, and the air will come out of the ceiling ducts. You cannot manually turn on the heat pump/ac blower when in either heat mode. If you are running the gas heat, the lp furnace will come on and the furnace blower should come on with it and the heat will come out of the floor ducts.

 

[Arch Hoagland]

Thermostat was replaced this summer for an A/C problem, so chances are that it is good. They did say that the thermostat signals are not reaching the furnace.  They said something about a secondary thermostat being in the system somewhere,,,, ??

Did the furnace ever in it's whole life ever work?

Did it work right AFTER the new thermostat was installed?

Who installed the thermostat and what was the original problem with the air conditioning?

 

[Peteyboy]

1. Yes
2. No
3. A local RV dealer.  Front A/C did not work at all.

 

[Arch Hoagland]

I think I'd go back to the RV dealer and explain that the furnace doesn't work and the problem started when they fixed the A/C.

 

[Peteyboy]

I think I'd go back to the RV dealer and explain that the furnace doesn't work and the problem started when they fixed the A/C.

Actually, I did do that and they said they couldn't figure it out.  I believe I stated that before.

 

[Arch Hoagland]

Actually, I did do that and they said they couldn't figure it out.  I believe I stated that before.

Didn't realize it was the same RV place.  What is the name of the RV dealer? I don't want to go there!

 

[Peteyboy]

Didn't realize it was the same RV place.  What is the name of the RV dealer? I don't want to go there!

Don't really want to badmouth them as they are good folks but just a bit short on diagnostic abilities.  They worked at it for 3 hours and did not charge me anything for their time.  Turned out to be a fairly difficult problem to find and they really had nothing to do with it.