Furnace not working

[JackMS]

My furnace will not come on at all. It is an Atwood 8535IVDCLP. I found the procedure to bypass the thermostat and it works when I do that. So I was about to order a new thermostat but I got thinking.

The thermostat is a dometic 5 button. I replaced the front a/c heat pump about 3 years ago and I had to order an additional circuit board to swap out on the unit to make it work with my thermostat. Both a/c's are dometic penguins. I think I used the furnace since then but am not absolutely positive of that. Is there anything on the roof a/c I should check first or is it most likely the thermostat? If the thermostat is the issue, would I be better ordering the updated thermostat and putting the original board back in the front a/c? Or is it even possible since the rear a/c is the original 2005 model?

I know a lot of questions but I know there are a lot of very experienced a/c guys here. Thanks in advance.

[enblethen]

The blue wires should go in to contact closure in the control board.
I would jumper the wires at the control and see if furnace comes on. If it does not, then odds are you have an open wire.

[JackMS]

So I did like Doug suggested and swapped boards from front to rear. Still no furnace so I figured I would just order a new board since the rear unit is the original- 13 years old. Got it put in today and furnace still doesn’t work. I ran out of time so didn’t get to check for power on the blue wires at the rooftop unit. Will try again next weekend. If I do still have power to one of the blue wires but not the other, what else could it be? 3 bad boards- not very likely but my luck does run that way sometimes.

[enblethen]

When you get into wiring by control box, connect blues together and test.

[JackMS]

enblethen wrote:
Are you sure the blue wires are connected to the unit that has the dip switches set for it?

Yes. Definitely

[enblethen]

Are you sure the blue wires are connected to the unit that has the dip switches set for it?

[JackMS]

Genius. Thank you sir. I was thinking about using the board off the old unit I took off a few years ago but it’s been sitting a while so who knows how good the board even is. Your option seems a lot better- will give that a try

[dougrainer]

JackMS wrote:
Thanks again Doug. The furnace dip switch was opposite from all the others originally and “furnace” did show on the 5 button but only had Power to one blue wire. I flipped the dip switch to test it and did here a click like a relay but still no power at second blue wire. So question is is the relay replaceable and if so, which one is it. If not replaceable then looks like I need to replace board right?

For me or a Layman, the relay is not replaceable. Dometic does NOT sell the parts mounted on the board. You must replace the board. BUT, there is a solution with NO money used. Take the Zone 2 board and mount it in Zone 1. Zone 1 goes to Zone 2. You just turn the Zone 2 DIP switch OFF and the furnace ON and turn on the Zone 2 DIP switch on the defective furnace board you are installing on Zone 2. Doug

[JackMS]

Thanks again Doug. The furnace dip switch was opposite from all the others originally and “furnace” did show on the 5 button but only had Power to one blue wire. I flipped the dip switch to test it and did here a click like a relay but still no power at second blue wire. So question is is the relay replaceable and if so, which one is it. If not replaceable then looks like I need to replace board right?

[MountainAir05]

Thanks Doug

[dougrainer]

JackMS wrote:
According to the link you sent, the dip switch for the furnace was wrong. When I switched it to match the others, that was the correct way- if pushed in at the bottom is off. But it still didn’t work. Do I need to reset the thermostat for that change to take effect?. If not, I will start working on cleaning connectors.

Lets make the DIP switch question simple. 99% of the time MOST of the DIP switches will be the same indent---OFF. On a 5 Button front AC/HP, you will have the Furnace ON, and will be the ONLY DIP switch you will have ON, for the Zone 1 board. So, it makes it easy, only one DIP switch will be opposite of the others---The furnace. Now, IF the furnace shows on the 5 button and the DIP was indeed OFF and you then turned it ON, you need to do the reset. ALL a reset does is verify the DIP settings for the Zone 1/2/3/4 AC boards. NOTHING ELSE. There is no magical reset like on a Computer or other appliance that will fix operational problems. IF the 5 button shows furnace AFTER the reset, then the 4 wire modular Communication cable or ends would not be a factor, like corrosion. But, yes, after the reset and you turn the furnace ON at the 5 button, BOTH blue wires should have 12 volt. IF NOT, I would suspect the relay on the board is bad. But that is extremely rare. The blue wires---12 volts comes FROM the furnace to the Board and is connected to one of the blue wires. It makes no difference which blue wire. The board closes the relay in furnace mode and 12 volts comes out back down to the furnace. Very simple system. Doug

[MountainAir05]

I would since its simple to do. If we were closer then I could give you new wire and connects. After you clean might want to reset again.

[JackMS]

According to the link you sent, the dip switch for the furnace was wrong. When I switched it to match the others, that was the correct way- if pushed in at the bottom is off. But it still didn’t work. Do I need to reset the thermostat for that change to take effect?. If not, I will start working on cleaning connectors.

[MountainAir05]

The switches have to be set for all the ACs and heater, not just any way. Will not talk to one another. The manual I link to you shows where they have to be. When I had this issue, I change the ends on the tele cable and clean the connectors that they plug in with Electronic cleaner. If you change them watch cause they are paired color for color. Put some Die Electric compound in then once you get it all clean and working.

I have also taken apart the thermostat and clean all the switches and put new button in. Use to be able to get a kit for new button at one of the place that will rebuild them at a high price, since all they do is clean and put in new buttons. check Solder connections on board.

[JackMS]

Ok finally got up on roof. One blue wire has power and one doesn’t. I found all the dip switches were the same direction, pushed in at the bottom, except the furnace. So I changed it’s position and heard a click but still no power out of the blue wire. Any more ideas?