Furnace or Thermostat?

jaycojay · 10-04-2020, 07:35 AM

While camping last week in Ontario, the one morning i went to take the chill out of the air and turned on furnace, fan started, as did the A/C unit on roof. Strange i thought, furnace fan ran for about 20 seconds than shut off. Shut system down. Now this Jayflight has the Dometic Thermostat, the small square one with blue LED, with mode button on one side, and up n down arrows on right. So, took out stat user guide, and when fan mode is set in low or high, the A/C fan will come on when furnace does ( not sure why u would want to ), and if u set to "Auto", only furnace will run. Set to Auto, turned furnace back on.... since A/C unit is right above my head, i heard the a/c unit click, than furnace fan came on, again ran for about 20 seconds or so, than shut off. I did not hear any clicking, like you do on the fridge when its attempting to light, which by the way, always seems to have a difficult time lighting, but all my Norcold Fridges have been like that. Who has that Dometic Stat ..... and am i missing something here? I turned on the stove, can hear the gas, hooked to shore power, took off the inside grille and flipped the switch on the top of the furnace on and off, which does make the thermostat on the wall go on and off. Heading out camping in two weeks, so chances of me getting it into a dealer and back before we leave are slim

RogerR · 10-04-2020, 09:21 AM

I think but cannot be 100% that your furnace fan turns on means the thermostat is working. But the culprit here is most likely the safety switch in the furnace that detects that the fan is running properly, vents aren't blocked, etc. This is called the "sail switch". It is located in the furnace and when the fan comes on and is up to speed it is supposed to move the "sail" or wind sensor enough to close a switch and allow the burner to come on. It is a safety precaution to keep the furnace from lighting when there is not enough air flow to prevent it from overheating and causing a fire.
Research "sail switch" along with your model number for location and replacement. If you can locate it and manually push it you may get the furnace to run, which proves that is the problem. Check all your fuses and connections while you are in there and make sure your air inlet and outlet are not blocked or the runs pinched or clogged.

Sail switch failure is not uncommon.

JimD · 10-04-2020, 10:34 AM

Quote:

Originally Posted by RogerR

I think but cannot be 100% that your furnace fan turns on means the thermostat is working. But the culprit here is most likely the safety switch in the furnace that detects that the fan is running properly, vents aren't blocked, etc. This is called the "sail switch". It is located in the furnace and when the fan comes on and is up to speed it is supposed to move the "sail" or wind sensor enough to close a switch and allow the burner to come on. It is a safety precaution to keep the furnace from lighting when there is not enough air flow to prevent it from overheating and causing a fire.
Research "sail switch" along with your model number for location and replacement. If you can locate it and manually push it you may get the furnace to run, which proves that is the problem. Check all your fuses and connections while you are in there and make sure your air inlet and outlet are not blocked or the runs pinched or clogged.

Sail switch failure is not uncommon.

You can be 99.9% on the thermostat. Its whole purpose in life, as far as a furnace is concerned, is to send a 12 volt signal to the furnace. As long as the 12 volts is on the furnace control line, the furnace will or should remain on. When the 12 volts is removed the furnace will initiate its shut down sequence. The only way a thermostat would be the problem is if it only pulsed the control line which is not common.

So, the furnace is receiving its turn on signal but only staying on for 20 seconds. Like you mentioned, this is very typical of a sail switch failure especially if no "click" is heard. The sail switch MUST work before gas and ignition is allowed. The "sail" must move freely and the switch must operate normally, meaning it is not gummed up or stuck.

Low battery voltage can often cause low blower speed which is not enough to move the sail as required.

RonkoCamper · 10-04-2020, 03:27 PM

Following

Same problem as mine. A buddy and I pulled the furnace unit out from under the fridge and tried to find the issue. We did get it to fire up! We chaulked it up to dust or spiderwebs in the firing chamber as some have posted. Reinstalled it and .... nothing!

I have a reliable RV Repair Tech coming over in the AM to look at it. I'll let you know what my issues is.

jaycojay · 10-04-2020, 04:17 PM

Quote:

Originally Posted by RonkoCamper

Following

Same problem as mine. A buddy and I pulled the furnace unit out from under the fridge and tried to find the issue. We did get it to fire up! We chaulked it up to dust or spiderwebs in the firing chamber as some have posted. Reinstalled it and .... nothing!

I have a reliable RV Repair Tech coming over in the AM to look at it. I'll let you know what my issues is.

I appreciate that!

jaycojay · 10-04-2020, 04:20 PM

Quote:

Originally Posted by RogerR

I think but cannot be 100% that your furnace fan turns on means the thermostat is working. But the culprit here is most likely the safety switch in the furnace that detects that the fan is running properly, vents aren't blocked, etc. This is called the "sail switch". It is located in the furnace and when the fan comes on and is up to speed it is supposed to move the "sail" or wind sensor enough to close a switch and allow the burner to come on. It is a safety precaution to keep the furnace from lighting when there is not enough air flow to prevent it from overheating and causing a fire.
Research "sail switch" along with your model number for location and replacement. If you can locate it and manually push it you may get the furnace to run, which proves that is the problem. Check all your fuses and connections while you are in there and make sure your air inlet and outlet are not blocked or the runs pinched or clogged.

Sail switch failure is not uncommon.

Where is this switch located?

Rustic Eagle · 10-04-2020, 07:06 PM

Quote:

Originally Posted by jaycojay

snip...... Set to Auto, turned furnace back on.... since A/C unit is right above my head, i heard the a/c unit click, than furnace fan came on, again ran for about 20 seconds or so, than shut off.....snip

The "click" you heard at the A/C is a relay linked to the furnace circuit, mine does the same thing and is normal according to Dometic.

As others have stated, what you are describing sounds just like a compromised furnace 'sail switch', I had to replace mine last winter.

What you can try is start your furnace from the wall thermostat, then immediately tap the furnace blower housing (or furnace housing) firmly while the furnace fan is running prior to ignition, sometimes this may free any debris (pet hair, dust, etc.) that may be keeping the micro 'sail switch' arm from moving freely thus allowing ignition. I did this with mine and it worked, but after a few weeks the switch failed all-together.

After three (3) unsuccessful ignition attempts the furnace will require a 'reset'. Turn the wall thermostat 'off' prior to preforming a furnace 'reset' via the reset switch (hold in 'off' position for 15 seconds, then switch back on).

The micro 'sail switch' on my Dometic is the located on the side of the blower housing with three (3) blue wires attached to it (reference following two pics). In my case I had to remove the entire furnace in order to replace my sail-switch.

While you have access to your furnace gather all the furnace identification numbers (model, S/N, etc.), you will need this info should you require a new 'sail switch' because there are a number of different switch 'paddle' arm configurations.

There is a 'high temp' pancake style switch which has two (2) two blue wires attached to it located within the exhaust port housing (reference following pic), but your symptoms seem to point more to the 'sail switch'.

Hope this helps....

Bob

jaycojay · 10-05-2020, 02:06 AM

Thanks for the info ..... must be a bad switch, because the unit is new ( purchased last October), furnace has never been used. I do remember now we were delayed about an hour when picking it up because they were having an issue with the furnace, but it was working when we did the walk thru, because it was toasty warm in the unt

Rustic Eagle · 10-05-2020, 03:50 AM

jaycojay,

If still covered under warranty, you may want your RV dealership handle the repair. I was a couple months out of warranty.

Last winter my particular sail-switch was on back order (everyone was "out of stock"), took a month before I could get my hands on one..., I purchased a spare. My electric heater was my back-up while waiting for the switch.

Bob

Jolvi · 10-05-2020, 04:51 AM

When we did the walk thru all was fine, well they did show us everything worked. Best part is that the dealer we bought from has a campground and prefers that you stay at least one night to maybe catch anything that "may malfunction" so we did. Good thing it was a bit cool at night and wife asked me to turn on furnace and test it. Our furnace lit one time just fine no problems then every other time the furnace would click run for a bit and the A/C would also run but not steady. The furnace would just run the bit to clear out any residual propane and do its safety check but no heat. Now the A/C running it was not the compressor just the fan part. Tech came by in the morning to see how it went so we told him even recorded it and showed him the video. First thing he does is go to thermostat to make sure it was on auto kind of a strange arrangement on them if the switch is on anything else both of the HVAC systems want to work. Some people say it is to keep the air moving to keep uniform temp, sounds good. Anyway ended up being the communication cable from thermostat both a/c and heat had not been completely plugged in and as such it was not functioning. Tech had never heard of it either so they called Jayco and got the answer. Hooray now all is good.

RonkoCamper · 10-05-2020, 01:35 PM

My RV Tech came by today and took a look at the system. Same as before- thermostat would trigger the furnace to run, fan would spin but not ignite.

Took the furnace out. Checked fit of connectors, inspected circuit borards, looked for dust bunnies, checked the sail switch, etc. Found nothing irregular. He hit the reset button and it fired right up! It works!

He has no idea what he did and felt bad for charging me a service call! Works for me! At least we'll have heat this weekend at Hershey Park.

jaycojay · 10-06-2020, 04:25 AM

Quote:

Originally Posted by RonkoCamper

My RV Tech came by today and took a look at the system. Same as before- thermostat would trigger the furnace to run, fan would spin but not ignite.

Took the furnace out. Checked fit of connectors, inspected circuit borards, looked for dust bunnies, checked the sail switch, etc. Found nothing irregular. He hit the reset button and it fired right up! It works!

He has no idea what he did and felt bad for charging me a service call! Works for me! At least we'll have heat this weekend at Hershey Park.

I tried the reset button last nite, no luck ..... we'll just use a couple space heaters next week if necessary ( as long as we don't lose hydro )

TweakerG · 10-06-2020, 07:50 AM

I just went through the same thing with mine, except mine would work on shore power and not on battery. Mine ended up being really poor splices at the furnace not allowing it to spin the fan fast enough to activate the sail switch.

Koppster · 10-08-2020, 02:11 AM

I've had the same issue with mine, it was the sail switch most recently but the igniter failed before that. The attached guide will be helpful now and in the future.

I would also mention that in my case I was initially able to get it working by checking the connections but it failed again within a week or so.

jaycojay · 10-08-2020, 02:26 AM

thanks for the guide .... very helpfull

AngieMike · 10-14-2020, 12:58 PM

I had the same issue, I pulled unit out and tested the sail switch, which was fine and everything was clean. I bench tested the furnace and all would work. Re installed in trailer and it would not fire up, it would turn the fan on and then shut off. There is a lock out switch or thermo switch which is also part of the safety system, this I found to be an intermittent problem, supposed to be a closed switch, allowing power to go through it, It was open. I installed a new one, still would not work. At the end of the day, the board was bad( Not sure if I damaged it by not being careful or not. There is normally a a red light that blinks on the board, this should steer you in the right direction.

Marmot · 10-14-2020, 01:09 PM

Quote:

Originally Posted by jaycojay

While camping last week in Ontario, the one morning i went to take the chill out of the air and turned on furnace, fan started, as did the A/C unit on roof. Strange i thought, furnace fan ran for about 20 seconds than shut off. Shut system down. Now this Jayflight has the Dometic Thermostat, the small square one with blue LED, with mode button on one side, and up n down arrows on right. So, took out stat user guide, and when fan mode is set in low or high, the A/C fan will come on when furnace does ( not sure why u would want to ), and if u set to "Auto", only furnace will run. Set to Auto, turned furnace back on.... since A/C unit is right above my head, i heard the a/c unit click, than furnace fan came on, again ran for about 20 seconds or so, than shut off. I did not hear any clicking, like you do on the fridge when its attempting to light, which by the way, always seems to have a difficult time lighting, but all my Norcold Fridges have been like that. Who has that Dometic Stat ..... and am i missing something here? I turned on the stove, can hear the gas, hooked to shore power, took off the inside grille and flipped the switch on the top of the furnace on and off, which does make the thermostat on the wall go on and off. Heading out camping in two weeks, so chances of me getting it into a dealer and back before we leave are slim

Sail switch

ALJO · 10-14-2020, 01:18 PM

Low battery is the first one to check. Older furnace had sail switch behind the fan and seldom gave problems. Now they install on the front and the dynamics of the flow changed. Installation design also has a effect. Sometimes with bench testing it wouldn't show the sail switch but as soon it is installed, it fails. For the DIY, just keep one in stock. Parts are usually cheaper than a service call.

Chad Horne · 10-14-2020, 02:18 PM

I believe your problem is that the setting on the fan mode (on thermostat) is required to be on auto for the furnace to work. I don’t know why this is but it is. If not auto try high. It’s been awhile since I had to figure it out. Dealer did mention this to me. Only way I would have figured it out.

Let me know if this helps. Cold wknd for know heat. We were up in mount Carleton Nb

Chad.

Dopalgangr · 10-14-2020, 03:04 PM

Like others have said the Sail Switch. My coach was brand new and it didn't work right until I replaced it. One thing to note however, it would not run and light properly unless the exhaust tube was inserted as well.

Here is a good video I found when I was troubleshooting mine:

http://www.youtube.com/watch?v=0t8vRuGdZLw

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