furnace blower motor when thermostat switched to on

[not_enuf_toys]

First, thanks for taking a look at this. I appreciate any help you can give me.

2007 forest river Salem 30QBSS
Suburban furnace
Dometic AC

Everything worked great at first. ac, furnace, auto, on, hi/low speeds.

the issue is, when I used to switch the thermostat from auto to on, the blower motor in the furnace used to kick on (when just on 12v). Now (still on 12v) when I click the switch from auto to on, the ac makes a one time click noise and the blower in the furnace never turns on. when plugged into 110, and switch from auto to on, the ac blower kicks on.

have tried a new thermostat and still the same results. I am thinking something goofy has happened with the furnace or analog controls but not certain.

Any thoughts? ideas? things to try?

[budwich]

you can likely solve the "mystery" by tell us how many wires are connected at the thermostat and what their colors are but the previous post is a good "line" on how things normally work.

[hohenwald48]

Yeah, it seems you might be getting AC fan operation confused with furnace fan operation. The only reason for the furnace fan to operate without heat being generated is for about a minute before the burner ignites to clear any accumulated propane from the combustion chamber and for about a minute after the burner shuts down to cool the components. When I cycled the temp up to 90 and back down to 50 all I did was fool the furnace into thinking it was going to light the burner, start the 1 minute "fume clearing" timer and then lower the temp requirement before expiration of said timer.

The A/C fan can be operated without actually running the cooling compressor for the purpose of moving air around in the RV. However, AC fan operation requires shore power. The relay that controls AC fan operation does operate on 12VDC and you are probably hearing that click.

I have never seen a furnace fan that will run without producing heat except for the short time before and after burner ignition. In fact, on my last motorhome I intentionally modified the furnace (with the addition of a switch) to provide fan operation without generating heat so I could move house heat produced by electric space heaters from the house to the storage compartments below the floor. That way the junk in the storage areas did not freeze in the winter.

[budwich]

yes but that's the A/C fan, not the furnace right?.... The a/c fan needs 120 power not battery power. Your "quote" is taken out of "powering context".

[not_enuf_toys]

now I know I am not crazy - straight from Dometic Manual. this is exactly how mine used to work

Continuous Blower Operation
1. Set the System Switch (3) to any position (including
?OFF?). See FIG. B3.
2. Move the FAN Auto/On Switch (2) to the ON position,
the unit fan will operate continuously at the fan speed
selected by the Fan Speed Switch (4). See FIG. B3.

[not_enuf_toys]

hohenwald48 wrote:
OK, here's what I just observed. With no shore power. I turn on the heat system to the fan position with the thermostat set around 50 and the furnace fan runs for about a minute. After it shuts off I can't get it to come back on by flipping any combination of switch ac/heat fan auto settings. However, if I run the temp setting up to 90 the furnace fan comes on, I then run the temp setting back to 50 and the furnace fan continues to run for about a minute and things start over.

Hope that helps. If not then let me know and I'll try something else.

It helps, what thermostat are you using?

[hohenwald48]

OK, here's what I just observed. With no shore power. I turn on the heat system to the fan position with the thermostat set around 50 and the furnace fan runs for about a minute. After it shuts off I can't get it to come back on by flipping any combination of switch ac/heat fan auto settings. However, if I run the temp setting up to 90 the furnace fan comes on, I then run the temp setting back to 50 and the furnace fan continues to run for about a minute and things start over.

Hope that helps. If not then let me know and I'll try something else.

[not_enuf_toys]

hohenwald48 wrote:
I'm not exactly sure what you want to happen? Regardless of settings, the AC fan will not come on without shore power. Is that what you mean? I don't think I've ever seen a setting that will only operate the furnace fan without operating the furnace too. I'm going to go out and check mine now.

Please let me know what you find out. I am certain that my furnace blower used to come on when just running off the batteries. Now I could be crazy, and It has been a long winter (spring and early summer for that matter too), but I swear that is how I used to cool off the inside of the camper when we first returned from the lake.

patiently waiting your reply

[hohenwald48]

not enuf toys wrote:
I am at the camper today, Checked the Thermostat switch (gas/oil or electric) and is on gas, tried auto/on with the switch from heat to cool (and vice versa) still nothing. when plugged into shore power and I switch from auto to on, the ac fan comes on. when unplugged from shore power (12v dc only) and I switch from auto to on, the ac makes a click sound and furnace fan does not come on. Checked all fuses and breakers (all good). Turned the heat on (switch to heat and increase thermostat) furnace fan comes on and heat starts.

What else can I try? would really like to have the furnace fan option on when just on DC (helps to blow out the hot air when we first come back)

I'm not exactly sure what you want to happen? Regardless of settings, the AC fan will not come on without shore power. Is that what you mean? I don't think I've ever seen a setting that will only operate the furnace fan without operating the furnace too. I'm going to go out and check mine now.

[not_enuf_toys]

something tells me that there is a relay in the AC unit that is deciphering 12v / 110 and which fan to turn on. and that relay is shot.

can anyone confirm? replaceable?

[not_enuf_toys]

I am at the camper today, Checked the Thermostat switch (gas/oil or electric) and is on gas, tried auto/on with the switch from heat to cool (and vice versa) still nothing. when plugged into shore power and I switch from auto to on, the ac fan comes on. when unplugged from shore power (12v dc only) and I switch from auto to on, the ac makes a click sound and furnace fan does not come on. Checked all fuses and breakers (all good). Turned the heat on (switch to heat and increase thermostat) furnace fan comes on and heat starts.

What else can I try? would really like to have the furnace fan option on when just on DC (helps to blow out the hot air when we first come back)

[not_enuf_toys]

budwich wrote:
sounds more like your thermostat has the "cool" function selected as opposed to "heat". The fan functon directs power to the appropriate fan unit based on that. Of course, the AC fan needs 120v to run which won't happen on 12v power. Hence, what you are observing appears to be normal.

I have tried switching from auto to on in all three modes (cool/off/heat), and still just a click sound from the ac

[not_enuf_toys]

Lazydude wrote:
The only way to tell if the curcuit board is bad is to replace it, so you might want to take it to a RV repair place and let them do it. It cost me 350 bucks cdn to have mine fixed.

I will take a look at the board. as a side note, when heat is needed, and the furnace actually kicks on, the blower works just fine. problem is only when wanting to run just the blower

[not_enuf_toys]

Lazydude wrote:
The only way to tell if the curcuit board is bad is to replace it, so you might want to take it to a RV repair place and let them do it. It cost me 350 bucks cdn to have mine fixed.

I will take a look at the board. as a side note, when heat is needed, and the furnace actually kicks on, the blower works just fine. problem is only when wanting to run just the blower

[budwich]

sounds more like your thermostat has the "cool" function selected as opposed to "heat". The fan functon directs power to the appropriate fan unit based on that. Of course, the AC fan needs 120v to run which won't happen on 12v power. Hence, what you are observing appears to be normal.