Suburban NT-16s Help

[camper8251]

Greetings:

Looking for some help. I have a suburban NT-16S furnace. I recently replaced the fan for my basement heat. Its a computer type fan that sucks warm air into the camper basement and then blows it out the other end. Since replacing it, when I turn the heater on it works for a while then goes into lock out mode. The Furnace shuts down immediately when the desired temperature is reached and does not run to cool down. Then when it goes to start back up the blower will run but the burner wont light unless I shut the thermostat off and then turn it back on. If I put the old basement fan back in everything works fine. Any ideas as to why this is happening? The new fan looks to be a lower amperage than the old. This makes no sense to me.

Thanks

["007"]

Here is a PDF for heater.
The 12V fan cycles with the furnace when its on and shuts off with furnace when heat level is reached.
The 12V for fan comes from control board of furnace is there a board on the old fan that you may need to place on your new fan and when furnace is on do you feel warm air coming from the heat hose?
The fan is only an aid to push more warm air into your lower compartments.
The new fan needs to be same current draw as the old one.
What is wrong with old fan?
I see nothing in PDF that mentions a 12V fan in duct tube may have been added by MFG.
Did you try reversing the wire leads to fan?

Welcome to irv2 by the way.

[camper8251]

007:

Thanks for the response. Yes the fan was definitely added by the Manufacturer. The fan is question is not the actual blower fan on the heater, it is an external fan for pulling air into the holding tank compartment.

Thanks for the PDF link, I have reviewed all those docs before and it doesn't cover my situation. The fan near as I can tell is just hard wired into 12vdc and doesn't even connect to the heater circuitry. The only connection is at the thermostat where an extra wire is added to the same terminal as the furnace. Very strange.

["007"]

I'll still ask, that lead to T-start maybe causing your problem with a battery or grounds from fan feeding to furnace control unit.
Maybe you need a diode to block what ever your feeding back through T-start to control in furnace.
OR you just need to reverse the leads on fan.

[Dave78Chief]

In truth it was a bad design by the manufacture. Connected at the Thermostat like that means the thermostat is most likely being asked to provide more current than it is designed for. This may be pulling down the voltage supplied to the control board in the furnace. Additionally, the repalcement basement fan may have ripple noise on it affecting the Furnace Control Board electronics. What you should do is insert a relay control for the external fan. Relay coil would be driven by Thermostat (much smaller amperage). Run a new wire from the DC Distribution Panel to the relay for the fan. Now you have seperate fan circuit which will not react with the furnace or possibly damage the Thermostat (reduced current draw). Concept is the same as adding fog lights to a headlight switch.

Dave

[camper8251]

I agree 100%, way bad design. Just confusing since it works fine with the old fan which I think is a cheaper lower quality fan and sucks more juice. I tried reversing the leads on the fan and it doesnt work, so that option is out. I believe when the thermostat shuts off it is causing something strange in the control board causing it to lock out. I guess I will have to go the relay route and see what happens with that. Thanks for the help guys!

["007"]

Thanks for your help Dave you sometimes wonder what some of these MFG's are thinking.
Also for all the PDF's you have been suppling me and others.
I know if when I give a maybe not correct answer to 0 posts just to draw out a answer from someone can count on you and others to set the OP on the right path.
You have a good back ground I'll keep you in mind when I move on down the old dusty road.

[Dave78Chief]

Thanks and your more than welcome. Internet searches are more a function of how to phrase a question to Google. The how to stuff I post is basically a extension of what I do for a living (technical documentation support for electronic equipment). I am just a technician, not an engineer getting close to retirement.

Dave

[camper8251]

Well I finally got around to adding a relay to the basement fan. The new fan works great and with the digital thermostat upgrade I did it is a huge difference. I can now control just the basement fan thru the thermostat solving my basement freezing woes. Thanks for the input guys.