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Limit: off and comes back on then it blows cold air, we..duo-therm…

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have a nt-40 furance it...
have a nt-40 furance it comes on for a while and blows heat and then shuts off and comes back on then it blows cold air, we change the high limit switch could it be are thermostat? we have a duo-therm model(NNN) NNN-NNNN015 if you have wire digram for it could you please email it to me thanks char
Submitted: 12 years ago.Category: RV
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Answered in 28 minutes by:
2/6/2010
RV Mechanic: gene, RV Mechanic replied 12 years ago
gene
gene, RV Mechanic
Category: RV
Satisfied Customers: 1,100
Experience: 28 yrs exp in the industry
Verified
Hi,
Before you condemn the thermostat there are several things that can cause this,
Does the heater heat,come up to temp,shut down and the next time the furnace cycles on it blows cold air.Sounds like the thermostat is working?
If this is not the symptoms,explain what is happening.
First thing to check is battery voltage.
Second thing to check is the lp pressure,needs to be 11.5 inches for the furnace to operate correctly.Quick check on the lp pressure is to light all four burners on the stove,turn on the furnace and watch the flames on the stove,they should stay at the high flame,if they start to fade down then you have a lp regulator problem.You can also remove the regulator and check for oil inside of it,if it has oil in it replace it,with oil in it the cold temp will effect the lp pressure,causing low pressure and the furnace will not operate correctly.
Check these things and let me know.
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Customer reply replied 12 years ago
we have checked those things and they r ok. we got up this morning and it was blowing cold air and we shut it off and tried it again it did the same thing. took it out and put in another hi limit switch. and it still did the same thing am getting really frusted with this. so what else could be the problem?
RV Mechanic: gene, RV Mechanic replied 12 years ago
Here is a manual for the nt series furnaces,sequence of operation is the same on all of the furnaces,only difference is the sizes-BTU's of them.
With a 12 volt test light and when the furnace malfunctions,check for 12 volts at the limit.
Sounds like the t-stat is fine.
Once the t-stat calls for heat it sends power to the time delay relay,time delay relay starts the motor,in turn power goes through the sail switch after it closes,power then goes to the high limit switch,goes through the high limit, sending power to the control board,the control board then sends power to the gas valve and the igniter starts sparking at the same time,thus the burner lights.The board should try to light the burner three times before it goes into lockout.
Turn on the heater,with the cover off,check for 12 volts to the limit,if no 12 volts present then the sail switch did not close or is bad.
If there is 12 volts to the limit and through the limit,which in turn powers the control board the furnace will have to be pulled and the control board tested.Most RV repair shops will test it for you.
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gene
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gene, RV Mechanic
Category: RV
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