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two things to try first to get you pointed in the right direction. First disconnect the R or RH wire from the T-Stat. If the furnace turns off (Stops firing w/ fan to turn offabout 90 sec. later) then you more than likely have a bad Thermistat. If it continues to run Pull the R wire from the Furnace circuit board. put door back on furnace if it stops you have a short in the wire. Still Running? Most likely a bad curcuit board. Hope this helps.
Short cycling, could be because of gas pressure, drain issues from inducer and exchanger (if 90% plus furnace), auxilary limit getting to hot (dirty filters, obstruction, dirty coil), or your flame sensor is failing. Does the furnace lock out? or does it just cycle on and off? If it does how often? Thanks
No idea what the problem is but - We have a manual vulcan gas wall furnace about 3yrs old and it has always been switching off since installed which is a real bummer on cold days. We had it checked out whilst under warranty but the technician did not fix the problem. Im not sure if it has anything to do with the gas supply but seems more of a product fault. Lucky weve got a combustion stove as well!
Most likely the cause is that the furnace is firing too hot. When a furnace is installed and set up, the manifold gas pressure and fan speed has to be set by the technician so that the temperature rise through the furnace is a maximum of 50 deg F (i.e. if air enters the furnace at 68 deg F it come out no hotter than 118). The target temperature rise on any gas furnace is anywhere between 30 and 50 deg F. If the furnace runs too hot, either the manifold gas pressure must be turned down or the fan speed must be turned up to keep in this range.
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