high limit switch for a york furnace

I have a York furnace. model # P2MPD14N08001C It seems as if the high limit switch is problematic. It has a rating of 170-30 It is made by Texas Instruments. patent no 5144274 the numbers 1NT17L-2303 L170F-30 025-29041-007A 9418
meaning it shuts off at 170 deg F and comes one when the temp drops by 30 deg F.
The problem seems to be that the downstairs of a 6000 sq ft house does not get warmer than 67 deg F on a 30 deg F night.
We have about 4 units, in a 2 story house.
Does anyone have any good and cheap suppliers for this part. I called Grainger, but they could not tell me the cut off temp on the York high limit switch in stock.
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The furnace does come on and seems to work. It is just that the house does not get warm enough. This is a 17 year old house in TN
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Since you said furnace I'm guessing gas? If it's running the problem is not the hi-limit switch. If it shuts down before the thermostat stops calling for heat then it might be the hi-limit switch. So is the temp at the thermostat reaching the set point? And the problem is that another part ofthehouse is too cold?
You don't mention any zone controls so we're talking about a sngle furnace, single thermostat set up, correct? A single furnace on that large a house must have a number of flow dampers on the vent ducts. They would be close to the furnace. They probably need adjusting. They will most likely need to tweaked everytime you switch from cooling to heating or back.
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Yes, it is a gas furnace. There are 4 such units for the house. Yes, it is a big house, but the cost of living here is cheap. I am out of work for a long while, so really tight with money. We bought the house when we both were working and got a great deal on it. I checked the vents and they all seem open and unobstructed (ceiling vents). I can feel a bit of air coming out, but don't know if that is enough. I changed the air filters. The room that is problematic is downstairs. I swapped out the high limit switch with another one, just to make sure that the one there was not faulty. The other one (from an upstairs unit) is Texas Instruments pat no 5144273 1NT17L-1875 L160F-30 025-29041-004 9413
It shuts off 10 deg F colder.
Maybe I should try to find one that cuts off at 180F
I watched the unit with the cover off. It seems to work fine just fine for the first 10 mins. The furnace turns off after 5 min, presumably when the high limit switch protects the unit from over heating. It blows air for about 2 mins, and reignites and heats up and shuts down. It seems to operate, but its just that after a while it stops heating up. I have the thermostat set at 74F, while as it only gets up to 68F. It seems to behave as if had reached the desired temp. Maybe I should check to see if there is still a call for heat by seeing if there is current across the C-Y leads [call for heat by the downstairs thermostat.
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It's not clear from your description exactly what the furnace is doing. Normally, when the thermostat is calling for heat, the burners come on and heat up the heat exchanger, then the blower comes on, and the burners and the blower both run until the thermostat is satisfied. Then the burners shut off and the blower runs for a short time until the heat exchanger cools down, and it turns off. Is this happening?? You say "I have the thermostat set at 74F, while as it only gets up to 68F." If this were true then the burners and blower would never shut off (with normal airflow) because the thermostat would never be satisfied. Are you sure it's not "cycling on the high limit switch"? You can tell because after the burner initially comes on and heats up the heat exchanger and then the blower will come on & stay on. The burners will heat the exchanger up past the maximum safe temperature, and the limit will open to shut the burners off, but the blower will stay on to cool down the heat exchanger (and therefore the limit switch) and then the limit switch will re-close and allow the burners to come back on. The cycling on & off of the burners tells you there is a problem. Probably an air flow problem as somebody else previously mentioned (weak blower motor, slipping blower wheel, loose duct liner, plugged A/C evaporator coil, etc.) You need to get a qualified tech to check it out.
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You can also test by simply maxing the thermostat and then observing the unit. I presume it has previously worked and this is a new problem. It is possible that the air flow is too low. Most units do not shutdown the fan when there is an overtemp situation, they just shut down the gas. You can also meter the overtemp, most commonly they are in series with the controls and open when there is a fault. That means you will measure no voltage across them when the unit is operating normally and there will be voltage across them when they have tripped. You need to look at the schemantic to see how they are wired. The most common wiring is also in series with the flame rollout switch if there is one so you want to make sure that is not the problem. You should find a schematic inside the unit. As others have mentioned the airflow needs to be correct as well. If the blower is starting up ok then about the only mechanical thing to check is the blower wheel, make sure it is not loose on the blower mtor. Usually they rattle when thisis the case but you can usually reach in and check that with the blower compartment cover off. Confirming the correct airflow is best done with a thermometer in the outlet side. The paperwork with the unit will give you a temp range that it should be inside of. You may find a hole taped over where the installation tech checked it. If it is too hot and you have confirmed that the blower wheel is ok and there are no obstruction in the ductwork you can change the fan speed, usually at the control board by plugging a different blower lead into it. But the fan speed should ahve been set properly on installation and there is normally no reason to change it. If you don't have a meter or you're not confortable following schematics then it might be time to call the service guy.
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