I have a Tempstar 4 ton gas pack about 9 years old with a hsp igniter system,on a call for a 2 degree temperature rise the furnace mostly works OK. When the thermostat is adjusted to more than 2 degrees,the unit works good for a while and then starts to blow cold air. I have relaced the programmable thermostat, rollout and high limit switches also the control circuit board [recommended by a service technician] cleaned the pilot sensor all to no avail.My current natural gas bill has increased 150% compared to the same
Period a year ago. Is it the pressure sensor now or what else could it be,what about the gas pressure being to high if the last service guy messed with it... Any help would really be appreciated, thanks in advance Tony B

First of all everyone's bill has gone up.
Do not look at how many dollars the bill is look at the usage from last year to this year. That is the only way to tell for sure.

Sounds like you have spent a lot of time and $$ for nothing.
I am sure you have checked your air filter. That will cause the furnace to over heat and trip limit if dirty.

Have you checked for a cracked heat exchanger ? Just curious.

I would replace the pilot sensor and while you are there clean the pilot orifice. Many times cleaning the sensor will not do the trick and they are not to expensive to replace. To small of a pilot due to a partial blockage of the orifice can cause problems also. Use a very small wire to clean the orifice.

You said a service tech advised you. Was he at your house? Or just on the phone ?

Gas pressure could be to high but it takes a gauge or meter to set. Use the label on the unit for the correct pressure to set to.

Post back with your results.

Thanks hvac 1000 for the very prompt reply.
Yes the filters are cleaned at least once a month,I have not checked the heat exchanger but had thought of the possibility of damage. Re the service tech he was here and quoted me a price of $573.00 hence the diy mode. Will replace the pilot sensor and clean the orifice.If the comb motor pressure sensor hose has a leak could that cause the problem?the temp at the nearest register reads 138 degrees when running normally and the return at the mid 90`s and thanks again
Will keep you posted Tony B

I((f the comb motor pressure sensor hose has a leak could that cause the problem?)) YES

((the temp at the nearest register reads 138 degrees when running normally and the return at the mid 90`s and thanks again ))

Return air is high should be just about the temp in the house

If you still have problems, post you units model number for us...

The model #pgme36g0905 and thanks

What fuel do you have going to this unit ? It should be firing at 3.5 inch WC on the manifold side if on Natural gas. As well, this unit uses a Honeywell SV9501 gas valve that has been know to be a problem child. The temp difference between the feed air and the return air ( rise of the unit) is from 35 to 65 degrees F, with a clean air filter it should run about 50-55 degress if properly set up in gas pressure and fan speed. So if that is correct, it should not be taking out any limits, unless they are defective. If you are able to observe what the unit is doing when you are getting only cold air, that would help ( at the unit) such as what is running at that time.. . kindly post your findings if you still have problems...

Have replaced the igniter and sensor,the pressure switch seems to be OK,but still get the occasional blast of cold air. While the cold air was blowing the units comb fan was running as was the recirculating fan,no hot igniter and no pilot.all seems to point to the gas valve.I have a quote for the valve and will order if you agree that is the solution.having never replaced a gas valve and no pressure gage to check the unit[nat gas 3.5#] would you suggest leaving this to pro

Blowing cold after a large temperature difference might mean problems with the second stage if it has one. The second stage could be controlled by the stat or the control board. It's possible it's a setup problem as well (control board or tstat configuration. I'm guessing at this point. Point to a location of the manual on the net.

Pay attention to cubic foot used per similar period (same time last year) to look at gas consumption, not dollars.

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Originally Posted by tsa7man

What fuel do you have going to this unit ? It should be firing at 3.5 inch WC on the manifold side if on Natural gas. As well, this unit uses a Honeywell SV9501 gas valve that has been know to be a problem child. The temp difference between the feed air and the return air ( rise of the unit) is from 35 to 65 degrees F, with a clean air filter it should run about 50-55 degress if properly set up in gas pressure and fan speed. So if that is correct, it should not be taking out any limits, unless they are defective. If you are able to observe what the unit is doing when you are getting only cold air, that would help ( at the unit) such as what is running at that time.... ...kindly post your findings if you still have problems...

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When the unit is blowing cold both comb&recirc blowers are running with no ign or pilot. I have just replaced the sensor and igniter assy and checked the pressure switch which seemed to be operating OK. The last service tech [ 5 days ago] got the unit to operate well
And said all he did was bypass the high limit switch [ which I had already changed out] and the cost to replace the switch would be $300+ which would suggest something else needed replacing, gas valve maybe? I will check out other techs in our area. Will keep you posted Thanks Tony B

You stated in your last post... that the tech "bypassed the limit switch" to keep the unit running..? This really has me wondering... he bypassed a safety to check a problem. The safety "limit" is doing it's job!! Or it is defective!! The next two questions is simple, as I stated before... have your tech put a thermocouple at the location of the main limit switch that is tripping "open" and read the actual temp that that switch is seeing. It is either tripping to early ( low temp) and defective or it is really sensing a high heat temp, and opens as designed to shut down the gas valve firing. The next question is why is the safety opening?? It is doing it's job to keep you safe. What can cause this problem, of an over heat condition... over fired gas valve / burners, dirty air filter, too low of a blower speed ( check the temp rise of unit as per ID plate specs.) time out of circulation fan too short at the end of a heating cycle, heat left in heat exchanger, and upon a new call for heat, the 30 sec. fan on delay causes the heat exchanger to over heat prior to the fan timer starting the circulation blower... = high limit opens now, burners go off... cold air blows... through your duct work...

And further... if that company was going to charge you $ 300.00 to replace the limit switch... YOU BEST SHOP AROUND FOR A SECOND OPINION... for that repair!!

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Originally Posted by tsa7man

You stated in your last post.... that the tech "bypassed the limit switch" to keep the unit running...???? This really has me wondering...he bypassed a safety to check a problem. The safety "limit" is doing it's job !!!! Or it is defective !!! The next two questions is simple, as I stated before....have your tech put a thermocouple at the location of the main limit switch that is tripping "open" and read the actual temp that that switch is seeing. It is either tripping to early ( low temp) and defective or it is really sensing a high heat temp, and opens as designed to shut down the gas valve firing. The next question is why is the safety opening ???? It is doing it's job to keep you safe. What can cause this problem, of an over heat condition.....over fired gas valve / burners, dirty air filter, too low of a blower speed ( check the temp rise of unit as per ID plate specs.) time out of circulation fan too short at the end of a heating cycle, heat left in heat exchanger, and upon a new call for heat, the 30 sec. fan on delay causes the heat exchanger to over heat prior to the fan timer starting the circulation blower...= high limit opens now, burners go off...cold air blows.....through your duct work....

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Today I was able to checkout the sequence of a misfiring,I had removed the high limit switch and installed a temp probe in the opening. On firing the unit started as normal, ran for about two and a half minutes but shut down before reaching set temp[the probe registered 145F at max]after a short delay the unit again fired up but shut down after running for about 10 seconds.I also was able to check the temp at the flue exhaust during the first firing and registered a temp of 240 + degrees which seemed like a total waste of energy. Since the heat exchanger does not have any overriding limit switches what sensor would tell the unit it is overheating? There is a slight flame rollout but I don`t think enough to acuate the rollout switch I`m ready now to get a service tech to check the gas pressure.

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Originally Posted by tsa7man

And further...if that company was going to charge you $ 300.00 to replace the limit switch...YOU BEST SHOP AROUND FOR A SECOND OPINION.....for that repair !!!

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I`m sure the service guy knew I could replace the limit sw myself, so it`s my belief he was also changing or adjusting some other component in order to get the unit up and running but did`nt want to tell me what ,just so I would not fix it myself.

Still got the same problem.yesterday I was able to see the misfiring,in the early morning the unit just misfired the first two times after being off all night,when the unit did fire I got on the roof and the unit cut off before reaching set temp, wihin a couple of minutes the recirc blower stopped and the unit was dormant for maybe 1 min,it tried to restart, the comb fan started the igniter glowed but the pilot did not light. Would this point to the gas valve as the problem. Thanks for the help.
Tony B

Quote:

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Originally Posted by tsa7man

What fuel do you have going to this unit ? It should be firing at 3.5 inch WC on the manifold side if on Natural gas. As well, this unit uses a Honeywell SV9501 gas valve that has been know to be a problem child. The temp difference between the feed air and the return air ( rise of the unit) is from 35 to 65 degrees F, with a clean air filter it should run about 50-55 degress if properly set up in gas pressure and fan speed. So if that is correct, it should not be taking out any limits, unless they are defective. If you are able to observe what the unit is doing when you are getting only cold air, that would help ( at the unit) such as what is running at that time.... ...kindly post your findings if you still have problems...

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Well replacing the gas valve did the trick and thanks everbody

If this is a 90 Plus unit I just had a problem that is about like this one. It was laid down horizonally they had the condisate drain in the wrong location. Allowed the fan to fill with water and make the pressure switches shut the burner down. It was installed for 3 years and having problems. Listen to exaust fan to see if you here water.