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heating system is shutting off before it reaches set temp

Customer Question
my heating system is shutting...
my heating system is shutting off before it reaches its set temperature? Also the temperature is going up much slower than normal. The code shows 3 shorts and 3 longs. I had the same problem last year and it was fixed by changing a new filter but it's not working this year. Is there a reset button or there could be some other problems. The system would shut off and goes back on right away.
Submitted: 7 years ago.Category: HVAC
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Answered in 2 hours by:
12/27/2010
HVAC Technician: Phil, Mechanical Engineer replied 7 years ago
Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,158
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Verified

Phil :

Hi,This is risky business, not responsible for damages on your end. We keep going until you're satisfied. I come and go. be patient

Phil :

Examine the flame on the main burners and tell me what you see. it could be that some have not ignited,

Phil :

Get back to me, we can go from there, Phil

JACUSTOMER-3u51wzwl- :

flames are all going......fyi: the unit was installed 3 years ago....

Phil :

Thanks, XXXXX XXXXX like there is a blockage in the air ducts somethere.. that was the case with the dirty filter.. but not now.

JACUSTOMER-3u51wzwl- :

yes..it happened last year but it was fixed when I changed the air filter. This solution is not working this time.

Phil :

What happens sometimes is that a piece of duct liner insulation comes loose and blocks air flow... or.... if flexible duct has been used in the return air ducts, it can collapse and cause such restrictions.

Phil :

to determine if there is a restriction the easy way buy a cheap thermometer, put it flush against the return air grill for a few minutes, then read it...

Phil :

then put it flush against a supply air grill and read it again.

Phil :

subtract the two figures and tell me what the difference is.

Phil :

With blocked ducts the unit will go off on its high temperature cut out.. and the stay off until you reset by turning the thermostat off then on again... sometimes its a manual reset by means of red button on the control.

Phil :

Look all that over and get back to me, Phil

JACUSTOMER-3u51wzwl- :

ok...i will be back

JACUSTOMER-3u51wzwl- :

I'm back...I went to get one of those infared red readers. The return duct/grill is getting me 69-72. the supply grill is anywhere between 106-117.

JACUSTOMER-3u51wzwl- :

I couldn't find the red reset button but turned the emergency switch off for 5 minutes.......did not work.....I also turned the ?thermostat off then on again....nothing......

JACUSTOMER-3u51wzwl- :

I guess the difference ranges from 37-45.

Phil :

Hello again, thanks for the readiings.

Phil :

117F at the supply grill, puts the heat exchanger temperature within viable ranges.. indicating no blockage in the air ducts.

Phil :

we need to look for some other cause.

Phil :

The fact that it shuts off then goes back on right away points to a loose wire connection somewhere.

Phil :

the cure for that is to replug all the gang connectors and spade wire connectors on the unit.. that burnishes potentially oxidized contacts.

Phil :

Do that, let me know how it goes, Phil

JACUSTOMER-3u51wzwl- :

I looked at all the connections and they seemed to be pretty tight. I don't see any oxidization around the connections/connectors.

JACUSTOMER-3u51wzwl- :

FYI: The quick off and on only happens when it tries to reach the set temp. Once it reaches the set temp, it goes back to normal running and code led would stay solid.

JACUSTOMER-3u51wzwl- :

On the panel: this is what it says with code 33: Indicates a limit, draft safeguard, flame rollout or blocked vent switch (if used) is open. Blower will run for 4 minutes or until open switch remakes whichever is longer. If ooen longer than 3 minutes, code changes to lockout 13. If open less than 3 minutes status code#33 conitues to flash unitl blower shuts off. Flame rollout swicth and BVSS require manual reset. Check For:

JACUSTOMER-3u51wzwl- :

Proper Vent Sizing....Dirty Filter or restircted duct system....Defective blower motor or capacitor.....Inadequate combustion air supply (flame Roll-out Switch open)....Defective swicth or connections....restricted vent...excessive wind...

JACUSTOMER-3u51wzwl- :

We know is not the filter and restricted duct... and I guess also connections?.....how can I check for other potential issues without killing myself or damaging the system.....

Phil :

Hello again, the fact that it only does the very sort cycles indicates it might be at one of the termperature sensors.

Phil :

Do this:

Phil :

find where the thermostat wires land inside the furnace, a red, white, yellow and green, jumper red to white and see if that cures the problem.. if so, the problem is in the thermostat.

Phil :

If not the problem is in the furnace controls

Phil :

**

Phil :

The oxide on the pin connectors (gang connectors and card connectors) is not visible, its slight, and the pins also fail to make good contact at times as well.. you cant tell by looking

Phil :

replugging them burnishes the contact though.

Phil :

Get back to me we can go from there, Phil

JACUSTOMER-3u51wzwl- :

I think this is getting beyond my skill set...I'm not sure if I fully understand everything hear

Phil :

Any time you try you learn something..thats good for the brain.

Phil :

Your situation is however a bit complex, calling a good onsite technican might be a good option at this point. Phil

JACUSTOMER-3u51wzwl- :

Thanks for your help...

Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 9,158
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
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