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I've been working with Billy HVAC. I had a limit switch

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I've been working with Billy...
I've been working with Billy HVAC. I had a limit switch problem relative to short cycling which turned out to not be a limit switch.. it has been between 2 to 13 degrees here in Chicago and house stays at 66.
Submitted: 7 months ago.Category: HVAC
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Answered in 11 hours by:
12/29/2017
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 7 months ago
BillyHvac
BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 23,151
Experience: Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
Verified

I am sorry to hear you still have issues. Are you getting a particular blinking pattern on the circuit board light?

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Customer reply replied 7 months ago
I will look..... My people here think it's a total Duct issue, and I'm not convinced of that of course. My son thinks that maybe I need to get in inline ducts, fans in the crawlspace to equalize the coldness because it's not a finished crawl space. I did see a drop in the elbow of the duct from 75 to 66 as it was going into the crawl space. It was a very big precipitous drop is 10 degrees. You think that could be maybe clogged. It does get pretty hot to 180 and then it stops as you know so that can't be a flame burner dirtyness could it be? Sorry about all the ideas I hope you can knock them all down and I will try to find this thing that you're telling me.
Customer reply replied 7 months ago
I am sure my HVAC guy would have seen that light no..who knows..
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 7 months ago

It could possibly be duct...but usually the duct is in place and the furnace ends up being too big. Same issue but different perspective.

Has the blower speed been increased yet?

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Customer reply replied 7 months ago
but it didn't show on same day, and now it's 14 to 15 degrees and it seems warmer now. These problems were occurring at -1 to 7 degrees outside
Customer reply replied 7 months ago
Same day, 68, now warmer outside, could that be it.
Customer reply replied 7 months ago
Warmer inside at 71.
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 7 months ago

Has anyone mentioned about just cutting a supply register in for the crawlspace? Not so much to heat the crawl space but to move more air.

Keep in mind....it is not uninsulated crawl space....this is the furnace running too hot.

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Customer reply replied 7 months ago
What about dirty gas sensors..You did not mention that. That was not checked....I do have a register that was cut just off the furnace out duct.
Customer reply replied 7 months ago
I also saw strange drop at an elbow point at the beginning of the crawl space as it went in from 77 to 65..want to hear your thoughts about the sensor idea
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 7 months ago

Dirty sensors will not have anything to do with a limit code.

LIMIT is a very common condition when filters are restricted, airflow slowed etc.

With your temp readings...you very simply are making more heat than what the system can get blown through.

Analogy - you have a water pump capable of pumping 10 gallons a minute.

Your water hose is only capable of 5 gallons a minute.

This causes backpressure on the pump....

This will not be sensors or electronics...it is just the relation of heat being made by the furnace to the amount of heat the blower and duct can move.

You need either less heat or more air......

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Customer reply replied 7 months ago
But sensors can deal with short cycling for other reasons?
HVAC Technician: BillyHvac, Journeyman HVAC Tech replied 7 months ago

Yes but these are simply heat activated bimetal switches.

The bimetal reacts to heat and warps..it is a open or closed switch...not a dc signal.

The temps you have mentioned are not low enough to be be shortcycled by a limit.

Lets say furnace is tripping at 120f discharge...that would be a limit switch that is bad.

Lets say you are tripping at 150-180...that is a limit switch acting as it should by limiting the heat load in the furnace.

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