# High Limit Switch Question



## pgh (Dec 6, 2010)

Hello,

My heat decided to stop working last night so I stayed up and figured out how to get it working again. I bypassed the High Limit Switch temporarily until the part is delivered, it's unfortunate that the supplier down the street won't sell me the part since I'm not a contractor. 

I have a Payne PG9MAB Furnace using a Carrier Bryant High Limit Switch HH12ZB250 (http://www.hvacpartsoutlet.com/carrier-bryant-high-limit-switch-hh12zb250.aspx) 

Here are my questions:

1) I know bypassing is not recommended however, I have no choice until the part is delivered. What should I watch out for in terms of dangerous situations? I'm going to run the heat around 65 for now.

2) The flame rollout switch is still functioning. Will this trip if overheating occurs?

3) I have a large attic fan that pulls air out of the house. Could this be the cause of the high limit switch malfunction?

Thanks
pgh


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## kenmac (Feb 26, 2009)

roll out switch doesn't take the place of high limit. Not a good idea to use the furnace until part arrives no matter what. something made the limit go out. It has been my experence that high limit switches just don't go bad . There's a reason it went bad


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## pgh (Dec 6, 2010)

Thanks kenmac. I'm trying to figure that out now. No money to hire a pro right now, wife and I are laid off and have little income. Changed air filter, the blower seems to be unobstructed, good air flow from the vents. Not sure what else to do. 

I wish Johnstone would just sell me the part.


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## hvaclover (Oct 2, 2008)

jumping a limit ???

DDDDUUUUUUDDDEEEE..that is so not right or safe


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## Technow (Nov 12, 2010)

In my experience, a properly running furnace and duct system will NEVER cycle a limit switch. The ones that do go bad are on furnaces that cycle on the limit (BAD) So chances are you have bypassed a safety control that will allow the furnace to overheat......."Danger Will Robinson"

It does not matter if you set the heat lower.....the furnace WILL Overheat.


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## hvaclover (Oct 2, 2008)

Technow said:


> In my experience, a properly running furnace and duct system will NEVER cycle a limit switch. The ones that do go bad are on furnaces that cycle on the limit (BAD) So chances are you have bypassed a safety control that will allow the furnace to overheat......."Danger Will Robinson"
> 
> It does not matter if you set the heat lower.....the furnace WILL Overheat.


You outta know by now that ain't no civilian gonna listen to a pro about a jumped out safety.


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## pgh (Dec 6, 2010)

Technow said:


> In my experience, a properly running furnace and duct system will NEVER cycle a limit switch. The ones that do go bad are on furnaces that cycle on the limit (BAD) So chances are you have bypassed a safety control that will allow the furnace to overheat......."Danger Will Robinson"
> 
> It does not matter if you set the heat lower.....the furnace WILL Overheat.


Understood. Heat off till new limit switch arrives. I'm sure if I don't fix the root problem, it will happen again.

How do I troubleshoot this thing to figure out why it is cycling on the limit? Need more advice.


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## pgh (Dec 6, 2010)

hvaclover said:


> You outta know by now that ain't no civilian gonna listen to a pro about a jumped out safety.


I don't want to listen but I have kids safety to worry about so, I'll be cold for a few days.

I just want to figure out how to troubleshoot the problem and find the root cause.


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## hvaclover (Oct 2, 2008)

pgh said:


> I don't want to listen but I have kids safety to worry about so, I'll be cold for a few days.
> 
> I just want to figure out how to troubleshoot the problem and find the root cause.


You'll have to have a thermometer and few other things. Get the new part in first.

It is possible for the limit to fail thru wear and tear also.


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## pgh (Dec 6, 2010)

Got the new High Limit Switch in the mail yesterday. Replaced and everything is working perfectly now. I would still like to figure out why this happened. Any help would be great. I do have a thermometer.


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## yuri (Nov 29, 2008)

Check the supply air temp in the duct about 1 foot downstream of the main bonnet where it branches off the bonnet. Run furnace for 5 minutes and take that reading and subtract the house temp. That gives you the temp rise thru the furnace. Compare it to the allowable range on the furnace specs sticker inside the furnace ie 35-65 degF. If it is too high then that is your problem. Rare to cycle a limit like yours which is 250 degF.


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## beenthere (Oct 11, 2008)

pgh said:


> Got the new High Limit Switch in the mail yesterday. Replaced and everything is working perfectly now. I would still like to figure out why this happened. Any help would be great. I do have a thermometer.


Check your air filter. Make sure its not dirty. Don't use restrictive air filters.
Make sure all supply registers are open. And that all returns are open and not blocked.

Check to see if your A/C coil is dirty, and restricting air flow. If it is, it needs cleaned. Or the new high limit will fail like the old one did.


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## Artco (Sep 23, 2010)

Do you have carbon monoxide detectors in your house? 
If not purchase one at HD or Lowes and install it ASAP.
If the system trips on the new limit you may have a bad heat exchanger
Check for blocked return air.
along with everything else all have suggested. 
YOU CANNOT SMELL CARBON MONOXIDE


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## pgh (Dec 6, 2010)

yuri said:


> Check the supply air temp in the duct about 1 foot downstream of the main bonnet where it branches off the bonnet. Run furnace for 5 minutes and take that reading and subtract the house temp. That gives you the temp rise thru the furnace. Compare it to the allowable range on the furnace specs sticker inside the furnace ie 35-65 degF. If it is too high then that is your problem. Rare to cycle a limit like yours which is 250 degF.


I did this the other night. The furnace spec gives an allowable range of 40 to 70 degF and I came within that range when I measured the intake temp compared to the air coming out of the registers.


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## pgh (Dec 6, 2010)

beenthere said:


> Check your air filter. Make sure its not dirty. Don't use restrictive air filters.
> Make sure all supply registers are open. And that all returns are open and not blocked.
> 
> Check to see if your A/C coil is dirty, and restricting air flow. If it is, it needs cleaned. Or the new high limit will fail like the old one did.


Filter replaced. Using a Filtrete 1900. 

Now we are getting somewhere. I did find that 2 of the 1st floor registers were totally closed, in the kitchen and by the front door. One register on the 2nd floor was totally closed too. That may have been the issue.


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## pgh (Dec 6, 2010)

Artco said:


> Do you have carbon monoxide detectors in your house?
> If not purchase one at HD or Lowes and install it ASAP.
> If the system trips on the new limit you may have a bad heat exchanger
> Check for blocked return air.
> ...


Got 2 CO monitors installed now. One on the 1st floor and one in the basement. Thanks!:thumbsup:


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## hvaclover (Oct 2, 2008)

pgh said:


> I did this the other night. The furnace spec gives an allowable range of 40 to 70 degF and I cam within that range when I measured the intake temp compared to the air coming out of the registers.


Can't measure at registers. Have to drill a hole in the supply trunk.

You get an uneven temps at the registers.


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## pgh (Dec 6, 2010)

hvaclover said:


> Can't measure at registers. Have to drill a hole in the supply trunk.
> 
> You get an uneven temps at the registers.


I was wondering what the small holes were in the supply and return coming off the furnace! Thanks. And the hole is about 1 foot from the main bonnet.


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## yuri (Nov 29, 2008)

Now we are gettin somewhere. :thumbsup:
Geez, you almost might think we do this for a livin.:yes:


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## pgh (Dec 6, 2010)

yuri said:


> Now we are gettin somewhere. :thumbsup:
> Geez, you almost might think we do this for a livin.:yes:


*yuri*: I know you guys do, and I thank you. I meant no disrespect 

Thank you all for helping me through a fairly stressful time an thanks most for helping me save a few bucks so I can feed my kids.


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## hvac122 (Dec 10, 2007)

I would stay away from the filterette filters. They are highly restrictive new and get worse as they load up. Most ductwork is already undersized and these filters can trip the limits by blocking to much air.


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## yuri (Nov 29, 2008)

The trouble with Yuri's internet humor is it can be mistaken/misread. I was glad it worked out and ironic that the test hole was "1 ft downstream from the bonnet" like he said.:thumbsup:


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## beenthere (Oct 11, 2008)

pgh said:


> Filter replaced. Using a Filtrete 1900.
> 
> Now we are getting somewhere. I did find that 2 of the 1st floor registers were totally closed, in the kitchen and by the front door. One register on the 2nd floor was totally closed too. That may have been the issue.


Along with filtrete filter.


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