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Old 01-15-2013, 09:36 AM   #1
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Any harm in increasing the control limit temp value?


Hey folks, my first post! Iíll try to be detailed!

So I bought an older house with an older furnace (early 90ís), the duct work I believe is original from the very early 1900ís when the house used to have a coal fired furnace so itís pretty restrictive. Since I got the house the furnace has been kicking off every now and then and I never had the time to investigate until now. Turns out, itís been over heating and the control limit switch keeps opening.

I changed the filter, cleaned the blower and inducer motor, checked the heat exchangers for cracks and lastly monitored the temperature inside the air chamber with my own thermocouple. My thermocouple isnít calibrated but the control limit trips at roughly 185F even though itís a L160-20. When I bypass the control switch the max temperature I saw is about 200F which is shown below in the graph. What harm is there in putting an L170-20 in as a replacement? It's a two family house and the other furnace is identical and uses an L170-20.

It seems like my options are:
-increase the control limit temperature value
-tone down the BTUs of the burners
-redo my ducts to allow better airflow

Am I missing something? Any input would be greatly appreciated


Temperature graph link: h t t p : / / i50.tinypic.com/hvapvp.png

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Old 01-15-2013, 09:47 AM   #2
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Any harm in increasing the control limit temp value?


If you change the switch, and the house burns down, it's YOURS to defend and buy. Not sure I would want that popularity. Maybe a smaller btu burner is the better option or add a duct to relieve some back pressure.

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Old 01-15-2013, 10:19 AM   #3
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Any harm in increasing the control limit temp value?


Understood, but as I said I have two identical furnaces, one uses an L160, the other an L170. I personally don't see there being any harm in allowing a furnace to run 20 degrees hotter. It's a giant metal chamber with nothing to catch on fire, perhaps the fan motors and sensors won't last as long but it's already an older, low efficiency furnace to begin with.

But then again I'm by no means an HVAC expert, there must be some technical reasons why I should/shouldn't use an L170 switch?
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Old 01-15-2013, 10:28 AM   #4
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Any harm in increasing the control limit temp value?


Quote:
Originally Posted by CzarDestructo View Post
Understood, but as I said I have two identical furnaces, one uses an L160, the other an L170. I personally don't see there being any harm in allowing a furnace to run 20 degrees hotter. It's a giant metal chamber with nothing to catch on fire, perhaps the fan motors and sensors won't last as long but it's already an older, low efficiency furnace to begin with.

But then again I'm by no means an HVAC expert, there must be some technical reasons why I should/shouldn't use an L170 switch?
Identical means symmetric in every way
Not your case if one is different! Not saying it won't work but could it be someone installed the 160 or 170 because that is all they had?
Call the parts house and ask for the limit switch by model/serial and see which one has which. Maybe the problem was corrected or created by a previous repair.
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Old 01-15-2013, 10:32 AM   #5
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Any harm in increasing the control limit temp value?


Touche, the unit with the L170 is heating half the sq ft with much shorter, larger duct runs. The unit with the issues, L160, has to go up two stories through less than ideal duct work to heat 2x the sq ft.

I'll do some digging regarding the proper part
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Old 01-15-2013, 11:03 AM   #6
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Any harm in increasing the control limit temp value?


The temperature diff between the two limits could be due to and change in design specs even though they are the same model furnace.

Or one of the limits was replaced by a tech using an old-school "rule of thumb". That is to say, if the tech did not have the exact temp limit on hand, he would use the old school rule of thumb that"Ten degrees over is okay".

It is not OK. The heat exchangers of now ...and in the 1990s are made of thinner metal so it is imperative that the proper temp rated limit be used.

I don't know how accurate your thermocouple is...but it is not uncommon to see variances in temp in the plenum. Position in the air flow, where you sense the air and the distance from the furnace all play a factor.


Just an aside concerning your duct work...coal furnaces of the 1900s were all gravity types and used no blowers. That being said I am having a hard time reconciling the size of the duct being your problem If anything the ducts would be oversized to accommodate the raising of the hated air to the living space.

Unless there has been new ducts added than I doubt you have a duct problem.
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Old 01-15-2013, 11:14 AM   #7
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Any harm in increasing the control limit temp value?


Thanks for the input! I'll try and get some photos of the duct work, I'd guess they're only about 6" wide. Perhaps they aren't original?

Also, you bring up an interesting point about the point of measurement in the plenum. I'm going to try and take different readings inside the plenum in different spots to see if that corner just happens to be a hot spot. The ducts exiting the plenum are placed in weird spots and I'm curious if this created dead spots for air movement causing heat to build up around the sensor.

Pictures to follow!
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Old 01-15-2013, 11:43 AM   #8
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Any harm in increasing the control limit temp value?


Another important point is how accurate and is your thermocouple?

Is it a dedicated digital thermometer, or just one of the functions on a multi meter?

I use a dedicated digital stem thermometer or my Fluke 289. I will not trust a $100 multi meter to be accurate.

I do trust my Fluke 289 because I have a calibration certificate and the specs show the temp accuracy to be .05* F* with in the rated temp range.

How ever, this is a $500 meter made in the USA, so I trust it completely...and I baby it death.

Get the duct pics and and you'll get plenty of input.
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Old 01-15-2013, 12:48 PM   #9
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Any harm in increasing the control limit temp value?


I'm using an Agilent U1253B meter with a bluetooth dongle to pole the data, hence the graph. My profession affords me some toys. I trust the meter but I'm not 100% sold on the thermocouple itself, it was whatever I could scrounge up.

When I get back from work I'll post some duct photos!
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Old 01-15-2013, 12:54 PM   #10
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Any harm in increasing the control limit temp value?


What am I missing? 160 limit and 200 actual temp. A 170 isn't going to help.
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Old 01-15-2013, 01:07 PM   #11
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Any harm in increasing the control limit temp value?


Well according to my multi-meter that 160F switch doesn't trip until after 185F on my meter. I'm also not going to make any claims that I put that thermocouple in the same spot as the switch, it honestly ended up roughly 1" behind the switch when I wrapped it around one of the legs so it was closer to the heat source. These numbers aren't perfect.

I will say though speaking relative to my thermocouple the switch tripped at 185F and the max operating temperature I measured was 195-200F. It's close and the L170 could very well not even fix it.
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Old 01-15-2013, 01:29 PM   #12
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Any harm in increasing the control limit temp value?


Quote:
Originally Posted by CzarDestructo View Post
Well according to my multi-meter that 160F switch doesn't trip until after 185F on my meter. I'm also not going to make any claims that I put that thermocouple in the same spot as the switch, it honestly ended up roughly 1" behind the switch when I wrapped it around one of the legs so it was closer to the heat source. These numbers aren't perfect.

I will say though speaking relative to my thermocouple the switch tripped at 185F and the max operating temperature I measured was 195-200F. It's close and the L170 could very well not even fix it.
Your Agilent ...great meter. By your above post it sounds like you are too close to the heat exchanger. Usual distance from the supply plenum is 3 to 6 ft to avoid radiant heat influence.

Get the pics up.
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Old 01-15-2013, 01:33 PM   #13
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Any harm in increasing the control limit temp value?


Quote:
Originally Posted by CzarDestructo View Post
I'm using an Agilent U1253B meter with a bluetooth dongle to pole the data, hence the graph. My profession affords me some toys. I trust the meter but I'm not 100% sold on the thermocouple itself, it was whatever I could scrounge up.

When I get back from work I'll post some duct photos!
I am lucky in that regard too...I got all top of the line Testo and Fluke instruments.

Some UEI stuff that is dead bang on temps, too.
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Old 01-15-2013, 05:51 PM   #14
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Any harm in increasing the control limit temp value?


Ok, I guess the L170 switch isn't going to help. I just ran the furnace again with the switch bypassed, this time with the thermocouple in a better location and it STILL climbed to almost 100C before I shut it off.

Photos of the ducts, there are two outputs. One comes out the left side of the furnace and the other comes out the back. The first photo shows the front of the furnace and the second one shows the back. In the second photo the duct on the right is the one that comes out the left side of the furnace. The duct work is kind of a cluster f, and yes that is in fact pink fiberglass insulation held on by old coat hangers and chicken wire.

What the heck are my options? Can the BTUs easily be cranked down (by a licensed plumber, definitely not me )?



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Old 01-16-2013, 12:11 AM   #15
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Any harm in increasing the control limit temp value?


Wow, thats quite the octopus goin on there LOL

anyways, lowering the output is a definite option, but that also comes at the cost of reducing efficiency. every heat exchanger has a specific BTU and airflow where it is its most efficient. too much fire and your blowing it all out the flue (and burning up the heat exchanger), same goes for airflow across the heat exchanger, too low of airflow and heat exchanger temp gets too high (heat out the flue and burning it up!) and as backwards as it sounds, too much airflow across the heat exchanger reduces efficiency as the exchanger cannot transfer the heat to the air at a fast enough rate

The biggest thing you need to do, is measure your return air temp and compare that to your hot air (getting a flue temp wouldnt hurt). Your furnace is rated for XXX* heat rise, which you are likely exceeding greatly. you need to either adjust the firing rate (BTU) or airflow to get the heat rise within spec.

Where is the air return ducted? is it possible that this unit is sucking in a fairly high temp return air which would cause the excessive heat?

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