Plenum temperature?


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Old 11-05-13, 05:01 PM
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Plenum temperature?

What's a good plenum temperature limit?
Since installation almost 20 years ago my furnace won't run continuously until the thermostat is satisfied. It hits a limit, gas turns off, it cools down & turns off--then immediately lights up again. It eventually "steps" its way to the thermo setting. I know this isn't right but I have had the clowns who sold & installed it out several times and still the problem persists. This is only a problem when I turn the thermo down--so I don't ever dial back. Problem solved (not).

Fast-forward to last month and I'm helping my daughter move into a new house. The old furnace needed to be lit so I got it going and right away I notice the plenum gets almost too hot to touch. This week they had a new high-efficiency furnace installed and its plenum is also very hot (compared to mine). So now I'm thinking I have a bad limit switch.

I have access to all kinds of test equipment, but I need to know what temp I should see.
 
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Old 11-05-13, 05:20 PM
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In general, limit switches are VERY reliable. It's unlikely that the limit switch is bad.


When limit switches open, it's usually because the furnace is overheating, and that usually occurs because one or more things are plugging up the air flow, reducing the air flow past the heat exchanger that allows the heat produced by the burners to be stripped away.

You can start by describing the limit switch for us. And check to see what the setting of the limit switch is. Some switches are fixed and the limit temperature is "L180" or whatever ----shutting off the burners at 180 degrees F.
 
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Old 11-05-13, 06:29 PM
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The fan on her furnace may not be coming on soon enough, something could be restricting the airflow, as S/P said, or the fan may be set on too low a speed.
 
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Old 11-05-13, 07:25 PM
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Well i havent felt many furnaces so which is correct, her hot plenums (old and brand new) or my lukewarm one?
 
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Old 11-05-13, 08:15 PM
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As a general rule, most furnaces operate between 40 & 70 degrees temperature rise from return to supply as measured just before & just after the heat exchager.
 
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Old 11-06-13, 06:32 AM
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So 140F would be a reasonable plenum operating temperature?

Since an overtemp limit is by design never supposed to be reached, what do you think about putting a jumper across the limit and monitor the plenum temperature to see where it naturally levels off? This sounds like a logical troubleshooting step to me. If it approached 180F I could just turn off the furnace.
 
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Old 11-06-13, 06:49 AM
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Why jump it? Just measure the temp. One step will tell you whats going on.
 
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Old 11-06-13, 09:07 AM
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Just thinking ahead to the next step. Verify it's capable of getting hotter if the limit is bypassed.
 
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Old 11-06-13, 02:42 PM
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You need to verify the limit is opening at the correct temperature. If it is opening too soon, replace it. If it is operating normally, you need to find out why the furnace is going over temp.
 
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Old 11-06-13, 02:53 PM
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Try following the directions you have been given rather than guessing about some later possible step.
 

Last edited by Gunguy45; 11-07-13 at 01:43 PM. Reason: Removed rude remarks...again
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Old 11-06-13, 07:53 PM
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I put a thermocouple in the plenum and set the thermostat up 2 degrees. The furnace short-cycled as always. The overtemp limit opened but the plenum temperature was stable at 120F. I jumpered the leads and repeated the test and now the furnace ran until the thermostat was satisfied. And the plenum never got hotter than 125F. The overtemp limit is marked L220F.
 

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Old 11-06-13, 09:39 PM
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The sensor is probably closer to the heat exchanger than the plenum.

I wouldn't leave that limit bypassed. Replace it if you are confident in your testing but if the problem persists inspecting the evaporator coil if one is present and testing return then supply ducting with a manometer would be a good idea.

Checking the gas pressure at the valve would also be a test that I would perform.

What is the make and model number of this furnace?
 
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Old 11-07-13, 01:38 PM
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Houston it's a Bryant 350MAV.
 
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Old 11-07-13, 09:21 PM
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That furnace has a secondary heat exchanger that may be (and probably is) causing the limit trip.

How long has it been since this furnace was professionally inspected?
 
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Old 11-10-13, 08:32 PM
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I have had the company that installed it out at least twice for this problem. Once they increased the fan speed and anothertime they cut a big hole in the cold air return so it would suck in basement air. So as far as "professional" service I would say never.

My replacement limit has arrived. I will install that tomorrow and I'm confident my furnace will finally run correctly.
 
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Old 11-10-13, 09:18 PM
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There is certainly no danger in replacing a limit.
 
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Old 11-11-13, 08:04 AM
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Re:secondary H/E...had a customer that limit kept tripping, had me scratching my head because it worked before. Pulled blower and looked up to find it was caked with cat hair. Turns out he was an older guy that had trouble getting filter in place properly all the time. Cat loved to sleep next to the R/A.
 
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Old 11-11-13, 08:58 AM
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All the suggestions mentioned in this thread are good things to check if a functioning high limit trips--BUT--my issue is it trips at a very low temperature so that's why I conclude it's the limit itself that's the problem.
 
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Old 11-11-13, 09:22 AM
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I was just commenting on Houston's post. Might help someone further down the line. Never did hear actual test results of temp across of what your furnace was doing. There should be a label inside your furnace that has actual specs of what is required. If it runs on the high end, there is an airflow problem.
 
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Old 11-11-13, 07:11 PM
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Guy48065,
For future reference, should the new limit do the same thing or the problem crops up again, the best way to check a limit is to put a thermocouple in thru a small hole as close to the limit as possible. The farther away the less accurate the measurement will be.
 
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Old 11-12-13, 06:49 AM
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I was wondering about that. So HVAC techs drill holes for temperature probes, manometers, etc? Does one put a piece of tape over the holes afterwards?
 
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Old 11-12-13, 05:25 PM
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I have temporarily removed the sensor and put a digital thermometer in the hole the the sensor was in to check the temperature.

Every time that I have done this my thermometer hit the limit temperature, if the limit was tripping.

Chances are high that the limit isn't the problem.
 
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Old 11-12-13, 06:30 PM
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Running fine now with new limit in place.

But now that I've jumped in with both feet I'd like to know how to clean & test the furnace. So far it has been suggested I test for the delta T thru the heat exchanger but how is that done on these furnaces with vertically long exchangers? I can sneak a TC into the plenum by the A-coil but where do I place the lower probe? What are the "plates" below the heat exchanger in the photo? Do they straighten the air flow?
Where do I locate the tubes for a manometer reading? I do have a Dwyer manometer with 3"WC full scale--is that an adequate range? What is the good/bad criteria?
 
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Old 11-12-13, 06:35 PM
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On the subject of cleaning I can't see how to clean the underside of the "A"-coil (more like an "N"-coil). I'll tackle almost anything and usually manage to R/R things without leftover hardware but I don't see how to do this without completely disassembling the furnace.

The topside is spotless but my blower is fuzzy so I assume the coil has some build-up after 10 years use.
 
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Old 11-12-13, 10:29 PM
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There is usually eight to ten 5/15 inch hex head screws on the triangular sheet metal panel on the left side of that N coil that you can remove to inspect or brush the coil. It would be very easy to cause a refrigerant leak trying to remove it though. A 5/8 inch hole saw bit and a milwaukee 12 volt inspection camera can let you see it.

A manometer can be used to read the external static pressure by taking a reading a few inches from the furnace intake and between the evaporator coil and the furnace. Drop the plus and minus and add the readings together (if reading the points separately like I do) or you can read them simultaneously.

.5" water column is the desired E.S.P.
So, -.24" before the furnace and +.35" water column between the furnace and coil would give you .59" water column external static pressure.

If you know the heating speed tap and the ESP you can look up the furnace cfm output.




You can measure across the evaporator coil to get a good idea of it's condition as well. .24" to .27" is an average reading across a box coil. The model number can give you a more accurate expected pressure drop across the evaporator coil if you can find the manual.
 
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Old 11-13-13, 07:12 AM
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Thanks Houston--I'll have another look at that coil & panel. If I use one of those spray foam cleaners won't all that gunk end up on the heat exchanger? There's no room in there to cover it or clean it.

Thanks also for the SP table--GREAT information to have at hand.

Did you see post #23? What's the purpose of the plates between the blower and heat exchanger? Besides being something else that catches gunk I knock off the N-coil...
 
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Old 11-14-13, 02:33 PM
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Worse than changing the plugs in a van

I'm STILL stumped how to remove that filler panel on the coil. Took out all 6 screws but the bottom securely snaps in and the top is trapped by all the tubes. There's nowhere to go with it--it's trapped in all directions by the tubing left, right and over and by the water tray beneath.

If an HVAC guy charges by the hour I'm looking at hundreds of $$ just to clean.

 

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Old 11-14-13, 04:26 PM
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But now that I've jumped in with both feet I'd like to know how to clean & test the furnace. So far it has been suggested I test for the delta T thru the heat exchanger but how is that done on these furnaces with vertically long exchangers? I can sneak a TC into the plenum by the A-coil but where do I place the lower probe? What are the "plates" below the heat exchanger in the photo? Do they straighten the air flow?

Plates = primary heat exchanger. Responsible for transferring around 80% of the heat produced to the supply air.

At least once a year you should check those "plates" for cracks/holes/separation. (...and pull+ clean the flame sensor with steel wool.

The coil under is your secondary heat exchanger - responsible for transferring the remaining heat in the exhaust to the supply air.

It's a good idea to pull and blower and clean the secondary heat exchanger if the blower compartment is dirty. (after that, if you use a decent filter, it should never need to be cleaned again) The a/c evap coil is unlikely to be dirty since you have a filter and secondary heat exchanger upstream.

Other things:
- Clock the gas meter to make sure that the gas input is correct (google how to do this)
- Check temperature rise once to establish a baseline, then once a year to see if the airflow has changed (a significant increase = new airflow restriction)

Everything else should be left to a technician.
 
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Old 11-14-13, 04:48 PM
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Aren't the large chambers that the gas flame shoots into the primary heat exchanger? That's the upper part. The thin plates below I can't see any connection to so that's why I thought they might just be vanes.

I think it's time I pull out the manual & brochure & get re-acquainted.
 
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Old 11-14-13, 10:04 PM
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Didn't realize you had a carrier/bryant furnace...

Smaller plates = secondary heat exchanger

Yours is unique - no fins.
 
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Old 11-15-13, 09:54 AM
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I looked over the packet of documentation that came with the furnace and my hopes of a DIY cleaning are getting dimmer. It does seem that to clean any part (primary HX, 2nd HX, AC coil...) everything needs to be taken apart. I didn't realize so many automotive engineers found work at the furnace manufacturers

Without any access to the evaporator coil I would need to gut the cabinet so the crud can be caught. Looks like I have to call in the cavalry this time.
 
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Old 11-15-13, 03:39 PM
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Do you have some reason to believe the evap coil is plugged or partially so? Unless there is evidence elsewhere such as a dirty blower? If I have to clean an evap coil I usually pull it out of the case taking care not to kink the lines.
 
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Old 11-15-13, 08:08 PM
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Ineffective electronic air cleaner, furry blower...just an assumption the coil is also furry.
 
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Old 11-16-13, 06:42 AM
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Can you use an inspection mirror to view the evaporator coil?
 
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Old 11-16-13, 08:54 AM
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No--that plate can only move 1/2 inch in any direction...so I bored a 1.5" hole in it. The coil is clean. So I guess all that fuzzy buildup on the blower is limited to the blower. My curiosity is satisfied and I can move on to other things.
 
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Old 11-16-13, 02:50 PM
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^Used to have an electronic air cleaner and had the same issue.

Once the cells get a little dirty, the charged particles pass right through and stick to the next available grounded surface -> the blower wheel.
 
 

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