one for the gurus

zeke

I performed the open door test suggested by Norm with the following results, comparing with normal operation



ret temp,supply temp, blower amp, diff coil p, p supply duct




normal: 70, 135, 5.5, .07", .05"




panel off 76 123 6.5 .05" .07"





From the delta T and the fuel rate (.75GPH, 74%eff) , using a heat balance I crudely got flow rates of 1450 cfm with panel off and 1048 cfm for normal operation. With the panel normal, I had previously gotten esp= .28" which should have corresponded to a flow rate of 1500 (vs the 1048 I get) according to mfr.

I almost forgot; the temperature at the limit switch location was 170 degrees and the stack temp was 580. Hope this helps in the analysis.

zeke

one for the gurus

I have this recurring problem with my furnace going out on hi temp that's puzzling me. Facts:

1)New hi limit switch replaced old one for this problem,so it is safe to say switch is OK (the replacement limit switch appeared identical to the existing, which was probably the original)

2)I measured esp, 0.3" H2O with fan on hi speed (according to the Mfr should deliver 1300cfm.

3) temperature just downstream of heat exchanger is 135 F,return temp is 70 F

4)Furnace is a split 85K BTU Carrier upflow w/ A coil, 15 years old.


Tech suggested that A coil may be clogged but if that is the case, how do I get a measured esp=.25" ? And why the large discrepancy between the 135 deg and cutout of 200 deg?
The blower checks out clean. and it freewheels easily.

Could it be that the blower motor is not getting up to speed?

Your comments and input would be greatly appreciated.

Ken_40

Oil-fired,

or gas?

The 'A'-coil does seem like a likely candidate, even with the "pressures" you state. The filter is too obvious for you to have missed, but sometimes the obvious...

Unknown

Have any changes been made to the supply or return ductwork between last year and this year?
Any major furniture changes making couches and things to be covering supply registers?
Any leuvers closed that were normally open?

Also how long does it take to go off on high limit after the blower comes on?

One thing I like to try is to put a thermometer in the ductworkas close to the supply (Heat exchanger)as I can and monitor the temperature. Then I can make air changes accordingly.

Its very possible the A coil could be your restriction. I would remove the panel to take a look at it. If there is no panel to allow you access to the A coil, be very carefull in cutting one in.

- Norm

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

zeke

oilfiired

Ken and Norm,

Thanks for responding

1)Furnace is oilfired

2)No ductwork changes ever made

3)I get the 135 F reading in ductwork just after heat exchanger

4) esp readings about same with or without filter

5) Norm, as far as how long it takes, I would say that it could be as much as 10-15 minutes, but I'm guessing here.


6) I also took pressure differential readings between the space between the A coil and the heat xchanger and found less than .05" which doesn't indicate A coil blockage. I am using a U tube manometer.

Thanks again for interest and your input

Unknown

Did you have an annual Maintenance where the tech may have installed a larger nozzle or increased the pump pressure to over fire the unit?



To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

zeke

nozzle


Norm,

Yes, I had a tuneup and the tech installed a 7580B nozzle - the specs call for a .75 gph 8o deg solid cone.
I can't say that he changed the pump pressure, but it is a good thought.

Thanks for thinking!

Zeke

tom_49

Zeke,

What is the model & ser. # of furnace?

Floyd

HX......

Check the HX... sometimes when it heats up it expands enough to open if it is cracked. Cools down and closes backup.
15 yr. old oil would be a great canidate for that...

Floyd

Ed_26

limit

Check your fan motor also - could br heating up & cutting out on thermal overload - if the timing is right, your limit will open.

tim smith

going out on limit

I would guess your temp rise is well with in limits on an oil furnace so I would rule out airflow. What I might suspect is the off delay on the blower is not working right and leaving too much residual heat in heat exchanger when it turns off causing the manual reset limit to trip. A 65 degree delta t is with in limits of most 80% gas furnaces and should be well with in your oil furnace but look on the tag. Maybe a bad fan timer. Good luck, Tim.

zeke

Floyd,
Thanks for responding.
But, if there were a crack, wouldn't the blower air flow into the flue gases, (and not the opposite) and, therefore, not be a cause for overheating the air outside the xchanger?
But, it may answer the other problem I have with this, namely the discrepancy between the measured temp, 135 F just downstream of the xchgr vs the cutout temp of 200 F, which may suggest a crack just near the limit switch probe.
Or am I all wet with this reasoning?

Arthur

Limit operaing.

Zeke,
Tim gave you a good clue, Does the limit only operate AFTER the furnace has been running and shuts down, Could sound like the fan is not running long enough to get rid of the residual heat before shutting off, should shut off at about 90 degs.

zeke

limit

Tim and Naroc,
Thanks for responding.
This is the sequence.
The furnace goes out on limit. The blower continues to run several minutes until the supply temp comes down to 90 F and then cuts off. So the residual heat idea is not happening.
But, again the difference in temp between the limit temp(200) and the 135 continues to puzzle me. Cracked heat xchger maybe?

jp_2

measure

measure the temp where the high limit measures its temp.

see if they asr the same, take a good look at the high temp location

zeke

jp,
I can't get in there without removing the limit probe. Further, I have no reason to suspect the accuracy of the limit value of 200 (and the 135 reading has been verified by two different monitors), since it is a replacement of the old limit switch and doing pretty much the same.

tom_49

Zeke,

I there a air deflector under the limit switch?

I believe there should be. If not, it can be causing it to trip.

Whats the model#?

Tom

zeke

hi lmit

Tom,

Thanks for responding. I don't have a clue on this. What is an air deflector? Model of furnace is carrier 58U085 series 100-CA, upflow w/A coil.(17 years old)

Zeke

Unknown

In the past most of this type of problem for me has been either blocked or out of service supply registers, or not enough return air.

If I were on the job I would investigate these causes after a CO test of the supply plenum and the stac with the blower running (I test CO first on every call)

I would take of the panel to the blower and tape the door switch open for my test and observe the unit to see if it shuts down on high limit with all the available return air. Then I would follow each of the supply s to the register removing the diffusers in the house to check for any obstruction. Along the supply ductwork sometimes there are balancing leuvers, and I would check the positioning of those also.


Ideally a furnace should run indefinately without reaching high limit provided the heat it produces can escape through the supply, enough return air is provided, and the heat source isnt too large for that kind of harmony to occur.


I have never performed CFM tests or anything like that. So to be honest I wouldnt want to interpret those tests that youve done in that respect simply due to my ignorance in that area.

Generally I feel warm air is simple, and sometimes over thinking can steer you down the wrong path.

Without actually being on the job there with you, Im stumped on this one.

- Norm


To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

zeke

limit switch

Norm,

I very much appreciate your continued interest and valuable insight.

I think I will do the open door test, but I am beginning to think it is a poor return system, but that may be partially responsible. Let me bounce this off of you. The esp I measured was about 0.28" or there about, which would seem to indicate good airflow. But,the pressure at the duct close to the input plenum is close to -.20" and the pressure just past the exchgr is about .05", leaving very little for the supply but the differential temp is 135-70=65F and the stack temp is 600.


This looks to me like three potential problems:


1) the motor is running slow (it is set on high). I have no way of getting RPM. Took a current reading of 5.5 amps fotr the 1/3 HP blower running without furnace.

2) there is blockage between the blower and the heat exchanger (caused by what, insulation??)

3) as noted, inadequate return ducting

Yours and others comments would be most appreciated.

Glen

another thought -

what are your differential air presures: across the furnace, and across the acoil? The high limit is doing its job for a very good reason. While your static pres looks good - if this is with a high differential - it would indicate no flow. Keep looking.

zeke

test

Norm,


Sorry, posted the "test" yoroposed under new topic (see below)


zeke

Rich W

Length of the probe may be your problem. 65deg rise seems within reason-low air flow would cause a higher temp rise. Check with Carrier for the correct limit.

rucomfy

Yes.. there is a way

> jp, I can't get in there without removing the

> limit probe. Further, I have no reason to suspect

> the accuracy of the limit value of 200 (and the

> 135 reading has been verified by two different

> monitors), since it is a replacement of the old

> limit switch and doing pretty much the same.

rucomfy

Yes.. there is a way....

During a training class with "my" Uncle Wayne. The proper way to check the limit is to use a Fluke 16 with a Type K thermocouple. Connect the thermocouple to the adapter and plug in your Fluke. You remove the lower screw and slide in the thermocouple in the screw-hole. Use a fresh 9V battery and you will read the temp that the limit is seeing. Bada-bing Bada-boom....
BTW for Carrier/Bryant Training you can not beat PeircePhelps and Uncle Wayne. Absolutely top-shelf.
Ask Jamie or Poppa Pompetti they know.
.

S Ebels

Static pressure........

Static pressure only measures what's goin on downstream of the blower. If you can't get any air into the blower your static will appear to be OK even though there is insufficient airflow. I'd check on the "suction" side of the duct system first. AFA the limit switches go, I've not found any that are very accurate or reliable. Darn near all of them are made in low wage land if you know what I mean. Check your actual air temp with a good thermocouple type meter or a digital thermometer. Remember also that temps in a plenum can vary a great deal with regards to the actual position/location of your measuring device.

GW

static pressure

seems low, I hate to ask...are you sure you know how to measure? When you say, "pressure doesn't change with or without filter" makes me go 'hummm'.

and what did you mean by .05" between the a-coil and the top of the furnace... perhaps i'm confused, where my eyeballs supposed to read "pressure drop across the coil"? If so, you must have a mouse eating the coil, that's very low pressure drop, or you have some nasty by-pass going on!

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

zeke

fan motor

No I have watched the limit switch rotate to the limit 200 degree position so I am positive about going out on limit.

Thanks for your response. Sorry I didn't see it earlier. Getting confused with the dates and entry locations.

zeke

Hvac64,
Sorry about not responding earlier. I'm having trouble tracking the inputs on this site.
The model is Carrier 58U 085101CA, serial no.3989U00315
Thanks for your interest.
Zeke

zeke

Tim,
Sorry about the delay in answering. Having trouble with the way this site picks up messages and dates.
Anyway, as I answered elsewhere, the fan stays on until the duct temp goes to about 90.
Thanks again.

Zeke

zeke

S ebels,
Thanks for responding. The suction pressure was about -.25".

zeke

SEbel,

Sorry, posted answer in wrong place again.

Zeke

zeke

delta p


Gary,

Thanks for responding.

What I meant is that I get .05" as the pressure drop across the coil, which seems low but I used both sides of a U tube manometer to get it. Maybe I have measuring issues, but I also get about -.25" at the input plenum and about .1" between the heat exchanger and the a point in the duct just downstream of the coil.nd I couldn't see any pressure differences with or without the filter in place.
Looks like my esp would be about .35" according to these new measurements.
Further, I also get a delta T of 65 degrees 135 to 70 and a stack temp of 580, so there is marginal flow even though I have the blower set on high.

Zeke

zeke

Rich,
Good point. I think I will pull the probe halfway out and insert a thermocouple probe in there to get some Idea of the true temp there.I wonder if it possible that the limit probe may be touching the xchger.

Zeke

zeke

Allan,
Thanks for the suggestion, but I'll have to go with poor man's method of moving the probe halfway out and sneaking the K thermocouple in the gap. Can't see buying a Fluke for this one-time problem. See what happens.

Z

clammy

High stack

Hey Zeke at a 560 degree stack for a oil fired hot air unit sounds a bit to hoigh have you pulled the flue collector off and cleaned the furnace ,i have in the past seen the same promblem and usually the heat exchanger has to be cleaned i would also re check the burners pump pressure and noz size the 560 stack is to hi in my opion fort a hot air furnce that's why the limit is going off ,also check the combustion chamber a for build up of debrie .I have always seen that guys will clean a oil fired boiler but feel that you don't have to clean a hot air unit just replce it and if the unit is not clean properly that's what you will do , i believe that carriers oil fired units are ducane also check on there web site and see if that unit has a baffle in the combustion chamber ,sometimes when theey are gone you will have a high stack and limit promblems also what size is the blower (cfm ex 3 ton blower 1200 cfm )Return duct size ,filter ect. Every oil hot air unit i have ever had a no heat call for was dirty and cycled on limit need a good cleaning just pull the flue collector off and brush and clean the combustion passes ,there's no short cuts to a cleaning change the limits till your blue in the face still will cycle on limit ,as stated check returns and clean if the stack is that high i hope the a coil does not have a plastic pan what temp is the discharge at ?peace and good luck clammy

zeke

Quote
" Date: November 19, 2006 05:47 AM
Author: CLAMMY
Subject: High stack

Hey Zeke at a 560 degree stack for a oil fired hot air unit sounds a bit to hoigh have you pulled the flue collector off and cleaned the furnace ,i have in the past seen the same promblem and usually the heat exchanger has to be cleaned i would also re check the burners pump pressure and noz size the 560 stack is to hi in my opion fort a hot air furnce that's why the limit is going off ,also check the combustion chamber a for build up of debrie .I have always seen that guys will clean a oil fired boiler but feel that you don't have to clean a hot air unit just replce it and if the unit is not clean properly that's what you will do , i believe that carriers oil fired units are ducane also check on there web site and see if that unit has a baffle in the combustion chamber ,sometimes when theey are gone you will have a high stack and limit promblems also what size is the blower (cfm ex 3 ton blower 1200 cfm )Return duct size ,filter ect. Every oil hot air unit i have ever had a no heat call for was dirty and cycled on limit need a good cleaning just pull the flue collector off and brush and clean the combustion passes ,there's no short cuts to a cleaning change the limits till your blue in the face still will cycle on limit ,as stated check returns and clean if the stack is that high i hope the a coil does not have a plastic pan what temp is the discharge at ?peace and good luck clammy
-------------------------------------------------------
Clammy,

Thanks foryour thoughtful reply. I had the unit serviced about a month ago and I have noticed this problem ever since. I'll try to answer your questions as follows

1) I have always had this high stack temp (This is my 3rd winter at this house)

2) The unit was serviced and cleaned about 1 month ago. The tech put in the right nozzle, I didn't see if he increased the pump pressure and I saw him clean the exchanger and removed the stovepipe, etc to get in. He spent about 2 hours here

3)The unit is rated at 2.5 ton and the blower is 1/3 HP,rated cfm 1320 at a esp=.5" . I measured the esp and get about .35" so cfm should be higher, but from a heat balance I did, I got an actual of closer to 1020 cfm

4) The return duct is 10x18 as it enters the plenum, only 8x18 for the main run. Only 4 feet to first return and 15 feet to second return. Total of 4 returns. Filter is 20x25

Zeke

Unknown

Not sure if you got my email zeke, but whats the MAX input of the furnace in GPH ?



To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

zeke

new test

I drilled holes in the intake plenum and fan housing and took new static readindgs

at fan housing: -.60"

at intake plenum -.25"

after exchanger--- .07"

supply duct--- .05"

Across filter is then .6-.25=.35"

So esp was .6+.07=.67" as opposed to my earlier erroneous posting of .35". So not surprising that the airflow was starved.I replaced filter with a more open pore structure, after which the esp went down to about .5" , the drop across the filter became less than .05", and the drop across the A coil was .1" . Also furnace did not cut out on hi limit, thankfully, but came close.

Now the stack temp is still near 600 and the supply temp is still near 135. I would think that my earlier 135 value was gotten near limit cutout so that it probably was actually higher.



Looks like problem and is at least partially solved. I'm sorry I didn't see this earlier.

Thanks to all of you who helped me through this . I am very grateful for your valuable suggestions and it was a good learning experience for me. You guys really know your stuff.

Zeke

zeke

> Not sure if you got my email zeke, but whats the

> MAX input of the furnace in GPH ?

>

>

>

> _A

> HREF="http://www.heatinghelp.com/getListed.cfm?id=

> 395&Step=30"_To Learn More About This

> Professional, Click Here to Visit Their Ad in

> "Find A Professional"_/A_



Norm,
Sorry but I thought I posted .75GPH somewhere in this maze. Having trouble following dates and responses in this funny site. Good thing I got your email.
By the way I just posted results of a new test and found the filter to be the problem after all you guys were screaming at me about that possibility.
Very grateful for your input and have a good Thanksgiving. Keep up the good work here. Guys like you are invaluable for us homeowners with limited skills in HVAC.

Zeke