Q:Inadequate Flow to prevent heat exchanger from tripping limit switch. How fix?

Tim1992 · February 2011

This is a long post. I believe giving all of the relevant information will better enable good responses.

My 2800 ft^2 house in Massachusetts built in 97 uses a Bryant 383KAV 80%, 154KBTU/hr input, 2 zone (pressure side only dampered)gas furnace, model #383KAV060155.

I have come to realize that when I run only on one zone and the system has to run for a while to reach thermostat setpoint (like on a cold morning, when I wake up and turn on/crank up the 2nd floor zone only) then the limit switch in the heat exchanger will trip prior to setpoint being reached. This creates a 3-3 code. The burners shut down, the blower will keep runnning as it should to cool the heat exchanger (so theres non heated air blowing out of the registers). The temp of the heat exchanger drops and then the burners relight and eventually the setpoint is reached or the process repeats until the setpoint is reached. Because the process will repeat until the setpoint is reached, it is not readily apparent there's a problem.

I've done some investigating to arrive at this and I believe the root cause is an inadequate duct system for running this furnace on only one zone. When only the 1st floor damper is open the system will run for about 7 minutes, 2nd floor damper only open then it will only run for about 4 minutes (this is in Jan/Feb), both dampers open it will run for at least a half hour, possibly indefinitely. The limit switch is a 160F. I've replaced it. I've also measured the temp in the duct just before the dampers. If both dampers open then it will go up to 154F, only one damper open then it will go to 160F and the limit trips. The temperature rise per the rating plate on furnace is supposed to be 50 to 85. 70room + 85 = 155 Max< 160. Further leads me to believe inadequate airflow.

I also checked out 2 neighbors houses. neighbor A has same furnace, different larger house, different duct work. He has no problem and air at damper is only ~130F. I checked out neighbor B's house. I told neighbor B of my problem and he said he hadn't experienced it. Anyway neighbor B has identical furnace and house. I measured temp at closed damper and I saw it reach 160F just like on mine, and then whaddya know, the burners shutdown, blower kept blowing and same 3-3 code. he has same issue and didn't even know it.

If I run the furnace temporarily with the door removed to the blower compartment (unrestricted air supply, door safety bypassed) then it runs fine and temps are fine.

I replaced the motor run capacitor (not a start capacitor, this is a PSC motor) to eliminate the cap as a possibility. New motor is like $400+. Won't replace that unless I'm 98% sure and I'm inclined to think it's running the same as when it was new.

I should mention that I've replaced the filter and its very non restrictive and none of the outlets are blocked. The blower speed for heat is the 2nd highest of 4 available. 1st reserved for AC use. Switching to the A/c highest speed doesn't stop the limit from tripping. I also "clocked" the meter to confirm that the unit wasn't being supplied excess gas.

At this point I'm pretty convinced that the root cause is that the ductwork doesn't allow the stock blower to push enough CFMs to adequately dissipate the heat produced, at least not for when only one damper is open (the majority of time).

MY QUESTION IS: what are my options for preventing the tripping without replacing the furnace with a smaller unit? Are their higher speed blower units available (if so from where)? Any merit to the idea of: adding a return register? Decreasing gas pressure? Capping one of the 7 burners? Any ideas? Thanks for any help.

Regards,

Tim

martin · February 2011

overheat

Some mfgs. will let you lower the gas press at manifold but would have check with tech help. I doubt if this would do it anyway. Even at almost 3000 feet thats a lot of furnace for a fairly new house it seems to me.

James Day_2 · February 2011

dampers

What damper system is being used. I know that on the EWC's you can install a supply air temperature sensor, which will cut low voltage to W on the furnace when the temperature reaches a certain setpoint. This would prevent your furnace from tripping out on High limit codes.

Tim1992 · February 2011

response to your comments

Thanks for the responses so far. The two dampers in my system are not EWC they are made by Honeywell. I think the control system is referred to as Trol a Temp. The dampers are similar to a shutter as opposed to a rotating disk. I think they are called opposed blade style. here's a link to the motor that's used in the system :

http://www.iaqsource.com/power_reviews/pwr/product-reviews/8709/Dampers/Replacement-Damper-Motors/Honeywell/p/113306-Honeywell-Trol-A-Temp-MST-Motor-Switch-Terminal.html

When the EWC system "cut low voltage to W on the furnace " is that essentially just shutting down the burners much like how my limit switch is doing it?

I agree that my system is oversized. The question essentially is how can I cost effectively deal with it? what are my options for preventing the tripping without replacing the furnace with a smaller unit? Are their higher speed blower units available (if so from where)? Any merit to the idea of: adding a return register? Decreasing gas pressure? Capping one of the 7 burners? Any other ideas? Thanks for any help.

Tim McElwain · February 2011

Try adding some more return air to the system

to see if you can reach a balance point on the system.

James Day_2 · February 2011

discharge air sensor

The discharge air sensor for the zone dampers prevents the furnace from overheating. Unlike your high limit which after 3 times of opening locks out, the discharge sensor will turn your burners on and off and will never lock out on saftey. It monitors your supply air, when the temperature gets to around 140 degrees it cuts low voltage to w and shuts of the gas valve, the fan will continue to run. As the temperature drops, it will kick the gas valve back in a relite. It is similar to a high limit.

icesailor · February 2011

Heat Exchanger

From what you describe, I think that your system has a serious case of emphysema. A serious shortness of breath. Your return ducts are inadequate. You said that when you open the return of the furnace and bypass the ducted return, the temperatures are OK. The return air is what cools the heat exchanger. Post some pictures of this furnace and especially the return. What are the sizes of the return grills.

Tim1992 · February 2011

Photo/Info file attached

Thanks for the replies

I've attached a PDF that shows numerous photos of the grills including sizes and of the main trunks and additional photos.

One thing I wonder about. Yes when I removed the blower door thus eliminating frictional losses in Return ducts, the overheating didn't happen. It would seem to me that the pressure side would be more of a bottle neck when a damper is closed. The system still has the larger return duct feeding it but only one pressure duct at that point. So I realize my taking the door off suggests adding a return would help, but I have to think that adding registers on both pressure zones would be more beneficial. If my logic is wrong please set me straight and explain why.

If it weasn't previously clear I have two zones, one damper on each pressure trunk. The damper is either fully open or fully closed. It is only controlled by whether the thermostat is calling for heat. There is no bypass damper or any type of system that send heated air directly back to the return.

Thanks in advance for any additional information.

Ironman · February 2011

Under Sized Ducting

Your trunk lines are sized to carry 1500cfm @ .1 static pressure. The 16x8 will carry 700cfm and the 18x8 will carry 800cfm. Just slightly less than a 4 ton A/C system needs (1600cfm) On the heating side, with both trunks open, you would be getting about an 83deg temp rise through the furnace at that rate of air flow (124,000btu output / 1500cfm = 82.66deg.) If only the 16x8 was open, you would have a 177deg rise through the furnace. Is it any wonder that the limit is tripping?

Some suggestions:

1. The least you can do is install a discharge plenum sensor that will turn the burner off before the limit trips. It will automatically allow it back on as the heat exchanger cools.

2. A bypass relief damper will also help some.

3. Adding more outlets will only help if there are not enough now (you should have about 15 six inch pipes or equivalent on the supply side). If you have this many, adding more is useless. As stated above, your trunk lines will only carry 1500cfm.

4. A better but more expensive solution would be to install a new 2 stage furnace with a variable speed blower that is properly sized to your house. A heat loss calc. should be done for sizing. With this, install the discharge sensor and a zoning panel that can be set to stage the firing of the 2 stage furnace with the number of zones that are calling. If one zone calls, the first stage fires. If 2 zones, the second stage fires. This stage could also be held off by a time delay even if the panel calls since full furnace capacity is not needed except in extreme cold.

5. Attempting to block off burners on your existing furnace is a bad idea as well as being illegal.

I'd recommend finding a competent pro in your area and getting his help.

P.S.

In answer to a couple of your original questions that I forgot to answer:

You can not reduce the gas pressure to the burner manifold below what the data plate requires. Probably 3.5 inches. And the 5 ton blower that you have is the largest that comes in a residential furnace.

Again, you have percieved your problem correctly. You don't have enough duct work when a damper closes to carry away the amount of heat the furnace is producing. It's not so much a duct problem as it is an over sized furnace for a zoned system. In fact, it's probably too big for your house. At present you're getting 44btu's per square foot. A newer house in Mass. probably doesn't need more than 25btu's per sq. ft., if that much.

Another simple thing that you could do to help some: Set your dampers to a 30% - 40% minimum open position. This will allow more air through even when the zone is not calling.

Tim1992 · February 2011

Will respond within next few days; new info (& questions)

Ironman,

Thanks for the detailed reply. I intend to reply back in the next couple of days with some questions about your reply and about some recent observations. I just took some manometer readings that I'd like to share. Also discovered at times the system is not getting so hot as to trip limit, even when only 2nd floor damper open. I'll report the details within next few days, I just wanted to let you know I appreciate the responses and have not given up.

Tim

Tim1992 · February 2011

more information and more questions for Ironman and anyone willing to respond

Ironman thanks for all the information.

If I add branches directly to the basement 1st floor trunk and a return off the basement return trunk, wouldn't that reduce the resistance, increase the flow and therefore lower the heat rise?

The dataplate says 3.2 to 3.8" for manifold prerssure, if it's currently halfway between them then reducing to 3.2 would be a 9% reduction. Not sure if the X pressure reduction means X or greater than X heat reduction, but it would seem to offer some significant heat reduction.

Unfortunately I believe the Trol a Temp dampers I have can only be fully open or fully closed. I could rotate the linkage on the motor shaft to not have it fully closed but that means it would not be able to be fully open.

You mentioned a bypass relief damper. When a bypass damper is connected to a return, the idea is that it will increase overall flow rate by reducing resistance, is that right? But it'll be raising the return temp quicker, which I would think I don't want, right?

I took some duct pressure measurements with an improvised water manometer. I suspect these confirm the ductwork being to small. The return was just upstream of the filter; the pressure was just downstream of the furnace housing by about 8”. This was outside of A coil, effectively downstream of it. The readings are with the heat running.

Both dampers open:

Return to atmosphere -7/16”

Pressure to atmosphere +5/16

Σ .75” Water

1st floor damper open only

Return to atmosphere -4.5/16”

Pressure to atmosphere +9/16

Σ .84” Water

2nd floor damper open only

Return to atmosphere -4/16”

Pressure to atmosphere +13.5/16

Σ 1.1” Water

I am confused by something I have recently observed. I mentioned in my 1st post that “When only the 1st floor damper is open the system will run for about 7 minutes, 2nd floor damper only open then it will only run for about 4 minutes (this is in Jan/Feb), both dampers open it will run for at least a half hour, possibly indefinitely.” Well recently I ran it with the 2nd zone damper only open and it ran fine for over half an hour before I shut if off. Same deal with running with only the 1st floor damper open. Later that night though in both scenarios the limit would trip but it still ran longer than when I started this thread.

Any idea why this is? It has been getting warmer out, but it was about the same temp later in the night when they tripped. Thinking about it, it doesn’t make sense to me that the outside temp should affect how hot the limit switch gets after a specific, set amount of time. I can see that the colder it is outside that the higher the heat loss to the outside and that the system would have to run a little longer to reach setpoint. That I can understand. However I wouldn’t think that the heat exchanger would be getting hotter after 4 or after 7 minutes if it is colder outside as compared to when it is warmer outside. Am I right on this or am I not considering something? If I’m missing something please explain what.

Thanks for any response.

icesailor · February 2011

Supply and Return:

Geez, I don't do duct work but, am I ther only one who notices this?

You have (say) two 10"X 20" supply ducts and one 10"X 20" return ducts. I know that warm ait takes up more volume than cooler return ait. But, there are only two ROUND 8" returns connecting into the 10"X20 " return. There is no way in my mind that this is correct. The returns are undersized. Don't look at the square returns at the furnace. Look at what it is being feed by. Add up all the supply areas and the return areas and see how well you can stuff all that return air in it. It may contract all you want in the square return. What is the pressure in the returns? I see this stuff all the time and scratch my head. Maybe you guys know something that I don't know. There are 7.5 gallons of water volume in a cubic foot. You can put the contents of 7.5 gallon bottles into a 12"x12" box. If my math is right (Ialways suspect at my age) there are 7.8 square inches in a 8" round pipe. There are 100 square inches in 10"X10" pipe. Measure the velocity of the air flow in the round pipe, Not the square return.

Ironman · February 2011

Chasing the Wind

Tim,

You're over-analyzing the the situation. Please think with me for a moment: When both dampers open, the limit does not trip. When only one opens, it trips. Therefore, the problem is obvious: not enough air flow with only one trunk open. It's that simple. The furnace and duct system were properly matched before the zoning dampers were added ( even if the original installer added them). Adding more run outs will not correct the deficiency of not having enough trunk when only one damper is open.

According to your pics and notations, you have 16 outlets on the supplies. That would equal about 1600cfm. The sum of both your supply trunk's capacity is 1500cfm at normal static pressure. When one damper closes, that is cut in half. Adding more outlets is useless.

It appears that you have four returns, each on a 10in. pipe. A 10in. pipe carries 400cfm. 4 x 400 = 1600cfm. Again, the problem is not the duct system, but a closed damper choking the system. Adding more returns will not help.

If you'll trust my 40 years experience in designing, installing and servicing HVAC systems, then please accept that the solutions that I and some of the others here have offered. Please look at the solutions in my previous post. I know of no others and I don't believe you'll find any that are proper.

Tim1992 · February 2011

Your 40 yrs of experience is 40 years more than I have

So I don't doubt your knowledge. I had pretty much thought my system was oversized and duct system inadequate. Being an engineer I like to understand the details. Do not mis interpret my asking questions as anything other than my wanting to understand.

I understand what you are saying but I don't understand why the problem doesn't happen 100% of the time when it's only operating on one zone, like I described in my last post. I think, most of the year it doesn't happen.

I also have another neighbor, I'll call him neighbor C who has the same house and swears he doesn't have this issue. He measured and tells me that his two pressure trunks and one supply are all the same size (including the return).

Icesailor, the two round ducts connecting to the return are only feeding one return register on the 1st floor. The other 1st floor return is fed by a panned 2X10 joist bay connected to the return trunk. The 2nd floor returns are connected to the attic trunk by two 12" flex ducts (rated for return as well as pressure). I'm away on vacation right now, I'll update and repost the previous attachment with more detail, to make it more clear.

Does this website have a referral service for people given their geographic location?

Thanks again for everyone's input

Tim1992 · February 2011

updated file attached

I updated and have attached the file with detailed information on my duct system. It now includes more detail about the size of the branches. Including that the two 8" dia ducts are only connected to one 1st floor return, 2X10 panned joist bay. There are 50.2 sq in in a 8" round (PI (~3.14) X 4"radius squared = 50.2). I don't own an anemometer to measure the velocity.

Although I believe my duct system is undersized I am at a loss as to understand why it is that sometimes the system will run much longer and not trip than other times. For example when I timed it in beginning of Feb: 2nd floor damper only open- limit tripped in about 4 minutes; 1st floor damper only open - limit tripped in about 7 minutes. This morning: 2nd floor damper only open- system ran for about 17 minutes, setpoint reached; 1st floor damper only open - system ran for over 10 minutes then other damper opened (2nd calling for heat again), eventually 2nd closed again and 1st floor only received heat for another 22 minutes until setpoint reached. Limit never tripped. It seems to be tripping far less now. If the system were tripping soleley because of the duct work being undersized then I would think that it would consistently trip, not just once in a while, since the size of the ductwork is not fluctuating. Can anyone explain why the system would behave this inconsistent way?

I mentioned in my last post a neighbor C, who has same house and same furnace but not this problem . He tells me that his two basement pressure and one basement return trunks are all 8 X 18. However he has 5 returns on the first floor (all wall mounted, four of them being two pairs of back to back registers). I tend to think that although his trunks are small the 5 returns (vs my two) are helping to offset that.

Regards,

Tim

Q:Inadequate Flow to prevent heat exchanger from tripping limit switch. How fix?

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