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  #1   IP: 141.154.226.27
Old February 4th, 2011, 08:03 PM
Timothy Timothy is offline
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Default Tripping furnace limit switch/ not enough air flow

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 raying 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 ideas?


Thanks for any help.


Regards,



Tim
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  #2   IP: 24.19.180.84
Old February 4th, 2011, 09:38 PM
suemarkp suemarkp is offline
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You may want to investigate your zoning dampers. You may need to reduce the minimum opening so it isn't so restrictive on just one zone. A zone dampered system can be difficult to get to work correctly because of the massive changes that could take place for different duct combinations.

If it works with the blower door open, you may either be just on the edge of enough airflow, or perhaps the return air is more restrictive than the pressure side. Try removing your return grills with the blower door closed. Is this enough of an improvement for it to work? If so, is it easy to add any more return openings somewhere?

If the return duct itself is too small, you could change just that to a larger one, or put in another opening in the return duct somewhere near the furnace. Should be rather easy to cut a rectangle in your return duct and then put the metal back with metal tape if it didn't work out.

Where are your return ducts compared to the pressure ducts? Best to put them on the same floor with similar square areas as the amount of pressure vents on that floor. If the furnace is in the basement, it may suck the basement door closed if there is excess return air compared to pressure vents. You may need to undercut the door if it works OK with the basement door open.

A 150 KBTU furnace is rather huge. If well insulated, that sounds too large unless you have a lot of windows or a big one story house. A furnace this big probably has the largest blower you can have (generally 1 HP).

Most modern furnaces are designed to push against up to 0.5" of water column. Some older ones were limited to 0.3". Some new ones can go up to .75 or even 1", but best to make the ducts bigger, slow the air down, and cut down the BTU. You could buy a magnehelic gauge, drill a hole in the duct work at the furnace at the input (between filter and blower) and output sides (past heat exchanger, before zone dampers), and measure that static pressure when dampered down to one zone. A gauge with two inputs would be easiest (return on one side, pressure on the other). A single port gauge will measure again ambient atmosphere, so you'll have negative pressure on the return side and positive on the output side. You add those two absolute values together to get the overall external static pressure drop.
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  #3   IP: 141.154.202.245
Old February 5th, 2011, 07:05 PM
Timothy Timothy is offline
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Mark, thank you for your detailed response.

The zone dampers are the opposed blade, shutter type, not the circular disk type. There is no mimimum opening so I don't think I can alter it. If it were a disk I suppose I could remove material from the center to create a bypass, but it is small blades. I suppose I could remove the dampers altogether but I don't consider that acceptable. I want two zones.

The return trunk is a large rectangular one in the basement next to the 2 pressure trunks all leading to the furnace. The return trunk eventually connects to a trunk in the attic via panned joist cavities to a vertical trunk . There are two registers on the first floor, connected to the basement return trunk by panned 2X10 joist cavites. On the 2nd floor there are also two return registers. These are in the ceiling, are connected to the trunk in the attic by way of 12" flex duct.

The return registers on both floors are located somewhar near the center of the floor plan. The much smaller pressure registers are located around exterior walls. I believe this is correct method.

All of the interior doors are undercut. Your comment about the potential of basement door being sucked shut is ironic. When I first moved in that used to happen. I found that the return trunk as it connected to the furnace was not properly sealed. I sealed it, then no more door being sucked closed, also no more outside air being sucked through rim joist fiberglass insulation. Although I clearly decreased the return air by forcing the blower to suck it from the registers via the ductwork instead of additionally pulling on the unheated, unfinished basement.

I am now just beginning the long term project of finishing a large portion of the basement. Eventually in order to heat the basement, I will be adding branches to the main trunks including connecting a return register. That will have a similar effect to your suggestion of adding a register directly to the return trunk.

I believe my furnace is oversized. I think that is the problem. Even if the ductwork were adequate, a furnace this size is larger than it needs to be for my 2800ft^2 house. The blower motor is .75Hp. It's supposed to be .2" "Certified External Static Pressure"/1770 CFM when in heating mode and .50" CESP/2055 CFM when in cooling mode per Bryant manual.

I have been considering making and using a water manometer. I think I'd have to hook it up above the A coil though. Not sure how much that will affect the comparison of the measurement to the Factory CESP.

Someone else has suggested considering changing the burner orifice size. Do you have an opinion on that?

Thanks again for any information.
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  #4   IP: 24.19.180.84
Old February 5th, 2011, 07:22 PM
suemarkp suemarkp is offline
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I don't know enough about gas furnaces to know if there is a way to reduce their btu output via orifice changes. On an oil furnace, that would work. I didn't think you could do that, and orifices were only for different fuels to get the right fuel-air mixture. Even finding a way to shutdown one of the burners would probably work.
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  #5   IP: 98.67.242.95
Old February 5th, 2011, 09:13 PM
buddylee buddylee is offline
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I install many zone systems and in the heating cycle they can be very finicky. Your ductwork is sized for your total system. When one zone is closed you can only move 1/3 1/2 1/4 of the total airflow depending on your setup. You could oversize each zone to account for the increased airflow when one zone closes. The problem with this is when all zones are open you keep the cfm's but decrease velocity. This can be a problem. Without proper velocity the air leaving the outlets will not be able to reach the far side of rooms and you will have more hot and cold spots. It is virtually impossible to have both with a standard zone system. You can accomplish this with a modulating zone system where the zone and bypass dampers open and close based on static pressure. No zone will ever truly close. These are far more expensive and need no maintenance.
Your system is fine operating the way it is. It will do the same thing in the cooling cycle if setup properly. When a zone is closed it should increase run time while cycling the system until it satisfies.
My suggesting is to decrease the output pressure of the gas valve. This will increase runtime but decrease cycling. There is a large round screw on the gas valve that hides another screw underneath. Turn the second srew counter clockwise and you will be able to visibly see the size of the flames decrease. Do not drop it too much or you will have problems with the flame sensor or with burners not igniting. Its best to use a manometer and stay within the specs of the system but you can do it by sight. No more than a half turn at a time. This is not hard to do, anyone can do this. Damn my fingers hurt. Sorry for rambling.
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  #6   IP: 98.67.242.95
Old February 5th, 2011, 09:15 PM
buddylee buddylee is offline
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Oh and sue is right you can block off one of the orifice's. I've bought caps and sealed them off on furnaces with a bad cell in a heat exchanger for customers who couldn't afford to replace the system or the exchanger. With a furnace that size where I live in a house that size i could cap two of them. You can get a cap at an appliance part store.
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  #7   IP: 199.46.198.233
Old February 8th, 2011, 03:26 PM
Timothy Timothy is offline
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Buddylee, thanks for your response. I have some specific questions I'm hoping you can respond to although I'm hoping that anyone else reading will feel free to put their 2 cents in too.

Sounds like you work as an HVAC tech, is that right? What better source for this.

You said "Your system is fine operating the way it is" you feel that way despite the fact that the limit frequently trips and I get an error code? Could you please explain? Is it just a matter of, if you have a dampered system this is quite common and the house still gets up to heat anyway so no big deal, don't be bothered by it? But shouldn't I be concerned about the furnace's life being shortened by this?

You suggest decreasing the gas pressure. If I do that should I reduce the orifice size also? The table in the Bryant manual which is geared towards adjusting for different altitudes, gives an orifice size with a gas pressure. These are selected based on specific gravity of fuel and of altitude. If I decrease the pressure but stay within the specs of the system as you suggest, is it a given that the efficiency won't be drastically affected?

You mentioned capping off a burner or two. Appliance stores in my area and those I've looked at online don't sell heating system parts. I've looked at online companies that do sell heating system parts and have not found one yet that carries caps or orifices for that matter. Do you (or anyone else reading this) know of any online sources that carry these? I'm sure a local Bryant distributor won't sell any parts to me.

What do you think of the idea of adding a basement supply register to increase flow/ deal with the problem?


Thanks again for any additional information. It's great that people are willing to help like this.
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  #8   IP: 98.67.219.151
Old February 8th, 2011, 05:43 PM
buddylee buddylee is offline
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Ok first off I don't want to mislead you when I said "your furnace is fine operating this way". You want to try and solve this issue but it is definitely no emergency. Many people feel that they should not operate equipment when there is the slightest fear of something wrong. But you don't have to worry about that. There is a chance that the heat exchanger continuously overheating and cooling will increase expansion and contraction which could leads to premature failure. That is why the limit switch is there to begin with. It keeps the system from getting dangerously hot. however it will turn off the gas supply long before damage can be done. It Is an automatic reset switch to eliminate nuisance tripping caused by dirty air filters, closed registers and temporary air flow issues. You wouldn't want to have to manually reset it or call someone out every time something like this happens.

You could in theory leave it this way and it could last for 30 years but why take the chance. Most of the zone control systems I install have a discharge air sensor mounted between the supply and return before the bypass damper. We can set the zone control board to monitor this temp. and turn the gas supply off before it gets hot enough to trip the high limit switch. Reducing the chance or damage or system lockout. This also happens when the air temp gets to cold. It will shut down the compressor and warm the coil before it allows the coil to freeze. Some brands have a limit sensor in the blower compartment, that is set lower than the high limit, this will shut the burners down if the hot air coming in is above the temp that would allow the heat exchanger to overheat.

Let me know what type of zone controller you have installed and I may be able to help just by changing a setting or two. Also you said that only the pressure side was dampered. Is there no bypass damper between the supply and return air sides of the system? If not the zone dampers should be the modulating type and controlled by static pressure in the duct work. Sometimes if like you said there is a basement or a two story area or any area where extra air is not an issue the bypass will run there as a waste area. in this case all the extra air will not return to the system but into the heated area of the house. This is the easiest to fine tune as you are not dumping the hot air back into the furnace. If there is a bypass damper it may be opening to much or not enough. These systems are hard to fine tune. Some have taken numerous visits.

I don't recommend capping an orifice because you do not change the temperature in the heat exchangerf cells that are still in use. You can drop the gas pressure without changing the orifice size. If you lower pressure and then increas the size of the orifice you are right back where you started. Changing to a larger blower will not help. Your blower has multiple speeds. it should be set at low or medium for heating. If you raise the blower speed it will drop the temp. in the heat exchanger but will then increase the static pressure and force more hot air through the bypass and still force the system off. They probably raise the blower speed to help it run a little longer before shutting down. In the cooling cycle this is not as noticeable. I have one system out there with 4 zones the largest being four rooms the smallest being one room. There is no possible way I could balance that system to be able to heat only that one room without having this same issue. To compensate the d.a.t.s. i mentioned is the best option. I try to design them all to have this capability. but each brand of controller has their own way of doing this.
Give me a little more info, even a picture or two and I could possibly point you in the right direction.
Just so you know using the d.a.t.s. to monitor the air temperature will still shut down the burners so technically it will be soing the exact same thing only at a lower temperature. So in my opinion the waste area outlet is the best to use but not as efficient since the hot air or cold air aren't being put directly back into the system which cycles the system more frequently but continues moving air across a hot exchanger or cold coil. But any engineer or tech. will tell you that a system is least efficient at startup. So opinions on this change drastically and rapidly. just like the do on programable t-stats and setting the system higher or lower when away.
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Old February 8th, 2011, 06:23 PM
buddylee buddylee is offline
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Oh and you ask about efficiency, if you lower gas pressure you are burning less fuel. Just think about a two stage furnace. it will burn less fuel but run longer cycles. If the temperature differential is not to great this will be more efficient. If the temp differential is large this will be less efficient. I see that you are an engineer so I'm sure you have had these debates yourself about run time, inout output and the overall goal. An electric furnace is rated at 100% efficiency since no heat is lost but it still costs more to operate than a 90% efficiency gas furnace.
No two situations are identical so I couldn't say with total confidence that it would help with efficiency but I also know that you have a furnace that is to large for the area it serves which is also wasting energy. Every day I talk to people about increased seer ratings, eer rating and afue ratings to help them make a decision on what type of equipment to install and when you account for payback period, my answer changes from day to day. In a well sealed house a high efficiency system will save less money albeit at the same rate than it would in an old drafty house. So I sell the higher end systems more on their increased comfort humidity control and features more than I push the dollars saved. After all your comfort is worth more than the 6 bucks a month you may save.
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  #10   IP: 151.203.201.197
Old February 9th, 2011, 09:33 PM
Timothy Timothy is offline
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Thanks for the new responses.

There is no bypass damper. The damper system is called Trol a Temp by Honeywell. Here's a link to a PDF I found on the web that describes it.
http://customer.honeywell.com/techli...0s/68-0101.pdf
Page 24 describes what I have. There is no temp sensor or pressure sensor. The damper is either fully open or fully closed and is a function of whether the thermostat is calling for heat for that zone.

My two thermostats are Honeywell Mercury Switch non programable.

I've also attached a Jpeg File of photos of the system with details.

Given this added information what do you think is my best bet, add a return, lower gas pressure or something else?

Thanks for any additional input.
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