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 there's 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 onlu 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

You have done a lot of good research, congratulations! Yes, it's tripping because of the heat buildup. I may be off base but that seems like a lot of BTU for 2800 ft2, particularly for a house built in 1997. So you could be underducted or overheated. Since it works with the front open, the problem sounds like it may be in return capacity. I would check to see if you could downsize the jets, if possible, or add more returns. We have used this approach when an older house undergoes a major upgrade with insulation, windows etc. downsize the jets and it keeps the house at a more level heat balance because you need less BTU. Do you notice it overruns the t'stat?

If the system will work with the blower door removed. The problem is in your return air. Not saying you don't have a supply air restriction but if it was the main cause your main limit would trip with the blower door off also. If it's a square duct return make sure the duct liner has not came loose inside the duct restricting the air flow.

Thanks,Yes I think furnace oversized. If I changed the orifice size would I need to adjust the gas pressure too? Would making these changes likely lower the efficiency? Any idea where I can buy the orifices? Y,it overshoots but isn't that normal?

Damn this 250word limit.I should note that tstats are Honeywell mercury switch.I set anticipators to .4. Gas valve is rated.3 amp.Isn't system supposed to overshoot setpoint? Overshoot, turn off then system back on when temp drops to setpoint right

Thanks for reply.I have galv rectlr trunks + flex + metal branches.Is liner specific to fiberboard? Any advice on how I would check for obstructions?I do have a flex return on 2nd flr that doesn't have much suction compared to adjacent flex return.

To avoid the 250 character limit, use the "answer" box, not "comment". Overshooting the setting is usually indicative of too much heat. The system shuts down but the residual heat keeps coming out. Anticipator should actually shut heat off a bit early and let residual heat bring the room up. Changing the orifice does not alter the pressure, that is the gas valve. This does not lower the efficiency, that's by exchanger design. As far as getting an orifice, you need to see if they are replaceable, I'm not sure on your model. I use a local ACE Hdwe store that has conversion charts for NG and Propane. You might try a HVAC supplier for a source. I saw on another reply you have a flex duct return from the second floor. Unless the duct is designed for a return, it may be collapsing and cutting off the flow. By turning up the fan speed, you also increase the return flow speed, under negative pressure. In a recent basement remodel, we installed a return grill in the return plenum just before the filter. Worth a try for $5. They way you don't have to keep the fan access door open and suck in basement dust.

Flex duct is well known for callapsing if on the return air. Any HVAC company worth their weight in salt would not use it unless it was a have to situation. Its Quick to instal its cheap to buy and in turn it's a headache for the home owner and the company that comes in behinde them. But That's my apinion. The only way to check for a callapsed duct is to feal and inspect the duct work.

Thanks for additional input guys.

ma0641,

I just set both thermostats to 67, house was at 65. The second floor damper stayed open until 2nd floor came to 68. That left only the 1st floor damper open. After a while the limit tripped. A few minutes later the burner relit, the burners kept on until the 1st floor was up to 71. Both Tstats anticipators are set to .4. I guess you would suggest I reduce the 1st floor anticipator setting to a lower #? It seems like if I adjust so they don't overshoot then the furnace will be cycling more. Am I missing something?

From looking at my manual, the orifices are replaceable. However it appears that one is supposed to adjust the gas pressure when changing the orifice. The table in the 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 were to swap out orifices should I go up or down in orifice #? When you changed the size you did not alter the gas pressure?

T-Top & ma0641,
The flex duct in my attic appears to be intended for use with either pressure or return.
It is a 12" size, class 1 form A (whatever that means). There are ratings for pressure and for max 3/4" WG Negative. From looking at it for its entire length (albeit from the outside) I don't see any indication of collapse. This particular one does not have an elbow at the register box, unlike the other, shorter one that seems to have more suction.

I am just starting to finish the basement. I will add a return branch to the return trunk as well as branches to the 1st floor pressure trunk. I think that the additional return will increase flow for any of the floors.

Thanks for any further information.

Regards,

Tim

Is simply reducing the gas pressure, so that the heat exchanger is producing less heat and requires less airflow to sink, a good option? I think adding a return possibly in the return trunk itself is a possibility but I'm wondering if the gas pressure is also a good option.

If I reduce the gas pressure should do I need to alter the orifice size also? Any idea where they can be purchased online?


Is it possible to post attachments here? I've got a PDF showing the details of my system.

I am out of town now but will get back to you ASAP.

Nov, you can add attachments, below the answer box click Go Advanced, then near the bottom you'll see manage Attachments. Good luck

I'm afraid I don't see "Go advanced" anywhere. The only thing I see below the answer box is the answer buttton.

Oh, your using the "new skin" I'll have to see how to post on that one. I'll let you know.

If you lower the gas pressure to a setting lower than what it is set to run at it will cause a chain reaction---soot build up on the inducer wheel not long after restricted heat exchanger and burners. One thought is to do away with some of the BTU, plug off one or two burner orifice relocate the flame sensor to the next burner over. But that's just me thinking outloud. You should never customise your heating and air system. It could be dangerous and cause injury.

I would only lower the gas pressure to as little as what the nameplate says. Nameplate says 3.2 to 3.8" Water, so 3.2" is as low as I would consider.

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 have another neighbor, I'll refer to as 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. He also has a finished basement heated by the same furnace. I guess all these additional outlets suggest that my problem might be diminished or resolved if I add outlets, but it does not explain the current inconsistent behavior.

I've noticed that my first floor will overshoot by 6 to 8 degrees. I've adjusted the anticipator (Honeywell T87F 3707 Mercury switch, round dial) numerous times and it doesn't make a difference. Can the anticipator some how not be working? Could it just be a bad wire connection (if so any idea which wire)? The first floor Thermostat is the one that controls whether system is on, fan or auto, A/c or heat; has 5 wires). 2nd floor only overshoots by 2F.