Hi everyone, I have been encountering issues with my furnace since I began using it this fall after taking possession of our new home. I know the Ngas furnace is grossly oversized as per the manual J it is 296% (EL296UH090XV60C) of the homes requirements. I brought this up as a violation of the codes and standards to the HVAC company and that it would cause high temp tripping due to the 3 zone design of my home. This was met with heavy resistance (and statements that 300% was allowed w/ variable units) and an insistence it was suited for the home based on their past experience in my area (North Houston). They provided a calculation of a smaller furnace at 196% (so just at the max allowable) but said they did not agree with the design and would do no further work if there were issues without cost (I reluctantly left the oversized furnace in place).
The system is controlled by a Harmony III unit and following its furnace startup process but after engaging the blower at the initial low speed and ramping up to the setting the DAS trips. It seems like the unit builds up too much heat in the combustion box before the fan engages as once it is going in cooldown mode and restarts things are more stable. That said, for some reason that I cannot get an answer to they either left in or installed the 140 DAS limit jumper and in monitoring the temp at the supply duct exit at the unit it is reaching 142F (slows down as it approaches 140) before the trip so I know the DAS is working.
I will be having another HVAC company assess the design and then deal with things directly but the things that sticks out to me is related to the condensation drain, DAS at 140, and the exhaust piping.
1. The condensation drain is installed completely backwards (input is output, output is input, cleanout is vent) causing negative pressure suction form the vent because their is no water lock.
2. The exhaust pipe is sloped away from the furnace (substantially) and I hear substantial water building inside it.
3. DAS set to 140 instead of default 160.
1 and 2 above are going to be addressed but I was curious if it would do anything related to the heat buildup (resistance on exhaust flow more specifically)? I'm waiting to figure out why the DAS is set to 140.
The unit is paired to a 3 ton AC (matched configuration) and the furnace DIP switches are set to limit cooling CFM for that design (low speed). I am starting to think this is part of the issue as with the Harmony III it seems to imply the max CFM is whatever the cooling CFM is set to and in most cases it expects more cooling than heating thus the heat reduction jumpers (but none for cool). For this particular unit the manual seems to say set them per the building need but then says leave them at high on this unit worksheet specifically so I wonder if during heat the CFM is limited too much but in setting it higher the AC wont operate effectively...
At this point I am pretty sure a smaller furnace will be required due to the smaller AC and the Harmony III complexity related to heating mode. Either that or the Harmony III just isn't suited for this design and the onboard 2 stage controller is the better option (but no per zone CFM, just 2 settings).
The system is controlled by a Harmony III unit and following its furnace startup process but after engaging the blower at the initial low speed and ramping up to the setting the DAS trips. It seems like the unit builds up too much heat in the combustion box before the fan engages as once it is going in cooldown mode and restarts things are more stable. That said, for some reason that I cannot get an answer to they either left in or installed the 140 DAS limit jumper and in monitoring the temp at the supply duct exit at the unit it is reaching 142F (slows down as it approaches 140) before the trip so I know the DAS is working.
I will be having another HVAC company assess the design and then deal with things directly but the things that sticks out to me is related to the condensation drain, DAS at 140, and the exhaust piping.
1. The condensation drain is installed completely backwards (input is output, output is input, cleanout is vent) causing negative pressure suction form the vent because their is no water lock.
2. The exhaust pipe is sloped away from the furnace (substantially) and I hear substantial water building inside it.
3. DAS set to 140 instead of default 160.
1 and 2 above are going to be addressed but I was curious if it would do anything related to the heat buildup (resistance on exhaust flow more specifically)? I'm waiting to figure out why the DAS is set to 140.
The unit is paired to a 3 ton AC (matched configuration) and the furnace DIP switches are set to limit cooling CFM for that design (low speed). I am starting to think this is part of the issue as with the Harmony III it seems to imply the max CFM is whatever the cooling CFM is set to and in most cases it expects more cooling than heating thus the heat reduction jumpers (but none for cool). For this particular unit the manual seems to say set them per the building need but then says leave them at high on this unit worksheet specifically so I wonder if during heat the CFM is limited too much but in setting it higher the AC wont operate effectively...
At this point I am pretty sure a smaller furnace will be required due to the smaller AC and the Harmony III complexity related to heating mode. Either that or the Harmony III just isn't suited for this design and the onboard 2 stage controller is the better option (but no per zone CFM, just 2 settings).
source https://hvac-talk.com/vbb/showthread.php?2223526-Harmony-III-w-oversized-furnace&goto=newpost
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