I did an inspection 6 months ago and my client has an HVAC issue that no one has been able to figure out. I’m hoping that if I can get enough eyes on this someone might have insight. The house is in Cedar Park, TX and the basics are: High efficiency Lennox gas furnace from 2011, a Lennox condenser from 2018 and a Bosch evaporator coil from 2022, 2 Nest thermostats and I didn’t get the manufacturer on the zoning motherboard but it’s not an Aprilaire, which is what I usually see.
On my inspection last October, I tested the zoning damper with the procedure I always use. I run the heat with the upstairs on and the downstairs off and then I run the A/C with the downstairs on and the upstairs off. The damper worked great with almost no bleed into the zone that was off. The only thing I noted on the report was that the furnace cover was being held on with tape and that I didn’t remove the cover because I was concerned about getting it back on so that it would stay on. At this point I realize that the cover fasteners were worn out because the cover was on & off too many times before I got there the first time.
The client has finally started moving in and he called reporting that the heat wasn’t working downstairs. By the time he called me, he had already had 2 different HVAC contractors out and so far, they have thrown parts at it but nothing has fixed the problem, so I went there last week and ran a series of tests.
The A/C works fine upstairs and downstairs regardless of which thermostats are on. The heat works fine upstairs and if the furnace is already running when I turn on the downstairs, the temperature at the registers increases to 105 degrees but there is no forced air. If I just turn on the heat with the downstairs thermostat only, the furnace goes through a normal start up but when the burners don’t ignite, it shuts down and tries again, with the same result.
The first HVAC contractor replaced the zoning board and the fan motor but didn’t specify if it was a blower fan or an induction fan. They ended up giving him some back some but not all of what they charged. The 2nd specialist replaced the downstairs zoning damper and again, no improvement. I spoke with him and he told me that the system was poorly designed and that a high temp safety sensor was reading 162 degrees after 2 minutes. I understand why this would cause the unit to shut down but I don’t understand why it would cause a cold unit to have a failed ignition or why it has no impact on the furnace when the upstairs is on heat. to clarify, when the heat is running upstairs and the downstairs is on, the zoning damper for downstairs opens but there is no forced air.
Sorry for the lengthy post, it’s complicated. I have 2 pages of typed notes with more detail that I’m not including, most of which is documenting the status of the lights on the zoning board when running all the tests. If you sufferd through reading all of this, Thanks!
Sounds like more than one problem.
If the furnace ignites and runs from 1 t-stat and not another, it isn’t likely to be the unit.
When you State “there’s no forced air”, do you mean the blower isn’t operating?
Addressing the above is where I would start. Shut off the upstairs thermostat altogether. Figure out why the furnace is not igniting when the downstairs thermostat calls for heat. This type of problem has to be broken down into smaller bites, in my opinion.
I would say it is a thermostat issue and that would be the first thing I would be looking at.
Is there a dump damper that has failed and recirculating the heated air back into the return?
Still waiting for an answer if the blower fan is on when the furnace starts. At the thermostat that doesn’t respond correctly, jump the green and white wire at that thermostat and see if you get air flow when the unit comes on. If it does come on with the heat, the thermostat is bad. You also may need to do the same thing with the Zone control. As for the furnace shutting itself down on Startup, a specific startup sequence is required or it will fail to ignite. For example, if the induced draft motor comes on but the pressure switch on the induced motor does not detect air movement through the heat exchanger, it will not light etc.
I am thinking that it is going to take a true expert in that particular zoning controller to figure this out. Perhaps with the right equipment (interface, software), someone could read an error code from the controller that may point to the problem.
If the downstairs thermostat is sending the “Heat” signal, then it is likely not your issue (i.e. most thermostats are exceeding simple ON/OFF controls)
The Zone dampers will not allow the gas to come on if the dampers are not fully in the correct position. If they’re not fully opened and closed as necessary, ignition will not occur.
There is a high temp sensor in the downstairs plenum that is somehow disabling the blower, and there is only one blower, from directing forced air to that zone, while the zoning board is opening the damper to that zone and the control board for the whole unit is allowing the system to still run, as long as the upstairs thermostat is on first. I know that more than two specialists have had their hands on this unit, with advanced training and diagnostic equipment who can’t figure this out. Personally, I rarely see high efficiency units, which is complicating the issue.
That may be the case, The HVAC guy I spoke with indicated that it’s a bad design but didn’t specify or I didn’t ask the right questions. I was under the impression that is was on the supply plenum and not the return.
Dominic, No forced air downstairs but the forced air upstairs continues to blow, as long as the upstairs thermostat is turned on first.
The dump damper is located between the supply and return. If one zone damper closes while the other zone is still calling for heat or cool, static pressure builds up causing the dump damper to open. It works similar to a barometric damper. If the counterweight is off, it can open completely.
David, I’m not at the house. I’ll suggest this procedure to my client so he can ask the HVAC guy to verify this. The only component in this house that’s original is the furnace. The evaporator, condenser, thermostats, zoning controller and downstairs damper have all been replaced at one time or another, with the zoning controller and damper replaced recently, as an attempt to solve the problem.
Jeff, I’m adding that to my list. Help me - would the dump damper in some way effect the high temp sensor that the HVAC guy mentioned. He told me that they had done extensive diagnostics and his equipment was able to tell him the exact temperature the high temp sensor was reading. I should have asked him if they tested the sensor to make sure it hasn’t failed mechanically. If I can get him on the phone again, I’ll ask him about this, the dump damper and also, the thermostat test David recommended.
Brian, One of the HVAC guys before me changed the zoning controller and I don’t know if he went with a new brand or a different brand, but the issue hasn’t changed.
David, The zone control is more complicated for me. Is there an easy way to tell me which wires to jump on it? I’ve looked at zone controllers before but never worked on one or been trained on one. I’m gonna go back over there for the thermostat test, and would like to test the zone control too.
There are a lot of possibilities on what is causing it.
If you have a140 degree return air temp, you are more than likely going to bump limit. This is just one theory. It came up on a job that another company designed and failed to remedy before that the company I worked for took over.
You cannot ask the furnace to do more than what it is capable of. On another project, one of the residential salesmen designed and sold a system on a huge house. It was a three-zone system on one furnace and one air-to-air heat pump. It was a complete failure. The major problem was that he listened to what the owner wanted instead of designing what was actually needed. The owner only wanted one furnace. He also did not want bulkheads in the basement. In order to accomplish this, the equivalent duct length ended up being close to 1,300 feet. Furnaces are only designed to handle 100 equivalent feet. We ended up having to go with two systems and totally reconfigured the ductwork. They ended up getting twice the system for free but had to suffer by having a bulkhead above their bowling alley.
Jeff, That’s helpful and I’ll try to convey that to my client. Are you OK if I share that verbatim? David’s post got me thinking and I just watched this video on youtube, which was very educational for me.
I think my client will really appreciate all of the effort and insight everyone in this thread is offering.
The advice that Dave and others are sharing is more about educating you and anyone else reading the thread. You have to be careful prescribing repairs or diagnosis. If they do not create a solution, who is going to pay for the time spent by the service techs? In other words, don’t do it.
If the homeowner has all the paperwork from the various changes over the years then write down the make and model of the components. For example three years ago the original thermostats were replaced with Nest thermostats, which is the time issues started to show up.
My WAG is the Nest thermostats are the problem. These thermostats likely contain built in software you can’t change. If your HVAC configuration doesn’t match one of the configurations preprogrammed it will likely default to one it chooses. NEST has an online configuration checker that may help.