After restoral of power following a power outage while my HVAC system was in Cool mode, the HVAC will restart and return to Cool mode. But the blower speed will only be 750 cfm and it won't ramp up. At that point I switch to Heat mode and wait until the Heat has been on for half a minute (blower is at 1267 cfm). Then I switch back to Cool mode. The blower speed starts out at 1623 cfm, then 5 minutes later, it ramps up to 1980 cfm. Normal on/off cool cycling will continue for days at that point.

The problem arises when I am away from home and a power outage occurs. When power returns, the system reverts back to Cool at 750 cfm and the evaporator coil will freeze over.

The Lennox model is an SLP98UH110XV60C with a two-stage furnace and multi-stage blower. The single-stage Air Conditioner model is XC17, a 4-ton unit. Thermostat is a Honeywell RTH9585WF1004. I have the two connections for cooling, Y1 and Y2, in place.

Does anyone have suggestions for what I should check to solve this absurd problem?

Thank you.

The highlighted text is something that needs to be resolved. There may be associated setting to go along with that, too.

 

In the thermostat settings, you have choices between 1 or 2 stage heating. And another setting for 1 or 2 stage cooling. (I have tried both cooling settings with no apparent change in the operation). There are no other settings related to that. There are no other settings relating to fan speed for example. On the Lennox HVAC board, there are four terminals for those wires. Insofar as cooling is concerned, there is a wire on the HVAC-Y1 terminal that goes outside to the compressor/condenser unit. There is no wire on the HVAC-Y2 terminal going outside (there is just the wire from the thermostat-Y2). I have learned that if I do not make a connection to the HVAC-Y2 terminal, there is nothing I have been able to do to make the blower ramp up from 750 cfm. So it seems to be obvious that even though only a 1-stage outdoor unit is being used, the blower speed is still affected by Y2 status.

Rather than to connect the wire from the thermostat-Y2 to HVAC-Y2, I have jumpered from HVAC-Y1 to HVAC-Y2 on the HVAC circuit board. When I do that (with the thermostat-Y2 wire removed), the cooling behaves in exactly the same manner as it does when the wire from thermostat-Y2 to HVAC-Y2 is in place.

What are you suggesting to "resolve" with the Y1 / Y2 connections?

Could the problem have something to do with the timing between the HVAC unit and Thermostat? As if perhaps each one has a timer to determine when to ramp up blower speed? But even if that is an issue, it would seem a ramp-up of blower speed should occur (?)

Thank you for your help with this.

 

As stated, we cannot help with DIY specifics because we don't know the poster's competency and the site wants no liability.

There are installation instructions from each piece of the hvac system that you need to understand and follow.
Please call a professional to assure you have properly working equipment.

 

This is not a DIY site. We are not allowed to help with how to fix.
You really should take this up with the installing contractor.

 

I had an HVAC technician come out a few days ago. He measured pressures which were fine. I showed him that the blower speed was too low for cooling. He determined that the blower motor had to be replaced. After he left, I experimented with the 'heat idea' because I knew that the blower worked fine during the winter. I then switched to cool and discovered the system ran fine. I called him and explained what I had found. He said perhaps killing the power and switching to heat and then to cool 'reset' the system. I had come to the same conclusion.

The next day, after the house had time to cool down, I killed the power again for a test and again, the cooling blower speed was low until I switched to heat and then to cool.

Yesterday the company called to see whether all was well and I explained the situation. They told me they would send someone out today to take another look at no cost. I thanked them for that and it would be great if they solved the problem. But I've thoroughly reviewed the three manuals involved. Very likely, the next step will be for me to install a communicating type thermostat, which is what the system had initially. I expect they will suggest doing so. I wanted to avoid doing that since two of them have failed during the past six years and they are expensive.

My hope by coming here was that someone would have come across this problem in the past or had an idea for what to consider. This isn't a problem that can be solved without experimentation and testing.

 

I had an HVAC technician come out a few days ago. He measured pressures which were fine. I showed him that the blower speed was too low for cooling. He determined that the blower motor had to be replaced. After he left, I experimented with the 'heat idea' because I knew that the blower worked fine during the winter. I then switched to cool and discovered the system ran fine. I called him and explained what I had found. He said perhaps killing the power and switching to heat and then to cool 'reset' the system. I had come to the same conclusion.

The next day, after the house had time to cool down, I killed the power again for a test and again, the cooling blower speed was low until I switched to heat and then to cool.

Yesterday the company called to see whether all was well and I explained the situation. They told me they would send someone out today to take another look at no cost. I thanked them for that and it would be great if they solved the problem. But I've thoroughly reviewed the three manuals involved. Very likely, the next step will be for me to install a communicating type thermostat, which is what the system had initially. I expect they will suggest doing so. I wanted to avoid doing that since two of them have failed during the past six years and they are expensive.

My hope by coming here was that someone would have come across this problem in the past or had an idea for what to consider. This isn't a problem that can be solved without experimentation and testing.

Pressures without temperatures, are worthless. It means nothing.
From here it sounds like you got a sales tech! Call the contractor back and tell him it’s still not working.

 

I apologize. He did measure temperatures as well as pressures. He used individual digital gauges of which he read values on an iPhone app. Something I had never seen before.

They are coming back out this afternoon. I'll update this thread with the results.

Thank you for the support!

 

You can go back to the other iComfort thermostat, but this equipment should work with a PROPERLY installed conventional thermostat AND properly configured control board, wiring and thermostat.

 

I installed a replacement S30 thermostat. I believe it exhibits the same symptoms that the original S30 exhibited back in November 2024 when we realized that the HVAC was not heating. The display is black. The LED on the Smart Hub is displaying alerts.

I'm now planning to replace the HVAC's control board. I am hoping it makes sense that I cannot access the Smart Hub with my mobile app if the Smart Hub, Mag mount, and display is OK and the Control board is bad.

Fortunately, my Honeywell thermostat is working well with the HVAC other than initial power up when the blower fan will not ramp up from 750 cfm. After briefly switching to 'Heat' and then returning to 'Cool', the HVAC follows the ramp up process just as explained in the manual.

The first HVAC Tech that came out said I needed to replace the blower motor. The second HVAC Tech that came out said I needed to replace the thermostat.

 

Keep messing with it!

 

I will, thanks. I did order a control board. That doesn't leave much that could be wrong.

 

A true sign of somebody that doesn’t know what they’re doing.

 

As equipment becomes more and more proprietary utilizing communication protocols between thermostat, boards, and motors this makes troubleshooting very difficult for even the best. I am assuming that the tech you had out was able to directly manipulate the motor and recreate the issue? Is this why he felt it was the motor? The motor's brain box could be scrambled/corrupt, but then it could be the board, too. Maddening for us techs, we may find we replace both and have the same issue. Many of us have very good dealer support and relationship with the manufacturer's local field reps and in situations like these can utilize them to resolve weird issues, such as this, if not a warranty issue, then the quicker and efficient means of remedying these issues is financially beneficial to the customer and not taxing on the relationship between the customer and the technician/repair company.

 

The first tech verified pressures were proper. He noted that the blower only did 750 cfm. He measured for proper voltages, he jumpered Y1 to Y2 thinking that perhaps the Y2 stage was not turning on for some reason. The jumper made no difference. He concluded the blower motor was bad and gave me an estimate to replace it. I knew the blower was fine in the winter. I removed the jumper, switched to heat, blower came up to speed, I switched to Cool, and HVAC runs fine until the AC power is shut off, then I go through the same process again to restore cooling. The second tech spent perhaps an hour and a half checking wiring and options and suggested reinstalling an S30, which I purchased and installed today, HVAC won't run with it. Reinstalled the Honeywell thermostat and ordered a control board. Ten year warranty runs out next month. If control board does not fix it, but it seems to me it will, I'll be out of ideas and will have a 3rd tech come out.

Thank you for your interest!

 

Pressures without temperatures are worthless!
The blow only did 750 as per what his test equipment or the controls?

 

A month or so ago I had a little blow out with a new member here and I decided to take a step back and realize that we are talking with home owners that have a true concern and are trying their darnest to express what their concern is, but they do not have the knowledge and experience to technically explain it to us. Thus, here the HO is talking about a tech checking the pressures, we know pressures are really useless, but we were not there to see exactly all this tech did and how he explains his findings to his customer, very few of us have a customer that cares about superheat, subcool, deltas, wet bulbs, etc., they just want their system fixed and if all goes well never see us again. Now if this thread was about a refrigerant issue, then yes, we should be banging harder on getting more that just "pressures were good", but this thread is simply about a simple blower or board issue. And this HO is simply reaching out to the masses, more so our group that live with this stuff day in and day out, if this is a common issue and if we have come across this phenomenon, and if so what was the remedy. Sounds fairly benign to me and though not a DIY site, he seems fairly knowledgeable with what is going on and explaining it very clearly where as, so far, none of us really have come across this issue, thus the lack of constructive replies.

To the HO, I would avoid your everyday service company, I am not sure what you have available where you live as for selection of service companies, but I would suggest one that is a Lennox dealer, their techs would be more familiar with your furnace, since they install and maintain these things day in and day out, in addition as a dealer, they have a much better relationship with the local distributor for assistance to troubleshoot odd stuff like this. The way equipment is going now, it is much like the automotive industry, days of old where your local garage could fix anything is nearly gone, now a car must be taken to the dealership. The days of the everyday HVAC tech being able to reliably and knowledgably repair any make and model of equipment is, too, coming to an end.

 

To the HO, I would avoid your everyday service company, I am not sure what you have available where you live as for selection of service companies, but I would suggest one that is a Lennox dealer, their techs would be more familiar with your furnace, since they install and maintain these things day in and day out, in addition as a dealer, they have a much better relationship with the local distributor for assistance to troubleshoot odd stuff like this. The way equipment is going now, it is much like the automotive industry, days of old where your local garage could fix anything is nearly gone, now a car must be taken to the dealership. The days of the everyday HVAC tech being able to reliably and knowledgably repair any make and model of equipment is, too, coming to an end.

I had a third "HVAC Tech" come out. (there was no additional charge for the 2nd and 3rd tech calls). This one understood Lennox products. He was great to work with because we had a mutual respect for each other's knowledge of the subject. There was no competition to prove who was the smarter person. There was no ridiculing. We stayed focused on the key issue. After perhaps an hour or so of trying various tests and making various measurements, he discovered there was 26vac on the "W2" terminal on the control board. Obviously, there should be no voltage on the 'heat' connection when the system is supposed to be cooling. He disconnected the wire to the W2 terminal and the system now cools after power restoral following a power outage. The HVAC system was being told to heat and cool at the same time. That is what led to the strange behavior explained above which was that I had to turn on the heat first and then change the mode to cool to get the blower to ramp correctly from 0 to 1000 to 1640 to 2000 cfm.

I then disconnected the 'W2' wire at the thermostat end. After doing so, I measured the voltage being applied by the thermostat itself. That voltage was .5 volts. So the thermostat is not sourcing the 26 VAC to its W2 terminal. So now, with both ends of the wire disconnected, I still measure 26 vac on the wire. This is not due to induction because there is plenty of current available... so it must be due to a nail, staple, or perhaps two bare wires shorting together. I haven't taken the time yet to determine the source of the voltage and to fix that issue because the system is now working fine. I'll wait until the weather cools down some to troubleshoot further. I will need to figure it out before the heat mode is required.

A question was asked about the settings on the control board. All DIP switches are 'off' which is default and are appropriate settings for this HVAC configuration. And no links are cut on the Lennox control board.

While this was a very unusual problem, I am posting this resolution explanation in case someone else experiences a similar problem and needs ideas for what to consider.

 

pecmsg: He did include temperature measurements when he checked pressures. The '750' came from an LED readout on the control board. I know it is low and probably accurate because the evaporator coil froze up twice at the 750. Further, after getting the blower to behave as it should, I am monitoring the current to the HVAC unit (not including current to the compressor). The current from the blower is about 98% of the inside HVAC load. The current is being recorded along with the three blower volumes (1000, 1640, and 2000), inside and outside temperatures and humidity, and the duration of the cycles: Cooling and Standby. The duration and volume values correspond with table in the manual that shows the ramp up and ramp down air volumes as measured in cfm over time. The evaporator coil has not frozen up since I was able to get the blower to perform properly and the house is staying cool. All appears to be normal until a power outage causes the HVAC to revert back to 750. Because of that problem, I have the AC power to the HVAC system set up such that when AC power is lost, the system won't start up again until I run it through the restart process to ensure the blower is running properly.

I still need to find the actual problem. I ordered a replacement control board today. I hate just replacing parts to repair something, but without specific information and experience, that is all I've got.

Thank you for the support !

 

slctech: I agree... I believe they are doing their best to solve the problem, but they don't appear to be familiar with the products. I hadn't realized that I should have searched out which company would best support Lennox in particular. There are quite a few companies in this area to choose from. I am glad you mentioned that issue.

From the start, I figured this was a weird problem. I came here with the hope that someone might have run across it before. I was fortunate to have found a way to temporarily circumvent the problem by switching the system to heat for a minute and then to switch to cool.

I would like to know if it makes sense that when I have the S30 connected, not only is the display black, but also it doesn't show up as a WiFi server so I can't connect to it. I now have two S30's and they both exhibit that problem. All they have connected to them is 24vac and the 2-wire data stream. The AC power is getting to each one, but neither responds. That sure seems to be a control board issue.

 

That reading on the board is worthless. It’s what the motor is commanded to turn bad ductwork lowers at CFM reading.

 

pecmsg - So would that cause the blower motor to start out running slowly and not ramp up?

This HVAC system does not have a sensor to measure the actual air flow.

 

pecmsg - So would that cause the blower motor to start out running slowly and not ramp up?

This HVAC system does not have a sensor to measure the actual air flow.

Sorry
You MUST find a contractor that knows

 

OP may have electrical gremlins caused by the power outage.

I am still curious about the control board wiring and configuration of dip switched and thermostat.

 

Voltage need 2 points for a measurement

W2 and where was the other lead?

 

The voltage is measured against the 'Common' terminal of the control board barrier strip.

 

Since the 5 conductor cable from the HVAC unit and the thermostat, that is two stories up, now only has 4 usable conductors, I was able to use the four conductors to reconnect the original S30 thermostat. I used one of the "shorted-pair" wires as the + lead. That left me with three conductors: 2 for the data stream and one for the - voltage. The S30 has been working for about a month now. So it seems that back in December of 2024, the two wires somehow were shorted together. That caused the s30 to stop working. I assumed a bad thermostat/hub and purchased the Honeywell thermostat, which worked OK until cooling season came. Then the odd behavior of having to start the HVAC system in the heat mode luckily worked to recover from power outages. But all good now! Thank you for the constructive support.

 

So in that theory you have one skinned wire not being used and one skinned wire still being used ?

I would try and pull a new wire, make it like an 8 wire or something that has one or two extra for bail out purposes !

 

The two conductors are shorted together. As I measured the resistance between the two wires, I wiggled and pulled the cable from each end with the thought that perhaps the short was close to one of the ends of the cable, but the short never waivered. The 5 conductor cable goes through walls and at least one ceiling.... several bends. No way to pull in a new one other than to perhaps pull one in through the ducts. But actually, since only four conductors are needed for the S30, it's OK. If I do lose another conductor, I'll relocate the thermostat to the middle floor, which is the floor just above the basement where the HVAC unit is. I can access inside walls on that floor from below such that I will run something like an 8 conductor cable so that I'm not locked into a Lennox communicating thermostat.

Thank you for the suggestion!

 

The difficult takes a little while
The impossible takes a little longer.

Replace the wire!

 

Yeah... I could do it. I've done quite a bit of sheetrock work and electrical wiring. But cutting and repairing sheetrock and sprayed texture ceilings isn't really something I want to get involved with when I don't have to. If I do lose another conductor, I'll look into running a new cable through the ducts to get upstairs. I've done that before, but not at this house. But the location of the wall where the thermostat is mounted is not near a duct. I'd have to get into the attic to access that wall from the top. Too many angled and irregular walls in this house. And the routes the ducts take is not obvious.