Gas and Oil Home Heating Furnaces - Trane XL90

Hi
First, thanks for any help you can offer. I have a Trane XL90 that is coming on often enough to keep us from freezing, but it doesn't light consistently when it should. When the thermostat sends the "go" signal, the furnace clicks, and then . . . nada. So far though, it doesn't usually let the temp drop more than 4-5 degrees below the set level. We're worried that's about to change. Any ideas on what the problem might be? Anything we can change, clean, try, . . . before interrupting a technician's holiday fun? Thanks.
Em

A 'real' techs holiday fun would be repairing your XL-90. I don't think there is anything you can do but others here may have some info to offer. If it is intermittent, it may be difficult to pinpoint the source of the problem. It is very helpful for you to gather as much info as possible to relay to the tech when you finally need to call one.

It could be a problem related to gas pressure or ignition. Neither has any parts you can check yourself. If it lit every time and went back off, you might suspect the flame sensor and you can clean that yourself with steel wool.

Ken

This is Emily's significant other. Could someone explain the basics of how this things starts up? The thermostat sends the signal, the gas flows, the igniter goes, and . . . What are the steps? We don't hear or smell any gas. Does that suggest one problem over another? Any other ideas?

By the way, for those of you considering parenthood, furnace woes are a much bigger problem now that we have two babies!!

So much for the "grab an extra blanket and worry about it in the morning" response. :)

Mike

A typical start up sequence is as follows:

The thermostat senses the need for heat and sends a call to the furnace – usually by placing a voltage on a control wire that leads from the thermostat to the control board in the furnace.

The furnace receives the signal and the sequence beings

The exhaust (draft inducer) fan which is used to exhaust the combustion gases and induce gas and fresh air for combustions begins to run. The successful operation of the fan is sensed by a pressure switch and reported to the control board.

After a preset period the igniter is activated. The warm up period of a hot surface igniter is much longer than one using sparks.

While the igniter is running the gas valve opens and gas begins flowing out. The gas then encounters the hot surface or the sparks and ignites.

A flame sensor senses that the flame is present by passing a current through the flame. This is reported to the control board that ignition was successful.

A while later the main blower comes on and air begins circulating.

If the internal thermostat senses the temperature inside the furnace is too high or the thermostat senses desired temp the furnace will be shut down. Typically the fan will continue to run for some time after the flame and exhaust fan have shut down.

If any problems are encountered at any point during this sequence the furnace will abort and re-try. If the problem keeps happening usually it will give up after a certain number of attempts.

excellent post rav

i would add that since it is a 90% that the flue is a drain and improper fall or sagging could cause lockout as well.

Hey all
Problem solved -- bad relay -- back of board burned -- could smell it. Tech is coming back in a few hours to replace. Any guesses on what the bill will be? :)

Thanks for the help everyone. Tech just left -- bad relay/board burnt on back. He went to get a new board. Anyone want to guess what the bill will be? Prize for closest guess: pride, and DIY.com bragging rights. :D

with me you would be looking at around 250 total unless the board is under warranty. i am the most reasonable guy in town tho!:)

Thanks hvac4u. Let me tell you what I did. One of the contacts on the board was burnt, so after the tech left, I put a little solder on it and VOILA! -- we have heat! So I called the company and they're sending me the bill for the visit -- about $115 or so.

They told me the cost of the board would have been $200, and that the total would have been $430. Seems a little excessive to me, especially when the problem was one poorly soldered connection. Is it standard operating procedure to replace these boards in such an instance? Also, if this fix doesn't hold, where can I order the board? Thanks all.


Mike

Well done!! Yes, to be honest in many cases the whole board does not need to be replaced. It is very rare for the whole board to fail - usually it is only one component. However, to troubleshoot these you normally need electronic tools such as oscilloscopes and a good understanding of the micro controller sequence which many techs may not have. So to save time and money they will replace the whole board and also charge you for this.

You should be able to buy parts such as control boards and other furnace parts online and replace them yourself if you have the confidence to do so.

in my experience the fix may not last. if it fails again get one online

Yes, quite right - the fact it was burnt indicates some underlying problem so you will probably be better off replacing the board anyway at some point in the future. Components that have failed can be replaced. However a faulty PCB is harder - maybe even impossible - to fix. Burnt boards for example could indicate tracks or contacts that are shorting - these are very hard to find and fix.

My sincere thanks guys.
I've been looking around online for the part, and I found it for $331 at one site and $170 at another -- hmm. Anyone know of a site with good selection and prices? It's actually harder to find this thing than I thought it would be. Also, if I want to check for it locally, and get the best price, what would I look under in the yellow pages? I assume I should go directly to the supply house to get the best price . . . Thanks again.

Mike

You can see if you can buy from Johnstone supply or Grainger if you have them there.;) ED

Unfortunately furnaces are not generally considered DIY items so the number of sources for buying spare parts is probably less and so you have less choices.

One site if you have not already visited you can try is heatcoolparts.com They seem to have a good selection although I don't know if they have what you want.

being a trane, i doubt you can get it locally. the trane suppliers are VERY protective of their dealers.

It seems that the relay that had the burned contact turns on the exhaust motor. If that's the case, how can I test whether that motor is drawing so much current that this is going to keep happening? I have a Kelvin circuit tester that I'm somewhat familiar with. Also, since the relay seemed to be working (it seemed to be clicking appropriately), that suggests to me that either the contact on the board went bad because of a poorly soldered connection, or the exhaust fan circuit burned it up because the fan was drawing too much current. Am I thinking about this right? Thanks again for the help.

Your reasoning on this problem is quite good. The contact is unlikely to burn due to a bad solder connection. Excessive current could cause this as you concluded. A motor with shorted coils could cause excessive current draw. This is certainly a possibility. You can check the current draw by connecting a multimeter in series with the motor and measuring current draw on the current setting. The motor should have a current rating plate and you can check to see if they tie up.

Is the burnt contact on the primary or secondary side of the relay.

Another thing that could cause this is an internal short on the circuit board. If the board is a multilayer board the supply and ground planes will be sandwiched internally. You could never see this.

Thanks rav12
How do I tell which side is primary and secondary on the relay? Also, when you say wire the meter in series, you mean disconnect the + wire from the motor, connect this wire to one terminal on my meter, touch the other meter terminal to the + terminal on the motor, and turn on the furnace. Is that right?

Thanks again for your help.

Mike

It may not be easy to see which is the primary and secondary side of the relay by just looking at the circuit board. You should have the original installation manual which should include a schematic of the relay. The terminals should be marked (letters or numbers) which will allow you to correlate the schamtic to the board. Are you good at reading circuit diagrams? The point I was getting at was to see if the problem was on the motor side or the control side. If the the burnt contact is on the control side it rules out a faulty motor.

To connect the meter in series you need to disconnect one of the motor terminals (can be + or -) and connect this to one of the probes. You then take the wire that should have gone to the motor form the control board and connect it to the other probe. Switch the meter to measure current, turn on the furnace and take the current reading. Exercise caution as the voltage will be 120V.

Thanks rav12
I'll give it a shot tonight or tomorrow and let you know what I learn.
Mike

I can't tell what the relay that had the melted connection does. There are two relays on the board, and both complete their respective circuits when the signal comes from the t-stat. Then the exhaust fan starts.

When I tried to measure the current that the fan draws, the fan didn't start. I had the tester wired in series, set to the appropriate setting, etc. The furnace didn't start immediately after I hooked it back up and sent the signal from the t-stat either. Perhaps it was in some resting mode of some sort? It came back on shortly after that and has run well since. Hmm.

The fan has three wires to it -- I assume two of different speeds and a ground -- red, yellow, black.

I checked the solder job I did and it still looks good. Is there any way for me to figure out what that relay does? What do they normally do on these boards?

Thoughts?

Thanks, Mike

Mike,

If you can scan or somehow get a copy of the schematic of the circuit board and either post or put it on a web site where we can see it I can give you an idea of what the different bits are doing. You should find the schematic in the installation manual. Also tell me which relay is the relay in question.

No idea why the fan was not starting. The meter in series will have very low resistance and should not cause sufficient voltage drop to cause problems. Meters are designed to measure current without causing problems.

Normally relays are used to allow a small voltage to control a large voltage. Relays or transistors can be used. Transistor solutions are normally called "solid state switches". When we use a transistor to convert a small varying current to a large one we have what is commonly called an amplifier.

Thanks rav12
I have looked through the user's and installer's manuals, and there are wiring diagrams for the unit, but no schematic for the board. Would this schematic be right in the installer's manual, or would it have been an insert of some sort? Perhaps it's lost. I found a few companies that sell the board. Maybe I'll see if they can email me a schematic of one. Thanks.
Mike

Hmm..I think you're right. Now that I think about it I don't recall seeing one for my furnace. I think I was confusing it with the system wiring diagram.

Two possible ways to proceed.

1. Contact Trane and see if they will provide a circuit diagram.

2. Do a bit of investigation yourself. Remove the board and look at the pins of the relay. They should connect to copper tracks at the bottom of the board. Trace through and see where they go. Make up a little diagram of what you see. We should be able to figure it out from that. Hopefully the circuit board is not a multilayer one with internal signal planes. If this is the case we need the circuit.

I'm guessing here but one end of the relay should go to something being controlled like the fan. The other end should connect to either a microcontroller or microprocessor - a chip in other words (black thing with silver legs). The connection may be through a driver circuit - another small chip or a resistor/capacitor network (for filtering etc).

This is another way to try to figure out what is going on.

Thanks rav12
I'll let you know what Trane and the merchants say.