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MattR757
MattR757, HVAC Technician
Category: HVAC
Satisfied Customers: 43
Experience:  18+ Years experience in residential and commercial HVAC service and repair
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Trane XE 90 is only blowing warm air and not getting the house

Customer Question

Trane XE 90 is only blowing warm air and not getting the house up to temperature. Therefore it's constantly running. Thee burners are working and the board code is blinking constantly with no error. Is the gas flow restricted or something inside the heat exchanger? I'm not sure why the air is not hot.
Submitted: 2 years ago.
Category: HVAC
Expert:  MattR757 replied 2 years ago.

MattR757 : Hello.
Customer:

Hi

MattR757 : Based on your description I suspect a duct issue. If your ductwork runs through an attic, basement, or crawlspace, or if the furnace is located in the garage, then air much colder than normal return air could be getting in through a disconnected duct. The furnace then can't generate enough heat to make that cold air as hot as it could if the return air was normal.
MattR757 : Take a quick visual inspection of the furnace and ductwork if you can and make sure all of the access doors are on the unit and no duct connections have obviously come apart.
Customer:

thank you for your reply, there is no sign of duct issues. the furnace is in the basement.

Customer:

the past few days the house temp would hover around 70 with a set point at 71. maybe get to 71. temp outside dropped to 15 last night and now i'm only getting 65 in the house.

MattR757 : If you have a thermometer you can take a temperature of the air at the furnace. Typically the supply air pushing into the house should be 40-50 degrees higher than the return air. If you aren't getting that you might have an issue with gas pressure or delivery. Also look for a big difference between return air temp at the furnace vs in the house which would be another indicator of duct leaks. Manysmall leaks can be as bad as one big one.
Customer:

i'll see if i can get a thermometer. what would be the cause of bad gas pressure or delivery?

MattR757 : Do you have natural gas or propane?
MattR757 : Apologies for my slow responses.. My internet connection is dreadful today. Gas pressure is regulated at least twice between the furnace and the source. Any one of the gas regulators could be malfunctioning, especially any that are outside in very cold.weather. Tiny amounts of moisture can freeze inside the regulator affecting the pressure on the diaphram. Low source pressure is less common with naturel gas than propane. Its also possibe that the furnace is too small. Every house has a "balance point" - The outdoor temperature
MattR757 : at which the building looses heat at the same rate the heat source can add heat. Below that temperature the furnace will run all the time and the house slowly gets colder. This same phenomenon is seen more o
MattR757 : seen more commonly in the summer when the AC can't keep up on very hot days.
Customer:

we have an older house with natural gas. the furnace has been ok all winter and previously. i did just have a gas line plugged and moved for my gas dryer. we moved the dryer. the new tap point is closer to the furnace. would that have changed the gas inlet pressure to the furnace? being a closed system, i would not think that's the case. i shut off the valve to the dyer and no change. i'll have to go out for a thermometer to check the inlet and outlet temps.

MattR757 :

As long as the change to the gas line didn't change the size of the piping, it shouldn't have any effect on the gas to the furnace. It is possible however, that the tee installed in the line is creating a restriction in the original pipe size. There are two different ratings for gas systems - pressure and volume. You could have adaquate gas pressure but if the line is too small - even in one point in the line - it will reduce the volume of gas that can pass through. (Sort of like a 3/4 water hose can carry more water than a 1/2 water hose even at the same water pressure.)

MattR757 :

Say your furnace is rated at 75,000 btu's - you get 1000 btus from a cubic foot of gas, so the line has to be sized to carry 75 cubic feet of gas per hour at service pressure.

Customer:

a tee was inserted but the original line was not modified.

MattR757 :

Generally speaking, I wouldn't suspect the gas line modification to create much of an issue.

Customer:

i agree, that's why this issue seems odd.

Customer:

could there be an issue with the solenoid valve not opening completely?

MattR757 :

The temperature readings will tell you a lot. Again - If you are getting big temperature differences between the air in the ducts right at the furnace and the air in the house at the registers then the issue is likely a duct leak issue. If those are close, but you're not getting a 40-50 degree difference between return air and supply air that means the furnace isn't performing to it's rating.

MattR757 :

I can't rule out a problem with the gas valve in the furnace if it's underperforming.

Customer:

if there are no leaks, is there an issue with the exchanger? does something need cleaned?

MattR757 :

If the issue were a problem with airflow, say a dirty AC coil or a restriction somewhere, if would cause the outlet air to be hotter than normal (usually more than a 60 degree rise) and the furnace would probably be cycling on temperature limit - You would see the furnace blowers running, but the burners would go out for a time and then relight without the blowers stopping. -

Customer:

ok, i see. the burners and blower are constantly running. the burners are not shutting off. that's where i thought it may not be getting enough gas flow or the heat is not transferring to the air effectively.

MattR757 :

I don't think I've ever seen an issue where the heat exchanger itself wasn't transferring heat. They taught us in a class (about a hundred years ago) that about 80% of all hvac issues are related to airflow.

Customer:

looks like i got 60 on the inlet, 78 outlet, 65 in the house and 21 outside temp.

MattR757 : Ok so you're only.losing 5° between the house and the furnace - That seems pretty normal. An 18° rise is really low. I'd say pretty confidently that the furnace is de-rated for some reason. You probably need someone to come test to make sure inlet gas pressure is constant and within rating and that manifold gas pressure is correct. I think we have eliminated just about all other options.
Customer:

so we are getting gas but maybe not enough to heat at a proper level? about how much should a service like this cost?

MattR757 :

Something else occurs to me... (Yes - I think about these things - I have no life) - When we put pipe together we usually use a thread sealant that's kind of a paste - If some of that got inside the pipe it could have made it's way to the furnace. There is a screen at the gas inlet of the furnace gas valve that quite possibly could be plugged up with thread sealant.

Customer:

ok, that was a thought of mine as well

Customer:

i guess i would have to remove the inlet pipe to the solenoid valve to check that though.

MattR757 : If we're talking about checking pressures and maybe cleeaning the screen strainer I wouldn't expect it to cost much more than the going rate for a service call. If it needs a new gas valve you're probably looking at another 150.00 or so depending on model.
MattR757 : and yes. Have to turn off the gas and remove the piping into the gas valve to see if there's a white pasty stuff stuck in there.
Customer:

ok, i think we may have ruled everything out we can here. thank you for your help.

MattR757, HVAC Technician
Category: HVAC
Satisfied Customers: 43
Experience: 18+ Years experience in residential and commercial HVAC service and repair
MattR757 and 6 other HVAC Specialists are ready to help you
Expert:  MattR757 replied 2 years ago.
Thanks and good luck. You really challenged me on this one and made me exercise my "out of the box" thinking. If you don't mind, I'd be very interested to know what you find.
Customer: replied 2 years ago.
Got a guy over here to take a look at the furnace. The strainer to the gas valve was clear but the meter was showing only half consumption at the rated unit 60K. He thought I should call the gas company to check the inlet pressure. They came and confirmed the pressure was adequate. Now there must be a restriction at or after the valve.
Expert:  MattR757 replied 2 years ago.
Apologies if I've led you down the wrong path. You really have a very uncommon situation - It's pretty rare to develop a problem with piping. I'd start with the new tee - Maybe the tee outlet is blocked with thread sealant instead of the valve screen... Also be looking for any place where the pipe itself might have been bent - Depending on the pipe material it could be kinked somewhere restricting the flow.
Customer: replied 2 years ago.
No clog in the tee. We removed that when inspecting the valve strainer. There seems to be nothing wrong in the piping? 7" WC in the line. No drop after turning on the stove, furnace and water heater.

Would the valve not be fully opening? Do they fail in that manner? The Trane manual shows the part as VAL 4335. The sicker on the solenoid is a 36E98-205. I'm having a tough time finding info on either.

Just this:

Models 36E98-205
Supplier White-Rodgers
Characteristics Slow
Pressure Settings 3.3 in W.C.
Convertible Yes
Valve VAL-4335
Spring KIT-1401
Spring KIT-1402

Is there a method to test the valve?
Expert:  MattR757 replied 2 years ago.
The only test for the gas valve is manifold pressure. If everything else checks out ok then the outlet pressure of the furnace gas valve should read a steady 3.5 Inches W.C. (which is roughly 1/10 of 1 psi) - I have seen valves fail where they restricted gas flow without stopping entirely - This would be caused by a mechanical failure of the moving parts inside the valve, not an electrical failure of the solenoid.

The valve is made for Trane by White Rogers so any gas valve that replaces the White Rogers 2 wire model would work as long as the piping connection sizes are the same. I'm pretty confident now that the furnace gas valve is your issue here - Again - very rare problem... Apologies for how long it's taking to figure this out.
Customer: replied 2 years ago.
Thank you Matt. I appreciate your input.

The gas inlet is 7 inches wc. Does that valve turn down the pressure that much?

Like I said, I had someone at the house look at it with unsatisfactory results. I have not been able to get a hold of him since I confirmed the inlet gas pressure with the gas company. Hen mentioned it might be beneficial to not invest in that valve and think about a new unit. that seems crazy but I have not been able to price out the valve. It has an inlet and outlet threaded pipe and two wire connections...pretty simple change out.

I want to bring someone else in but want to make sure they are able to do the proper diagnostic on the valve since that seems to be the case. Would you hook up a pressure gauge to the outlet side of the valve and make sure it's reading 3.5 to test and confirm?
Expert:  MattR757 replied 2 years ago.
Right, A gas pressure gauge measures in very small pressure range and should be able to test the outlet pressure to the gas manifold. If you're sure you have at least 7" inlet pressure and the outlet pressure is not 3.5 then it is for sure a bad gas valve. There should be a small plug you remove with an allen wrench at the outlet side of the valve to test manifold pressure. The furnace gas valve has a little regulator built in that steps down the pressure from inlet pressure (which should be between 5 and 11 in.w.c) to the 3.5 in.w.c for proper operation.

I wouldn't suggest a new furnace unless your current one is more than 12 years old or so. Again - I can't really assess the overall condition of your furnace so it's hard to comment on that with conviction. The best I can offer is this: Would you buy your current unit if someone was selling it for the price of the repair?

I just did a gas valve locally and the total invoice was $380 for service call, parts and labor. I don't know the labor rate where you are but it should give you a point of reference.

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