# Manifold pressure setting on new furnace



## kb3ca (Aug 22, 2008)

Just had a new Trane XV95 natural gas furnace installed. The installer's guide calls for a second stage max pressure setting of 3.5"W.C. but the installer used his manometer and changed the factory setting to 4.0"W.C..
Will this have any adverse effect on the operation of the furnace? Will it use more gas with this higher setting? Should I make them change it back to the factory setting? The gas valve is a White-Rogers.
Appreciate your comments. Thanks.


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## beenthere (Oct 11, 2008)

Thats past what the factory allows.

I'd have them come and reset it. And check what the pressure is in first stage also.

Too much gas pressure can burn out the heat exchanger.


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## kb3ca (Aug 22, 2008)

I'm going to have them check both stages when they come back. 
Does the higher pressure setting cause the furnace to burn more gas?


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## beenthere (Oct 11, 2008)

kb3ca said:


> I'm going to have them check both stages when they come back.
> Does the higher pressure setting cause the furnace to burn more gas?


Yes, it can. Since the heat exchanger is only design to get so hot. The air being blown across it won't pick up as much heat as the heat exchanger is trying to give off. And the extra heat will just be blown out the flue pipe.


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## yuri (Nov 29, 2008)

Too much gas pressure will SERIOUSLY damage the heat exchanger and will overheat it and void the warranty. They do inspections on failed heat exchangers and can tell that by the color etc. Will also trip the flame rollout/spill switches in the burner box. I would ask him why he did this in the first place.


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## beenthere (Oct 11, 2008)

It was easier then reading the instructions on how to set the DIP switches for proper temp rise.


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## yuri (Nov 29, 2008)

We must be psychic twins, ever get the craving for garlic or vodka? :laughing: I wuz thinkin he did it to get a hotter supply temp as he may have got complaints from other customers and decided to be pre-emptive. Trane is goin to luv him in the future.:yes:


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## beenthere (Oct 11, 2008)

Garlic yes. Vodka no. LOL


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## yuri (Nov 29, 2008)

Ukrainian


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## kb3ca (Aug 22, 2008)

I contacted my HVAC company about changing the manifold pressure and here is their answer:


If you would like your gas pressure set back to 3.5 that is fine and we will. But, it lowers your temperature split, produces more CO, and will end up using more gas therms. The higher gas pressure is much better all the way around. The reason the factory setting is 3.5, is so that they furnace is not over fired. But when we raise the pressure, we check the temperature split to be sure that you are still with in the recommendations there. If you are then we run them al at 4.

They claim the furnace is more efficient at 4 because the heat exchanger gets hotter and the furnace ends up running less using less gas overall even though it is using slightly more gas when the burners are on. They said TRANE heat exchangers are built really well and can tolerate the slightly higher heat than some cheaper models.

Any comments? BTW - My previous furnace was a TRANE XE90 and the pressure was set to 4 on that one by a different HVAC company and never had any problems with it and it was 14 years old.

The reason I asked the question in the first place was to try and get a handle on gas consumption. So, what do you guys think?


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## kenmac (Feb 26, 2009)

I would set to within factory specs.


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## beenthere (Oct 11, 2008)

Then ask them for a letter from Trane that says exactly what they said. And tell them you would like it on Trane letter head.

Also, you would like to see the combustion analyzer print out from the readings they took at 3.5" and the read out at 4" to prove that they are correct.

They are BSing you. All they have to do is set the blower for the temp rise they want. Not a increase gas pressure input.

Trane rated that furnace at X BTU. Not for what ever they think they should make it.


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## yuri (Nov 29, 2008)

EXACTLY. An experienced tech can set the proper temp rise with adjustable DIP switches on the circuit board. Trane programmed that circuit board for a specific temp rise to not melt their heat exchangers and plastic collector boxes. Overfirng it defeats their programming and can definetly void the warranty later/good luck when they are out of business. I'm sure GM and Ford allow their field techs to alter the fuel mixture/computer settings to what ever they feel is good, Yah right!


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## kb3ca (Aug 22, 2008)

I'm certain they didn't do any combustion analysis, just operating from their experience. They are coming in the morning to replace the O.D. sensor which is way off and I will have them reset the pressure to 3.5 and check the temperature split, adjusting the blower speed if necessary. Right now the blower is set to med/low for both stages of heat so can't go much lower and effectively push the air around.
I'm a little concerned about the accuracy of their manometers. Do they require periodic calibration or are they always accurate to within + or - some value?
I'll have them check both stages and set to factory specs.
Thanks


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## beenthere (Oct 11, 2008)

If they didn't use a combustion analyzer. the they have no way to know if its producing more or less CO at 4" then at 3.5".

Normally. First stage should be at a very low speed. And second stage at a slightly higher speed.

Setting second stage to 3.5", may allow them to speed up the blower.

In heat mode. You really don't want the air blasting out of the registers. if they are in the floor.

Also. What size is your furnace?


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## yuri (Nov 29, 2008)

Manometers if properly taken care of in a padded case and not bounced around loose in a service truck should stay accurate. Few guys own a water tube one/ I don't but they are the most accurate. I would be a bit uncomfortable with their attitude and the way they setup your expensive furnace now. May want to get a 2nd reliable co to check it out later. It is a very complex procedure to check temp rises/adjust blower and I would meter clock it on all 3 stages to make sure it is not overfiring. There is a special procedure in the Trane manual for setting the manifold pressure and I hope they follow it.


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## kb3ca (Aug 22, 2008)

The furnace is a XV95 two stage unit 100K BTU at 95% efficiency. The service data indicates that first stage heating with blower on Medium Low the CFM is 971, temp rise is 58 at 0.1 static pressure. Second stage is 1464 CFM, temp rise is 59 at 0.1 static pressure. These numbers are probably based on the recommended manifold pressure of 3.5 and since mine is set at 4.0 I don't know what the temperature rise is on either stage of heat but we are going to find out in the morning.
Changing the blower setting to Medium would increase the CFM to 1146 with temp rise of 49 in first stage and CFM to 1631 with temp rise of 53 in second stage. FYI, the Medium speed is factory setting so they must have changed the blower setting while adjusting the furnace during the install. The temperature rise range for this furnace is 35-65.
Beenthere - Is this what you were asking for in your previous post?


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## beenthere (Oct 11, 2008)

Yes.

Trust me. Your furnace is operating against a lot more then a .1" static pressure.

Your air filter. Even if it is a cheap see through fiberglass one, will have a PD of .07" min.
Your supply registers will have a min of .03" PD. And your return grille will also have a min of .03" PD. 

Thats .13" static, and I didn't include any of the duct work yet. Or an A/C coil, if you have A/C on the furnace.


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## kb3ca (Aug 22, 2008)

Yes, it does have an A/C coil. I just gave you the first set of figures listed for static 0.1 to give you an idea of the difference between MediumLow and Medium blower settings. It does list CFM and temperature rise for static pressures of 0.1, 0.3, 0.5, 0.7, and 0.9. 
Since they set the manifold pressure to 4.0 I would think that the temperature rise at any given static pressure would be greater. So, why would they lower the blower speed to med/lo instead of leaving it at the factory setting of Medium?
Should I set the blower to the factory setting?


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## beenthere (Oct 11, 2008)

kb3ca said:


> Yes, it does have an A/C coil. I just gave you the first set of figures listed for static 0.1 to give you an idea of the difference between MediumLow and Medium blower settings. It does list CFM and temperature rise for static pressures of 0.1, 0.3, 0.5, 0.7, and 0.9.
> Since they set the manifold pressure to 4.0 I would think that the temperature rise at any given static pressure would be greater. So, why would they lower the blower speed to med/lo instead of leaving it at the factory setting of Medium?
> Should I set the blower to the factory setting?


Need to check actual temp rise first.

Might want to wait until they reset the manifold pressure back to 3.5" then see what temp rise is and adjust it then if need be.


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## kb3ca (Aug 22, 2008)

My HVAC guy came this morning to replace the O.D. sensor and to check out my furnace manifold pressure. While he was here I had him check the starting and running amps on the XL15i condenser because I suspected it was drawing a lot more power when running in the heat mode versus the cooling. The starting amps in heat mode were 25 and running amps were 12.5 which is well within the specs for the unit. He did a manual defrost and the running amps were 9.0 without the condenser fan running and it is rated at 2.8 amps so looks like all is well with the heat pump.
Next, I had him check the manifold pressure and temperature rise on the XV95 two stage furnace. Manifold pressure in first stage was 1.2" and second stage was 3.3" which is within specs for this furnace. He said his gage reads a little low. The temperature rise in stage two was 60 degrees. The range for this furnace is 35-65 degrees and I thought it was a little too close to the upper limit so I had him change the dip switches on the blower to run on Medium speed which is the factory setting instead of MedLow which it was set to during installation. After increasing the blower speed the temperature rise was 47 degrees and I'm leaving it there for now. Air temperature out of the registers is nice and warm and everything seems to be set up correctly.
Want to offer my sincere thanks to all who responded to my post and provided good advice on this matter. I feel good now that I have confirmed that everything is operating properly.


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## beenthere (Oct 11, 2008)

Glad you recognized that 60 was too close to the upper limit.

I shouldn't say this. it might make you wonder too much. But. I wonder how he knows his gauge reads a little low. And how he knows how low a little low is. To know if your manifold is set right.

Next year. When you have it serviced. make sure the tech uses an accurate manometer to check it.


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