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kb3ca 11-13-2010 07:06 PM

Manifold pressure setting on new furnace
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.

beenthere 11-14-2010 05:25 AM

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.

kb3ca 11-14-2010 08:26 AM

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?

beenthere 11-14-2010 08:34 AM


Originally Posted by kb3ca (Post 533989)
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.

yuri 11-14-2010 08:39 AM

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.

beenthere 11-14-2010 08:43 AM

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

yuri 11-14-2010 08:59 AM

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:

beenthere 11-14-2010 11:02 AM

Garlic yes. Vodka no. LOL

yuri 11-14-2010 11:43 AM

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kb3ca 11-15-2010 12:22 PM

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?

kenmac 11-15-2010 01:33 PM

I would set to within factory specs.

beenthere 11-15-2010 03:24 PM

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.

yuri 11-15-2010 04:41 PM

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!

kb3ca 11-15-2010 04:47 PM

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.

beenthere 11-15-2010 05:03 PM

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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