Gas piping

4Johnpipe

At A Loss

Rob I thought you were quoting something a manufacturer rep told or instructed you about the Navien. My bad I assumed Navien to have a 1/2" gas inlet. Rinnai does not. The equation I posed earlier was based on an available 7.5" of water column at the connection to the gas. An example would be a run directly from the gas meter.

Example: If we have 7.5" WC at the meter and the appliance requires 5" the 1/2" iron pipe has a .067" WC pressure drop per foot at 200K BTU's ~~ .067" WC x 24' of pipe = 1.608" WC pressure drop in that section. Take the starting point in this case of 7.5" WC subtract the 1.608" WC = 5.892" WC delivered to the appliance. That said I am full aware of velocity issues with negative pressure combustion chambers and would not install smaller than the opening of the gas valve.

PS quite a few boilers (at least natural draft) I install for residential have a 1/2" gas connection.

4Johnpipe

Job Example

Take a look at this job. This was submitted with the permits. The 2" was existing but ended up needing to be changed anyway. The worksheet shows pipe type, BTU's, section pressure drop and delivered pressure. This may clear it up. This was all rigid pipe.

Ironman

Tim

Any word back from Navien?



We know they monitor this site. Why don't they reply?

Tim McElwain

No I have nopt heard from them

as of today. I will be busy with classes starting tomorrow so will not be able to pursue this.



As for why gas valves and other hookups to appliances are a certain size say 1/2" or 3/4 " it has to do with several things one of which is related to ANSI Standards testing. All gas valves have charts giving you the allowable pressure loss through the gas valve at various sizes ex a gas valve 1/2" x 1/2" has a different loss than a valve 3/4"m x 3/4". Most of us in the gas industry allow for a 1" pressure drop through the gas valve when sizing. That is in most cases much more than is needed in fact a Honeywell VR 8300 A 3/4" x 3/4"  24 volt, standing pilot valve does not reach a 1" drop until it is fired at 200,00 BTU's or more and then the drop increases as you go up in BTU. So the 1/2" x 1/2" size assumes (manufacturer assumes) the installer will know enough to supply adequate pipe size to that 1/2" valve to make sure the burner fires at full input all the time.



Think about this most meter bars are 3/4" so does that mean we run 3/4" pipe to all the appliances. I hope not and in fact you may see that increased right at the point of delivery to 1 1/4" or 1 1/2" pipe.