Tankless Water Heater gas line question

[chetwynd]

Hi Guys,

I've got some pretty solid plumbing experience but it's been a while since I've done much with it, and I'm running into a problem with the installation of a tankless water heater.

First issue is that the flow meter on the remote control for it (Takagi, nat gas) doesn't show the flow that it should when we turn on the faucet. Maybe the flow sensor...but, that aside.

We ultimately determined that the gas line we have running to it isn't big enough. Given the distance and BTUs, we need a 1" line going to it. See the attached image. As you can see, it starts at the meter as 3/4" (the only outlet from the meter is 3/4"), then goes into a long 1" pipe, then drops back down to 3/4".

It basically needs to be 1" the entire way. BUT, it seems to me like unless the outlet on the meter is 1", then the initial 3/4" pipe the gas travels through would bottleneck there and make the 1" pipe beyond it basically useless...no? Meaning, if only 3/4" worth of gas capacity can feed the 1" pipe, doesn't that mean the 1" pipe isn't capable of moving as much as as if the meter outlet was 1"?

See what I'm asking?

Thank you.

 

[havasu]

Hopefully someone will chime in soon that for every 90 degree elbow you use, you lose x amount of volume. I don't know the formula though.

 

[CT18]

You need to get someone who can calculate the volume you have and what you need. I dont have access to that anymore but there are charts.

 

[Mitchell-DIY-Guy]

In my old neighborhood, the gas line pressure at the exit of the meter was, I believe, 1 PSI. All gas piping in the home was black iron; 1" from the meter, branches off that for the appliances, generally ¾" but some ½". That's old school. When some neighbors had either pool heaters, or tankless water heaters put in, there was not enough gas being able to be pushed through those lines: so they had to call the gas company and upsize the meter, and upsize the lines to the "offending appliance". That seems to be the situation you are facing.

NOW I live in NC. Here they have hybrid gas systems. I believe that the output from the meter is 2 PSI. That goes to a black iron manifold, with a 2 PSI to 1 PSI regulator on it. So, now there are two sides to this manifold: 1 and 2 psi. The high volume devices pull off the 2 psi side, the low volume off the 1 psi side. Soft copper, not black iron, is used for the runs. Each appliance has a plumber installed regulator IN ADDITION to whatever regulator may be inside the device.

So, right now I have a 50 gallon conventional hot water tank, which is about 40,000 BTU. If I switch to a tankless, say 195,000 BTU, I'll have to move the gas line from the 1 PSI side of the manifold to the 2 PSI, change the regulator. Then, the gas piping will be able to deliver the gas needed by the tankless.

See the attached. You may not be able to solve your issue w/o upsizing your meter or similar.

 

[fixitron]

FWIW, any fluid (liquid or gas) traveling down a pipe or duct will experience friction with the pipe/duct and lose energy. Since fluids move by either gravity or a pressure difference, in this case the gas pressure will drop, as will the flow rate. NFPA 54 has gas sizing tables that show that for a given size of pipe, the BTU/Hr capacities decrease as the length of pipe increases. Different tables are for natural gas or propane, different pressures, and different piping materials, keeping in mind that every fitting equals a certain number of additional feet of piping. The 1" pipe is not useless. The capacity would be calculated starting with the capacity of the 3/4" pipe at 10 ft., then look at the capacity of the 1" at however many feet (or equivalent feet). Your IOM for your water heater will also have a table for how far you can run exhaust piping, with various configurations and vent pipe sizes, based on the same physics.

 

[breplum]

The issue of bottleneck: Don't worry about running a 1" off of a 3/4" meter stub. The pressure drop issue is over distance.
We have done hundreds of tankless WH gas runs (PG&E), and start with basic 3/4" meters (AC350).
Technically, you should notify your gas supplier of the increased demand because in many cases, esp. in homes with other demanding appliance loads, the utility will want to provide a bigger meter and wants to keep updated on utility loads in the specific neighborhoods.
Imagine as people go tankless, it can really affect peak demand so that is why they want to know.