# Calculations for tankless water heater gas supply

### Users who are viewing this thread

#### Sluggo

##### Member
I'm considering switching to a tankless gas-fired water heater when my conventional gas-fired water heater starts to die. The tankless models I've been looking at require about 155K Btu/hr, compared to 75K Btu/hr for the water heater I have now, and there is another load of 95K Btu/hr in the same location for my hydronic boiler. So I am looking at a peak demand of about 250K Btu/hr in this location. My main concern is that I have a long run of pipe from my meter to this location...about 90 ft. The piping at the meter starts out as 1-1/4" but, after about 60', it reduces to 1", which is what I would be hooking up the tankless water heater and the boiler to. My meter is sized to theoretically supply all of the gas demands in the house, but I want to make sure that the 1" supply line is big enough over this distance to deliver at least 250K Btu/hr to the tankless heater and the boiler. My power company told me they supply gas at 6.0" WC, but they weren't able to answer my question about the line sizing. Does anyone have familiarity with this who could either answer this question or refer me to a calculator or table that would be useful? Thanks.

#### Jadnashua

##### Retired Defense Industry Engineer xxx
The total pipe length needs to be figured as the equivalent length...any elbow makes it 'appear' to be longer. This calculator seems to say that size pipe on an equivalent length of 90' is about 159K BTU. If you just google gas pipe sizing chart or gas pipe sizing calculator, you'll get several ways to check out the capacity and how to determine the equivalent lengths. Here's one...

http://www.engineeringtoolbox.com/natural-gas-pipe-calculator-d_1042.html

#### Sluggo

##### Member
Thanks. I suspected it was in that ballpark, but I saw various tables that used different pressures drops and I wasn't certain what was what. This also tells me that the current demand I have now - 170K Btu/hr - is pushing it already! It looks like if I had 1-1/4" all the way, I wouldn't have a problem. Is there any leeway I have since 2/3 of the line is 1-1/4" before it drops down to 1"?

#### Jadnashua

##### Retired Defense Industry Engineer xxx
A short section of pipe may not have a huge overall flow or pressure drop (Venturi effect), but once the run stays small far enough, that is what limits the flow. I think you'll find the length of pipe after reducing will drop your pressure and volume. If you assumed you had enough volume where that reduction was made, rerun the calcs using the smaller pipe diameter and length and see what happens.

#### Sluggo

##### Member
Thanks for another helpful response.

#### hj

##### Master Plumber
There is a specific way to apply a gas piping schedule, and many people do not know how to do it.

#### Jadnashua

##### Retired Defense Industry Engineer xxx
Another thing to consider...assuming that you may want actual hot (or even just warm) water from a vanity sink, consider that tankless systems (that don't incorporate a buffer tank) first, have a minimum volume before they will actually turn on, then, the burner and heat exchanger doesn't reach full operation immediately, then, that cold water sitting in the supply line must be purged before the hot water will ever get to the fixture. If that distance is very long or the diameter of the pipe is moderately large, it could literally take minutes to get hot water there since it also takes some time to warm up the pipe.

A tank and a recirculation system will typically mean nearly instant hot water and less wasted water down the drain and may also add to your sewage bill. A well designed tank system with recirculation can have similar energy use, and might actually be less expensive to operate if that's a major factor to you. Size it properly, and it can still give you many of the benefits, with lower installed costs, and easier, nearly universal availability of service options.

### Similar threads

Replies
19
Views
548
Replies
0
Views
118
Replies
7
Views
174
Replies
6
Views
307
Replies
5
Views
141
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.