Enough gas supply to install tankless?

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SF_TD

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I need help determining whether I can install a tankless heater with my existing gas line setup.

I have a 17 year water tank heater (75 gallons) that is in the attic (it was there when we bought the house) and that is going to be impossible to remove without cutting it into pieces (incredible how they set this up, but that's besides the point of my question). I want to be proactive and start planning on how to replace it.

We cannot replace the current heater with another water tank heater so I am exploring whether we can install a tankless heater in the same location. Two companies told us we need a dedicated gas line, and two companies told us the current gas line is ok.

We have a 1 inch gas line going up to the attic, being reduced to 3/4 to feed one of our furnaces and the current water heater. This main gas line has a total of 2 furnaces connected to it (1 in the attic to heat the top floor, another furnace in the crawl space to heat the lower level, and one gas stove in the kitchen (5 heads).

We have 3 shower heads and 2 1/2 bathrooms. We live in the San Francisco area, so the 2 furnaces don't run at the same time very often.

Can I install a tankless with the current gas line setup?

Thank you for your help!
 

Stuff

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Most likely you will need new piping. More info is needed for a better guess. What is the total length of the 1" line from the meter to the attic? What is the length of the 3/4" line to the furnace? Can a T be installed at the 1" to feed the water heater? Distance from there to heater? What is the BTU of the furnace? BTU of the new tankless?
 

Dana

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Most tankless installs for whole-house sized burners require a dedicated 1-1/4" gas line to operate properly. Navien claims that their NPE series tankless units can do it with smaller gas lines, but it's not clear how that really works.

natural%20gas%20pipe%20chart.jpg


These numbers are in 1000s of BTU-in, and the lengths include the "equivalent length" adder for every ell, tee, & valve along the path, not just the straight runs.

equivalent-length-flanged-fittings-meter.png


Teeing-off the run to the tankless to another load of any size results in pressure fluctuations when the other burner turns on/off that can interfere the proper operation of modulating burner such as a tankless, and adds to the BTU load for the pipe sizing. Given the ridiculously low space heating loads in your area you might consider scrapping the furnace, replacing it with a hydro-air air handler (properly sized for the space heating load) running off the tankless. Most exisiting furnaces in the Bay area are more than 4x oversized for the space heating load, which takes a toll on comfort. A right-sized hydro-air system would run fairly long cycles when it's cold out, but at a lower air velocity than the oversized beast puts out- a warm summer breeze rather than a scorched air blast followed by a chill when the thing shuts down.

What is the nameplate BTU/hr for the furnace currently sharing the feed to the water heater? And the BTU/hr of the other furnace is...?

Describe the size & construction of the zone the furnace sharing the water heater is serving.

Use the existing furnaces to calculate an upper bound for the heat load at the 99% outside design temperature in your town (every municipality in CA is listed in that document) using this methodology.

Any air conditioning coils in those furnaces?
 

WoodenTent

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Do you know you water inlet temp, and your house Electrical Service Amperage? All but 1 of my apartments in San Jose where tankless electrics. It's a warm climate, so the delta Temp you need might not be that high. On the other hand the water comes out of Yosemite, so I'm not sure how much it warms up. I never measured the temp of tap water when I live there.

you say 3 heads and 2 1/2 baths, so I take that to mean you have a 2 head shower in there. You might be able to offload one of the showers/baths to a electric tankless, and that could free up some gas supply to a tankless gas for the rest. Plus get you some redundancy. Or even go full electric but that could be a stretch. Still, 2 sources of hot water could be nice.
 
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