On tankless hot water heaters and sizing a natural gas pipe

Researched tank-less hot water heaters and their voracious demand for natural gas. Decided to go for the Takagi T-K3-OS because: 1) it is for outdoor installation — no fancy exhaust flume required, and 2) its *minimum* BTU rating of 11K was one of the lowest I’d seen (considering that the maximum side was more than capable).

I worried about the minimum rating because there’s a dirty little secret about tankless hot water heaters: they completely shut down when the flow is too low and you get no hot water. Yup, your water conservation efforts of running a trickle of water will be for not with most gas fired tankless heaters. And after having the T-K3 in use for a few days now, I’m glad I did worry –0.4 gpm doesn’t quite go low enough for my liking. It’s probably just me, but the small stream of water I use when washing dishes is not enough to keep the water heater from turning off due to inadequate flow. You don’t read about it often, but, this is one of the cons to a tankless hot water heater.

A second negative to the tankless setup is the amount of natural gas they consume at full water flow — 190,000 BTU’s for the T-K3. That’s more than my furnace, oven/range, clothes drier, and old water heater… combined. Most installs are going to need to upgrade the gas plumbing to provided enough gas flow.

Here’s an online natural gas flow calculator that takes into consideration more than the standard 0.5″ water column pressure drop that most flow tables use.

Here’s is a good paper that illustrates the application of the low-pressure gas flow formula [1] used in the above calculator, and gives some helpful information about how to account for discontinuities. For example, a 90° elbow adds 2 linear ft of “equivalent pipe length” of resistance. Things that make you go, “huh”.

[1] found here.

1 comment to On tankless hot water heaters and sizing a natural gas pipe

  • Leo Butler

    Jeez… that made my brain hurt. I thought I already knew a thing or two about the tankless water heaters, after I’d researched ‘em a bit… but it’s even more complicated than my feeble brain can grasp. Oh, wait… it’s probably due to the wine consumption on this fine Saturday evening. Yeah, that must be it. :-D

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