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I have two tankless natural gas water heaters (Takagi TK-510U) in a connected parallel cascade configuration. They are served by a 1" gas line and each heater takes a 3/4" tap. They work fine the vast majority of the time. The home has a much larger supply, it's just a dedicated 1" branch (150' from the meter) to these heaters.

Usually there are no issues and they work together as expected, even when both units are running full blast. But occasionally, once or twice a year, when the secondary unit kicks on to ignite it will cause an ignition fault in the other primary unit.

I think this is because the initial inrush of gas can momentarily starve the other unit.

It's annoying because these particular water heaters do not self recover from this problem. I need to manually push a reset button to clear it.

I have many gas appliances and they can all be running simultaneously. I haven't seen any correlation with utility total gas usage and this coincidence.

How can I prevent this problem?

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    No manufacture or model numbers? What kind of ignition? Feb 22 at 16:43
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    Tankless water heaters use a huge amount of gas. Are you sure your gas provider can provide that much gas at once to handle the needs of two tankless heaters at the same time? Do you have other gas appliances that might be using up some of that supply capacity in this failure scenario? It's not just the size of the pipe at your appliances, but also what the utility can provide thorough the meter.
    – Milwrdfan
    Feb 22 at 16:44
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    What is the make and model? How long is the 1" pipe from the meter? Is it natural gas or LP? What is the line pressure? I've seen manufacturer instructions for piping relative to ganged tankless water heaters, though I think parallel configurations are more common than cascade/serial. Of course the gas supply problem is the same regardless of how the water is plumbed. If natural gas at 4 ounce pressure, 1" pipe strikes me as too small to serve two tankless water heaters.
    – Greg Hill
    Feb 22 at 16:56
  • If this was water, I'd suggest something like a pressure tank to provide make-up flow to cover that surge. But I don't know if anything similar exists for gas.
    – keshlam
    Feb 22 at 21:10

1 Answer 1

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The manual for the heater is available from Takagi. I'm assuming that you're working with natural gas at 4 ounce pressure -- if the pressure in the long feed line is higher, or if the fuel is propane, smaller piping would be appropriate.

Per the pipe sizing procedure outlined in the manual, that branch serving the two water heaters should be 1-1/2" minimum size because they lie 150' from the meter and together consume 398 cu ft/hr of gas. This leaves only 83 cu ft/hr of headroom before the step up to 2" pipe would be triggered. If other appliances also are fed from this same 150' of piping, the bulk of it almost certainly should be 2" with the branch that feeds the heaters down-sized to 1-1/2". There's a sizing example given in the manual (though I believe it contains an error: it seems 3/4" would suffice for the 15 foot length going rightward from point B in their diagram).

There are some instructions in the manual relative to ganging multiple heaters for higher heat capacity so that the heaters can communicate amongst themselves. I would assume this might enable them to stagger their firing and/or modulate their gas flows to minimize problems with gas supply surges. If yours aren't linked as described in the manual that might cause trouble.

So, what can be done to avoid the problem? Ensure that the heaters are linked correctly, then examine the sizing of the gas piping. If it is found to be lacking then you could correct it by converting your system to run at higher gas pressure or by replacing existing and/or adding new piping to supply the gas demand better.

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  • Thanks for the answer. It seems strange that, based on the gas values in the manual there shouldn't be enough gas to even operate one heater, nevermind two. Yet they operate just fine 99% of the time
    – Matthew
    Feb 23 at 7:40
  • They probably work fine while modulated down - i.e. full blast probably isn't - but if for some reason one is running flat out when the other kicks in, that's beyond what the supply can handle. The requirements in the manual will have some margin built in. It's quite likely that they don't often run at full power, but that they start at full power (@Matthew)
    – Chris H
    Feb 23 at 19:38

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