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Thread: Does running Furnace cause Tankless Water Heater to not heat water ???

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    DIY Junior Member cody21's Avatar
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    Default Does running Furnace cause Tankless Water Heater to not heat water ???

    We had a very strange problem last night. Got home to a very cold house and turned on our forced air furnace. We have 2 Tankless Water Heaters. We went to take a shower and one 1 of them, after about 30 secs of very hot water it turned completely COLD ... went to the other shower (different Tankless heater) and flipped on the HOT water ... NOTHING .... About 10 mins later, went back to the 2nd one and tried it again - we got HOT water for 1 complete shower. In the middle of the 2nd shower it went totaly Cold again.

    So my question is : Is there a possible gas pressure issue when running the forced air furnace and trying to generate HOT water ??? I mean the fact that BOTH tankless heaters bhaved the same way leads us down that path of gas pressure ... Thoughts??

    Thanks in advance

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    Forum Admin, Expert Plumber Terry's Avatar
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    Some gas meters are good for 325,000 BTU units.
    Assuming everything is correctly sized up. Most tankless units are 199,000, leaving 126,000 for something else. Assuming that's the gas piping is the correct size, and assuming that the gas meter really has an available 325,000 to use.

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    Engineer Furd's Avatar
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    While SOME gas meters are okay up to 320,000 BTUs/hour many will be hard pressed to pass more than 250,000 BTUs/hour. It also depends upon how the tankless heaters are piped from the gas main. If they are just tee'ed off a central main you may be experiencing a severe pressure drop and THAT keeps the tankless burners from firing.

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    DIY Junior Member brewdesign's Avatar
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    FWIW, I teed the incoming 1" service line immediately after the gas meter to feed my 140K btu water heater in the detached garage (used for heat) and ran the parallel 1" line to feed the house furnace, water heater and appliances. There are charts that give maximum BTU output by pipe size and distance at x"WC. I'd assume you are running 7"WC but you'd need a manometer or a fabricated clear hose to check it. Also unless the instant water heaters are mechanical standing pilot, i'd expect some sort of diagnostic flash code to diagnose the reason the heater didn't fire, such as low gas supply pressure.

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    In the trades Dana's Avatar
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    First, it's highly unlikely that a normal-rate shower would hit anything like max-fire, instantly overtopping the output of the regulator at the meter at typical CA incoming water temps. If the hot-air furnace is oversized (probably is) with over 150KBTU/hr input ratings you could bang the 250K limit with two showers running, but hitting hard up against 325K is unlikely.

    But unlike hot air furnaces tankless HW heaters can be quite sensitive to fuel pressure fluctuations (or worse, oscillations) or big step-changes from another burner suddenly drawing down during operation, since it's constantly modulating the flame to maintain a steady output temp. That gets funkier when you have more than one modulating burner drawing at the same time. The configuration of the plumbing counts- if it's a tree configuration with branches to the combustion appliances it's a worse case for interaction between the burners, which can cause one of them to shut down with competing modulators creating an oscillation. A star configuration, where the lines to the appliances tee-off very close to the meter with no-branches is preferred, since the regulator at the meter can null a lot of those pressure changes, limiting the interactions.

    Smaller piping that is rated for the burner sizing only makes it works- often MUCH worse in a branched plumbing configuration, as they're all trying to suck harder through the same skinny straw. In most homes would take 1-1/4" piping per branch for the tankless units to get rock-solid & reliable performance at the high end of the modulating range, depending on lengths of the runs and number of ells.

    Bottom line: Yes, having three burners on a the same time could easily cause a burner with modulating controls to shut down.

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