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Tankless water heater venting and gas supply pipe

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I have a 90% furnace and would like to install a high efficent Navien tankless water heater that also uses pvc for venting.  My question is can I "Y" the water heater into the furnace exhaust vent?  They both vent using 3" PVC.  Also my furnace exhaust vent runs horizontal for about 20ft.

Another questions is from my gas meter to my furnace I have 3/4" black iron gas pipe and that is the only appliance it runs.  The Navien takes a 3/4" gas line also.  Would it be sufficiant just "T" the gas line to both appliances?


asked in Gas Water Heater by tanzuki (120 points)
edited by darekrudy
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1 Answer

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Hi Tim,
You cannot vent any condensing type appliances into a common vent. What that means is that a high efficiency furnace and a tankless type condensing water heater require dedicated vent pipes that terminate individually / they cannot be spliced together with any kind of connectors.
Tankless water heater gas pipe
Tankless water heaters require significantly more gas than the regular ones. Also, they have more restrictions as far as the gas line pressure. Navien tankless water heater manufacturer suggests to install the appliance as first from the gas meter to ensure proper operation and the natural gas line pressure to be between MIN 3.0” WC - MAX 10.5” WC
So it would be wise to have that line pressure checked by a professional before purchasing the water heater.
The “T” installation would be fine but arranged in such way that the WH is closest to the gas meter. 
All the details for the Navien water heater are here: Installation manual  and pages 21 & 24 partially cover your questions.
answered by darekrudy (21,730 points)