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Marc, Plumber
Category: Plumbing
Satisfied Customers: 1127
Experience:  Plumbing heating & home maintenance contractor
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I have a GSW T-H3-DV / 540H tankless water heater to replace

Customer Question

I have a GSW T-H3-DV / 540H tankless water heater to replace an existing hot water tank. The existing unit has 2, 2" holes to vent thru the side wall. My vent run length will be less than 6 feet with 3 elbows. Can I run 3" pvc from the heater to the wall and then reduce to 2" to go out thru the existing holes in the concrete basement wall? It will be very difficult to increase the holes from 2" to 3" or to use a 4 or 5" concentric vent option.
JA: Did you double-check the thermostat? And what about the unit's filters?
Customer: What?? Not related to my question
JA: Do you plan on doing the work yourself?
Customer: Some. I have a neighbor who is HVAC tech.
JA: Anything else we should know to help you best?
Customer: No
Submitted: 1 month ago.
Category: Plumbing
Customer: replied 1 month ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call.
Customer: replied 1 month ago.
Let me know if you need more information, or send me the service offer(s) so we can proceed.
Expert:  Rick replied 1 month ago.

Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.

If the install instructions call for 3" you must run 3" full size (no reductions) through the wall ro the outdoors. I'm sorry but there's no way around this.

Customer: replied 1 month ago.
Pretty simplistic response. We know that the longer the run, the larger the pipe required. The instructions say a 3" pipe is good for a 70' run, while a 4" pipe is good for 100', so a 2" pipe should be good for 40'???? There should be a formula for that. The instructions also allow for a concentric pipe where the exhaust pipe in the middle is just over 2". So we both know that a 2" pipe 6 ft long should be able to move just as much air as a 3" pipe 50' long.
I guess I expected a more thought full answer.
Expert:  Rick replied 1 month ago.

You can't extrapolate to a smaller size. The instructions do not list 2" as a venting option. You must follow the manufactuere's install instructions. It is a code requirement. Sorry if my original answer didn't seem "thoughtful" but there is a simple answer to your question. You can't use a 2" vent.