I would recommend installing the hot water heater outside, then noise is much less of a concern. also, it will cost you much less since you will not have the expense of the ductwork.

the higher volume heaters make more noise than the lower volume heaters.

are you planning on one hot water heater or multiple hot water heaters?

I have three Noritz tankless gas hot water heaters. two are installed outside and they are large commercial heaters and they make more noise than the residential model that is installed in an interior closet.

will you have gas or electric? electric tankless hot water heaters typically are quieter than gas.

I would be more concerned about the placement of the heater or heaters in respect to where you need the hot water output. do avoid installing tankless hot water heater in a laundry room. lint and tankless water heaters do not work well together. I recommend going with a certified installer for the brand that you decide to purchase.

also, given the water shortage that we have in California, I would recommend a recirculating pump. this should cut down on water waste while waiting for the hot water to make it to the spigot.

the recirculating pump makes the hot water heating process less efficient, but it will save on the amount of water that you use.

Thanks much. Great points.

1. If I install outside, does it generate enough noise that could be problem to my neighbor? Both of us have 5-ft side yard between our homes?

2. Another option is installing in attic (above main foyer leading to family room). This is a perfect place for our home as it is in center and we can install one 12 gpm tankless heater. Do you see noise an issue from attic?

sorry that I never responded. no, the noise should not bother your neighbor.

We put one in a rental home and I don't consider it to be that noisy. I do hear a click or something when I turn on the hot water in the kitchen and am about six feet away from the unit. If you could locate it in an area that is already noisy (kitchen or laundry area), it shouldn't matter.

@Kelley Dockrey are the 3 installed in series or do they serve different outlets in parallel? Did you face any problems due to which you had to upgrade to 3?

Two heaters are installed in a series. The other heater serves a different area. I started out with three. There was no upgrade. Proximity to outputs is very important. Right size the gas line is essential or the heaters won't have enough gas to work properly. I made the assumption that you have natural gas heaters. Larger gas lines require larger gas meters. Follow the manufacturer's instructions for gas line size.

I positioned the pair of tankless in closest proximity to my largest output ares, the master bathroom. The other tank less heater is more centrally located for the areas that it serves.

I have to say that I prefer outside installations. The pair is outside. The other one is inside the house

Any installation should allow for the units to be removed easily from their brackets and water connections especially to accommodate any cleaning or service.

Also, They can be noisy. The larger commercial units are noisier than residential units.

Was the positioning important because you did not have re-circulation? Or did you not want to have a long re-circulation loop?

I do not have a recirculating pump. The installation location was selected for that reason. One issue that I have had with the commercial tankless is that the low flow faucets and showerhead can delay the heaters hot watrer output. I do not have the same issue with the smaller residential heater

@nauna The solution to your problem is the recirculation line with pump. Tankless and Tankful water heaters should have the same behavior as far as time to hot water. From your description I get the feeling that your problem started only after shifting to the tankless water heater. One reason could be that tankless water heaters require a minimum flow before the heating kicks in. The flow rate could be adjustable and setting that to a lower value may be a simpler solution to at least getting it on par with tankful water heaters.

@Kelley Dockrey there may be a similar setting on the commercial tankless. Ironic that you likely paid more for the commercial heater and are less satisfied with it !!

Joe, the tankless unit was installed in a brand new home. I'll forward your comment to my son. Thank you

Hmm .. new home with no recirculation line = wasted opportunity. There are options to retrofit the a recirc pump on existing piping by connecting a special device at the farthest sink. This may be a cost effective option. A good plumber should be able to advice him.

Joe, thank you I'll forward this to him as well. I guess it would be considered a spec home as they bought it when nearly completed. You know builders, they'll save a penny everywhere they can.

We‘ve had a tankless for 23 years. The 2 minute wait for the hot water to reach the 80 feet to the master wastes all of 5 gallons. The issue is not the tankless. It’s the 80’. Site location is usually the issue with wait times. But it would be an issue with tanked hot water production as well. It’s the distance. Not the methodology of production. A centralized plumbing location with a PEX manifold system would fix the issue. It’s not really an issue to brush the teeth with cold water though. I promise.

Joe Macker - I needed the volume of hot water that only two commercial tankless could produce.