We just replaced our water heater in the attic with an external tankless water heater on the first floor mounted on the hardi siding of our house outside our back door.

We had an inspection done just to see what we were missing on things that needed attention - turns out the vent pipe for the water heater in the attic was scorching the attic roof where the vent touches the roof opening - the water heater is gas and we also didn't like having it out of sight where we NEVER notice it (or smell).

We are terrible at monitoring and maintenance of our home so I think moving it out of the attic was a very good thing for us to do. If you are someone who is going to visit your attic every month or so and you know what to look for and you are going to watch over it, might not be too bad. But we are VERY glad we switched though wasn't cheap. Hope it will help sell the house whenever that happens, but hopefully selling is at least 10 years away. (One thing I'm told about tankless water haters is that they DO last a whole lot longer than regular water heaters - hope that is true with us.)

Weight and the eventual failure and leak.

You need a pan and drain under the heater, and a place to route any T&P valve (it cannot go into the pan).

The wight mounts up quickly since water weighs over 8 pounds per gallon.

The ONLY pro is that you won't be taking up conditioned space or using conditioned air for combustion. Both of those issues can be planned for at design phase by having a designated mechanicals/laundry room, and using supplied air for the combustion if the heater is gas.

Personally, I'd go tankless, on the exterior in a warm climate or on an interior wall in a cooler climate, and save the space and manage to have endless hot water. If you are in a cold climate, then a boiler that also produces hot water would be the preferred method.

Down in the sunbelt where there is little to no chance of a waterline freezing in the attic space it has become a common practice to put the water heater in the attic to conserve livable space, especially in contractor turnkey constuction, but due to the physical problems and additional code requirements it is generally not considered economical for a retrofit.

In addition to all the regular code requirements for a water heater, when the water heater is installed in an attic you must first have an attic access hatch that is a minimum of 24"x36", and even more for larger water heaters. Remember, the water heater must fit through the hatch to permit future change outs.

The final location for the waterheater may not be more than 20' from the access hatch.

There must be a 20" wide catwalk from the hatch to the water heater location, and the waterheater must be placed on a plateform that provides a minimum of 30" work space directly in front of the water heaters firebox or control section. You must then have a 24" wide walk surface on both sides and the back of the water heater.

You must consider the total combined weight of the plateform, waterheater, the water, two service men/women and their tools.

By example,For a 60gal water heater you have;
Plateform & decking 60lbs
Water Heater 250lbs
60gal water 500.4lbs
2 service ppl 450 -500lbs
tools 200lbs
total 1460 to 1510lbs

Generally in new construction the water heater platform is placed over a load bearing wall with additional supports for the water heater.

If retrofitting you can be assured that your trusses are not currently designed to accept this much additional load.

You must also have a light fixture and a 110v outlet in the near proximity of the water heater. (most codes limit it to 6')

The switch for the light must be within arms reach as you enter the hatchway and before you are actually into the attic space.

The water heater must be equiped with an approved pan, and the pan must have a dedicated drain line which is run down and discharge outside the structure at not more than 24" above grade or into an approved waste receptor.

The T&P valve must also have a dedicated drain line that is run down and dischages in the same manner as the pan lines.

The point of discharge must be in plain sight, (may not be concealed behind shrubs, vegitation or personal items.)

If it is a gas fired water heater the flue must be type B flue pipe and it must have a minimum 5' rise in the attic space.

Most local jurisdictions are now requiring both a hared wired smoke detector and a hard wired combustible hydrocarbon detector within 10' of the water heater and in some jurisdicitions those detectors must be linked with the detectors in the living space or they must be fitted with a remote annunciator in the living space.

And last, but surely not least, most jurisdictions also require...

what? last but not least they require what??

HWH's have grown in diameter since April 2015 as the insulation jacket is now required to be thicker. You may or may not be able to fit your new HWH up the pull down attic stairs.

FWIW, 90% pf Texas HWH's are in the attic. Mine is in the attic. On any given summer day the attic temp is commonly 120º or more, providing free hot water. In Houston summer lasts only 11 months a year. We do have to pay for hot water between Christmas and President's Day.

First this is an old thread. Second, cpmcmahan, you will never get your answer as lazypup passed away some time ago. Third, it's a water heater, not a hot water heater.