You don't really have to worry about crossover of the power cord. It's not an issue. If yours is an 82% you want to keep it away from the flue is all.

As for mounting, I'm in Central Indiana - the block wall above the ground can get cold when it's -15F outside...so I put an inch of foam (2 pieces of 1/2" RMax) between the wall and the 5/8" plywood that was the mounting pad for the heater. The plywood was lag bolted using lead anchors in the wall.

The result is a nice, firm mounting pad that's thermally insulated from the cold block. Even though there's a built in electric heater for freeze protection, I don't want to chance a frozen heater.

If you don't have to worry about the cold, you can mount it directly to the block.

Thanks for the response. It's a condensing unit (90+) with PVC 0-clearance vents, so no worries about heat from the flue. Also, it's in Georgia and just below grade, so (recent weather notwithstanding) unlikely to be significantly colder than the basement itself. A little water makes wood rot (or steel rust) over time, so it's worth handling now.

I like the idea of foam for vibration dampening, too and I have some 1" Dow Extruded foam boards I use for thermal breaks against the sill plates. Did you have to do anything special with the fasteners with the foam sandwich? I would think over time the joint might sag.

Dan

mine is held up with lag bolts and lead anchors... isn't going anywhere, even if the foam went away. Been fine for 5 years.

I prefer to see a good quality piece of 3/4" plywood painted and mounted to the wall. Make it larger than the footprint of the water heater. Reason being, you will want pipe anchors/clamps secured to the plywood so everything is going to stay where you put it.

My tankless is a Rinnai and they come with a 6' power cord attached. General pipe layout is from l to R hot cold gas. All 3/4" Make sure the service valves are included in the install.

Nationwide the number one mistake in tankless installation is inadequate gael one sizing. Be aware that the CSSt flexible gas line has substantially greater pressure drop than black iron. Make sure the length of CSST is going to properly carry the load.

One other thing and most important to your comfort. Make sure the water heater is located as close as possible to the primary points of use and pay attention to the pipe sizes to the individual faucets. There is no benefit to running 3/4" most of the way to a faucet that draws .7gpm. It is a time, distance and volume issue and affects your wait time for hot water.

Thanks, Jack. I had a Noritz tankless at my old house and am familiar with the placement implications. The new one is a Rheem (re-branded Paloma) and is has lower initial activation and continuous flow sense requirements than any of the others. That was one of the few complaints I had with the Noritz (no hot water with very low demand fixtures operating alone). The unit's inlet and outlet are 3/4" and the supply trunks that connect to the current tank (not far away) are as well. No sense in deviating for less than 10 feet of path.

I already know the local gas utility needs to upgrade the meter to handle my simultaneous demand (199k BTUh for the tankless alone...), but I believe the distance and diameter of the black iron already serving the vicinity of the install will be adequate. BUT...maybe not. That's why I'm hiring a pro to calculate and warranty that part. Also, the gas company won't even touch the work until after it's permitted and signed-off by a licensed plumber (fair enough).

I was thinking of a 3/4" board large enough for all the mounting of what's pictured in the attached image. I have a combined tpr valve outlet on my (hot) service valve, but everything else should be nearly identical to that image. My current tpr outlet is actually slightly higher than the new heater will be installed, so I may have to re-route that to a floor sump a few feet away instead. Also, I plan on hard-wiring mine to a box with a power switch and an accessory receptacle. Not sure if I need the hard-wired portion to be GFCIed or just the receptacle.

Use solid pipe and not flexible lines. If you're having a pro do the pipefitting, why pay for flex lines? A 3/4" flex line does not have the same capacity as a 3/4" black iron pipe. It's the bushing effect. Any pipefitter that uses flex lines is doing a DIY-quality installation.

Good point, Aidan. I was planning on sweating all the hard copper lines all the way up to the union fittings on the service valves. No reason the pro can't turn a few new threads in steel :)

I have a feeling he may just be wanting to use a couple feet of the flex stuff to dampen vibrations (which are common with the direct vent condensing systems). We'll see what the detailed estimate says when I get it.

Dan

"If you're having a pro do the pipefitting, why pay for flex lines?"

In my jurisdiction and another I checked (in California), flex water and gas lines aren't required but are recommended. Maybe this is only for tank units?

Both of my heater tanks, each installed by a different but equally well respected plumbing contractor, have flex connections for water and gas. Both are also strapped to the wall.

Tankless water heaters use so many BTU's, the size of the gas line is critical. We're talking 200K BTU/hr. That's 2 to 4 times more gas than a central furnace. A flexible line reduces flow rate by the bushing effect. That may be compensated by stepping up to the next size, but then you're adding even more cost to the project.

Flex water lines are more likely to leak than a hard pipe. Flex gas lines can be accidentaly crimped or damaged which could cause a flow restriction or gas leak.

I don't use any flex lines with a tankless. They're too demanding to tolerate bushing effected flow reductions. Besides, they are installed on a wall, which makes hard-piping quite easy.

Flex lines are handy for tank units. I use them all the time. I also live in California. We have unique circumstances like earthquakes. A tank water heater that is improperly strapped (like most residential installations are) may be better off with flex lines in a minor quake than a hard-piped installation.

I'm n CA and hard piped my Rinnai on the water side. I used an appliance connector for the gas side. I do not like flex connectors on water as they are the first thing to leak in my experience. I do have it piped so the last few feet of pipe kinda float. I ran pex under the floor and the 3' risers are copper. The pipe system itself will flex as it is piped. I haven't had the final on this yet so we'll see hoe the inspector feels about it. i also put a swing joint on the gas line so it has some flexibility.

I just noticed that each of my two forced air furnaces (also each installed by a different contractor) have flexible line for natural gas.

Maybe the practices vary by region? I'm SF Bay/Cent Coast.

The issue with gas appliance flex connectors and tankless water heaters is the BTU capacity. The 3/4" flex connector must be rated for well over 200,000 BTU/h. A forced air furnace only uses a fraction of the BTU's as a tankless water heater.

FWIW, I'm in ATL and our Noritz is mounted outside on the wall. It's mounted to a pair of pressure-treated 2x4 rails. That basically leaves an air gap to the siding for most of it.

I'm not sure I followed your idea of using foam board in your mounting. I would just make sure you do it in a way that isn't a fire code concern. Given how ours is mounted (and that it's outside), I don't think you need the foam.

Ours has a switch/receptacle weatherproof box beneath it. I have a heat tape for the piping that I plug in for the winter.

FYI, The install is complete (took until the holidays to have a few days to finally do it).

Mounting is 5/8" ply over a sheet of 1/2" polyiso held by 6 tapcon through-bolts and into the cinder block. All supply plumbing is is 3/4" sweat except the unions at the service valve assembly. TPR assembly is CPVC. It and the 1/2" poly condensate line go to an existing sump drain to the left of the install. All venting is 3" PVC. Wall penetration is through a "foundation vent" opening with a custom plywood/polyiso sandwich blocking plate. All gas and venting penetrations are sealed by grey fire caulking (overkill, but the inspector liked it).

The new gas branch line is a 3/4" ID flex system with a smaller flex tail into black iron at the end. The gas is the only part the plumber did and he warranted it for 199k btu/h. The inspector approved of the method.

Dan