Now a few thoughts on Pex pipe - it's certainly more expensive that PVC, however it's less expensive than Copper. The real benefit comes in its flexibility and ease of installation. Pex uses a black coated, copper crimp ring, tightened to fittings to form a joint. You cut the pipe to length using plumbing cutters (these are the same as what you typically use when plumbing PVC or CPVC). You next slide a crimp over the end before pushing the "nipple" of the fitting into the tube. The ring is then slipped over the fitting "nipple" and a crimping tool is applied to the ring, which applies even pressure inward. If the crimp is successful the joint won't leak when water is pressured back into the system. Crimping can be a bit tricky - if the ring isn't on straight or if the action of the crimping tool is precise, the ring can be twisted a bit on the fitting. It's my experience that usually these poorly crimped fittings won't leak, however you do increase the chance of leaks as there's less surface area for the ring to apply consistent pressure on the "nipple" of the fitting - you're basically taking a chance with these when improperly crimped. The solution is to cut off a little of the end of the hose, grab a new fitting and try again. You can carefully remove the bad crimp by cutting and re-crimp, however once the Pex tube has been compressed it becomes suspect, so you may as well cut off and start with fresh.
Some of the other advantages of Pex - once a 90 degree joint is made the pipe can still be twisted without causing a leak - something you can't do with CPVC without adding additional fittings and pipe (or cutting and re-placing the rotation angle with a slip coupling). Another advantage has to do with the flexibility of the pipe itself - it can be carefully bent to go around obstacles and the material won't split if the water in the lines freezes - making it an appropriate replacement pipe for a supply line out to your meter. If you've ever dealt with copper piping you know there's preparation involved in sweating the joint with solder - and if you aren't successful in making the joint it becomes problematic after water has been added to the line. With CPVC you have to deal with primers and glues that can get a bit messy (don't spill the primer on a good linoleum floor - ask me sometimes how I know). With Pex it's all about the line, the crimps and the fittings. There's some additional cost involved with the crimping tool - I bought one that has both the 1/2" and 2/4" in the same tool - it was purchased while I was working on my own plumbing so I was grateful for the opportunity to use it again - I think it cost about $65 or so.
All the Pex was run back from the unit back to the location of the supply line (cold water) and system tie-in for the hot. By reusing the existing we didn't have to worry about blocking off the old hot water and making a connection elsewhere - however you may want to rethink that based on your own circumstances. If you can tie-in closer to the unit you will probably have a faster delivery time for the hot water, depending on the placement of the spigot. All the water lines were run and any additional parts, bits or pieces were noted. Next careful measurements were made to run the gas line to the unit. This is where it can get tricky - since the gas line has to be 3/4" all the way to the unit you need to find that size pipe to tie-into - also you can't use the flexible connecting pipe as it reduces the line size to 5/8" or smaller (there is commercial flex line available but it's a bit expensive). Since it all runs through galvanized pipe the measurements need to be very precise. Also you'll need a slip-coupling or two to make sure you have room to place the final couple of pieces - the slip-coupling uses a compressed o-ring so you can turn either end of a pipe with a wrench, providing some flexibility in install. We saved the gas-pipe install and actually connection to the house for the third day after making a list of all the fittings we thought we'd need - we also added plenty of extra to prevent an extra trip or two.
On the third day we purchased everything we'd need to complete the job at Home Depot - we also made sure to buy extra nipples (in plumbing the short pieces of pipe that are threaded on either end are called nipples - don't ask me why) in various sizes in case we needed them. One tip - if you buy this stuff at Home Depot - use the same credit card for all purchases on the project - when you're done you can take back anything unused without a receipt - the do a reverse look up based on the credit card number - it makes it much faster and convenient and you don't have to hunt down receipts (I believe they keep these records for several months which is real handy when your projects get extended for whatever reason).
There was one additional task before hooking up the water - the control pad was mounted to the wall and tied-in (there's an image above that shows the control pad mounted). I found it interesting that extra pads could be added and placed about the house providing some heating customization depending on the task - basically if you want the water to be hotter for the dishwasher, for instance, you can add a control to the kitchen so that the water temperature can be adjusted just before use. That's something you can't do with a tanked water heater.
Finally the Pex was tied into the house lines using Sharkbite fittings. I mentioned these before - Sharkbites are a product that allows for the connection of dissimilar piping. I discovered these while working on my own home - I had Pex coming from one direction where it needed to attach to 1/2" CPVC at a right angle and Copper going straight out. The Sharkbite allows for the end of each pipe to come together as one connection. In Allen's home the majority of the piping is copper - using the Sharkbites allowed the Pex to tie-in directly to the copper. If you've ever had to do this in the past it usually takes several screw-fittings and a sweat joint (if copper) so the Sharkbite cuts down on install time and works great.
The last task was to fit up the water filter that was recommended to Allen - apparently if you filter out as much as you can before water enters the unit you save having to descale the unit (water in Atlanta has quite a high level of mineral content) - this serves to extend the life of several parts. The filter was added between the cold water supply, extending outward as there will be a wall added to section off the home's systems from the rest of the basement. Finally, we turned on the water and checked for leaks - surprisingly there weren't any (rare when doing any plumbing) - the gas was turned on at the unit and the power was set to "on" - the final test was to go to a sink and turn on the hot water. I think the most amazing thing was in how quiet the unit is - you can only hear it when standing right next to it.
Here's an image of the final install - note that the drain line for the pressure relief valve was moved to the front - it was a bit too stiff to be workable in the back and still make it into the washer drain. Allen set the temperature to 120 degrees (see the read out) which seemed to be optimal for their household. In all, the installation wasn't too bad - I can totally see why the installers charge so much though - lot's to think about here and many systems that need to be "touched." If you aren't a very handy person I'd leave the install to the pros - of if you feel confident on parts of the install you could do what you know and bring in someone to do those bits that are uncomfortable to you. On a scale of 1-10 I'd call this about an 8. . I'll update again once the unit has been in a while to reveal how it's working out for Allen and Susan.