You can certainly do a hybrid or combination system. Doing your proposed L/R lay-out, is the only way to go in my opinion, I would still suggest a pipe tying the two sides together. I would put and accessible ball valve in that cross connection pipe so, if for any reason one system went down you could open the valve to have one side support the other, at least temporarily. Do you have a basement under this home or only a crawl space? If you need hot water quickly and not shower volumes in the downstairs rt lav, perhaps a small undercounter electric would do the trick. I hate to suggest that, but you are dealing with geography here. Time, distance & flow rate . The Master Bath will be best serviced by tankless. Your hi flow shower head needs to have a specific flow rate attached to it. A standard shower head will provide 2.5 gpm. In my new shower that means 2.1gpm of 120f water mixing with .4gpm of cold to arrive at my nice comfortable shower temp. I have also seen 20 gpm shower heads. Put a number on the flow of the single shower head please. What is the volume of the tub. What faucet set are you putting on it and is the tub valve going to be a 1/2" or 3/4" connection. With 1/2" pipe you will not get much over 5 gpm. With 3/4" you can get 10-12 gpm. Some tub faucets, the waterfall type, require 3/4". Otherwise the visual effect is not as good. Get the tub volume and compare the flows to the time you are willing to wait for the tub to fill. Yes, you can put tankless in the attic, but understand that you need a pan below them to prevent water damage from any possible future leakage. That is just good sense. Tankless water heaters are compact. As a result people tend to install them in really small spaces. Please keep in mind that this is a mechanical system and must be accessible for inspection and service. I long ago coined the term, "Just because it fits, doesn't mean that is where it goes." You would not believe the places people have installed these. It is a good idea for you to have the "service space" discussion with all of your subs. A lot of duct work gets destroyed by the technician the first time he has to service the heat pump, furnace or air handler. Build all your system to perform well, but make sure everything is positioned for accessiibility. These types of agreements tend to be forgotten when the equipment arrives on the job and the contractor wants to "get it done". So the question to your General and each of the subs is, "How are you going to build and position the equipment so it can be easily serviced". That is a good discussion to have with your architect too. Make your expectations clear and let them know you will be following up to monitor this "agreement". Sorry to get off topic there, but you have what, as I look at the floorplan is going to be a really lovely home. I just want to make sure the cardiovascular systems work well for you too. Another thing, the gas line will have to be sufficiently sized not only for what you are planning for the initial job, but you want capacity for future expansion. I have seen so many places that get undersized gas lines and the owner is blocked from any possible future expansion. This is a big project and even with great care, things pop up.

asolo: you are so so right. At the same time this water heater thing is happening, a leak developed in the corner of my dining room. The plumber came over and couldn't figure it out; a handyman came over and opened the wall a bit and couldn't see any pipes. But he kept insisting the water was coming from a pipe. We knew it wasn't a normal water leak because the meter wasn't moving at all. Well my DH and I were discussing this leak and suddenly we made the connection between the bum water heater and the dining room leak. We both realized that the leak must be from the water hater drain pipe. It was leaking down the slab outside instead of coming out in the proper location through an open pipe in the wall. The pipe must be plugged up. Anyway some of my parquet floor is ruined, not to mention having to repaint where the handyman put the hole in the wall. So yes, you are right. This can cause lots of damage. Here's the kicker, though. Tonight we tried to drain the water heater. We shut off the cold intake and open the pressure release on top of the heater, and hooked up a garden hose to the heater to drain to the outdoors. And turned on hot water spigot in the shower to release pressure. And very little water actually came out of the water heater - certainly not 40 gallons. So what did we do wrong? Is there so much sediment that the water cannot come out? I don't see how the tank can be empty. I am so worried now. Any thoughts on what to do next?

I am facing the same problem as John. John, If you see this or anybody else who knows which switch to flick and where it is located that would be great if they can let me know. Thanks, Andy

A couple months ago I replaced a 20 year old A.O. smith LP WH that was still working properly. I replaced it because I wanted to get the last of the "old" style WHers and not the "new" style ones down the road when I needed a new wH. My water is 45 gpg hard and the water is softened. Get a comprehensive test of your water at the house not a years old water quantity report from the water authority. with those results you'll see what needs to be treated and then decide IF your present softener is treating all the problems and is correctly sized for your water conditions and water usage and is actually working. When you have water quality problems call a water treatment professional and when the plumbing is leaking call a plumber.