bave my house is over 100 feet wide and I ran into this exact issue. When I did the calculations of how much water is in a 100 foor long run (with the pipe sizes I had) it was over 5 gallons of water. Look at the gallon per minute ratings on the faucets and you can calculate how long it will take hot water to get to the sink (a long time). I used gas tankless but it really doesn't matter what the hot water plant is. What does matter is how close (in pipe run) that hot water source is to the sink that or tub it is feeding. If your houses is like mine where you have master suite on one end of the house and kitchen sinks on the other end, you might think about putting one hot water source near each end. I thought about a compromise (the middle of the house - in the basement) but then everyone suffers. I ended up with tankless under the master suite (our house - we get hot water first) and use a recirculating system for the kitchen. One day I will add a hot water tankless on the other end so I can remove the recirculating system. I have a basement which makes this much easier. If you do end up going with something in the attic (not recommended), make sure you have a pan and drain beneath the hot water plant in case there is a leak and put a water alarm on it which will sound off if there is a leak. A good buddy of mine built his house in Florida where there are no basements (similar to texas I understand). He put his hot water tanks in the garage - and they were a long way from his tub. On top of that, the water pipes ran underground where the hot water would cool off before it reached his tub. He absoutely hated it because he has to run his tank at a very high temperature do get decent hot water temp at the tub faucet. And there is not much he could do about it. I helped him design a hot water recirculation system which helps get hot water to the tub faster but he pays for it in the monthly gas bill. Plan carefully.

After ours was installed I stumbled across a company that makes an all in one unit called Eternal Hybrid (link below). You may want to contact them and see about getting the energy efficiency specs to go that route. I think Navien also makes an all in one system now too. I would think either company would be able to give you more accurate energy specs and better answer your question. The other thing to keep in mind is that without the loop (& pump), you are in effect wasting gallons of water waiting for the hot to kick in--think showers, sinks, etc. How much does the water wastage cost in terms of both water & sewer bills + the unnecessary waste of a resource? In addition to washing machines not pulling enough water through at a time to trigger the heater and then remain on long enough to actually get the water down the line to the machine, some dishwashers are having problems too. Here is a link that might be useful: Eternal Hybrid Hot Water Heater Site

This post might be old but id like to respond. HI, Big Bias I'm seeing here... I would like to show some transparency so people know the actual costs. I would also like to note that i'm not including Markup to pay for things such as both types of insurance working and worker, Municipal licencing, Over head for a shop, Gas and insurance for a vehicle, the time it takes to order products as well as picking up/ delivering products. Expect a mark up of 26-37% on everything to pay for these items. If it's a Small one man company out of their home figure on the low end. If it's the largest plumbing company in the district expect to pay more. On the flip side they may get a bigger discount on the unit and hide that mark up in the unit it's self. My location is Alberta Canada, So keep in mind CND$$$. A typical 40-50Gallon Natural Draft hot water tank is 36-50k BTU's . A Rheem or Renai or Navian tankless is typically 140-205K BTU's . The Tankless will need a larger gas line. Usually needing permits. In my Jurisdiction The inspector gives a 4 hour window for inspection. So you'll be paying for a plumber for 4 hours. On top of that Tankless units DO NOT use the same venting as a Natural draft old style water tank. Thus a Venting inspection by the same inspector. Permit needed. So you will be changing two Major components of your homes "gas" and "venting" systems. Typical install time is about 4-6 hours. This includes, Turning off the water supply, Draining the house and tank, Unhooking the venting, Unhooking the old water supply. Then Mounting the new unit to a wall or Optional Metal rack carrier, Running new water supply lines complete with new valves and possibly a new Humidifier valve if the old one was above the hot water tank. Once the new lines are run, then they can Cap the old Vent. In some Jurisdictions this vent may be removed all together with a general contractor removing and capping the roof. Again more permits needed in this case. After the old venting is capped. then running new venting out of the side of the house. which has to be 12" from other venting or 3' from any opening door window or gas meter. If it's an older home you'll need to hire a Coring contractor to Core a hole through the Cement foundation. If the home uses TGI joists you can typically exit through there. Once the venting is complete the contractor can move on to the gas line. Cutting gas to the home for up to a couple hours Hopefully using "lock out tag out procedure" IE locking the riser while they work. They will open the outside of the gas line close to the meter Then Purge. Meaning to leave it open to air for 10 minutes. Some companies use a manual pump from the inside to push the gas outside. Once purged properly the contractor can start to take the gas line apart to add a tee. Remember he was supposed to "Size the gas line" at the time of Estimate to make sure your home has a large enough line for the added BTU's If he doesn't show the inspector this Calculation he could instantly fail inspection even if he did a good job. Most contractors will install a hard Tee then run Flex CSST Gas line or "Gas Tite" similar product. Once the gas is hooked up they do another purge the other direction. removing air from the line from the outside in. once gas is at the fixture it's time for commissioning. another 20 or so minutes till he's done. In my Jurisdiction the venting materials are to be Gas 636 PVC piping minimum. In multi unit dwellings it will need to be a higher spec. Cost of venting materials depends on length but the average is about $210 2" Double that for 3". Parts, correct type of glue Primer and glue. (the inspector checks), 2 45's 4 90's , 2 couplings, TY, 4 lengths of pipe. Seems rare to have an up and out easy unit. but costs obviously for that $60 or so depending on elbows needed. Gas permit $212.65 Materials gas $85.00 15' CSST, CSST adaptors, Nipples, Black 1 1/4 x 3/4 tee, Tape, jet lube, Materials Venting $210 4" B-vent Cap, 636 pvc piping & elbows Optional Rack $310 1/2" Plywood and screws $32.00 6 hours labour $510 Inspection time say we're lucky and only wait 2 hours $170 Owner supplied Tankless install 1219.65 Navian NPE-240A Wholesaler price $3044.22 Including Service kit. Some companies may get a higher discount through volume. Navian NPE-240A Amazon price 2800.00 No service kit https://www.amazon.ca/Navien-NPE-240A-Premium-Condensing-Tankless/dp/B00FAMAB2U/ref=pd_lpo_sbs_60_t_0?_encoding=UTF8&psc=1&refRID=P40Z2FYMVNSYE4DJ1A24 Also don't be fooled when it says 1/2" gas line the actual fitting is 3/4" it's just saying they have a special valve inside that allows for lower gas flow but in turn the burner does not run at full capacity. Total Canadian Dollars $4263.87 Transparent Fair pricing

There is a LOT of disinformation on these forums about on-demand tankless units, ESPECIALLY the electric units. First, energy cost savings is NOT a reason in itself to make the switch. No reason alone is sufficient. It is the total package, in light of your particular lifestyle, that will dictate the pro-con outcome of the decision making process. First, tankless units save space -- a lot of it, especially electric units that do not need to be vented. In Texas we have no basements, so that can be an important issue. Second, an electric unit is unlikely to blow up, unlike anything using natural gas. Third, there is an endless supply of hot water, which in a successive-morning-shower household like mine, is a really important issue. And finally, yes there is some documented comp fuel savings (electric tankless over electric tanks, or gas over gas), that is really offset by the added initial cost of the unit plus upgraded electrical service or gas supply line sizing. Moreover, there ARE full-house electric units out there that provide plenty of water, unless you plan on taking two showers simultaneously while also doing your laundry and running the dishwasher! And if that's your bag, you can run 2 or more electicals together. Or get a really big gas unit. Bottom line, nearly every nay-sayer of tankless units ignore the pros and focus on a single con - cost, cost, cost. Well, we all don't drive a Ford Focus for a reason. Some of us enjoy our Buicks, Porsches or Camrys even though another car would get us from point A to point B cheaper while using less fuel etc etc etc. Full disclosure: I have a Steibel Eltron Tempra 36 whole-house electric unit. Love it. My electric bill went up nary at all.