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.

Paulbm has it right: There are two separate issues here: 1. That the furnace is using all of the combustion air for combustion -meaning that it is pulling the air into the furnace and then sending it out the flue. 2. The furnace has a problem with missing air return - so it's pulling the air from the closet, heating it, and sending it out to the rooms. 3. Both things are happening All of these situations are serious and are an indication that the job wasn't done by a qualified installer. The fact that it sucks the door closed indicates that it's probably at least #2 The closet must provide combustion air intake by having vents of at least a specific size that are a combination of requirements by the furnace and water heater manufacturer. Alternately, you can provide outside air intake for the furnace through a pipe - also spec'd by the furnace manufacturer. (you would still need venting for the water heater). You may not have a disconnected air return on the furnace. That is very dangerous in that it will draw carbon monoxide from the water heater flue into the house. In a way, you are fortunate that the water heater is being blown out. Far worse would be for the water heater to stay going and kill you while you sleep. Get this problem resolved immediately. Also get a Carbon Monoxide detector. Carbon monoxide is colorless and odorless. It's possible that this problem was a legitimate error... but not likely. I would be very suspicious of the person who did the install. What was done here can kill you and your family.

The disadvantage of tankless is that you may not have enough hot water to supply multiple simultaneous uses (running 3 showers at a time, for example). So what I'm thinking is a tankless and a tank heater feeding into a mixing valve such as a Watts LFMMV. Set the tankless to deliver 110F and the tank to deliver 120F and set the mixing valve to supply 110F. Then under normal conditions the the mixing valve calls for "cold water" of 110F from the tankless but if the tankless can't keep up and falls below 110F then the mixing valve mixes in hot water from the tank to maintain the 110F. Would this work?

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.