I think in your case, supply line hot would be the only option. For us, the old water heater was in the kitchen so we just transfered the wait time from the kitchen to the bathroom. We're having a tough time changing our water use habits. Prior to the new on-demand, turning on the shower and waiting seemed part of the routine. For handwashing after a potty stop, you knew to turn on the water when you entered! Now, in the kitchen the wait seems so long. A quick rince of dishes or a handwash doesn't seem worth the wait so everything but washing dishes is in cold water. I'm trying to figure out if the few times we need unlimited hot water (hot supply line) is worth paying for heating a small amount of hot water to sit in the pipes each time you use less than 3 gallons in the kitchen. I'm really leaning towards the cold supply line and seeing if 6 gallons will meet our needs. A dishwasher, which we do NOT currently own, uses 5-6. I've heard that hand washing wastes more water but I need to actually measure. Then the consideration is the cost. We thought we were going to heat water with gas and now are considering electric for the kitchen. I'll post what we finally decide to do.

I doubt very seriously if the failure of the humidifier supply tube is a result of heat. First of all, let us consider the temperature of your incoming cold water supply. The mean average of the earths crust is 55degF (geothermal temperature of the crust). Soil is a rather poor insulator so the surface soil then cools proportional to the ambient air temperature. While the crust is 55degF the actual surface of the soil will freeze for a short depth which we define as the "frost depth". While the frost depth is the deepest point at which freezing commonly occurs it must be understood that from that depth the temperatures increase gradually from freezing temp(32degF) to the natural geothermal temp(55degF). Typically geothermal temp occurs at approximately twice the frost depth. In order to protect our water lines from freezing the codes require that we bury the line 6" below the frost line thus when the dirt is frozen to the frost depth the average temperate at the water line may be only slightly above freezing (34degF+). Fresh water reaches a maximum density of 231cu.in per gallon at 39degF. There is a very coplex formula to determine the rate of linear expansion but for the sake of illustration let us use the maximum here. When water is expanded from its maximum density of 39degF to the boiling point it expands by approximately 40:1 thus we could say the physical volume of a 40gal. water heater expands by one gallon. To put this in perspective consider that a 1/2" pipe has a volume of one gallon for each 98 linear feet. We must now keep in mind that water is not compressible therefore as the volume expands it must go somewhere. Normally the increase in volume is compensated by physically pushing some of the water back to the municipal main or to your well pump pressure tank however if we install a check valve in the line there is no place for the additional volume to go. In this case the excess volume would by necessity be relieved by discharging through the water heater Temperature & Pressure Relief Valve (T&P valve) BUT let us consider another potential problem here. Imagine for the moment that your T&P discharge line discharges out through the wall of your house. The discharge end of the line is then exposed to atmospheric temperature and in a worst case scenario the outside end of that line could freeze shut. At this point there would be no place for the additional volume to be released and it could result in blowing out fittings or valves or rupturing a pipe or the water heater vessel. Now keep in mind that the T&P valve is a spring loaded valve that is set to release when the pressure reaches 150psi and most likely the plastic line to your humidifier is failing slightly before the internal pressure is sufficient to release the T&P valve. On the other hand, if you did not have that plastic line there is a very strong likelyhood that the increased volume could be relieved by blowing out a compression fitting at a fixture angle stop, a pipe joint might fail or premature failure of Bibb Washers in your faucetts. The proper solution is to install an expansion tank on the water heater side of the check valve to absorb the increased volume. Installing an expansion tank is quite simple. You simply install a Tee on the cold water line with a male thread adapter on the Tee stub out, then screw the expansion tank onto the thread adapter.

Depends on the construction of the valve body. Have you tried calling the manufacturer? If you do not know who made it, post a picture on the Terry Love Plumbing Forum and ask the Pro's there.

Nothing wrong with flex pipe on a water heater. 99% of the problems come from people not knowing how to install it properly. If you look at a new water heaters supply and outlet nipples. You will notice a red and blue plastic insert. Those inserts are mainly for sealing the connections. Now when you use a nipple to connect two lines you do not have the plastic inserts to protect the rubber gasket in the hose connecter flange. Thus making them very easily to damage and causing leaks. Once the rubber gasket is damaged you can never get it to stop leaking no matter how tight you tighten it. Been using them for years and have had very little problems.