Thanks for your photos and replies. Yes, we want the soap dispenser also. That's why we want ONE faucet for the instant hot and filtered water. That would be at least three "things" in total on the counter behind the prep sink: one faucet for instant hot and filtered water, one regular faucet, and one soap dispenser. Maybe four...disposal air switch as well? I was thinking of an undermount prep sink that is about 15" by 15" inside. I think it will work on the counter; I'm more concerned about underneath. I figure the rest of the cleaning supplies will be under the main clean-up sink and the dishwasher air gap will be there, too. Maybe I need to just buy all of the products ahead of time and see how it fits.

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.

We went with a 4 gal. Bosch Ariston GL4S Electric Mini-Tank Water Heater, which is close to 2 years old at this point. https://smile.amazon.com/gp/product/B0006GVO1C/ref=oh_aui_search_detailpage?ie=UTF8&psc=1 It needs an emergency drain. Ours is pushed through the exterior wall. It's the pipe to the left of the hose bib: This is one of my favorite things in the new kitchen. Yes, it's a bit of a space hog, but I don't regret losing the under sink storage a bit. You know all the times you just need to wash your hands or that one little thing you're using? Or wash the raw meat off your hands and cutting board so you can do the next thing? You need hot water, but don't want to wait 60-180 seconds for the warm water to show up? This is the answer. We hooked ours into the hot water line. It provides hot water until the hot water from the tank in the garage shows up, which only happens when I have pots and pans to wash. Meanwhile, I can just wash my hands without a major production. I was recently using DD's kitchen making chicken nuggets for the kids' dinner and was reminded how much I love this water heater at my house. I had raw chicken, raw egg, flour, and Panko crumbs all stuck to my fingers and I needed to get a move on. I'm pretty sure it took 3 minutes standing there watching the water run down the drain waiting for warm water to show up, wondering, "Have I got the handle pushed the right way for hot? Maybe cold is to the back. Maybe I should flip it the other way." Five minutes later when I needed hot water again - well, I settled for cold. How often do you settle for cold?

I know this is 4 years later but the missing grout is obviously why water has leaked under the tiles. Yes floor tiles will have the odd pinhole in the grout and also as you learned grout does absorb water and moisture/water does inevitably get under the tile but a large chunk like that would increase the amount of water under ur tiles. Even with weep holes, everytime your shower water will soak under the tiles and it'll never dry off. Your base is waterproofed but it'll also increase mold etc bc it'll never really dry. What I would've done was not use the shower until everything was dry. Maybe even use a shop vac and suck out as much water as you can from that one large area. Then regrout all the areas with holes and voids. But this would've been a potential temp fix as the entire grout job could've been bad and powdering if it wasn't mixed properly. I have mosaic floor tiles in my shower and I regret not getting larger ones because I hate all the grout lines. More grout lines means more maintenance and cleaning of it. 😑 But I do wonder how many people have shower floors that are constantly wet under the tile and not know it. I'm sure more people than not. Most people don't even realize whether or not their grout is in good shape or not. Btw. 4 years later, what did you end up doing?