Tankless Heater on slab house

[Nukedaddy]

I was a plumber for many years but got out of that industry to fix nuclear plants. During my absence, gas tankless waterheaters finally improved to the point of popularity. I now own a Rinnai that is hung on the OUTSIDE of my house! You can imagine the shock to my poor Yankee Plumber's heart to see that! The house is 22 years old, and the Rinnai was installed before I bought the house (in Vicksburg, MS) and replaced a 50 gal propane tank.
It is and was common practice to NOT put in a main water stop valve where the service enters the house. (Down here they apparently never had to dig through a few feet of snow to find a meter stop or curb box.)
Anyway....The piping appears to be all copper, and headered for splitting off to all baths, but hidden in walls or behind tub-showers. (No access panel)
The tankless works great, although it takes a good while to get hot water to the master bath. Also, the dishwasher never seems to fill at a rate to get the tankless to turn on and provide hot for dishes.
So, two questions for you cats without a 15 year gap in your experience:
I read that there are ways to set up a circulating system using a small thermostatically controlled pump between the furthest hot and cold lines. Physics seem sound and no new piping. Does anyone have an opinion on them?
Also, the dishwasher. Has any one run into such an issue before? Is it possible to obtain a solenoid inlet valve with a faster flow rate that will cause the tankless to run? Or would I be begging for a flood? I just dont remember if dishwasher fill is determined by a time or a float switch.
Thanks,
Nukedaddy

 

[phishfood]

I have no personal direct experience with the crossover recirc valves. But from what I have heard from those that do: They work. Expect some warm water from the cold side when you first turn the cold on to the fixture with the crossover valve. If you ever experience crossover from the cold to the hot side, or vice versa, suspect the crossover valve, they can and do fail.

Regarding the dishwasher, I would check for any filter screens or the like on the dishwasher. Most tankless that I am familiar with require 1/2 gallon per minute flow rate, which a dishwasher will more than demand. What is the model # of your Rinnai? Different models have different minimum BTU, but once again, I would expect a dishwasher to more than meet that demand. What temperature is the tankless set to provide?

 

[Nukedaddy]

Rinnai set for 140F. I know that is pretty toasty for safety, but the hot has to traverse about 70 feet of un-insulated underground copper to get to Master Bath. I would have to go outside to get the model number, but it is a biggun. 180KBtu or so, easily supplies 2 showers and a sink or lav at the same time.

 

[pikachu4u]

What I have done before and will fix both your problems is installing a small 10 gallon electric water heater very close to the tankless and install a circ pump on the outlet of the electric heater. Then install the supplied crossover tee at your farthest fixture and your good to go.
If you need I can post a pic of a hand drawing.
Hope this helps