Get a recirculating pump. Not a tank. And never put supply lines inside a slab.

Better still, create a better home design all together. It’s a bad design that doesn’t take into account the mechanical room location vs the uses of the home

Also, check to see if he turned the circuit breaker back on.

6 gallon 2k watt pos shouldn't suffer momentary drop at a basin but might suffer slightly in a shower but not enough difference to make it uncomfortable. You have some options but shouldn't be attempted with instructions on the web. (1) If a new circuit must be run for heater,make it 240 volt and use an oversized 3.5k to 4k element. That's enough for unlimited continous use. If you are using an existing 20amp/120v circuit and nothing else besides lights are on it, the t-stat can be adjusted to come on a few secounds sooner IF NEEDED. Before readjusting stat,clamp a meter on to wire and see how long before stat kick's in. It should be very soon on a well desighned heater. All that's required is aiming cold inlet directly at stat sensor.

Mention was made of recirculation. I would never retrofit for continous circulation,it wear's lines out. New build have specailized lines for continous circulation. I've had satisfactory results installing pump wired to occupancy sensor or a manually operated wall switch that times out after 10-30 minutes. The down side to retro circulation is that some facuets will have hot water on cold side when pump is operating and shortly thereafter.

In the sink you will never notice it.

You generally wash your hands for about a minute at a time and a sink is about 1 gpm.

The shower is 2.5 gpm - so you might notice it just a bit.. but you are mixing 1.8 gal of cool water in with 6 gal of warm water.

Be SURE you are insulating the hot water run in the slab.

I hate supply lines in the slab, for lots of reasons. If you can avoid it, you should.

I think that will likely be the solution. Put a POS unit under the vanity for the 2 vanity sinks only in series with the main HW heater and let the shower receive water directly from the distant HW heater.

I think we'll appreciate the quick hot water at the vanity but will be OK with waiting 2-3 minutes for the shower to warm up.

Yep, will also specify insulation on the in slab run.

I looked again and short of a considerably re-design on the floorplan - there really aren't any options to put a tankless in the middle. The builder has mentioned putting the tankless on the outside wall which is common practice in this area. It saves on having to do ventilation and a roof penetration. A) the middle of the house is where the front door and windows are and the patio out back is - so exterior middle doesn't really work and B) after the big freeze in Texas where some folks had problems with frozen tankless heaters externally mounted - not sure I like that idea.

So for now I think we'll plan on the solution above - POS water heater for vanity sinks only at the distant master bath.

Thanks! This has been helpful