Andy, staggered studs?

I do know that it was argon filled thermopanes. The house is fully tubed, with an air handler for A/C, and general fresh air exchange.

The loading looked to me to be all handled by std 2x4 framing to the inside of the wall. Some points were carried by posts, there were a few hips. End studs at door, or window locations were 2x6 stud, and headered as a 2x6 wall. Basically looked like a 2x6 exterior walled construction 'til you were about 8 foot away. Then your eyes would cross a little. I scratched my head a few times.

"Andy,
I just abbreviated the air line tubing to "Pex". What I have chosen isn't the pipe sized type used for water/hydronic but a tube sized rated at 220 psi working pressure.

I did spec in a W/M Ultra but am looking at others too. I also what to listen to what the local heating contractors have to say. I think the Ultra has a recirc pump for the primary loop as well. I'll check it out.

With the hot water, I haven't really looked at many of the tankless brands very much. I want to hear what the contractors have to say. The side arm HWH indirect is a option I've already put in my spec sheet. For one bathroom it may be more than needed. (It's going in the house addition though!) A efficiency comparison between running the boiler all summer for the hot water and a less efficient on demand would be interesting. The tankless will be within 10' of the lav and directly behind the shower so the circ pump really isn't needed.

I'll take a look at the Taco - sounds interesting! Is it for HW or a hydronic zone loop control?

Thanks"

If your only useing hot water for a single bath, and your needs permit, you might be able to get away with a small electric water heater. Say 20g unit, only because of the shower. Easy maint. The maint. on the new style boilers, and tankless water heaters is really pretty basic. I'm quite positive you could do it in under an hour. The only thing you may need someone else to perform is a flame analysis. Even then only needed if your unsure of the unit. Most will tell you if they're unhappy.

The internal recirc on the Navien also serves a small buffer tank.
Try to picture this as I try to describe it in type.
You run hot water in the house. The supply piping then will have hot water in it. You turn it off, and you begin to lose a little heat. You turn on the hot water again. It will take the unit a small amt of water to fire again. Creating a "cold sandwich" if you will. The internal recirc/tank will hopefully end this small problem. That coupled with he possibility of 98% doesn't hurt.

I've got a few Takagi units that have been in for 3+ yrs on well water. 2 of which are used only for hydronic. Both are on seperate wells from the house. Both guys insisted on only useing a heavy sediment filter for supply water. 0 repairs.
Maintainence is just that. Quick clean of the combustion chamber, and flush on both sides of exchanger. Useing a mild acidic to help in releasing the build up, if any.