Best Of
Re: We Got Steam Heat - wish I never read this book
I'd have to see real data to believe this. Pressure differences within the house is not the same as pressure differences outside the house.
I would say that the vacuum caused by the exhaust of combustion gases greatly overrides any possible difference due to forced air blower.
at any rate I hope we can all agree that it's misguided to think that steam heat provides better humidity
Re: HVAC recommendations for a new build
Are you doing it yourself or hiring a contractor? If hiring, take a look at what they recommend and how they back up and service their product. This should go hand in hand with each other.

Re: Need more consistent domestic hot water
After reading your post a few times has me at a slightly different take.
When you take a shower, is this the place where you notice your problem the most. Think about it. Its important because this could be an issue at the shower only.
You mentioned something about having a replacement coil siting in its box. Replacing one can be a rough undertaking and probably a last resort.
So at this point and hopefully this hasn't been answered yet, but what mixing valve do you have now? What is the one you plan on replacing it with?
Im asking because this and or your shower valve could be your issue. Is the "mixing valve" a "Thermostatic mixing valve?" Or is it a tempering type mixing valve? Answers to these few question should help eliminate a few trouble shooting points.

Re: HVAC recommendations for a new build
Cheapest would be scorched air. (Which is why it's installed everywhere) Next,and much better would be mod-con hydronic and maybe mini splits for AC.
I'm looking at 2 pipe steam for a new build,but my driving parameters are different.
Re: We Got Steam Heat - wish I never read this book
was going to insulate the near boiler pipes. They would do it for more money though.
This is reasonable. They aren't going to quote insulation as part of their base quote when they know the competition isn't going to either. Of course they are going to charge money to do more work.
Although I love the humidity and charm of it
Steam heat doesn't provide any different humidity than forced air, unless your system is full of leaks, which would be a Bad Thing™
Re: New return line to boiler feed tank, problems?
Umm. Weld 3" tapping to the tank and run condensate line full size?
Do I win something?
Re: Chimney question: what is this?
Yes, that is an old hot air heating outlet. Look around the basement and you'll probably see more signs of the old round duct work. Commonly installed in the 'teens and often replaced by steam in the 1930s. You may have more outlets covered up in other rooms.
Re: HVAC recommendations for a new build
balance between lower on budget and pretty good efficiency
So, you want a Maserati but at a Kia price!
Budget and efficiency do not play well with each other.
Efficiency: Your best return on investment is tightening the envelop. Keep the heat from escaping. Spray foam insulation, 2 X 6 walls. 2 X 12 Roof, 4" Foam under slab. Now you can heat with a candle! But not cheap.
No basement then slab on grade. Insulate below the slab and the perimeter and use In Floor Radiant for the heat. The more in seal up the living space the less fuel you burn.

Re: Embasy Onex Condensing Boiler
Gas pressure (static reading, drop at ignition, and if it lights off measure the high lockup when burner shuts down)
can you tell if it lights off at all?
How does the flame sensor look?
pictures please showing gas line, venting etc