Best Of
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
Re: Need more consistent domestic hot water
I love the informative views and experiences of you all wrt water heating. The biggest issue, as some mentioned, is how much water a household needs or wants. A 2 person household when they only do a quick shower a day is a candidate for a simple electric wh.
As far as tankless vs tanks, it seems my customers like tankless better than I do!! For instance, my neighbor with 8 children loves his tankless and that one gets USED. As for maintenance, the simplicity of tanks is appealing to me but then there is anodes and failures also. We have seen a fair number of failures with stainless indirects of various manufacturers. We will not put tankless in if a customers' water is ugly and untreated, so most of them I put in see very little scale/dirt buildup even after several years. If you count the hours runtime on a tankless you typically will find they run but a fraction of a boilers' run time, thus they do last.

Re: Need more consistent domestic hot water
Hi @hot_rod , I like to think of it as the plumbing training the people. Tank-type heaters with modest reheat capability teach us that back to back to back showers while washing the dishes will give somebody a cool/cold shower, so we adjust. A tankless unit, sized for multiple uses just delivers, more or less endless hot water. This is the plumbing training us that we can be wasteful. I see endless hot water as needed only in some commercial or industrial applications… It's sorta fun. I'm beginning to sound like one of those old guys, yep!
Yours, Larry
Re: Is there a way to save this boiler?
wow how thin must burnham be making those castings?
And wow that boiler looks big. Ed’s advice above is very good
Re: Need more consistent domestic hot water
Hi, Thinking is good🤔 Have you considered adding an indirect tank? It would give you more consistent temperatures.
Yours, Larry
Re: New return line to boiler feed tank, problems?
I guess they will have backed up condensate in the system. I don't understand why the return tapping's in the tank are so small but the installer decided that the pipe need be no larger than the tappings.
The 1 1/4 strainer is probably already plugged.