It seems that you are looking to get custom-built brick fireplaces. It is very difficult to give information about that in the abstract. What you need to do is contact at least 3 different fireplace builders, tell them what you want, and ask them to bid. The more detail you can provide the better. Otherwise, you can fall into the trap of expecting to spend XX dollars and then finding out that it will cost a lot more because you didn't specify certain important details. Obviously, such an effort means that you will have to have a pretty good idea of what it is you are looking for. Just a few words of "common wisdom:" In a conventional wood burning fireplace, most of the heat goes up the chimney. They are not energy efficient. Washington has in place a program of tax credits (not deductions, actual credits) for the installation of systems that meet certain ENERGYSTAR standards. Since you are planning such a large project, it might pay for you to look into that program. You should also consider putting in gas fired direct vent systems. These are prefab units, and some of them are really attractive. I do not think they are eligible for the tax credit because they use a non-replaceable fuel. But they are highly efficient. We have one, and it's almost like a blast furnace. You do need to get a remote thermostat to regulate the heat output. If you google direct vent fireplace, many options will appear for you to review. One more suggestion: the website hearth.com. They have a forum devoted to people for whom wood burning fireplaces is an all-consuming passion. Good luck with your project.

If you try to put a 12 inch supply line off of that 10x6 trunkline, then yes you will have airflow issues. Look at it this way. Your 10x6 trunkline can handle around 260 CFM (cubic feet per minute), but that 12 inch pipe handles about 700 CFM. So, you could drop the size of that pipe you are running to an 8 inch piece of pipe and still keep your current static pressure. Do NOT go any bigger than that 8 inch pipe because you will lose a lot of pressure. It would be the difference of using a pressure washer with a good tip on it to clean your drive way or a hose with your thumb on the end of it trying to do the same job,

Andy, I understand your frustration. Hydronic heat specialists are getting more difficult to find, I understand. Just be glad that you don't have steam. If you ever decide to replace the boiler, your engineering background might help. There a several variables and lots of calculations that need to be done to be sure you end up in a working envelope -- water temperature, heat loss from the structure, heat transfer via the radiators, circulation rate, temperature drop in the individual loops,... You could have a new hobby. The boiler heads in those two fora will be able to help you out a lot, I think. Someone there, maybe several people, will know that boiler well. Just tell them you are tired of the excess heat and give them your boiler and indirect hot water tank details. Be sure to ask them about scheduling the boiler on and off when it is only needed for water heating. Look at that this way, if you have your boiler off 16 hours a day while working and sleeping during the week, and 8 hours on Saturday and Sunday, you've solved nearly 60% of your problem. The flue can go in two ways and depends on what kind of boiler you have. There is subtlety that I am skipping. Condensing boilers have low temperature flue gas, and use plastic pipe going horizontally out of the side wall of the house. They are usually using outdoor air for combustion eliminating another heat loss point. Condensing the steam means heat recovery from that steam that used to leave boilers up the chimney. Now it runs down the drain after giving up its heat. All old fashioned residential boilers used to be non condensing. They will have a metal flue pipe that goes up to the roof line in some kind of double wall, insulated metal, or a metal joined to a masonry chimney. The problem is a mismatch between boiler and flue. Obviously a high temperature boiler will melt/burn plastic pipe. I've never heard of that happening. Exhausting wet (and acidic) flue gas in a masonry, or inadequately corrosion resistant chimney will cause a much slower failure and that happens even though it should not. Particularly bad are very old, unlined masonry chimneys no tile liner. The acid gets to the mortar and eats it up leaving gaps. Sometimes, with a change to a more efficient condensing boiler, a plastic flue can be installed within an old-fashioned chimney and go up through the roof if going out sideways is not convenient or not eye-pleasing.

Thanks so much everyone for weighing in. I definitely think LVP might be the winner. I realize it doesn't conduct the heat as well as tile but maybe that's not a big deal. I think a wood look would be my preference, maybe with a couple area rugs to define spaces within the wide open rec room, make it feel cozier and provide extra cushioning for kids. I grew up with a basement with wall-to-wall carpeting and my sister and I did a lot of roughhousing down there (mostly pretty safely, although once she accidentally broke a friends' collar bone with a wrestling move!) so I was just picturing all that on a tile floor and cringing.