Yeah, I am not a hydronic engineer, but I do hacking, yes I am a software developer and I will do all the installs on my own. Honestly, I saw so many contractors out there having no clue about and/or asking fortunes to get this "proper" installed, so I am taking the risk doing by myself. At least if something goes wrong I will blame myself not crying for the money I spent on so called "contractors".
The think is I am trying to understand why do I need that manifold, since I have only one single loop which delivers heat on the thermostat call. I do not need even a bypass or mixing valve since is a combi boiler separating on its own the DHW and Heating, and all radiators has TRV's except one which allows the flow going through all the time. Simple as is. If there is no call for heat, there is no flow or any other moving part/s.
Sure there's only flow when there's a call for heat but is it the
right flow for both the radiation and the boiler?
Without looking up the specs or doing any of the math...
Lets say you need 4 gpm on the radiators to get the right amount of heat into the system with the given radiation, and the heat exchanger on the NCB needs to operate between 1-3 gpm. How are you going to do that with one pump, and no hydraulic separation?
Conversely, lets say you only want 1 gpm on the radiation and the minimum flow requirement for the NCB is 2 gpm. That might be do-able without hydraulic separation, but you have to design it.
Then let's say both the radiation and boiler are happy with 2-3 gpm. How do you know if you'll be able to achieve that flow with internal pump in the NCB without doing the math on the pumping head for the radiation loop?
To date I have never done the math on pumping direct with the NCB with either it's internal pump or a different pump, and I don't know the actual specs on what it really needs. But as with any water-tube boiler I absolutely WOULD do that math if I were contemplating pumping direct, to be
damned sure it would both deliver the necessary heat and not burn itself up.
Also, dealing with big (150K+) modulating burners requires doing the math on the gas plumbing and getting that right too. Pulling a permit? Are you even allowed to install gas fired equipment in WA without a gasfitters license? (I don't actually know the answer to that.)
Seriously, if you're not going to do the math, use a condensing tank type water heater and a flat plate heat exchanger, an approach highly tolerant of gross hackery-tinkery (unlike water tube mod-cons):
A smallest 50 gallon
HTP Light Duty PH76-50 with the 76K burner is fine for most 2 bathroom houses with a sub-30K heat loads. (Westinghouse rebrands those and sells them through box stores too.)
Whether combi-boiler or tank type water heater, even if you DIY the installation it's important to have a qualified tech do the final review and commissioning, testing your gas pressure drops at different burn rates, verifying the combustion efficiency etc. Installing modulating burners and heating systems is way more than just a "hookin'-up-the-plumbin'& wirin' " type project if you want to work efficiently & safely, and last for more than a few years, even if that's what you did with your original water heater version.