Replacing an existing monoflo system

[wjc4]

Hi All,

I recently purchased my first home and have slowly been updating it to this decade. My next project is to deal with the existing 1-1/4" monoflo hot water heating system, but I have almost no experience in the design of a hot water heating system. The house is currently all on one zone and almost every monoflo tee in the system is leaking and are severely corroded (the water we have was extremely hard but I have already installed a water softener). The exiting loop is also not very conveniently placed and required a box out to be put around it all around the perimeter of the house and with the floor to underside of joists only 7'6" putting that box out back is not ideal.
The plan is to make a 2 zone system (upstairs and downstairs) while reutilizing as much of the existing system as possible. I have a ton of 1/2" pex that was given to me so I want to use it in place of the existing 1-1/4" copper pipe loop. This way I can tuck it between the joists and save head room. I know 1/2" pex isnt ideal for hot water heating systems but I am planning on running separate supply lines to each section of baseboard to help mitigate the fact that I'm using 1/2" pex. Each supply run isnt that far either (about 30' at the longest) which Im hoping will help. The house also has a small wood stove and vents in the second floor to allow heat from downstairs to make its way upstairs and heat the upper floor.
I have attached a photo of my plan and am looking for some advice on whether my existing circulator pump, expansion tank, etc are adequately sized for the new system they will be supplying and any other general advice on the design.

Thanks in advance!

 

[Dana]

The hardness of the water doesn't much affect heating systems, since it shouldn't be constantly needing fresh water.

Half inch PEX for home-runs to manifolds on a micro-zoned system is fine, but micro-zoning has consequences to the boiler efficiency & longevity. Odds are the boiler is ridiculously oversized for the existing load or radiation even at a single zone, but would likely short-cycle itself into an early grave if you break it up in to multiples. Take the time to do it right.

Start with an AGGRESSIVE ( not conservative) heat load calculations on room-by-room basis using Manual-J or I=B=R method (if I=B=R, use conducted heat only, not infiltration.) Use only favorable (but still reasonable) assumptions about the thermal performance of the walls, windows, etc.. If you're planning insulation or window upgrades, only use the "after upgrades" performance numbers.

Then, make a spread sheet of the heat load and baseboard length per room, and calculate the ratio of load/length.

Take a look at the boiler's nameplate, and the BTUs in/out numbers, and compare the ratio of DOE-output to the total house load (the oversizing factor), and the ratio of the output to the total amount of baseboard. This will tell us how close to the line it is on short cycling, and how likely or how bad the short cycling will be (or already is.)