Advice on hydronic unit heater install

[AkNeedsHeat]

Hi, I have an older cast iron boiler that’s still working like a champ. There’s a loop not controlled by a zone valve that is stubbed out in the garage, so water is circulating any time one of the other zones is calling for heat. This loop also pops out in a somewhat large basement room to a ceiling mounted fin tube and fan box heater.
The stubs in the garage are 3/4” copper. Supply and return are 4” on center and each have a gate valve before they connect.
I would like to use this loop to finally install a hydronic unit heater in the garage. I picked up a modine hot dawg, and have a design on paper, for which I’m hoping I could get your feedback.
Diagram below, but here’s my explanation:
The stubs are about 8” off the concrete slab in the garage. The unit heater will be mounted off a 10’ ceiling and has 1/2” fittings. My idea was to use two Venturi tees to adapt from 3/4” to 1/2”, spaced 14” apart, followed by ball valves, followed by drain valves, and then an air bleed valve just before unit heater (not in diagram). Everything in garage would be wrapped in foam pipe insulation.
I would like to do this all in oxygen barrier pex just because it’s easier to work with, I’d avoid a lot of hard angles, more seismically friendly. I’m not strictly opposed to doing it all with copper tho. In any case my plan is to secure assembly and vertical runs to wall with unistrut. If wondering, the unit heater is way oversized for my space but has an adjustable fan that will lower btu output appropriately.
I would appreciate any feedback you have.

 

[Fitter30]

Pex for he heat is rated at 180° different manufacturers could have a different spec. Your 14" between tees might not be enough pressure drop.

Comfort Calc

 

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[AkNeedsHeat]

The oxygen barrier pex at Home Depot is rated for 200F at 100PSI. My boiler runs 180, and I have a run of about 25' rigid copper between the boiler and where this stubs out. I figured that would keep the water within spec, but do you think it's too risky? Is this because it's too close to spec (90%) or other reasons? I'll read anything you want to say on this matter (experiences with it, observed failures, etc).

Appreciate the note on the distance between the tees. I had been thinking 12" min was recommended, but I see people talking about 18-20" online. I'm not too limited on space for this, so I can open up that distance. I should mention that I've looked at this document from Taco, https://www.tacocomfort.com/documents/FileLibrary/100-3.6.pdf, explaining how to calculate the hows and what fors, related to pressure drop and distance, etc., but haven't done the math yet. Kind of winging it, on the assumption that the bypass setup should work well enough. I think that with this information from Taco, I can calculate the expected flow rate through the bypass and that would help me understand expected heat output of the unit heater, but I'm not sure if it can tell me an ideal distance between the tees.

I appreciate your feedback and any further advice on the matter.