Help Altering System with mixed Radiant and Baseboard

Robot_Tolstoy

I moved into a house with a incorrectly plumbed hydronic system, It is a 2000 SqFt home and it has a non-condensing Crown AWI128 cast iron boiler running LP. The basement is in-slab radiant heat and the upstairs is baseboard. At some point the radiant heat was disabled because the water in the loop was running too hot. This also dumped a lot of cold water into the boiler on start-up. Flue and damper were replaced at some point from the condensation. A sketch of the layout is shown in the first image and it looks like a supply and return manifold system with the main circulation pump, taco 0014-f1 medium head medium flow, on the boiler return. There is a second circulation pump that pumps into the cold port of the mixing valve, which doesn’t make sense to me. The two pumps in the system try to force water through the 3-way mixing valve. When the other two baseboard zone valves are closed, the main pump is dead headed except for the path through the 3-way valve. In this scenario, the main circulation pump wins out at the mixing valve and forces unfixed boiler supply water through the radiant loop. The expansion tank is on the return before the pump, and whoever plumbed it made the return manifold higher in elevation than the supply and put the expansion tank at the high point with an air bleeder there. There are no circulation pumps on the baseboard loops.

I would guess the radiant heat was originally set up with a 4 way mixing valve based on the physical piping layout. I am wondering if the middle solution in the second image "zone valve hot water bypass would work? Do I need the differential bypass valve? Would people suggest adding a 3-way anti-condensation valve between the main manifolds, or just one on the return of the radiant heat loops?

Current system

considering piping the radiant loop following the middle layout "zone valve hot water bypass"

or I could use a 4-way mixing valve, somewhat like what is piped in this diagram but my circulator is on the boiler return.

hot_rod

If the low temperature radiant is 20% or less of the total load, which in a basement it probably is, then you don't need boiler return protection. A simple dual temperature system could look like this

A boiler primary loop.

Hi temp loop

Lo temp loop with 3 way thermostatic.

If the basement is one zone, you could use a manual 3 way and build a proportioal mix system.

This would be most efficient as both hi ad lo temperature modulate based on outdoor temperature

Robot_Tolstoy

https://forum.heatinghelp.com/discussion/comment/1905715#Comment_1905715

I would guess the radiant is close to half the load. It is about equal square feet on the two floors and half the basement is above ground (walk out basement). There are two radiant zones but I don’t know how they are distributed. There are two baseboard zones upstairs. Right now the system is a direct pumping scheme, not primary secondary. Are you suggesting to re pipe the entire system? It’s original boiler and piping from 2003.