Recommendations for replacing a baseboard radiator with toe-kick radiator

I honestly recommend you try to consider something else. I ran a toe kick in my one bathroom, hated it so much I disconnected it. Because we super insulate that bathroom on a remodel, turns out we didn’t need heat at all.
Without knowing your btu needs, or the layout, Are you sure you even need heat to replace the radiator?
Is there an option for panel radiators? I had a customer do two tall, narrow, flat panel radiators on each side of a dining room slider in an open kitchen and it provides plenty of of heat. You could even do all toe kicks with flat, radiant baseboard.

Your situation is completely different. You are removing heat emitters and replacing them with a different type of heat emitter in a large room. You are not working on a small bathroom with other rooms adjacent to it. let me look a little closer at your total situation and see what is best for you.

Thanks Alan. I was quite busy today and started to answer this at about 3:59 PM then came back to it three times. That's why I was napping. LOL

@jsantoku asked "

1. Regarding the valve vs MonoFlo® tee:
   1. Wouldn't a globe valve be better than a ball valve for adjusting flow?
   2. What about putting this valve in the return pipe of the kickspace radiator branch (i.e., between the radiator and the tee connecting to the main pipe) to increase the resistance through this branch instead of putting this valve inline in the main pipe?

A full-port valve is best. When completely open, the path of least resistance is straight through. A ball valve is less expensive than a gate valve. Common globe valves are not full port. Placing a valve in the branch circuit is not necessary if there is a full port valve between the tee thru fittings, because there would be little or no flow to restrict when the full-port valve is fully open.

Finally, the restriction in a diverter tee is fixed; there is no adjustment available when using that arrangement.

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2. Is my thought that I can avoid the need to wire anything into the boiler controls but instead buy a low-temp (110ºF) aquastat (such as this one) and connect it to the K42 Twin-Flo III kickspace heater. And then I just need to supply 120V to the radiator? Do I even need a dedicated branch circuit?

I believe that aquastat is already included and prewired in the heater you selected

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3. Is my understanding that supply to the current kitchen radiator on the side with the elbow, and that would align with the main flow (see attached image).

I am not sure what you are asking here but here is what I would do:

1. Drain the boiler system
2. Remove the kitchen radiator.
3. Cut the risers to the kitchen radiator in the basement.
4. Connect the kitchen supply to the kitchen return using 3/4" pipe so the piping is the same as if the radiator had never been there.
5. Install two 3/4" × 3/4" × 1/2" tee fittings with a ball valve between the two tee fittings in the pipe installed in Step 4.
6. Connect the kickspace heater to the two 1/2" branch openings on the tee fittings.
7. Fill and purge the system as you would any baseboard loop.
8. To remove all air from the kickspace heater, continue purging that loop with the valve installed in Step 5 closed.
9. Once all air is purged, open the valve installed in Step 5 and operate the system.
10. After the system reaches operating temperature, slowly close the ball valve to about a 45° angle and determine whether that provides sufficient restriction to allow proper flow through the kitchen kickspace heater.
11. Observe the system for several days If the kitchen is too hot open the valve slightly.
12. If the kitchen is too cold, close the valve slightly.
13. Do not close the ball valve more than about 80% (approximately 75° from the fully open position of 90°).

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4. To avoid letting air into the system at any point, what are your suggestions on the following?
   1. Initial stage (prior to install/ drain where the radiator is):
      1. Should I close the last valve for the supply part of the loop (after the pump in the upper right of the photo) and close the first valve for the return (bottom right by the expansion tank I think)? And then cut the tubing to and from the kitchen radiator I'm removing, and install ball valves inline with the main pipe there?
         2. You are overthinking this
   2. Final stage (recharge/ purge through the radiator):
      1. Should I also install a boiler drain valve off of the return pipe after the new radiator and before the ball valve inline in the main? And hook a hose up to this and into a bucket? (See attached image).
         1. Again over thinking. just purge as usual with one more step on the Kickspace loop. purge with valve open until air free, then with valve closed until air free.
      2. Should I flip the fast-fill level on the pressure reducing valve (below the boiler) until the gauge reaches 12-15 PSI while the gate valve between the radiator branch tees is closed and the new radiator drain valve (downstream of the radiator), to force air out of the new radiator?
         1. You can go above 15 PSI, just don't get too close to 30 PSI.  I like to stop at about 25 PSI, but if you go over by accident don't worry, the relief valve will release any excess pressure so you don' damage anything.   …and it is only water
      3. And then open the gate valve between the radiator branch tees and and open the drain valve near the expansion tank, to force air out of the rest of the system?
         1. already explained above
5. "The PEX connecting to the kitchen radiator is actually 1/2" PEX and not 3/4" PEX, that seems to be the case for at least a few of the radiators, but I'm not sure where or why it converts between 3/4" and 1/2"."

This can be a problem on a series loop, if that is how the system is piped. It is usually not much of a problem on a 3/4" single-pipe loop with diverter tees.

With a branch system: Think of the 3/4″ pipe as the main highway, and the 1/2″ pipes as the exit ramps that feed each radiator.

on a series loop: If you have a highway with construction and all the traffic needs to funnel down to one lane, then there is a traffic jam

Using a rule-of-thumb calculation, the flow through 1/2" pipe, to stay quiet and avoid velocity noise, should not exceed about 1.5 gallons per minute (GPM). Based on a 20° temperature change (ΔT), 1.5 GPM would only be able to move about 15,000 BTU per hour through that loop.

So, a series of pipe sizes that includes any significant length of 1/2" pipe would limit that branch or loop to about 15,000 BTU/hr.

If there is a 3/4" main pipe that stays 3/4" from start to finish (from the boiler supply to the boiler return), that pipe can move about 4 GPM without creating noise. At a 20° temperature drop, that would move about 40,000 BTU/hr.

So having a series loop like this with any 1/2" pipe could limit how much heat can be delivered from the boiler to the radiators.

However, if the 3/4" pipe is the main loop and there are 1/2" branches coming off of it, then the 3/4" main can still move about 40,000 BTU/hr from the boiler, while the 1/2" branches can supply up to about 15,000 BTU/hr to each radiator without a problem.

Does that make sense? For more about your system here is a simple book that is easy to understand when it comes to what is happening inside your heating system Zoning Made Easy.