I found a similar post on this subject, and I'm also looking to convert a baseboard register to a zone of in-floor radiant heat, but have a little different situation. I have a split level with about 4' of crawl space open to floor joists below with finished ceramic tile floors above. I want to keep all the tile floor intact. It is in a dining room under a bay window. I'm replacing the bay window with a patio door, so it needs to go. I'm wondering if I can simply tap into the same tee used for this baseboard and install a separate "zone" of radiant heat. Total floor area would be about 16' x 8'. I imagine a separate thermostat and valves to control it. Is this a DIY project? Is some kind of a kit available? I've installed hot water heaters, various plumbing and electrical home improvement projects, but would like some advice so I can do this right the first time.

Yes, what you suggest can certainly be accomplished and even as a DIY project. Just recognize that an under-floor hydronic radiant emitter is as inefficient (high required supply temp) and as expensive (requires heat transfer plates) as you can get. You will also need under-floor insulation...at least R11 to only reduce heat loss to 10%...and R22 to reduce heat loss to 5% which would be better (assuming 3/4” plywood subfloor and 1/4” ceramic tile). You will ultimately need to determine the PEX spacing, supply temp and flow rate that will generate the required heat gain at your outdoor design temp. So the first step would be to determine the heat loss at your outdoor design temp (e.g., accomplish an ACCA Manual J heat loss analysis or something akin to that).

We have a heat loss calculator and a hydronic floor heating design calculator on our website which could sort all this out if you are so inclined. You will also need to sort out how and if this will play with your existing heating system. John Siegenthaler’s Modern Hydronic Heating would be a good resource for you. You might want to try BlueRidge Company as they sell prefab hydronic radiant components and plates.