Small Radiant System Design Help

Hello all,

I am new to the site but have already learned a lot from paging through some discussions. Great information here!

I have a small (1300 sq ft) brick ranch that currently has a gas, forced-air furnace. I would love to switch over to a hydronic radiant system, but unfortunately, this is not really a possibility for us (we have two small kids and running the pex in the main areas of the house would be a significant undertaking).

It is, however, a possibility for us to have radiant in our two bathrooms and kitchen where there is or will be ceramic tile. Here are the stats for the three rooms that will be on the system:

1. Basement Bathroom: 5' x 8' (40 sq ft), 7.5' ceiling height. No windows. 1 entry door. When I redid the bathroom I had to move around some of the plumbing so I just busted up the whole floor so I could put pex in for radiant heat. I went down pretty deep (6") so I could put 2" of XPS foam board, then 4" of concrete. PEX (Type B, WITH oxygen barrier) is roughly 1 1/2"-2" deep in concrete spaced 12" apart. The PEX has about a 12' run to our current hot water source. Unfortunately, I forgot to measure the PEX before I poured the concrete so I don't know exactly how long the loop is, but my best guess is around 70'.

2. Upstairs Bathroom: 7' x 6' (42 sq ft), 8' ceiling height. 1 Window: 12" x 30" (2.5 sq ft), 1 entry door. When I redo this bathroom, it will have pex with 1/4" cement board, thinset and ceramic tile above the PEX. Garage is below but will be well insulated after bathroom is done. Bathroom is located about 18' from hot water source.

3. Kitchen: 11' x 13' (143 sq ft), 8' ceiling height. 1 Window: 30" x 30" (6.25 sq ft), 1 door to basement. The one wall is completely open to the dining room/living room. Flooring will have pex with 1/4" cement board, thinset and ceramic tile on top. Kitchen is directly above current hot water source so only a few feet away.

In the kitchen and upstairs bath I planned to use a plywood type track system with aluminum plates to lay the pex down.

The plumber I work with regularly (I am a carpenter/GC) has given me suggestions for this system but he is trying to talk me in to an "open-loop" system and I'm not comfortable with that and nor is my wife (for safety reasons). I would prefer to have the radiant system and dhw completely separate.

We currently have a 40 gallon gas-fired hot water tank/heater. It is in it's last few years of life so I am trying to have a game plan for when it dies how I can have both domestic hot water and this small radiant system supplied as efficiently and simply as possible.

These are the options I was considering:

1. 95% efficiency, 40 gallon tank water heater for domestic hot water, tied to a heat exchanger for the radiant system.
2. Tankless water heater that is dual-purpose for both dhw as well as the radiant system (I like this option less because I've heard horror stories about the maintenance involved with tankless. I also may have difficulty getting intake and exhaust vent pipes to the location where it would need to go - I currently have a 3" steel vent going out through the chimney)
3. Two completely separate systems: One for dhw and one for the radiant. I'm open to suggestions as to what each of those might be. What do they make that would be small enough for such a small system?

I'm open to absolutely any ideas and would love to get your feedback on this system! Obviously, this is not my area of expertise, but I do find the science behind it fascinating and really want to make sure I get it right. Again, my goals are efficiency and simplicity.

Thanks in advance and I look forward to hearing from you all!

Aaron