I have some basic DIY plumbing skills (radiator & hot & cold water pipe fitting) and have considered installing a cheap DIY underfloor heating system run off a normal radiator type system, by running regular 15mm pipes under my suspended timber floor in our new kitchen. The layout of the kitchen will only allow for a wall radiator (which I'll prob install anyway, but the BTU output on these is quite low, and below worktop level will not benefit).
Has anyone tried this, i.e placing a good lot of relective foil between the concrete floor and the pipes so the heat in theory only rises to the above floor boards and makes the floor a little more toasty on the feet ?
I'm afraid your approach would be very inneficient. Most of the heat would be dissipated away from the floor.
At the very least, you need substantial insulation below the pipes and 15 mm pipe work would not give efficient heat transfer. The pipes need to be embedded in a layer of drymix screed (preferably 70mm thick) with the insulation below the screed and the kitchen floor lying immediately on top of the screed. Don't run the u/f system under the kitchen units. The number of joins you would need with 15mm pipe gives too many opportunities for leaks and the corrosive effect of the screed on the copper would not be helpful. Taking the hotwater feed from an existing radiator is not practical, as the maximum temperature for the u/f flow is 40 C, much lower than your radiator requirements.
If you want to use copper pipe, try 10mm, or even 8mm microbore encased in plastic etc. No joins necessary and you can form it to very small centres. The smaller the centres the better as you need the maximum pipe run in view of the limited open floor area of a kitchen.
Also, use more than one circuit, otherwise the temperature drop will be too great.
Have you considered the fan assisted/water heated finned-core heaters as pioneered by Myson, for fitting under kitchen cabinets, flush to the front of the kick-panel;-- to complement these, skirting-radiators were available ?!
How about using a "conventional" underfloor heating system with flexible PEX pipe & a mixing valve.
I agree with the suggestion of using several circuits & embedding the pipes in a dry mix to increase the thermal mass & conduction but would certainly not even consider using copper pipe.
Let me know if you need help with that.