I am new to radiant heating, but believe in its value. I will be installing some version of it in my proposed contruction of a single story ranch w/walkout basement. Total 1st floor sf approx. 1700.    One idea I have received is 7/8" pex under the joist w/alum heat transfer plates. The heated water either coming from a Polaris (50 gal) or a tankless type.  The other idea has been use 1/2" pex in the floor, stay with the Polaris heater.  The trade offs appear to be less line with 7/8" but would there be slow 'heat up' as well as slow 'cool down'?   The 1/2" would be more manageble to install, and I could run at cooler temps, but would be using at least 2x times of tubing, maybe using smaller pumps?  I want to do as much as possible installing either system, but will certainly have professional help if needed.  Is there a preferred system? or is it pretty much fielder's choice?  Any advice will be glady appreciated.
Thanks,
Ray

pex pipe is pretty cheap at ~25cents a foot for 1/2inch w/oxygen barrier (hint, check ebay) so that should not be such a concern.
your heating choice on the other hand, well...i would continue to do some more research if i were you. there are better choices
out there.

Without being specific, (your choice however) should I be considering a condensing boiler instead? What other type of heat source would be considered a 'better choice'?

OK,
1/2 inch pex pipe is right, oxygen barrier, heat transfer plates.
7/8 is a nightmare to work. Why anyone would recommend this is beyond me. We design 3/4 pipe for commercial slab work only.
7/8 on a staple up, nuts. zero savings here.
Next if you want to keep your efficiency low, consider a surface mount like our RHT floor panel system, or quick tracks. You are already committing to heat transfer plates, the additional cost for the RHT floor panel is about .65 cents sq ft and your delivered water temperature can hit about 20 degrees less than an staple up. Direct fuel savings and response time improves.
Dan

there is zero benefit to the huge pipe unless you are running your domestic hot water through it, which I strongly advise against. Plus you would want heavy plates, not the lightweights, if you're in the joist cavity, in the vast majority of cases.

site built overfloor sandwich is cheaper for slightly better output and more work.

I would consider a Phoenix Evolution or a mod/con boiler with indirect.

"I would consider a Phoenix Evolution "- this seems like a great solution but also pretty costly for something that's new and untested. has anybody had any experience with these units? my local pace supply rep had the same concerns and told me so. if they can deliver as promised tho they seem like a terrific reduction in installation time and complexity.

untested? the phoenix has been out for quite awhile. the "evolution" part is simply an injection mixing station on the side of the tank. Very similar to the Taco X-block in goal and a strategy used by combination water heaters for a long, long time. nothing magic about that. nothing untested about any of it.

it is expensive; and so is the Polaris with comparable hardware, and so is a mod/con boiler and indirect. I prefer the mod/con route because I can do really good things with them and it can be cheaper, but for a situation without any good design behind it, the evolution does a lot of good stuff.

We like the LAARS Mascot for our designs, straight forward application, your home would likely need 2,400 feet of pipe (1,700 sq ft X1.4 (8 inch pattern).
This can be driven by the pump in the boiler. a side arm tank can be added for domestic H20.
Dan