This will be a long post, with lost of background information and a couple of questions.  First I'll list my questions, so you can keep them in mind as you read the background stuff:

1. Which is a better choice for a radiant panel system - Warmboard, Roth, or Infloorboard (or some other option I'm not yet aware of?)

2. Is it possible to use a residential water heater (on-demand or high efficiency storage tank unit) to heat the water for my system, or do I have to go with a boiler?

3. How difficult is it for an experienced do-it-yourselfer to lay the panels and run the tubing on a concrete slab (I would have a contractor handle everythng else -- ie, layout design, installing the boiler or hot water heater, and the manifold, etc.)?

Now for the background: I recently bought a 1950's house.  It's 1800-sf, single-story, slab-on-grade throughout.  I live in Northeast Ohio, and our winter low temps can be in the teens -- although it's not unusual for us to have snaps where we're at below zero for a few days at a time.  I'm seriously considering retrofitting with hydronic radiant floor heat for a couple of reasons.  My current heat is forced air (gas furnace), with the ductwork installed in the slab.  I've always hated forced air heat for comfort reasons, and the forced air system in this house is problematic for other reasons, too.  The vents are badly located and are really limiting my furniture placement choices.  Also, the ductwork is 50 years old now, and although my pre-purchase inspection detected no rust issues in the ductwork, I've had colds and sinus problems constantly since moving in last November.  I'm guessing the old ductwork is to at least partly to blame.

Another factor is that I'm planning an addition - it would be a 350-sf master bedroom space (I would also be converting an existing bedroom into a bathroom).  It will be slab-on-grade.  I need to choose a heating option, and since I've been really impressed with hydronic floor heat my sister used in her new house, that's my first choice.

What I plan to do is install the radiant system in the addition and the remodeled bathroom now (and possibly the other bedrooms now, too), and phase in the rest of the house as my budget allows.  Fortunately, my current furnace is in good condition, so hopefully it will last for a few years and I can continue to use it in the parts of the house I haven't yet phased in.

If it's relevant, I'll add that my flooring over the radiant heat will be ceramic or stone tile in the bathrooms (and kitchen, mudroom and perhaps also the family room once I phase those areas in) and engineered wood or laminate in the other rooms.  I won't have any wall-to-wall carpet in the house (although I do have several area rugs).

In case you're wondering about my plans for air conditioning, I realize that switching to radiant heat and abandoning my old ductwork will mean I have no a/c.  Currently the house has a very old central a/c unit but I'm not a huge a/c user anyhow.  In my old house, I had window units in the second-story bedrooms only, and I used them infrequently. I'd just turn them on for a few hours at bedtime to cool down the rooms, then shut them off.  If we got hot during the night, I'd open a window.  My plan for the current house is to live without a/c in the rooms I've retrofitted, and eventually I'll get around to installing a new a/c system with the ductwork running through the attic.  And I'm thinking about installing a whole-house fan in the near future so the a/c may never become an issue at all.

Now to some details on the radiant heat options I've considered.  Since I have a concrete slab foundation, the two options are installing pex tubing on the slab with the thin-set method, or using panels over the concrete.  I've talked to a contractor certified to do radiant heating, and he thinks I should go with panels.  He also said I should use panels in the addition, even though I could theoretically install tubing in the new foundation when it's poured.  He said it would be complicated and expensive to design the system to accommodate in-slab tubing in one room, and panels in the rest of the house.  If anyone disagrees with this advice (either the part about using panels as opposed to thin-set, or about using panels in the addition versus embedding tubing in the new foundation), I'd love to hear your comments.

Assuming I go with panels, it's time for my question about which panels.  The contractor recommends Roth.  I'm wondering what people here think of them.  I'm guessing that when you apply panels over a concrete slab, it's important to insulate undeneath the Pex tubes, to prevent heat loss into the concrete.  The Roth panels have a base made of foam -- would that provide better insulation  than the wood and composite bases of Warmboard and Infloorboard?  The Roth panels also appear to have aluminum covering the entire surface, including the tubing channels.  I believe Warmboard also has this feature, but Infloorboard does not.  Is that an important factor?  Does anyone know how the cost of Roth panels compares to Warmboard and Infloorboard?

What about the boiler vs. water heater issue?  The contractor I talked to says he always uses boilers.  One reason is he doesn't recommend sharing water with the drinking/bathing plumbing system.  But would it be possible to install a second water heater and use that exclusively for the radiant system?  The would certainly be a lot cheaper than a boiler.  My sister runs her system (in Massachusetts) on an on-demand water heater (in-slab tubes on the ground floor, radiators on the second) and it works great.

Finally, I'd like to know how easy it would be to install the panels and tubing myself.  The contractor said he'd be willing to have me do that, if I choose to go this route.  I've laid tile and put down click-and-lock laminate successfully, and I feel comfortable using all the tools I'd need for installing the panels.  I would consult with the contractor (or some other pro, including the panel supplier if they offer this service) on layout, so the do-it-yourself part of the project would just be the actual, physical installation.

If anyone can help me on these questions, I'd really appreciate it!

Search this site- there's a lot of discussion about whether & when using a hot-water heater makes sense or not. Without knowing your actual heat load it's hard to figure much of anything, but if you're using the same hot water heater for both domestic hot water (DHW) and heating, DO use isolating heat exchangers. Your contractor is right that "open systems" that run DHW and heating water in the same plumbing is asking for trouble. (If that's what your sister is doing, she may be in violation of MA code unless she cycles the heating system pumps multiple times even when not heating on a schedule prescribed in the code.) If your heat load is in a reasonable range, (do a manual-J type heat calc), yes, you CAN use a hot water heater- with some caveats... (and some design skills.)

If you're pouring new slab, YES you should insulate it from the ground (2" of XPS minimum, more around the perimeter edges for slab-on-grade). But embedding the PEX in the slab is easier/cheaper than Roth panels on top, and puts the thermal mass of all that concrete inside the thermal envelope. for retrofitting over the old slab, if you can take the hit in headroom, add what XPS you can.

As for AC, and the whole-house fan issue: You're probably better off boosting the insulation and air-sealing the place if you can. Whole house fans tend to be very hard to seal & insulate around, which is bad both in summer & winter. In winter it sucks the heat & moisture up into your attic space to condense & rot stuff while boosting your heating bill. In summer it allows a chimney-effect to draw hot humid air into the living space all day. If you have a tight structure, good insulation values throughout, and can manage exterior shading on the E/S/W sides, central AC can be replaced with small-duct heat-recovery ventilation, controlled to push higher ventilation levels at night if/when you want to lower the interior air temp. Leave the windows closed all day, open them at night if you like, but close 'em at night and let the HRV controls handle it if you don't get up early enough to close all the windows before sunrise.

It makes a lot more economic sense to really go over the thermal & pressure boundary issues, tighten up and insulate the place before embarking on high efficiency or super-cushy heating systems. The size & cost of the heating system shrinks somewhat in proportion to the actual design heat load, and the operating costs go WAY down (even using so-so efficiency heating/AC systems.) I improving the envelope is usually a better investment than going straight to high-efficiency heating, since it lasts pretty much forever, and will work no matter what the eventual heating system is. Any additions/renovations you make, think "superinsulation", not code-minimum- shoot for at least 2x minimum. Consider retrofitting to a "hot roof- spray foam insulation between the rafters, sealing off all attic venting- that's usually the easiest retrofit way to guarantee a good pressure boundary, keeping all structural wood on the inside of the thermal boundary where it can be humidity & temperature controlled to avoid mold. You can still further insulate at the attic floor with blown cellulose (preferred, since it's a hygric-buffer- keeps condensation in check) or fiberglass batts (less preferable, but doesn't take special equipment to install.)

If you can, digging in 2' around the old slab perimeter and putting 2-4" of XPS vertically, glued to the foundation can often cut a significant heat loss. If it's the typical '50s 2x4" 16" o.c. constru with R11 batting, retrofit dense-packed cellulose can cut the infiltration considerably while giving a modest R-value boost (odds are 40% of your heat loss is infiltration anyway, not R-conducted heat.) If you're ripping up intererior walls, add at least 1/2" of foil-faced iso-board under the new finished wall to act as a thermal-break against the thermal short-circuit of the studs. (If you're re-siding the exterior, go for even more, but make it breathable fiber-faced iso, not the foil-faced stuff.)

In the meantime, as you plan to continue using your furnace, take a weekend day and seal all duct joints & seams that you have access to with duct mastic (you can get li'l tubs of it at your usual box-store. (If it's all subterranean, it might not be much though.) Use FSK tape (2" aluminum, also available at box stores) to seal all joints & seams on the furnace itself. The buried ductwork may not be rusted out, but it may occasionally see condensation, and may be a mold-spore spreader. It seems that was all the rage in the '50s (I'm currently dealing with a ca. 1955 slab-on-grade institutional building with similar buried duct issues. I'm trying to get them to abandon it for a healthier cheaper to run more efficient hydronic system.)

BTW: Did your sister have her system designed by a pro, or was it an internet-store "one size fits all" system? (You can use the private messaging system on this forum to respond.)

You might consider an electric solution. Operational costs run a little higher than natural gas (in most areas of the country) but you save yourself the trouble of installing a boiler and plumbing entirely. Specifically, Calorique makes a paper-thin heating element. Lay a layer of insulating foam down over your slab, then Calorique on top, then your flooring. That would be perfect for an existing slab. Dana1's comments about insulating the slab and blocking infiltration are spot on. For a newly poured slab, INSULATE including the perimeter. Nexans is a good electric radiant solution for a fresh pour. Hydronic is great for new construction and large areas, but I favor the simplicity of electric for remodels and additions. Of course, I am a material supplier, so I'm biased that way.

A LITTLE higher operating costs than natural gas?

If you live in the land of $0.05/kwh electricity, or have ultra-light heating loads go for it.

In my neighborhood ($0.18/kwh $1.58/therm NG) Nexans, Z-mesh, Calorique or similar would be a dubious choice indeed...

Electric radiant would be relatively cheap to install, but 3x as expensive to operate as NG fired hydronic using a mid-efficiency boiler in my market. A few grand a year difference in operating cost buys quite a lot in up-front installation even on a short term 3-5 year net-present-value basis, let alone an equipment lifecycle cost. YMMV.

Since there are ~large ducts in the slab I am wondering if it would be feasible to run flexible plumbing through the existing ductwork and then install radiators (powered by a central hot water boiler/furnace) at the forced air ducts? I suppose the flexible plumbing could be self-iinsulated--as I doubt it is wise to blow insulation in slab on grade ducts--in case they would get moisture.