I am about to start a new ICF home in Tn and will be using radiant on the (2400 sq ft) ground floor. Where can I find an explanation of exactly how to set up a hot water radiant system so that each zone can be controlled by it's own thermostat? I installed my own system 15 years ago for warming tile floors and wound up (mostly) heating the house with it, but it did not have any zone control and sunny rooms were too warm. On that simple, self designed system I circulated from the hot water heater through several equal length Pex circuits with the pump controlled by a line voltage thermostat set in the tile floor. It was great and never caused any trouble.

(1) You can either contact Fred and he'll design it ALL for you (www.radiantdesigninstitute.com for quality and affordable radiant designing) for 20-40 cents/sqr ft, or go at it alone. I've read too many horror stories of "radiant heating that doesn't" on places like the Garage Journal forum to risk doing it on my own; once the concrete sets, it is either right or a huge waste of time and money. I'll let Fred do this one. (2) Now for your question: On my son's house it was a simple matter of having the thermostat turn on a transformer (Honeywell aquastat?) which sends juice to the circ pump AND a Wirsbo zone valve simultaneously. Pump comes on, that zone valve opens, and warm water hits that area. Any plumber will be able to show you in seconds. ( have a manifold of however many zones you want, one Wirsbo valve/zone, and one circ pump to pressurize the manifold. That said, you better also include a mixing valve to keep the water temp below about 140* (I read various numbers there), as concrete does not like water at the temp that baseboard radiators do (180* or so). Good luck, and be sure to install some kind of wood (??) in the corners of those ICFs so you can lock the sheet rock down if a plastic vein is not w/in reach. later. john

You start designing a radiant floor heating system buy choosing a fuel source; floor coverings and heat loss, per room. These numbers dictate tube size, length, spacing and delivered water temperature. Then you can pick a boiler to match the total heat load and don’t forget to install an indirect fired water heater.

We help with this kind of system, we have pipe lay out, pump curve, heat loss and on. In a 2400 sq ft house zoning is typically accomplished with pumps, generally 1/2 pex pipe is fine. Insulation, windows, floor covering are all factors. A condensing boiler with side arm tank works well but different areas of the country will determine fuel selection/cost. Here in the North West our best prices are for electric or Natural gas. Geothermal or reverse cycle chillers are also a option but can increase the up front cost and are more complex for self installers. I have seen a considerable number of Fred Seaton designs, he favors a 500 foot 1/2 loop on a 18 inch spacing, I would be very cautious of this type of design.
Dan

I also need help! I'm building an guest space on the end of a 78' X 38' X 16' tall shop building. It will be just under 500s/f on a heated slab first floor (kitchen, full bath, laundry, small eating/seating area) with a 340s/f sleeping loft. Financial constraints require me to build it in phases unfortunatley. Ultimately the remaining shop area (64X38) will have radiant floor heat - wood fired. My first phase is to complete the quest quarters (to temp. live in) and some of my questions are,
1. Can I use an on demand (tankless) HWH for the slab heat and domestic hw ?
2. I like to put an air handler in the attic to blow down hot air for heating the sleeping loft. I'm not a HVAC guy so my terminoligy may need explaining... pipe hot water thru a coil, blow air thru! with the necessary other coil for AC. I am aware that this may not be as cozy or even heat as radiant in the loft but would like to have the AC wihtout the added expense of doing the radiant under the wood, this is not going to be an occasional use space after the "real house" is built.
3. My walls are framed and should have about a R30 (3" iso foam with 4" fiberglass) or so insulation value with 9 Anersen windows in the whole space. The ceiling will be insulated heavilly also.
4. Unfortunatly the orientation of the long wall is to the east (only a short time of early morning sun)
5. Eventually this space will be tied in to the wood fired boiler for the shop.
6. I have read that It's good to stay with equal 300' loops. I figure only 1 zone for each floor.
7. I'll have at least 2" ridgid foam under the slab.
8. The tankless HWH will be fired on LPG. (I'll need about $25,000 to get the electric in)(phase 20?)
9. How does the radiant perform under/in a wood framed floor condition? with carpet?
10. I had/built a car wash a few years ago and we installed floor heat for de-iceing, we just tied the tubing to the conc. wire (making 12" centers of the tube spacing) and just had a threaded black pipe assy. (tees and close nipples with ball valves)for the manifolds, no problems !
11. How much difference is there with the tubing, alum center, oxy. barrier? size, what should I look for.
I'd appreciate any suggestions or answers. From reading other responses it sounds like there is a wealth of knowledge available from some of you guys out there! Thanks!

Tankless water heaters are not boilers, but boilers can safely and efficiently heat hot water. You too need design help. This is a complex design taking many hours to put together, starting with a heat load analysis per each room.

Tankless HW heaters aren't boilers per-se, but some models are even fully ASME certified as boilers (eg. Rinnai 98i ASME, or the Nortitz NH series, Noritz 0931 ASME , etc.), and many others still perform adequately as boilers.  Most (all?) of Takagi's product line is marketed (and warranteed) for use as space heating boilers.   Navien has designed, tested & is marketing a product specifically targeted toward tankless based combi or pure heating system designs. (http://www.navienamerica.com/content/heatingbox-training-manual.pdf ) They're boiler-like enough to work as boilers, with caveats...

The differences between tankless HW heaters and standard low-mass copper tube boilers aren't huge.  Tankless heat exchangers are designed to be more tolerant of condensing-temperature return water and higher delta-Ts, and the internal controls aren't designed with heating systems in mind, so you have to work your design around those quirks.  Most have somewhat higher head (needs more pump) than standard copper tube boilers, and they're not a "one size fits all" simple swap-ins for existing hydronic systems.  But in systems designed from scratch they can be better than they look at first glance.   There is quite a bit of study of tankless combi systems going on in CA right now, both by academic types and in programs by utility companies.  Expect to see actual performance & reliabitity of various configurations published in the next couple of years.  Configurations under study include hydronic baseboard, coils in air handlers, as well as radiant slabs & staple-ups.

But as a DIY project...  don't want to go there- too many factors to consider.  Heating systems designed by hacks often both LOOK and PERFORM like they were designed by hacks.  Go with a pro who understands your design goals- preferably a local pro with references.  Generic system packages sold via internet can be of dubious value without professional installation & tweaking of design factors. Debugging a failed system in the dead of winter via email isn't the stuff of happy dreams either.

If you go with a tankless as the heat source, make sure there is technical competence on using them in heating applications and spare parts in your area.  But the same advice holds for any moderately complex boiler too (all mod-cons apply).  The days of dumb Keep-It-Simple-Stupid cast iron boilers are numbered.  Any electronically controlled modulating boiler is bound to have more issues than dinosaur boilers, but with that complexity can come higher overall efficiency.

But taking a half-step back...

The main situation where a tankless HW system might be a reasonable value compared to a modulating condensing boiler is in systems where for the bulk of the heating season the return water temps are above condensing temps (or barely into the condensing range).  When this is the case the mod-con only delivers ~88% combustion efficiency, compared to a tankless' 84-86%- not exactly blowing it away on performance.  This condition may be true in many staple-ups, but almost NEVER in radiant slabs!! In a radiant slab the in-situ performance of a mod-con is likely to exceed the boiler's AFUE test rating.  Unless the annual heating requirements are small, the difference in up front cost will likely have a very reasonable payback in fuel savings with a better-performing mod-con (and it'll be easier to find pros to design & maintain the system for you.)

The other situation where a tankless could make sense is if your design-day heat load is small (<25KBTU/hr).  Most mod-cons don't modulate anywhere near low enough to not lose some efficiency from cycling more often under those heat loads, whereas most tankless heaters can modulate under 25KBTU/hr out with little or no loss in efficiency. Many tankless HW heaters go as low as 9-10KBU/hr out, so with any reasonable buffering (or even none, if it's a single zone) they can still perform quite well.  But here again, the high thermal mass of a radiant slab is like having a large buffer, and a mod-con won't take the same performance hit that it would in an ultra-low heat load staple-up.

Bite the bullet on this, hire a pro and buy a mod-con.  (And NRT.Rob faints... ;-) )

So lots of talk about mod-con VS tankless VS old iron and a healthy buyer be ware. Heat loss is essential. You are looking at something like 3,500 sq ft if my math is close, most a large open shop.
Here are the key point's I spot, you mention a fan coil unit for forced air, typically that is using delivered H20 at 145, what ever mechanical configuration you use, likely it will be primary/secondary loop, primary will need to be 145 min to satisfy the air side. The secondary loop will satisfy the floor at lower temp. So this would imply that the mod con will be held at 145 to satisfy the primary loop, secondary temp will be the in floor reduced by a 4 way or 3 way valve. so much for condensing.
Old Iron boiler set low 145 deg and a 2 coil 75 gallon buffer tank, extract the in floor and forced air from coil A (now you can also dump hot air in to the shop untill you finish the build out with the slab heat several years down the road. Next throw some solar on coil B to cover the tank in fair weather, domestic h2o is a 75 gallon storage capacity should the 3500 sq foot space become something else other than a shop in the future. Tax credit now available on the buffer tank and panels and labor to install 30%.
Or forget the air handler or run it low temp for supplemental and air movement/filtration and stick with the modulating condenser.
You mentioned power constraints. If are you off the grid we see the hi-tech computer controls not to compatible with varying electrical signal, you likely will want to place a transformer in line to buffer the signal.
Pipe install is straight forward, If you built out a car wash there is not a lot of difference here. Forget the Iron T's there are proper pump manifolds and pipe manifolds that simplify all of this. You are already familiar with fluid movement, unistrut, and mechanical rooms. A good gas/boiler technician is necessary/essentiial.
Once a design approach is settled on mechanical lay out and pipe lay out falls in place.
I am not a fan of open loop, or tank-less water heater systems as boilers. We don't design to this. Some of the people I have seen use Takagi's on the scale building you mention have swapped them out over the years to replace with a proper condensing or non condensing unit. Perhaps there are some better units out there or coming.
Dan