Radiant for new slab_need advice for an easy cheap way to supplement F/A htg in 3 rms

We are building a two-story home at 4500' elevation in Northern AZ (zone 4 to 5). We'll have a ducted, 6-ton geothermal F/A system, but would like to add comfort to three of the downstairs, slab-on-grade rooms by adding a simple, low-cost radiant system (the footers will be poured in a week or so). The two important rooms are about 400 and 200 SF, and the less-often used room for the radiant system is 200 SF (we would do a fourth area, about 400 SF, if the system would remain simple). I'm thinking something like three to four separate runs of PEX, and maybe just ball valves to regulate the flow from a water heater (but plan ahead for possible thermostats?). If so, what PEX specs (diameter, etc), what type and specs for the water heating unit, etc? (i assume a gas heater is the way to go - current winter rate is around $1.17/therm.) Or is there a better way to do it? The key thing is to keep it low-cost and simple, since it's just adding a layer of comfort to complement the 6-ton F/A system (which could be a tad undersized if it gets really cold). As for comfort in the two infrequently-used guest bathrooms downstairs, I'm thinking an electric mat in each. Thanks for your help!

The most commonly used diameter for pex is 1/2". Whatever you use, it must have an O2 barrier. A typical pex loop run should never exceed about 200'. You need someone to determine the spacing and you'd need the room layout to determine the best routing for the amount of heat you may need. A general guideline is to have the beginning of the run towards the perimeter since that tends to cool off the most, and needs the most heat. It is critical that the slab have insulation and a water barrier...no insulation and/or wet earth (may not be that common in AZ, but still potentially an issue) just sucks that heat out into the earth rather then keeping it in your slab.

Thanks for the input! I'm unsure about the barrier, though. We will need to bring in fill material to bring our elevation up a bit before pouring. In order for a barrier to keep the soil dry, it would have to be under the compacted fill, not just under the insulation. Right? We do plan on putting down 2" of XPS.

A 6" layer of 1/2" or 3/4" compacted screenings under the foam provides both a capillary break and a drainage path, and is pretty standard fare in most places. (If you have radon issues it provides a flow path for active or passive slab depressurization strategies too.)

XPS is blown with HFC134a, which has ~1400x the lifecycle global warming potential (GWP), of CO2. If it's not too late, see if you can switch the spec to EPS (Type-II EPS is sufficiently compression resistant for residential loading under 3" & 4" slabs, but some locations require higher-density Type-IX.) EPS is blown with pentane (GWP= 7x CO2), and is far greener. In your location & loads the greenhouse payback on 2" of XPS under a heated slab is FOREVER- longer than the anticipated lifecycle of the house, even assuming it hangs in for a couple hundred years(!). But with EPS you can take it on up to to a foot if you like, the "payback" in greenhouse gas terms is but a few years. But in dollar-payback 3"/R12 is more appropriate.

Some radiant installers prefer XPS so they can use staples for securing the PEX, but that's not the only method. If it's DIY it's your call.

A 10 mil (preferred) or 6mil poly vapor barrier needs to go between the foam and concrete. It could go between the foam and screenings, but placed below the foam it collects excess-water moisture from the concrete pour in pockets, and takes forever to dry through the foam and any finish-floor.

Getting it right requires something better than a WAG for the heat loads of those rooms, and unless your hot water heater is a condensing version running high-mass low-temp slab radiation may shorten it's service life severely without doing real hydronic design to minimize destructive condensation on the center-flue heat-exchanger. You can't just run 120-130F hot water heater output into a 74F slab and call it a day- it has to be a real design, if not a complicated design, and heat exchanger isolation of the hydronic loop from the potable is highly recommended, since the water in the slab PEX will be stagnating, most of the time.

Excellent tips, thank you. I'll look into EPS instead of XPS - I had thought that EPS shouldn't be used underground, that XPS was the only way to go.

You mention a heat exchanger - why not have just a water heater dedicated to this radiant system, so no exchanger? Also, do you have a recommended condensing water heater?

EPS has a higher fraction of closed cells than XPS, and is a preferable material for use in wet environments. When submerged it initially takes on water more rapidly than XPS due to the interstitial spaces between the macroscopic beads, but it pretty much stops at ~7-9% (retaining over 90% of it's R-value even when soaked) once those interstitial spaces are filled. The microscopic closed cell structures remain intact, and while they're semipermeable to water vapor, the surface tension of liquid water prevents rapid adsorption into the interior of the cells. While XPS takes moisture on more slowly, it dries MUCH more slowly after the tide goes out.

In most states using a water heater solely for space heating constitutes a code violation, but combi heat/hot-water systems are usually allowed. Some (but not all) states allow "open" systems that use potable water in the heating loops, others do not. (I don't know what code is on that in AZ.) In MA open systems are allowed, but for those system code explicitly requires a minimum amount of flow to occur even when there are no calls for heat, to mitigate stagnation related health risks. But with heat exchanger isolation there are no such requirements.

With radiant slabs the operating water temps are well below the dew point of natural gas exhaust from ~80% combustion efficiency hot water heaters, and unless you design the hydronic end of it to protect against condensation in the heat exchanger a cheap gas hot water heater will suffer an extremely short service life. If using hot water heaters for slab radiant heating (either open or isolated systems) a condensing hot water heat is by far the preferable heat source.

The A.O. Smith Vertex is specifically designed with side ports to be used for space heating applications, but has no internal heat exchanger. The Polaris is a very similar HW heater that is also ported for space heating. They differ in price and construction details- the Polaris is probably good for ~20 years, the Vertex for ~12 in typical applications, either can work well in your application if you get the hydronic design right. (BadgerBoilerMN who posts here has a great deal of experience on designing slab radiant heating systems around the Polaris. Were I to embark on that path I'd run any proposed system design in front of him before diving in, and pay him for the consult. Hydronic heating design isn't rocket science, but it's WAY more than just a plumbing exercise.)

Sorry for this late response, Dana - thank you very much for your help!