The interwoven world of radiant design: A masters class for installers

I recall a story told by an installer about a husband and wife who were building their dream home. The husband was a meat n’ potatoes guy who, when they turned their attention to the mechanical system design, said he wanted only the basics.

The installer could’ve given them just that. But the woman of the house wanted something special for herself and their growing brood of kids. Hmm…that was four against one!

Turns out that a friend of hers spoke warmly about an experience with radiant heat. The installer knew just the place to take them: to his own home. As soon as the family entered his home, the installer asked them to remove their shoes. Socks, too! They were puzzled by this, but quickly learned why they were so comfortable on his tiled floors: the radiant heat was fabulous.

Needless to say, the plan to install a nine-zone radiant heat system was settled that day.

As hydronics professionals, we often talk about the “magic” of radiant heat, but the true magic lies in the careful crafting of a system design that ensures optimal performance and efficiency. It's not just about putting tubes in the floor; it's about understanding how each design component interacts.

Tube spacing, water temps, pipe size, loop lengths, flow rates, installation methods, and floor coverings are all interwoven and connected. A change in one ripples through the entire system.

Tube spacing: beyond the inch markers

The system design that you constructed (or received from someone you trust, of course) provides a lot of important information. But the key question is: where do those numbers come from, and what exactly do they mean? “Why this spacing, or that tube size and loop length?” These are questions I get frequently from installers, and they have a great impact on system performance (or lack of it). For instance, tube spacing isn’t a mystery, and it’s not arbitrary: it’s all about ease of installation.

If the system is going to be a typical residential with a 4-inch concrete slab, the variable for tube spacing chosen may be tied (quite literally) to a reason you may not have thought of.

Are you ready for it? Tube spacing is based on the installation of the wire rebar that’s required for the concrete’s structural stability. The rebar (also referred to as “rewire”) will typically be a six-inch by six-inch grid.

If a design is calling for 6-inch on-center spacing, we choose that. When you’re zip-tying the tubing to the rebar, you follow each bar out and then turn right back. If the design calls for nine-inch centers for the tube spacing, you follow one rebar down and then turn around and come down the middle of the next grid. And if it’s designed with 12-inch on-center (OC), you get the picture: the tubing is tied to every other rebar. That’s it, folks – tube spacing demystified.

Heat output - vs. - cost:

There’s a little more to it when it comes to a designer choosing the spacing. I usually start at 12-inch OC and go closer if needed. There are cost implications. Six-inch OC spacing uses twice as much pipe and takes twice as long to install; it’s often that simple. Now, why would I want to change the spacing to something closer? It’s all about lowering the target water temperature and simplifying the grouping of water temperatures to make the installation and layout of the mechanical room easier.

Now remember: the designed water temperature for a room is based upon heat loss, installation method and the finished floor covering. Just randomly changing the spacing without reviewing these factors can result in rooms/spaces not performing the way the customer expects. Changing a room that had a 6-inch OC spacing to 12-inch OC to save on material could result in easily-perceived thermal dissimilarities – or “striping” – on the floor with hot spots directly above the tube and cool spots in between.

Ask the designer about this. Check the design carefully. Rooms with a high heat loss or a large amount of glass on one wall may have a tighter spacing for comfort reasons. Tighter spacing may also be deliberately tailored to eliminate the cold washing your customer could feel while standing in front of those windows. Elsewhere, a fully enclosed basement with low heat loss can easily have wider spacing.

Installation methods + techniques: fitting the system to the structure

Installation methods and techniques are based upon pipe size, but even more so based on the construction of the house. Make the radiant fit the house; don’t change the house to fit the radiant. What this means is that a designer will choose an installation method based on the construction of the house. Use what you have; don’t make it harder, less expensive, or complicated. All of these factors go into a smart radiant design.

The different installation methods are:

• Slab-on-grade: tubes embedded in 4-inch thick structural concrete
• Thin slab/thermal mass: over subfloor (e.g., Gypsum concrete)
• Overjoist (sandwich plate): tubes in grooved subfloor systems or panels
• Underjoist: tubes stapled to the underside of the subfloor with or without heat emission plates

Remember, radiant heat is the goal. By giving the customer the best heating system possible, you can mix and match all of these in one house. As I commented earlier, some of these installation methods govern tube spacing.

With thin slab spacing, like in kitchens and bathrooms with tile floors and a place where being barefoot is a little more common, I like to design at 6- and 9-inch OC. When looking at a sandwich style system, the panels or the systems will already have the spacing manufactured into the design. And when you’re designing an underfloor system, with or without plates, the tubing spacing is regulated by the joist spacing: i.e. 16-inch OC joists make the tubing spacing at 8 inches.

Again, let’s make the radiant fit the house, not the house fit the radiant.

I was working with an installer who wanted me to design a system with all thin slab; the finished floors were to be tile and hardwood. Designing a project for all thin slabs creates challenges if the customer wants a hardwood floor. There’s nothing to attach the flooring to unless you add another layer of plywood. Sounds fine, right? However, once the system was completed, there were saddles and transitions from one room to the next that the customer wasn’t expecting. The hardwood areas – because of the extra layer of ply – were higher than the tiled spaces.

If we’d designed a panelized system for the hardwood areas, we could have gotten the finished floor heights much closer, making transitions smooth and easy. Make the radiant fit the house!

Water temperature: the Goldilocks zone

You’ll recall my earlier comment: tube spacing affects water temperature. A given heat loss needs a certain btu/sq. foot. Closer spacing in a slab lowers the water temperature, while wider spacing increases the water temperature. A system designer may change tubing spacing to ease installation requirements in the mechanical room.

The design will call out water temperature requirements for each room. You do not need a separate mixing valve for each room! Instead, consider combining rooms of similar water temperatures to one mixing valve.

One thing I watch out for is different installation methods – such as concrete slab and underfloor with plates – because even if they are similar in design and water temperature, they must be on different zones. These rooms will respond very differently: the slab room will heat up slowly and give off its heat to the space just as slowly; the underfloor heats up very quickly.

Combine up to a 20 degree difference and consider a modulating mixing valve (such as a Taco i-valve). The design may call for a 20 degree ∆T (temperature differential) on a design day, but when the outside air temperature is higher, that difference in design water temperature actually gets closer.

So why consider this? If there was a fixed temperature mix valve used, and the design for each room called for 140° and 120° water temperatures, you’d need to set the valve for 140°F. When the room that needs 120° only gets a call for heat, the water temp moves to 140° because that’s what the mix valve is set for, permanently. It doesn’t make the room get too hot, or not work – but the thermostat will satisfy faster than the other room. Again, that’s not horrible, but now the floors start to cool down – yet, the air temp is satisfied.

Of course, in this scenario, the homeowner here isn’t satisfied. Remember – they wanted warm radiant floors, not just a heated space. Modulating valves add a level of comfort to a space that’s hard to describe.

Now you might think: Well, the boiler is modulating, why bother with a mixing valve that’s also modulating on outside air temperature? The boiler working on an outdoor reset curve is great, but that’s also assuming that the boiler is perfectly sized to the heat loss and there’s only one zone in the house. We call that boiler reset.

When you have several zones working with a modulating mixing valve that is also working on its own outdoor reset curve, I consider that load reset. Combine the two – boiler reset, and load reset – now you’ve got system reset, and the result is going to be the most comfortable heating system you can design, and also the most efficient.

Again, pay attention to the spacing and the installation methods to manipulate the design water temps to minimize the number of mixing valves (three temp ranges is my maximum). Your designer may not have called for this deliberately to not “add more tubing” to the project or make it “less expensive,” but to design the best possible heating system.

Lower required water temperatures may allow for direct connection to a low-temperature boiler or heat pump, simplifying the mechanical room and reducing equipment costs and complexity. This is a significant design advantage.

And when it comes to modulation, consider the modulating circulators that are out there – not just the ones that will speed up and down based upon the number of zones that are running (Delta P), but for almost the same reason as above: a circulator that modulates temperature (Delta T).

Another design feature (more for comfort than installation): most radiant floor systems are designed on a 10-degree temperature drop. For example, if the room calls for 120°F on a design, the return temp will be 110°F. This temp drop allows for a nice, even surface temperature in the space, which adds another level of comfort to the space.

Of course, this ideal temperature drop only occurs when these variables line up like dominoes: on a design day, if the circulator is perfectly sized, and when all of the zones on that manifold/circulator combo are on. Okay, it’s a rarity – like having your birthday, Christmas, New Year's Day, and a month-long vacation all rolled into one!

Now imagine allowing the circulator to speed up and down all season long to maintain that 10-degree temperature drop, while also maintaining that temperature drop when the zones satisfy. Talk about the perfect alignment of dominoes.

A delta T design also allows for a system to have a circ that automatically sizes itself based on the number of zones calling while also varying its performance with seasonal change. Now imagine what this can do for your water temps that were combined with the mixing valves we discussed above to smooth out those temperatures with different rooms. Near perfection.

The art of harmonious design

Thinking about your radiant designs is sometimes about the preferences of the designer, but also the construction of the house, as well as being cost-conscious. However, remember, a lot of the aspects of the design are connected to each other and changing one thing randomly affects other decisions the designer made. I’m not saying that you can’t change anything, but choices were made based upon a lot of factors.

Ultimately, successful radiant heat design isn't about mastering one single component; it's about understanding the symphony of interactions between them. When we approach each project with this holistic view, we create systems that deliver ideal comfort, efficiency, performance, and long-term satisfaction.

Still want to learn more?

Taco's residential hydronic system design software, specifically FloPro Designer, is a free, user-friendly tool designed to simplify and accelerate hydronic heating system design. The software allows users to create system schematics, calculate heat loss, size components like pipes and circulators, and generate a bill of materials.

Taco's commercial hydronic system design software, part of the HS2 Design Suite, is a collection of tools that helps HVAC designers create and analyze hydronic systems. It simplifies the design process, allowing for quick system sizing, equipment selection, and detailed documentation. The software aims to reduce design time and errors by automating calculations and offering features like drag-and-drop functionality and automatic pipe sizing.

ABOUT THE AUTHOR

Dave Holdorf is a technical trainer, residential, for Taco Comfort Solutions.