Radiant heating can often be a hard sell when uneducated potential customers have an unrealistic expectation of the installed costs. 

When that happens, I strive to educate them regarding good, better and best installation methods, knowing they will likely be receiving estimates from contractors who themselves lack the proper education and training to understand the ramifications a poorly designed hydronic radiant system presents. 

Spacing the concrete-embedded tubing farther apart to lower the installed cost will require hotter water. Using thin aluminum metal flashing plates instead of thick extruded plates attached to the underside of the subflooring also requires hotter water, and most likely will add irritating ticking noises from expansion/contraction of the PEX tubing. 

Staple up, where the tubing is attached to the underside of the subflooring, is one of the poorest and least effective means for transferring heat energy to the living space, and requires substantially hotter water than a well-designed radiant heating system. It requires significantly more energy to heat water to 180° F than it does to warm water to 87° for the upper limit of an outdoor reset curve. 

For my part, I cannot begin to design a radiant heating plan without first executing a heat-loss calculation. My design programs are based upon the ACCA Manual-J heat loss/gain platform, which is recognized by all authorities having jurisdiction, and is code compliant. 

Richard Trethewey (plumber from “This Old House”) stated during a keynote speech that for every 3% we can lower the hydronic water temperature, efficiency is increased by 1%. I had three retrofit hydronic radiant jobs installed in engineers’ homes where they accurately tracked energy consumption both before and after. While not an exact 3:1 ratio, Richard’s claim was quite close to being accurate. 

I have years of training via the Radiant Professionals Alliance, most of which came from classes taught by longtime pme columnist John Siegenthaler and Robert Bean. Through their trainings, I attained designer, installer and instructor certifications. More recently, I attained the ASSE 19210 hydronic heating/cooling installer certification. The end result is designing radiant systems that perform as promised, are miserly in their energy consumption and deliver the ultimate in comfort for customers.

The business of educating

A few weeks ago, I was contacted by a potential customer interested in exploring the possibility of installing a hydronic radiant heating system for a new slab-on-grade home. He wanted a budget price, but did not yet have any house plans. I hesitated because guestimates almost never pan out and absent critical data such as insulation values, slab thickness, window types and square footage of glass, doors (French wood, sliders, solid wood, metal, etc.), any attempt would be a shot in the dark. He insisted and I, against my better judgment, complied. He was officially interested and said he’d contact me when his plans were ready. 

When we met to go over the house plans, I spied a full wall of French wood doors in the great room and lots more glass than I had included in my guestimate. In addition, he now added a large office on the second floor, so naturally my revised highly accurate pricing created a stir. I suggested he reach out for other estimates and got this reply. 

“One question: Per your suggestion, I reached out to a couple other contractors for bids. One guy recommended against in-floor radiant (though he installs such systems), suggesting a conventional ducted heat pump with a high-efficiency propane furnace, using the heat pump on mild days and the propane when it turns colder. His ‘logic’ was that given the time delay (as an engineer I’d call it hysteresis) of heating the slab and then having it release that heat and cool down on mild spring and fall days where it’s cool at night (and you need heat) and warmer during the day, you’ll find yourself running the air-conditioning during the day because the slab continues to release heat you don’t need. Any truth to this, or does the outdoor reset (and cooler water circulating on cool – not cold – nights) minimize this problem?”

Hope springs eternal! One thing leaped off the page: “As an engineer, I’d call it hysteresis.” An educated customer equals huge relief. My reply:
“One advantage of utilizing a top-notch design program is knowing the hottest water temperature required on the coldest day of the year, which is 112° for your application. 

Yes, the slab is considered to be ‘high mass,’ but the outdoor sensor will react to changing weather conditions, and the boiler’s controller alters water temps as needed. Even if you have a wall of south- or west-facing glass, we can reposition the outdoor sensor to minimize the flywheel affect that causes indoor temperature overshoot. During those shoulder seasons for the cool nights, your water temperature won’t be more than a few degrees above room temperature, so the potential for room temperature overshoot is minimized. With the boost feature, we can utilize a more aggressive outdoor temperature reset curve knowing that an overly long call for heat (this timing is programmable) will begin a step-series of increased water temperature adjustments until the thermostat is satisfied. High winds will alter a home’s heat loss, and boost can offset the boiler’s response since the outdoor sensor doesn’t react to wind. 

“I added carpeting in several areas with padding when inputting the info to ensure we’re more than covered regarding supply water temperature, so in reality your final floor covering of LVT will likely result in tweaking the program and result in a lower water temperature. I design all our radiant systems for the lowest water temperatures possible to drive down operating costs.The lower the upper water temp limit, the higher the operating efficiency, which can exceed 95%.The lower the required water temperature, the lower the fuel consumption, too. That’s also why we utilize ECM circulators, which use 70% less electricity than induction-motor circulators. 

“Contractors who drive down installed costs often do so by reducing the amount of tubing (wider distance between tubes) and raising water tem-peratures, which burns more fuel. It’s also fairly common for contractors who don’t do their homework to guess the reset curve (set it high and that leads to indoor temperature overshoot). Or, they don’t utilize outdoor reset at all while giving the boiler program an upper water temperature limit of 180° — just like a conventional cast-iron low-efficiency boiler. 

“With the outdoor sensor connected, we can also program warm weather shutdown (WWSD), which disables the heating side. We typically set WWSD for 70° outdoor air temp. 

“Our home is 100% radiant because I know firsthand how much more comfortable radiant heating can be. Radiant heat is also healthier than other types of heating systems. We also have Fujitsu inverter mini-split concealed air handler heat pumps that replaced our conventional ducted AC units about 12 years ago. This enables us to set the radiant heat back if we’re away and have rapid recovery for indoor air temps, and then let the radiant take over. I can control all systems via my smartphone or Wi-Fi-connected computer/laptop/iPad. 

“Standard heat pump systems are yesterday’s technology. There are now inverter driven-heat pumps for central ducted systems that can provide heating at their nominal rating well below 40° outdoor air temps. Virtually all of the unitary manufacturers now have inverter units, and most have also partnered with mini-split manufacturers to include them in the lineup. For example, Rheem partnered with Fujitsu to private-label the Rheem mini-splits.”
His response:

“Thanks for the explanation… and it all makes sense to me.” 

Revised pricing absent the second-floor radiant brought this response:

“Well, that’s certainly much better… and likely within our budget. I’m going to mull my options for a bit and will get back to you soon with a decision — one way or the other. Thanks for the effort you already have invested in my project.”

Some quick advice: Never stop educating yourself and your customers.

Merry Christmas, Happy Hanukkah or Happy Kwanza everyone.