If you’re a subscriber to PHC News — as you should be — and reading this right now — as you most certainly are — you’ve probably been involved with a radiant floor design, radiant floor installation or, at least, wish you had. And I think it’s safe to say most of us wish we had radiant floors.

Radiant floors for basements, garages, living rooms, dining rooms, bedrooms and offices don’t usually pose design issues as there is usually ample square footage to get the necessary BTUs per hour to meet the room heat loss. If they’re covered with wall-to-wall carpeting, that’s another story, a sad one at that.

Kitchens are usually the problem child requiring a bit of extra work. Next to the sunroom with its three exterior walls, all loaded with glass, the kitchen stands out as the room needing special attention when designing a radiant panel project. Rarely will you find the necessary usable floor space to squeeze enough BTUs per hour out of it. Kitchen cabinets, islands, and appliances abound, limiting heat output. 

I have good news: The solution does not include hydronic kick-space heaters or baseboard. When my customers want radiant panel heating, I make every effort to abide by their wishes. 

Figure 1 is a project I did about 25 years ago; a good-sized kitchen tricked out with all the latest and greatest of everything. It even included a cooktop fan that required me to run a 12-inch diameter metal duct up through the home’s cedar shake shingles. A huge island was in the middle of the room, and three of the four walls were laden with cabinetry of some sort.

Unfortunately, the tubing and the 1 1/2-inch gypcrete overpour weren’t enough, and the home had an extravagant ceiling that was off-limits to supplemental radiant. I ended up installing cast-iron baseboard toward the back under the windows. Options are almost always options to get the job done, even if it isn’t in your sweet spot.

I recently finished one design where I was able to add a second stage of radiant heat to a kitchen by way of the ceiling, and the one I started today will likely require the same. Here’s how I approach this type of project.

The math

Similar to every design I do, I start with a Manual J heat loss load calculation. And by running those numbers, I know this kitchen is set to lose 7,440 BTUs/hour on the coldest day of the year. I also have these facts to work with: The total kitchen square footage comes in at 306. Then, when I add together the square feet from the island and the lower cabinets that sit on the floor, I get 84 square feet. 

Total square feet: 306

Total cabinet/appliances square feet: - 84

What’s left for radiant heat: 222 square feet

Do you see where I’m going with this? Just because we have 306 square feet in the kitchen, it doesn’t mean we’re going to be able to use it all. The cabinets and the appliances will only act as a show-stopping insulating blanket on our radiant panel, so we’re left with the usable floor space.

Kitchen heat loss:        7,440

Usable square feet:        ÷ 222

Required BTU/hr/ft²:        33

From the get-go, I’m not comfortable with that number and here’s why. First off, did I mention the kitchen has hardwood floors? No? Well, the kitchen has hardwood floors. But it’s not the only thing giving me pause. Going by the Uponor chart in Figure 2, you’ll see that if the kitchen needs 33 BTU/hr/ft² and the room setpoint is 70 degrees, we’re going to end up with a floor surface temperature of approximately 86 degrees. 

It clearly exceeds the maximum of 80 degrees for hardwood floors and comes close to the maximum of 87.5 degrees for human comfort. Your dog might be good with those numbers, but the oak floor, not so much.

So, here’s what we do. We slide to the left on the BTU/hr/ft² scale until we arrive at 20 BTU/hr/ft² because it gets into the safe zone of 80-degree surface temperature for hardwood. Given what we now know, we’re going to quickly re-engineer this a bit.

We have 222 square feet of usable kitchen floor, which will provide 20 BTU/hr/ft²:

Usable floor     space:        222 square feet

Heat per square foot:         x 20

Total floor output:         4,440 BTU/hr/ft²

We’re falling short of meeting our heat loss by using only the floor — 2,960 BTU/hr/ft² short, to be exact. Now what? We could install a hydronic kick-space heater in a cavity beneath one of the cabinets. But then, if we’re thinking about potential service down the road, it would probably be a good idea to install an access panel on the bottom of the cabinet. 

At some point, the tangential blower wheel is going to collect its share of dust and dirt, and someone is going to expect you to clean it. Or we could slap some hydronic baseboard along an outside wall. Do you think the homeowner is going to sign off on that when they asked for radiant heat, the ideal type of heating that you can’t see and shouldn’t hear? Maybe? Depends, I guess. 

If you want to get homeowners’ attention and get them what they asked for, mention a radiant ceiling to supplement the radiant floor. Their toes will still be warm, and you’ll know they’ll be comfortable when the digits drop to bone-chilling numbers.

Supplemental heat 

Referring to Figure 3, we’ll find out how we're going to make up for those 2,940 BTU/hr. With the ceiling, we don’t need to worry about the island or the appliances, just some can lights and maybe a ceiling fan we’ll have to avoid. Other than that, the whole space is ours to work with. Here’s how I try to do it and I’ll tell you why I prefer this method, although it’s certainly not the only option.

I start with the 2,940 number and then I consider the 20 BTU/hr/ft² we’re getting from the floor. Then I consider the R-value of the floor (1.6) and the water temperature necessary to generate 20 BTU/hr/ft². Referring to Uponor’s Appendix E, we find that at a 10-degree delta T, we need a 126-degree supply water temperature to achieve our goal using Joist Trak at 8 inches on center. 

That’s the key for me when moving on to my plan for the ceiling. I prefer to go with a minimum number of supply water temperatures; for the ceiling, I’m targeting the same 126-degree value. If you see a wall of circulators lined up with a mixing valve for each, you can bet your next supply house order that somebody didn’t think it through. The simpler the design, the better.

Looking at the radiant ceiling graph where 126-degree supply water temp intersects with the ceiling’s R-value of 0.6, we’re able to get 30 BTU/hr/ft². Not too shabby. Again, we have unlimited space to work with, but we only need 2,940 BTU/hr; 2,940 divided by 30 equals 98 square feet. That’s all the ceiling surface area needed, but I like round numbers, so I’ll bump it a bit to fit the configuration of the layout. These numbers will vary from manufacturer to manufacturer as all plates are not created equal.

Figure 4 is a radiant ceiling project of mine; in this one, I used 8-foot lengths of ThermoFin U plates, getting optimum plate contact with the drywall ceiling. Sneaking around the lights is easy, as you can see. 

My preferred method of design is to approach these kitchen projects as two stages of heat. The radiant floor panel would be the first stage of heating, managing the load through the bulk of the winter. And the ceiling would be the second stage, banging on only during the coldest days. This is easily accomplished with a two-stage thermostat and a couple of relays; don’t forget to use a floor sensor with projects including hardwood floors. I prefer to use them on all my jobs, but that’s just me. 

I think radiant ceilings are too often overlooked as an option for primary heat and supplemental heat if needed. Once you know the heat loss and the area of space, the rest is simple. And you get to install it standing up. No knee work, no bending over, no crawlspaces.