Adam Emter is building a new house in North Dakota, a Climate Zone 7 location with some 9,500 heating degree days a year, and temperatures that fluctuate from 30 below zero in the winter to a humid 90 degrees during the summer.
“My family and I plan on living here for many decades,” Emter writes in a Q&A post at GreenBuildingAdvisor, “so I’m very focused on building an efficient and comfortable house. I am also trying to keep a reasonable budget and simple design.”
The house will be a two-level split-entry with 1,200 square feet per floor, and Emter is leaning toward the REMOTE construction technique, which places most or all of the rigid foam insulation outside of the sheathing. An insulating concrete form foundation has already been built, with the basement slab due in the next couple of months.
While the house is under construction, Emter and his family are living in an adjacent 30-by-40 garage heated by a radiant-floor system built around a Polaris water heater. Emter plans on using the same type of heating system in the new house, and it will be an “open direct” system in which there is no separation between his domestic hot water supply and the water that heats the house.
With that as a backdrop, the discussion turns to the potential for problems with his heating design, and how Emter will keep the house cool in what can be very muggy summer conditions. Those are the topics for this Q&A Spotlight.
Is a radiant slab heating system the right choice?
Writing from northern Minnesota, also a Climate Zone 7 locale, Steve Vigoren says he’s planning to build a similar house, and also was considering a radiant slab system.
“From what I have been hearing,” he says, “if you have an airtight superinsulated envelope, with that size home, you would be better served by installing a ductless minisplit heat pump. Cheaper — and it will supply air conditioning also. Since you haven’t poured your slab, you may want to investigate this option soon.”
Richard McGrath doesn’t agree. “Don’t eliminate an option without the right information,” he says, “radiant can be done and done well.”
Using a water heater to handle space heating is a great start, he adds. Getting double duty out of the water heater will knock several thousand dollars off the price of a radiant system because the boiler and flue won’t be needed.
Plus, standby losses in the heater will be reduced to a minimum because the heater will be active more than it is idle. “Very low water temps also expected in these homes at design temps and outdoor reset makes it all the better.”
The decision to go with radiant floor has been made, Emter replies. His experience with heating the garage confirms he’s making the right call, although he has been considering a minisplit for shoulder season days when the house needs a quick blast of heat.
Don’t mix potable and heating water
While McGrath is a fan of radiant heating systems, the idea of a direct connection between drinking water and heat tubing sends up a red flag because it adds the risk of a bacterial contamination. The perfect breeding temperature for a bacteria called Legionella is between 90 and 120 degrees, he says, “the most common temperature range for a properly designed radiant heating system.”
“I know at first glance it sounds like a good idea,” he says of the planned heating system, “but I assure you it’s not. Legionella is no joke and 80 percent of the cases are misdiagnosed as pneumonia every year. It is everywhere and even in 50 percent of public water supplies.” (For more information on this issue, see Using a Tankless Water Heater for Space Heat.)
McGrath also is critical of Radiantec, a Vermont company that sells radiant-floor components. Emter has purchased some of his supplies from Radiantec.
“Be wary of online design outfits that do not offer their own errors and omissions insurance policies,” McGrath writes, “for when and if there is an issue. I have not been involved with nor have I heard of a single instance where these type of folks stood behind their design when it failed, believe me, I have been called upon to address many.”
Michael McArdle is another critic of radiant floor systems in which potable water is circulated. Most health codes forbid such systems, he says, as they would prohibit the use of a non-lined boiler for domestic hot water.
“If you’re out of the house for 48 hours and the heat is on, could Legionnaires grow?” he asks. “Are there specific times or temperatures? I don’t know. I’m not a bacteria specialist, but if it were me and my family’s health I would opt for a heat exchanger or some means to separate them.”
Emter, however, has done his research. “As for Legionella,” he says, “…under the conditions that I will be operating the system, I don’t consider it a problem.”
Other problems with the proposed radiant system
McGrath also is concerned with other details of Emter’s heating system. One is that when there’s a demand for domestic hot water, the water pump will pull cold water through the entire tubing system, “robbing heat already stored in the mass.”
When there’s no longer a call for domestic hot, he adds, that heat will have to be replaced before any space heating can occur, which “contradicts everything radiant is about.”
Plus, he adds, potable water entering the system contains a lot of entrained air, which will damage all ferrous metal components in the system.
But Emter has included an air separator in his system, and because he’s on well water there’s no chance he could contaminate a public water supply should bacteria contaminate his own system — another of McGrath’s worries.
On this issue, Emter and McGrath are bound to disagree. “All things considered,” McGrath says, “not very green nor efficient. But you saved a load, or did you? Enjoy your system and good luck, Adam.”
Dealing with summer heat
Emter wonders about cooling as well as heating. He thinks he will need air conditioning for about 30 days a year. Whether he’s better off with a ductless minisplit or a through-the-wall AC unit is still up in the air.
Shading and humidity control are the most important factors in keeping cool, writes Nathaniel G, and managing moisture with a dehumidifier is cheaper than air conditioning when you don’t need to control temperature.
Low-e windows and wide roof overhangs will help, as will having a well-insulated attic and a reflective metal roof.
But with the R-60 insulation Emter is planning for the attic, says GBA senior editor Martin Holladay, whether the roof is reflective won’t make any difference. It is, in fact, irrelevant.
Further, using Nathaniel G’s own calculations for potential cooling loads, the difference produced by a metal roof would be only 400 Btu/hour. “For a house with a 2-ton cooling load (24,000 Btu/h), that difference amounts to 1.7% of the cooling load,” Holladay says.
The added cooling loads due to a non-reflective roof are negligible, says Dana Dorsett. “The 400 Btu/hr difference in cooling load between 1,000 Btu and 600 Btu is the cooling load equivalent of one adult human doing yoga,” Dorsett says. “It takes about 20-25 watts of minisplit power to remove that much heat.
“There’s plenty of good reasons for going with a metal roof in that climate,” he adds, “but neither peak cooling load nor total energy use is among those reasons.”
Dorsett also suggests that Emter’s plan of using a minisplit for quick heat in the shoulder seasons may not be his best option.
“Using a mini-split to bring the house temperature up quickly during the shoulder seasons means you’ll get at-best a COP [coefficient of performance] of 2.5 out of it,” Dorsett says. “If you let the thing modulate with the load rather than playing catch-up running at full speed, a better-class minisplit will average a COP of 4 or better whenever the average daily temp is in the mid-40s F or higher.
“From an operating cost point of view that’s usually going to be cheaper than condensing gas.”
Our expert’s point of view
Here’s how GBA technical director Peter Yost sees it:
I decided to check in with fellow Green Building Advisor Michael Chandler on this one, since he is both a master builder and plumber, and has deep experience on mechanical systems, particularly radiant heat. Michael is a detail, hands-on guy, as you will see in his response:
“Basically my motto about open radiant systems is ‘you only have to kill one granny to ruin your whole day.’ No one really knows how prevalent Legionnaire’s is because it looks just like a chest cold in people with robust immune systems. I live in a house with an open system and it really doesn’t scare me all that much. But I don’t install them in my customers’ homes because the cost of an extra pump and a heat exchange combined with the ability to eliminate all check valves from the system make the whole deal so much more trouble-free and safer for a very modest cost increase.
“The costs on radiant heat are so all over the place and the price of propane is so high that it really doesn’t make all that much sense unless you can get a plumber with a wet heat license to do it at a reasonable price and you have access to affordable natural gas.
“Let’s say you have a 2,000-square-foot house. If you put radiant in all 2,000 square feet and modulate the water temperature low enough so you won’t overheat the home, then you won’t really have warm floors at all — you just won’t have cold floors. But in a superinsulated house with plenty of foam under the slab and around the perimeter you can easily think of the radiant floor as a ‘virtual wood stove’ and just warm the areas where you really will appreciate the warm floors.
“That means the tile floor in the bathrooms (100 square feet times the number of baths), the front hall and mud room (maybe 200 square feet), maybe the area under the dining table (100 square feet) and the kitchen floor (150 square feet) and the spot in front of the fireplace where you read the paper on a cold day (50 square feet). So put in seven 100-foot loops of 3/8-inxh PEX @ $20 each because short thin loops stay scoured and clean.
Splurge on a $234 brazed flat-plate heat exchanger that will actually move enough heat into the floor to do the job with a $168 bronze circulator on the potable side and a $67 three-speed Wilco cast-iron pump with a pair of $8 flanges on the radiant side with enough (10-15%) glycol in the water to inhibit corrosion in the cast iron.
“We roll the loops out and fold them in half and install them on a cross of ladder wire or #3 rebar on the foam and run them back to the manifold so that each loop is exactly the same length — so I don’t have to worry about using valves to compensate for different lengths of pipe in the floor having different resistance to flow.
“The material to do a radiant system for a 2,000-square-foot home would be $831. I’d pair it with a $1,680 AO Smith Vertex condensing natural gas tank water heater and we have a solid radiant heat system for $2,500 in parts.
“I’m just saying that when folks talk about how pricy radiant heat is, they are over-complicating things. And the savings of skipping the heat exchanger and extra pump? A measly 300 bucks! Why bother?
“With the cooling the real issue is dehumidification. I really like the GE zoneline vertical PTHP for that but it doesn’t get a SEER so you have to go by EER and the comparison makes the PTHP look like less than it really is. I just bought one for $1,215 but with the plenum and grille it came up to $1,805 with freight and $115 for a return-air grille that I didn’t really intend to order and will not use. So it should have been $1,690.
“It connects by a 10-inch flex duct to two or more supply registers so it can distribute air to multiple rooms. It typically gets a filter in the plenum grille which can double as an access panel so we can use MERV-13 extended media filters with them. I like it better than a minisplit but they are sold as hotel units and not intended for residential use.”
Like I said: Michael, the detail guy. And no complaints here. Thanks, Michael!