Image Credit: Image #1: Moppet65535 via Flickr In Ottawa, Canada, just 6.1% of the total number of hours in the year would be deemed "comfortable" without any active heating or cooling. (Illustration: Climate Consultant)
Image Credit: Images #2 through #8: Climate Consultant With space heating added, the total number of comfortable hours rises to 92.5% of the time. (Illustration: Climate Consultant) With active heating and cooling, the house will be comfortable 97% of the time. (Illustration: Climate Consultant) Active heating and fan cooling means the house will be comfortable 97.9% of the time. (Illustration: Climate Consultant) Active heating and natural ventilation cooling also produced comfortable conditions 97.9% of the time. (Illustration: Climate Consultant) Active heating and dehumidification delivers comfort 95.5% of the time. (Illustration: Climate Consultant) If you want to be comfortable 100% of the time, you'll need active heating, active cooling and dehumidification. (Illustration: Climate Consultant)
Radiant-floor heating systems are unobtrusive because the plastic tubing that distributes hot water around the house is buried in or under the floor. Homeowners like that. But because there are no air ducts with a radiant-floor system, air conditioning must be added separately.
That’s the situation Lance Peters faces as he plans a new, two-story house in Ottawa (what Peters assumes is the U.S. equivalent of Climate Zone 6 or 7). He’s currently planning to use hydronic in-floor heat on all three levels of the house, plus either a heat-recovery or an energy-recovery ventilator with its own ductwork.
“The house will be superinsulated (~R-40 walls, R-60 attic, R-20-30 below grade) with southern exposure and proper window shading to limit solar radiation,” Peters writes in a Q&A post at GBA. “Finished plans should be around 2,500 square feet. A large portion of the second floor will be open to the first floor (above the living and dining room areas and main staircase), leading me to think ductless minisplits would be a bad idea.”
Peters would like to use the ducts for his heat-recovery ventilator to supply cool air in the summer. “It would be nice to avoid doubling up on the amount of ductwork running through the house,” he says.
Is that plan feasible? That’s the topic for this Q&A Spotlight.
Use an air-source heat pump for both heating and cooling
One of several types of air-source heat pumps would be a good choice for a superinsulated house, GBA senior editor Martin Holladay suggests. There are three basic varieties: a conventional air-source heat pump with ductwork to distribute the conditioned air; a ducted minisplit using a smaller, more localized duct system; or a ductless minisplit.
“Once you’ve installed your air conditioning system, you can use the heat pump for heat as well,” he says. “That means you don’t need any in-floor radiant heat. This approach saves many thousands of dollars.”
Other GBA readers advise Peters that ductless minisplits work well in houses with open floor plans, as well as those with spaces that open from one level to the next.
“Typically, an open floor plan is favorable for a centrally located ductless mini-split because there is so much natural air circulation around the house,” writes Reid Baldwin. He adds that if cooling loads are less than 10,000 Btu per hour, Peters might be able to use a Minotair multifunction air exchanger for heating, cooling, and ventilation.
Dana Dorsett agrees that a minisplit should be effective for cooling in a house with multiple levels. He cites one house in Minneapolis that is cooled with a single 1.5-ton minisplit head on the upper level near the top of the stairwell. The house is heated with hydronic radiant floors using an electric boiler (at time-of-use rates.)
“Even though the minisplit can deliver a large fraction of the heat, it isn’t distributed well,” he says. “When the owner is going to be away for more than a day he turns off the radiant, lets the minisplit run as freeze-up control.”
Dorsett adds that determining cooling loads with a Manual-J calculation or its equivalent is important for getting the equipment sized correctly. “Superinsulation does a lot more for reducing the peak heating loads than it does for the cooling loads, which are often dominated by solar gains through the windows,” he says.
Dorsett adds these thoughts: “The sensible cooling loads aren’t very severe in Ottawa, with a 1% outside design temperature of 83°F (28°C), but there is also a real latent load (21 grains @ 50% RH). The higher the ventilation rate with the HRV, the higher the latent load, and it will be important for comfort and dust-mite control to dial back the ventilation rates a bit when the outdoor dew points are north of 56°F (13°C), which is most of the time in the July-August time frame, since the sensible load may not be high enough that the cooling system will dehumidify sufficiently. Right-sizing a modulating AC for the load will make a real difference on that front.”
Have you considered a ground-source heat pump?
For Andrew Bater, there’s nothing new about the advice at GBA to steer clear of hydronic heat. “Welcome to the loneliest fraternity here on GBA,” he tells Peters.
Peters hasn’t mentioned what the source of hot water for his radiant-floor system will be, and Bater offers his own experience with a ground-source heat pump.
“Our house is heated and cooled by a ground-source (geothermal) heat pump,” he writes. “We make hot water in the winter and chilled water in the summer. We have two levels of in-slab heat and one second-story area with panel radiators. Like your home, our second story is partially open to the first floor.
“For summer air-conditioning, chilled water runs through a Unico high velocity fan coil that feeds a main trunk spine,” Bater continues. “Most rooms have one, two, three, or more flexible takeoffs from that spine.”
Bater believes high-velocity air conditioning and radiant-floor heat is a “somewhat common” approach in parts of the western U.S.
Steven Knapp, however, adds a cautionary footnote about ground-source heat pumps (GSHPs): they’re expensive, and they don’t always work as well as homeowners would like.
“My development requires spec homes to be GSHPs, and most of the new homes are specs,” Knapp says. “Based on conversations with neighbors, there seem to be quite a few comfort and reliability issues. I considered installing a GSHP myself but opted for an air-source heat pump at one-third the cost. This savings meant I could do a bit more to improve the performance of the structure, which also further reduced my heating and cooling requirements.”
A heat pump has lots of potential
If Peters heats the water for his hydronic system with a heat pump (air-source or water-source), it can be reversed in summer to produce chilled water, says Charlie Sullivan. In that case, a single central fan coil could distribute conditioned air via ductwork, or he could use mini fan-coil units, such as those made by Chiltrix, HTP, and Jaga.
“You have to insulate the chilled water pipes well to prevent condensation on them, and finding an installer who gets that is hard, but once you get over that hump, installing the hydronic distribution system is easier than installing ducts,” Sullivan says.
“Given that most of the load is latent (dehumidification), it’s actually quite attractive to put a chilled-water coil on the incoming HRV air,” Sullivan continues. “When that incoming air has higher humidity than your interior air, you can remove a kilogram of water vapor from it at lower energy cost than removing a liter of water from the interior air. And it’s satisfying to stop the humidity from entering, rather than removing it after it enters. That’s probably all you need for cooling.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost added:
Given that I live quite comfortably in southern Vermont with hydronic baseboard heat and no real air conditioning, I decided to see what Climate Consultant 6 (CC6) would show in terms of space conditioning strategies for Ottawa, Canada, about 6 hours to the north and west of us. If you’re not familiar with the free software, take a look at this blog or simply download the latest version here.
Each of the screenshots from CC6 (below) shows delivered thermal comfort (per ASHRAE Standard 55); hourly temperature/relative humidity dots that are green represent “comfortable” and red dots are “uncomfortable.” The difference from screen to screen represents the highlighted Design Strategies in the upper left-hand box.
For the first screenshot (see the first image on the left at the bottom of this page) with just “Comfort” highlighted, the house will be deemed comfortable only 6.1% of the time without any active strategies.
The second screenshot (Image #3) is not surprising for Ottawa; adding space heating adds 86.4% to the green-dot comfortable total, resulting in 92.5% comfortable hours. The radiant floor heating system will deliver on these hours.
Here is where it gets interesting:
The next screenshot (Image #4) shows active heating and cooling, while Image #5 shows active heating and fan cooling. Image #6 shows active heating and natural ventilation cooling. All of these HVAC strategies deliver around the same level of comfort: between 97% and 98% of the time.
Image #7 (active heating and dehumidification only) delivers a total of 95.5% comfort. Presumably the added 3% for dehumidification only is shoulder season moisture removal, when you don’t need cooling for comfort.
Image #8 (active heating, active cooling, and dehumidification) delivers 100% comfort.
So, for year-round comfort every hour in the year in Ottawa, you may need both stand-alone dehumidification as well as active cooling. Active cooling alone does not do any better than either type of ventilation cooling — natural or fan-generated. Depending on your budget, the house design, and your own standards for thermal comfort, maybe you don’t need active cooling at all.