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Q&A Spotlight

Adding Air Conditioning to Radiant-Floor Heat

Planning a superinsulated house in Canada with hydronically heated floors, a homeowner looks for the best way to add air conditioning

In a hydronic radiant-floor heating system plastic tubing distributes hot water around the house from a central manifold like this one. The complexity of the system adds to the cost, and air conditioning must be added separately.
Image Credit: Image #1: Moppet65535 via Flickr
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In a hydronic radiant-floor heating system plastic tubing distributes hot water around the house from a central manifold like this one. The complexity of the system adds to the cost, and air conditioning must be added separately.
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.

9 Comments

  1. Li Ling Young | | #1

    CERV - just the thing for a little AC and a lot of ventilation
    Want to distribute AC through your ventilation ductwork? A heat pump ventilator does what an HRV/ERV does, but with a heat pump instead of a passive heat exchanger. In the winter, the air that is supplied to the house can be warmer than the air extracted from the house, and in the summer the supply air can be cooler than house air, and cooler than ambient air. Ductwork is similar to standard ventilation ductwork, but you may need to design to satisfy cooling loads, not just ventilation needs. The capacity, both heating and cooling, is low, so you couldn't heat your house all year with one of these, but for a home with low cooling load, a heat pump ventilator might be just the thing.
    Two products I'm aware of - CERV (conditioning energy recovery ventilator) from Build Equinox, and Boreal 12000 from Minotair. The Minotair product is especially designed to manage humidity. The CERV has a recirculation function that pairs well with mini-split heating.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Li Ling Young
    Here are links to two relevant articles on these appliances:

    A Balanced Ventilation System With a Built-In Heat Pump

    Another North American Magic Box

    -- Martin Holladay

  3. Beth Freeland | | #3

    Adding AC to Radiant Floors
    I am building a home in Oklahoma and want to use radiant heat and AC. So, are you suggesting a heat pump for the AC and back up heat? My floor plan is open as described in your blog with 2600 SF.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Beth Freeland
    Beth,
    This "Q&A Spotlight" article reflects a variety of opinions and viewpoints. My own opinion is that if you want air conditioning, you might as well let your heat pump provide heating as well. That way you don't need to install a radiant floor heating system, and you will save many thousands of dollars.

    If your house has an open floor plan, a couple of ductless minisplits (or one ductless minisplit plus a ducted minisplit) will probably meet all of the space heating and air conditioning loads of your home.

    For more information on these issues, see these two articles:

    All About Radiant Floors

    Just Two Minisplits Heat and Cool the Whole House

    -- Martin Holladay

  5. Alec Shalinsky | | #5

    Living in a super insulated house
    Lance, We have a super insulated house (passive compatible) in a similar climate. We have no active heat source in the house. And no cooling either. We depend on solar radiation (big windows) for heat and cool breezes in the summer for cool. We installed a hydronic system, but it has never been turned on. We depend on high performance windows (www.vettawindows.com) and lots of insulation in the walls (like you are doing).
    The problem with using a ductless for heat, is that the OBC does not recognize this unit as a primary heat source, so something else needs to be present. This could be baseboard heaters, for example, which may not be ever used. The other problem with hydronic in a superinsulated house is the lag time. The slab will stay warm for hours after the thermostat has said "off". The problem is overheating the house. In a well built home, a heating system that turns off "now" is far more effective.
    Also remember that high wall insulation increases transmittance time (the time required for heat/cold to penetrate the building envelope), and proper shading in the summer, and "summer bypass" on the HRV will all help with the cooling situation at little or no extra cost.
    My suggestion? Try the house for a year. Every situation is a bit different, and you may find that your house performs very well sans AC. Or install the tubing for a ductless or hydronic in the event that it might be required in the future.
    Good luck on your project.
    Alec

  6. Kevin Camfield | | #6

    Response to Li Ling Young
    Why do you say that the Build Equinox CERV pairs well with a mini-split?

    We are in the process of trying to decide on a heating system and are finding all of the standard tradeoffs. I would like to hear your opinion on how you would use the CERV with mini-splits. Also, do you have an idea of the average installed cost of the CERV?

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Kevin Camfield
    Kevin,
    I urge you to read A Balanced Ventilation System With a Built-In Heat Pump. In my article, I reported that the installed cost of a CERV system is between $6,500 and $9,000.

    -- Martin Holladay

  8. Ross Elliott | | #8

    City of Ottawa
    One issue with point source heating in Ottawa is that the City is very strict about requiring a heat source in every room with an exterior wall, even if its an open plan. This means that if a ductless mini split on each floor will be able to handle the heating load, something will need to be added to every room.

    One way around this is to add heat to the ventilation system with an inline duct heater, and electric radiant floor heating in the rooms without a ventilation air supply diffuser.

    Dehumidification using the ventilation air would be excellent, if that is all that is required for us to stay comfortable in Ottawa.

  9. Kevin Camfield | | #9

    Response to Martin Holladay
    Martin,

    Thanks for reminding me about the article. Actually, I've read it. Frankly the title was so interesting it is what got me to subscribe to Green Building Advisor. I had forgotten that you had quoted an installation price. I'll go back and have another look.

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