A comeback for radiant floors heated with air tubes
Radiant heating systems are very common in concrete floors now because they provide a very balanced and comfortable distribution of heat. Those systems are most commonly hydronic, meaning heated liquid is pumped through tubing in the concrete which then radiates that heat into the house.
Hydronic radiant tubing for a slab-on-grade floor © Ecohome
Alternatively, you can heat floors with electric wires and this is often the most affordable after-market solution for happier feet. Even in new homes with forced-air heating, electrically-heated mats are sometimes installed below the tiles in bathroom floors for added comfort. What you don’t see a lot of, are floors heated with air.
Electric wire radiant floor heating © Ecohome
How to make air-heated floors work:
Since the first forced-air heating systems were introduced into homes, there have been attempts to distribute that heat through flooring systems, but they often failed miserably. The most problematic of those were attempts to heat concrete floors with standard metal ducts from forced-air furnaces.
Collapsed duct © Inspectapedia
Embedding ducts in concrete that were not intended to bear weight frequently caused them to collapse, rendering the heating system completely ineffective. Ducts that didn’t collapse were at risk of filling with water or at the very least, moisture. This led to corrosion and blockages, so even systems that worked at first were at risk of a short functional lifespan.
Corroded duct © Inspectapedia
For the systems that did work, and during the time they were working, they would have offered at least some success in warming floors, but doing so brought air quality concerns. These makeshift floor-heating systems functioned similarly to any other forced-air ducted system, where air from the home was circulated through heat vents and cold air returns.
Circulating air from the home though these floor ducts led to the normal dust and debris that can be found in any home being deposited in these floor ducts, and overtime created a buildup of organic matter.
Corroded vent image via Inspectapedia
In summertime when systems were inactive, dust-filled ducts embedded in cool, humid concrete floors provided the ideal environment for mould to develop. In the fall when systems were activated again, air circulating through dirty and mouldy ductwork would distribute particulate and mould spores throughout the home.
During inspections with remotely operated cameras, some even less appealing discoveries were made, including snakes, insects, dead rodents and their excrement. If you really want to, you can read more about that here.
How these flaws can be remedied:
- Make it a sealed, closed-loop system so that air in the heating system is not exchanged with the air of the house.
- Install non-corrosive duct work.
- Cure the concrete to remove moisture before systems are activated.
A company called Legalett that originated decades ago in Sweden now manufactures and distributes heated-floor systems across North America. They are the only company we know of that provides this system; if anyone knows of others, please let us know in the comments section at the bottom.
Why the Legalett system works:
Legalett air-heated systems are entirely embedded in concrete, including the heater box. It is a closed-loop system where air is moved through either 2 or 4-inch PVC tubes (non-corrosive), so that no dust from the air in the home is introduced into the system. With no open vents there is no access point for rodents and insects.
After the concrete is poured, Legalett requires floors to be cured with an open-air construction heater (provided with the system) that maintains a temperature of 28°C for 3 weeks before permanent heating units are installed. This acts to remove moisture from the concrete and prevent corrosion of metal components in the heater box.
Advantages of air-heated floors over hydronic:
- Legalett heating units are embedded in the floor along with the tubing, so they do not require a wall in the mechanical room to house boilers and manifolds. Units are accessed through a small hatchdoor flush with the concrete and can be placed anywhere in the home – in the mechanical room, under appliances, or in a closet.
- A metal box frame is hard-wired and embedded in the concrete to house heater units, the heater units themselves plug directly into the box to allow for easy removal for maintenance.
- Heater units use standard parts, so, long into the future when the warranty has expired, units can be easily removed and repaired by any electronics repair shop using commonly available parts.
- The first 10-12% of each zone output is fully-insulated, the second 10-12% is half-insulated – this forces the hottest air further down the tubes to more evenly distribute heat throughout the floor.
- With the system being entirely embedded in the floor, heat is also entirely contained in the floor, so there is no buildup of heat in the mechanical room as is common with hydronic systems.
- The reduced capacity air has to hold heat compared to water can reduce the risk of overheating.
- When installing a hydronic tube system you need to be very cautious, since accidentally piercing a tube will render the system ineffective. An air-heated tube system is not pressurized and there is no liquid to leak out, so an accidental screw put into an air tube is of no consequence.
- Each system and zone layout is custom-designed to deliver heat evenly. A detailed cut list and directions are provided, making installation simple and quick.
- Systems can be used in conjunction with thermal-solar air heaters, which are less risky (and less expensive) than thermal-solar hydronic heaters.
Insulating air tubes near the heater box output to better distribute heat © Ecohome
Can a radiant floor provide all the necessary heat for a home?
In theory, yes, but it will not meet the requirements of building code. According to code, ventilation air cannot be introduced into a building at a temperature below 17°Celsius at floor level, and not below 13° Celsius from vents located high on walls or the ceiling. Because of that, every fully ducted ventilation system in a home with any type of radiant heat will require supplementary heating to heat ventilation air back up to ambient temperatures.
Worthy of note: this is not about whether or not a heating system has the ability to provide the entire heat load of a building, this is about comfort, and making sure we don’t suffer the effects of walking past a vent delivering air at a bone-chilling 16°Celsius or less.
With traditional heating system like forced air furnaces, ventilation air is typically mixed with the already-heated indoor air to achieve the temperatures listed above. Because that option isn’t available with radiant heating systems, a touch up heater must be used with ventilation air.
Can air-heated radiant floors deliver enough heat?
Yes, they can. While it is true that much more heat can be transported by water than air, that doesn’t mean there isn’t enough. More power can be delivered by a Boeing jet engine than a 4-cylinder Toyota engine, but if you were purchasing a Toyota and you were given a choice between those two engines, which would you choose? Bigger isn’t always better. Any home-heating system needs to be designed and sized to meet the heat load of a specific building.
With an airtight and well-insulated bungalow equipped with an ERV or HRV using an inline fresh air make up heater (as required by code), a Legalett floor-warming system can provide all the additional heat a building requires under normal use. They have provided the entire heat load (excluding makeup heat) for buildings up to 35,000 square feet.
The floor heating system is only part of what the company offers, they provide engineered raft slab kits and have installed over 2 million square feet in North America.
2-inch air heated tubes © Legalett
Heating with electricity but avoiding peak rates:
An additional benefit that can be realized with any heated concrete floor (air, electric or hydronic), is that the floor itself acts as a thermal battery. A warmed slab of concrete will take a long time to cool, long enough to last through peak hours.
Floors can be kept on a timer, set perhaps to go on no sooner than 7:00 PM and off again by 7:00 AM, so you can avoid daytime peak rates. A well-insulated warm concrete floor can easily keep a house warm for 12 hours, allowing you to heat your home affordably with electricity in regions with high peak-electricity rates, without using fossil fuels.