Every day when I step out of the shower I get a rush of joy when my toes hit the warm floor. I want to share this joy with you – thus this guide to underfloor heating.
What is Hydronic Heating
This guide is about hydronic heating, which has become popular these last several years. Hydronic heating is energy-efficient and uses tubing to run a hot liquid beneath the floor. It can also be used to heat along base board heaters, or through radiators in your home, thus it’s 2nd name – radiant heating. I found this great articl from hydronicheating.net that really explains it better than Doug or I could:
Introduction to Hydronic Heating Systems
The hot water, or alternative heated liquid, of the hydronic radiant heat system is circulated throughout the home through loops of plastic piping. Most often, these tubes are installed within your homes concrete slab or floor joist system as radiant flooring, and allow the heat to radiate evenly across the entire floor surface. In addition, baseboard heating units and radiators can be used as sources of hydronic heat.
The liquid in a radiant system begins in an energy efficient boiler, where it is heated before flowing to a plumbing manifold system. Your plumbing manifold acts as the control center by connecting to the thermostat and directing the correct temperature of water to the various heating zones of your home. This allows each zone to maintain a personalized, steady temperature. Pumps work to constantly circulate newly heated water into the tubing, while cooler water is returned to the boiler to begin its journey again.
This closed loop system of heating allows for flexible temperature customization, while maintaining an incredible level of energy efficiency.
Comfortable Room and Surface Temperatures
Radiant heating provides superior comfort because the entire floor radiates heat up from the ground in an even and consistent manner. Traditional forced air systems work by blowing warm air through a series of duct work in the floors, walls, and ceilings, and finally to its destination by way of one or two vents in a room. This method can lead to hot and cold spots due to poor air circulation, as well as temperature spikes and dips when the system turns on and off. Underfloor hydronic systems eliminate this issue by producing a steady heat that radiates from the floor throughout the entire room.
Hydronic heating allows you to say goodbye to cold tile floors. Your kitchen and bathroom tiles will never be chillingly cold to the touch, but instead, radiate a gentle warmth that makes the morning walk to the shower easy on your bare feet.
Do Hydronic Radiant Heating Systems Limit My Flooring Options?
Traditionally, hot water radiant floor systems have been installed in the concrete slab of a home, but advanced designs and installation techniques now make it possible to install hydronic tubing in floor joist systems and below hardwood floors, carpet, laminates, and vinyl flooring. This means that all traditional flooring options are available to homeowners with radiant flooring.
With this said, it is essential that both the hydronic system installer and the flooring installer are aware of the others work, so that they can make any needed adjustments to the materials, and protect the hot water tubing when the flooring is installed. As an example, carpet selection for underfloor systems is important because thicker carpets and pads can create an insulation layer that lessens the effect of the radiant heat. Speak with the hydronic heating system manufacturer and installer for advice on flooring materials that will ensure your system functions at its best.
Now that we have a basic understanding of residential hydronic heating systems, we need to go into more detail on how hydronic heating works.
Heat Transfer
To understand why hydronic heating is so effective it is important to understand how heat transfer takes place. The transfer of heat occurs in three ways:
Conduction – This is the movement of heat through objects that physically touch, where heat moves from the warmer object to the colder. Standing barefoot on a beach with hot sand or a cold kitchen tile floor are good examples of this process. Density affects an objects ability to be conductive, which is why liquids are much better conductors than gases. This is the reason why we get colder much faster immersed in 60 degree water, as opposed to standing in outside air temperatures that are the same.
Convection – This occurs when fluids or gases transfer heat while they are being circulated from one area to another. Traditional heating systems that use forced air are perfect examples of this type of heat movement. Just picture the warm air that flows out of the vents in a room to understand this process. Unfortunately, air and other gases, as mentioned above, are poor conductors of heat.
Radiation – Thermal radiation is heat that travels in invisible waves through empty space. It is not something that can be blown away by the wind or moved. It is simply absorbed by the person or object that is in the path of the beam of energy, and is a far more effective means of transferring heat.
Hydronic heating works so effectively in keeping your family and home warm over conventional forced air systems because it utilizes conduction and radiation in heat transfer along with a superior conductor of heat in the form of a liquid. On the other hand, forced air heating relies solely on convection and air. Air is such a poor conductor of heat that it is often used as a means of insulation, as can be seen by its use in between the panes of energy efficient windows.
Hydronic Heating Systems in Action
The effectiveness of hydronic radiant heat is produced through 3 steps:
The boiler system uses conduction to quickly and efficiently heat the water.
The hot liquid being pumped throughout the house heats the concrete, tile, or other type of flooring through conduction as well as radiates heat into the room.
Lastly, the flooring surfaces them radiate the heat they gained from the water into the room as well.
The ending result is that a consistent, comfortable temperature radiates from the floor as well as the solid objects around the room. And, a surprising effect is that the radiation of heat in a hydronic system will actually allow you and your family to feel warmer at lower air temperatures than conventional heating. This allows you to further lower your heating bills and energy usage, while increasing your comfort level.
With an understanding of hydronic heating and how radiant systems work, we will now take a look at the different types of hydronic systems, so that you can find the one that best fits your needs.
Types of Radiant Heating Systems
Heating designs for a radiant system can be tailored to most residential needs, whether it is a new custom home, or a remodel in a historic area. The flexibility of this type of advanced home heating system, as well as the high levels of energy efficiency, have made radiant heat an increasingly popular choice for contractors and families alike.
Basic Equipment
Every residential hydronic heating design will require the following basic elements to heat the home:
Boiler – The means to heat the liquid.
Liquid Medium – Traditionally, this has been water, but some systems allow for other liquids or water and antifreeze mixes.
Manifold/Thermostat – Connected to the thermostat, the plumbing manifold is the hub for directing water flow and room temperatures.
Tubing – PEX or some other form of plastic tubing is pretty much universal with all hydronic systems, with very little in the way of copper or other rigid piping used.
Heat Exchanger – The radiator, baseboard heater, or in floor element that transfers the heat into the home.
Pump – To keep the heated liquid in circulation, so that the warmth is constantly replenished.
With this understood, there are many different options within each element that are used depending on the type of hydronic radiant system you want for your home.
Boiler Options
The cornerstone of any hydronic system is an energy efficient boiler that heats the liquid with little to no heat loss or wasted energy. In addition to the standard boiler for radiant heating systems, there are a few different boiler options, as well as components that work in tandem with your boiler.
Tankless Hydronic Water Heaters – Much smaller than the traditional tankless water heater, these heaters are specifically designed to allow on demand, energy efficient hot water for radiant heat.
Combined Hot Water and Heating Systems Boiler – Originally more common in commercial building applications, combined boilers that allow for hydronic heating and home hot water usage are starting to appear for residential use. These can be designed for water tanks and tankless water heating alike.
Solar Water Heater – A solar heater will not provide enough heated water by itself, but it can increase the energy efficiency of the system and work in tandem with your boiler. A panel on the roof heats a liquid that runs through tubes inside a water storage tank. This creates water that is naturally pre-heated and requires less energy from the boiler to bring to the right temperature.
Geothermal Heat Pump – Similar to the solar heater above, geothermal heat pumps can preheat the water by using the natural warmth found several feet below the soil. This reduces the amount of natural gas, electricity, or oil that your boiler will need to heat the water.
Liquid Options
Traditionally, hydronic heat consists of hot water as the method of transferring heat, and while this is still the norm, modern radiant systems can also incorporate other liquids to heat the home. Glycol is a common antifreeze that is added to a closed loop system. This can help to protect the water from freezing if the piping runs through an exposed area, as well as increase the waters boiling point. The need for antifreeze or other liquids other than water will be explained by your installer and designed around your specific heating needs.
Heating Exchangers and Their Locations
Underfloor Radiant Loops – Radiant flooring has become very popular for new home construction, as it is much easier to add into the concrete or floor joist system during the original building of the home. It can provide consistent comfortable heating evenly throughout the home.
Baseboard Heaters/Radiators – Hydronic baseboard units and radiators can be much easier to install for home remodels because they require less tubing below the floor. They can be inconspicuously located along the walls of a room, but you should make sure they are not blocked by furniture, so that the heat can radiate out into the room.
Walls and Ceilings – Similar to installing a radiant floor as above, wall and ceiling radiant panels are available to be placed behind walls, and can heat a broad area. These panels are most often run with electric radiant heat, as opposed to hydronic, due to water damage possibilities. They can be great additions to residential hydronic heat, especially in spaces where large areas of floor are covered by cabinetry, such as kitchens.
Outdoor Radiant Heating
In addition to meeting your indoor heating needs, outdoor radiant heating systems can be a very helpful addition to your overall HVAC design.
Snow Melt – For those who live in areas with a good amount of snow fall or ice, a radiant heating system for melting snow on the sidewalks and driveway can make you home much safer. Additionally, radiant roof heating can help protect your roof from ice dams and future leaks due to snow and ice buildup.
Heated Pools – Anyone who likes the feel of a comfortably heated pool will appreciate radiant heating loops beneath the pool walls that allow for an energy efficient way to keep the pool warm and extend the swimming season.
Electric Radiant Heating
Hydronic systems are not the only means of radiant heat. Electric heated floors are an alternative method of radiant heating that can be suitable in some residential heating designs. While this website’s focus is about using a whole house hydronic system, it is important to know all of your options so that you can make a decision on which configuration or combination of systems will work best for you. Here are a few things you should know about an electrically heated floor:
Design – Electric radiant floors work in a similar fashion to an electric blanket. Typical installation is done by rolling out a thin layer of matting or mesh that contains the low voltage electric heating coils laid out in a pattern similar to hydronic radiant tubes. They are glued or stapled to the subfloor and a thin layer of self leveling cement is spread over the top. After this sets up most flooring types can be placed above the electric flooring with tile being the most popular choice. Finally, the system needs to be connected to the home’s electrical supply and a thermostat. You will want to make sure that the electrical connection is done by a licensed electrician, but the actual floor installation is a great project for a do-it-yourself homeowner.
When to Use – Floors that are heated electrically are not usually used as a whole house heating system, instead electric radiant flooring is used as a secondary system to focus on specific areas of the home. Their easy installation flexibility allows homeowners with forced air heating to complete a small remodeling project and add the luxury of radiant heat to the areas where they most need it. Bathrooms, kitchens, basements, and garages are popular areas to install electric heated floors because they allow you to have evenly heated warm floors at the specific times you and your family enjoy that space.
Cost – An electric radiant floor system can have a less expensive initial installation cost when compared to hydronic heat. This is due to the ease of laying down the electric coil matting instead of hydronic loops, and the fact the electric heated floors can be readily connected to the home’s electrical system. On the other hand, operation costs for electrical radiant heat is much higher than that of a hydronic system, which is why most people prefer their whole house heating solution to be hydronic, and save the installation of electric radiant floors for targeted areas only.
Radiant hydronic systems can be designed around these many different options as mentioned above. This will make sure that the final heating plan will meet your family’s needs of comfort and long-term affordability.
Whether you are trying to add a super energy efficient radiant flooring system to a new custom home, or maximizing the space you have in a home remodel with tankless hydronic hot water units and radiators, there is a hydronic heating plan that can work for you.
I also found the following article on inspectapedia.com, which I think is brilliant as it is a true case study on what not to do if you are installing this yourself. I’d even print this and show it to your contractor… you don’t want this happening to you!
How to Really Foul Up a Radiant Heat Concrete Floor Installation – Mistakes to Avoid
One exception to the general order of priorities of where to insulate in buildings concerns homes built with slab-on-grade construction, particularly homes which have used radiant heat in the floor slab.
The contractor (Nightmare Construction) for a small cabin in the North combined being opinionated and a bully with dismal ignorance of how to construct a properly insulated radiant floor slab. (See Slab Log Cabin Siding for cabin photos and other comments).
Not only did the owners have to battle with the bully to put insulation under the entire slab (he thought that Mother Earth would be warming the home from “ground heat” (which is below 40 degF in winter there).
Owners also lost a battle to have the contractor install proper insulation around the slab perimeter with a frost wall before the floor was poured (he insisted on a floating slab with no inside-perimeter insulation plan).
Worst of all, the contractor also pushed the radiant heat tubing so deep into the concrete (ranging from 7″ deep to more than 18″ deep) that the entire radiant heat system was not useable at all. Running the radiant heat pushed heat faster into the ground than it did up into the building, even with foam insulation under the slab. We had to abandon the (expensive) radiant floor system and install alternate heating.
See SLAB INSULATION, PASSIVE SOLAR for a discussion of proper insulation below a heated floor slab.
Don’t permit your contractor to make the (many) mistakes this one did. Insist that radiant heating in a poured concrete slab have these attributes:
Critical Design Details for a Radiant Heated Concrete Floor
Insulate below the entire floor slab. Sketch at left courtesy of Carson Dunlop.
Insulate the slab perimeter, making sure that the insulation design does not rely on foam placed against the slab perimeter and extending above grade up to siding where it will invite termites or carpenter ants into the structure
Place the radiant heat tubing at the industry-recommended depth down from the surface of the slab. Typically the maximum depth that tubing should be placed in a concrete floor slab is 2″ down from the finished floor surface.
Supervise: If you cannot be present at the job site at critical stages in construction, find someone knowledgeable who can inspect for you before the work continues
If your contractor is an opinionated bully like ours who ridicules standards for good workmanship and proper radiant heat floor design, find someone else to do the work. Most contractors are conscientious and are glad to hear about good design.
Radiant Heat Floor Slab Design Specification Details
After reviewing photographs taken during installation of the radiant heat floor slab described above, here’s what we wrote to the owner and to the contractor:
I am doubtful that we can successfully and economically heat the cabin with radiant in floor heating as the current system is designed and installed, and it is unfortunately the case that the cost-to-cure is prohibitive as the slab would need to be completely replaced with one using proper insulation and tubing placement.
The bully contractor, who originally estimated the monthly heating cost for this small and otherwise well-insulated building, had said the owners would face winter heating bills of about $30./month based on his prior experience. Stunning heating bills arrived, exceeding $400./month or more than ten times the estimated amount. That’s when we began digging into the installation details of this project. The floor slab and radiant heat tubing had been placed by the contractor while we were unable to attend the jobsite.
When the heating bills were excessive and when the heat, running 24-hours a day for weeks, was unable to raise the interior temperatures above 60 deg .F., the contractor offered to “correct” the problem by installing larger capacity circulator pumps.
The “option” of adding larger pumps for this radiant heat floor was not a proper solution for several reasons:
Forcing a faster hot water flow in the radiant tubing would only correct a boiler operating problem if the boiler internal temperature were running too high and causing a shutdown due to the thermal over temperature sensor – this is not what was happening (I measured input and output temperatures).
The boiler was already operating at spec in that it was producing more than 20 degrees between the input line and output line – so the problem is not flow but rather the inability of the boiler to handle the heat loss through the slab due to the slab tubing placement and insulation design.
Installing a boiler of higher capacity might permit delivery of more heat to the slab and raise the indoor temperatures and slab surface temp but at a higher heating cost
The current heating costs if we turn on the radiant floor system run $300. to $440 a month which is 10 times what the contractor originally estimated – doubling these costs by adding a larger boiler or faster water flow is unreasonable.
We didn’t have the option of taking advantage of reduced electrical rates because the electrician did not install the electrical service to our specifications – leaving out a separate service and meter at the entry point.
The most economical fall back is to install electric baseboard heating or possibly hydronic heating using the existing electric boiler which was installed to pump heated water through the radiant tubing in the concrete floor.
Meanwhile we shut down this unfortunate radiant slab heat system, installed a few portable electric heaters, and given the tight, well-insulated construction, we found we can keep the little cabin comfortable for a fraction of the cost of heating the earth underneath our floor with the contractor’s heating installation.
Here are the details of the errors visible in photographs taken during installation of the radiant floor:
Photographs of the slab and radiant tubing installation for the cabin show that the guidelines for radiant heat slab installations were not followed.
Tubing is at a depth greater than 2″ from the top of the slab and at some locations is considerably deeper than that, in some areas more than 12″ deep in concrete.
Insulation is incomplete around the slab perimeter and cannot be added outside due to 1. insect damage risk and 2. would not extend below and under the slab edges
Insulation is incomplete within the slab where tubing installed at a lower level is stepped down from the upper slab level and heat transfer is permitted into the gravel fill below the main slab area.
Discussion of the Above Radiant Slab Heat Performance Case Study
James Darling, General Manager of Preferred Heating LLC, in Eagle River, WI commented on this article that the contractor’s promise of heating the building for $20. a month was an unreasonable promise not to be relied on – one that could make the article above misleading. We agreed that the description of the failure of this installation needed some clarification, and added the following information that should be considered:
Actual Heating Costs for the Building Described Above
Keep in mind that this was a small new structure (624 sq.ft.) whose construction details, methods, materials were unusually well documented as a project. So the insulation, air tightness, materials, heating details were known.
The building was super insulated, tiny, airtight, with double-glazing throughout, leading to an expected low heating cost. If the owner’s actual heating bills for the structure had been even five times what was promised for this building that was occupied only part-time, the owners would have been happy. Heating bills weren’t the arm-waving promise of $20 per month, they were not $200. per month. They were more.
In fact, the utility cost to heat this tiny cabin resulted in bills that more than doubled the corresponding costs of the nearby 1960′s vintage two story large old, comparatively poorly-insulated house on the same property, exposed to the same conditions. And the exploding heating costs were observed when heating the building well before the coldest part of the heating season.
Heating Capability Limitations of an Improperly-Installed Radiant Floor Slab
The effects of putting the tubing deep into the slab created a problem of heat transfer losses to the ground, not just a matter of longer response time to warm the building. Even if money had been no object, the system simply could not heat the building to an acceptable temperature.
The problem with very deep radiant-heat tubing, combined with incomplete insulation, is that even with just 12 to 18″ of concrete above the tubing, heat flowed enough into the ground below the building that even with the thermostat set to maximum, and running heat continuously for a week solid, in moderately cold weather (in the 40′s in Northern Minnesota where in winter it can drop to 20 deg F below zero) we never ever could get the indoor temperature above 59 to 60 degF. And this was in a new, small, airtight one-story well-insulated building.
Even if we had continuous solid foam insulation under the slab, say R-10 for simplicity, if we have enough inches of concrete above, even though the “R” of concrete is much lower than the insulation, it’s the total heat resistance by the total inches that comes into play.
If we have enough thickness of concrete above the tubing (Where 1″ to 2″ tubing depth is the best design and 6″ is considered a lot, in this building we are looking at 18″ or more at least in many areas, maybe 24″). With radiant tubing at those depths, the concrete begins to offer not just a lag time in heating (Mr. Darling’s point) but also an actual resistance to heat transfer until we begin losing at least some heat into the ground.
The contractor and others tried to improve the system’s performance by changing the boiler settings from those set by the manufacturer on its integrated circuit control board, upping the circulator size and capacity, checking flow rate through the system, checking the thermostat controls.
What Caused the Failure of This Radiant Floor Heating System?
Our photo shows where we found the radiant heat floor tubing when we later broke open a section of the floor slab. Radiant tubing was at the bottom of the slab, in this area more than seven inches down in the concrete, and set atop the foam sub-slab insulation.
Our photo above on this page shows that tubing was in some sections more than 18″ deep, and adjacent to a large area where sub-slab insulation was simply omitted by the contractor.
We also measured floor temperatures in different areas of the building, mapping clearly where the radiant heat tubing dropped to the bottom of the footing-portion of the monolithic-slab footings! That deep run, probably combined with the incomplete insulation at the level drop between slab bottom and the integrated footings, were almost certainly the prime cause of the failure of this system to heat the building.
As our reference document(s) below show by calculation and model, ultimately, the heat flow into the ground for tubing really too deep in the slab can be significant, even if there is insulation below all or part of the slab. In the structure described here, not only was some tubing 12 to 18″ or even more below the slab top, the insulation below the slab was incomplete, inviting ready heat flow into surrounding soils.
Despite varying opinion by some radiant floor installers, consumers, and installers as well should be wary of ignoring the advice of the radiant heating design experts and heat transfer engineers about tubing depth in radiant floor slabs shown just below.
Worse than too-deep radiant floor heating tubing, in this case, because the contractor put NO insulation at the area of soil where he stepped the slab down to the depth of the monolithic integrated footings, we have heat transfer from some of the tubing through concrete right into the cold soil, not just through concrete up into the room through the ceramic tile floor.
In this egregious error, even worse than putting radiant heat tubing too deep in the slab, insulation was simply omitted where the floating-slab monolithic footings were poured. The R-value of concrete is roughly .08/inch (US DOE). The builder located sections of the radiant tubing so that there was about 6″ or less of concrete (in the 12″ footing section”) between the tubing and the cold soil, giving us a heat transmission path (tubing to soil) of R 0.24 or less. This is a likely area of heat loss at all four sides of the building: where the slab dropped down to form footings. (See INSULATION R-Values & Properties)
As an aside the ceramic tile on the finished floor slab was set in mastic – leaving some air spaces and mastic that is a poor conductor compared with tile set in concrete (optimal) – but we doubt that’s nearly as important in the system failure in this case.
