Warmboard Tubing Spacing

We are often asked why Warmboard is only available with 12" tubing spacing. These questions typically come from people familiar with the limitations of low conductivity thin slab systems which often require tubing spacing of 6" o.c. (on center) or closer. The short answer is that because of its high conductivity, Warmboard at 12" o.c. equals the performance of a thin slab system at 2" spacing. Therefore there is no need to ever use tubing spacing closer than 12" o.c. with Warmboard.

The long answer begins with an analogy. Imagine a frying pan with a solid aluminum handle (highly conductive) compared to a similar pan that has a wood handle (a poor conductor). When the pan is hot you would certainly not grab the aluminum handle near the pan itself. You would move your hand nearer the end in the hope that the handle would be less hot further away from the pan. But even then, unless the handle was very long you would probably still need a potholder to keep from burning your hand. But if you were to grab the wooden handle version, the wooden handle may not be very hot at all a few inches from the pan. This is a good intuitive way to understand the power of conductivity to deliver heat far from its source.

Now let's remember what is happening in a radiant floor. The heat supplied by a radiant floor is directly proportional to its average surface temperature. The floor surface is always the warmest right above one of the tubes carrying hot water. The question is how warm is it halfway between two tubes? With 12" o.c. Warmboard the drop off between tubes is between 1˚ - 3˚ depending on floor coverings. In a 12" o.c. thin slab system it is between 5˚- 10˚. If we shorten the distance between tubes to 6" o.c. in the thin slab, we may reduce the drop off to between 2.5˚ - 5˚, still not equal to Warmboard's performance. The thermodynamic formula below defines the relationship of conductivity and tubing spacing to heat flow.

When we use this formula to compare Warmboard to gypsum based thin slab systems, the laws of thermodynamics tell us that a Warmboard system at 12" o.c. roughly equals the heat flow of a thin slab at about 2" o.c. The graph below will help you visualize the benefits of Warmboard's superior thermodynamics.

Because of the low conductivity of slab systems, the floor temperature directly above a tube averaged with that half way between two tubes is significantly lower and therefore the output of the floor also drops significantly. The only way to increase the output of these systems is to tighten the tubing spacing.

Complex thermodynamics aside, Warmboard's high conductivity provides many benefits, not the least of which is greater comfort through more even floor temperature. This even heat is also one of the reasons that Warmboard works better with hardwood floors. Wider tubing spacing lowers labor and materials costs throughout your completed system by requiring less tubing, fewer manifolds and controls, and less labor to install all of these components, which also ensures greater reliability. Warmboard's superior conductivity also lowers the required supply water temperature which will save a significant amount on your heating bill year after year, decade after decade. This is why we say that in radiant floors, conductivity is king, whether we are talking about tubing spacing, comfort or energy savings.

“Fast Radiant” Demystified

One of the most significant benefits of Warmboard is its ability to quickly adjust to temperature changes and keep a conditioned space at the desired comfortable temperature. We call this “fast radiant”, because unlike competing radiant solutions, Warmboard reacts faster. This unique benefit is a result of Warmboard’s low mass and high conductivity.

A radiant panel has a very simple function to perform; conduct heat from warm water in a tube to the surface of your floor. Warmboard performs this simple function so well because:

* Our unique aluminum lined channels make excellent contact with the tubing
* That same aluminum is highly conductive and covers the entire floor surface
* Finish floor goods are in direct contact with the aluminum.
* The total mass of the floor assembly is very low.

Warmboard is a simple concept, which maximizes the speed with which heat is delivered under all circumstances. The key to this is Warmboard’s low mass. When a heating system needs to react quickly to the demand for more heat, or conversely less heat, a low mass system will always outperform.

Here’s how: The aluminum in Warmboard has a similar specific heat to traditionally applied thin-pour material, meaning that an equivalent heat input will raise equal amounts of material (mass) at the same rate. However the mass is not the same. Warmboard weighs 3.1 lbs per square foot. Because it’s the structural subfloor, there’s no additional mass added in framing or construction. A thin-pour of 1 ½” weighs 14.5 lbs, and because this is installed above a subfloor, we add 2.5 lbs per square foot for a ¾” plywood subfloor. Assuming the same finished flooring materials are used, Warmboard’s 3.1 lbs per square foot has 5.5 times less mass than thin-pour’s 17 lbs assembly. Heating less mass takes less time and this means that all things being equal, Warmboard will react 5.5 times faster.

But, things are not equal. Warmboard’s conductivity also contributes to our fast delivery of heat. To understand this, let’s look at the basic equation for heat flow: Heat flow (F) equals the difference in temperature, or delta T (ΔT) times the Coefficient of Conductivity (K) times cross sectional area (A) divided by the length (L) over which heat must flow.

Comparing thin-pour to Warmboard in this equation, ΔT remains constant and L (tube spacing) is 12”. K is 360 times greater for Warmboard (aluminum compared to a gypsum-based thin pour) and A is 60 times lower for Warmboard (.025” of aluminum to 1 ½” of gypsum based thin pour.) Based on these facts, Warmboard will have heat flow 6 times faster than traditional thin pour systems. If you were to change the thin pour tubing spacing to a very tight 6”, Warmboard’s heat flow is still 3 times greater. Greater heat flow equates to faster radiant heat.

A structure’s heat loss, starting point temperature, system water temperature, occupancy rate, and much more, significantly affect any heating system’s ability to respond. But when these two floor assemblies are compared side-by-side, Warmboard’s superiority for heat transfer is clearly evident. Based on the science behind Warmboard, it can deliver at a rate 5 to 10 times faster than traditional thin-pour systems.

For the homeowner, this means that floors become noticeably warm within minutes. Ambient air temperature in most cases can increase from 2° to 5° degrees per hour when the need for heat arises. The important features of low mass and high conductivity combine to make Warmboard one of the fastest radiant systems available.

How to build a home that if it were a car, would get 100 mpg - Part I

Energy efficiency is, and has always been, one of the key reasons for building a radiant floor heated home. Given the rapidly escalating cost of energy, the efficiency of radiant has never been more important.

For the expanded version please click here.

What makes radiant heat so energy efficient?

- Parasitic losses.
- Lower ceiling temperatures.
- Zoning reduces energy usage.
- Lower air temperatures for the same comfort.
- Blowing hot air paradoxically can cool us.
- KSU study. A study of all of these effects was done at Kansas State University in conjunction with the American Society of Heating Refrigeration and Air-conditioning Engineers (ASHRAE) that established that a more or less typical radiant heated home in the US can expect 25% savings over a conventional forced air home.

Warmboard drives these energy savings numbers higher for several reasons:

- Warmboard is the most conductive radiant panel assembly.
The basic equation for heat flow is: heat flow (F) equals delta T (ΔT) times Coefficient of Conductivity (K) times cross sectional area (A) divided by the length (L) over which heat must flow.

It is a principle of thermodynamics, established by this formula that as conductivity goes up, water temperature can go down. It is always less expensive to heat water to a low temperature han a high one. It is well accepted in the boiler industry that for every three degrees that you lower the water temperature, you save 1% of the cost to heat that water. This means that compared to the least conductive radiant floor systems (staple up) our water temperatures are as much as 60 degrees lower for the same heat output. Compared to the commonly used thin slab systems, Warmboard uses as much as 30 degree lower water temperatures. This means an additional 10-20% savings over these more antiquated radiant panel assemblies.

- Warmboard maximizes the efficiency of condensing boilers. See Part II of this blog for specifics.
- Warmboard’s low mass allows the efficiency of temporary temperature set back.
- Warmboard’s low mass prevents overshoot.
- Warmboard’s low water temperature requirements are ideal for alternative heat sources.

Save thousands on fuel costs each year.
Modern homes are well insulated, have excellent glazing, low energy usage lighting and many energy efficient solutions that make them seem more like the 40 mpg gallon economy cars we see on the road today than the gas guzzlers of the past. But if you add a radiant heat system to that otherwise efficient home and combine it with a fast response, low mass, highly conductive Warmboard radiant subfloor, in effect, you can start approaching the ownership of a home that behaves, by comparison, more like a car that gets 100MPG. We say this not only because of the theoretical savings detailed above, but because we have many homeowners living with the comfort of Warmboard heat in their homes who report heating bills as much as 60% lower than similar sized homes in the same community. If you might normally expect to pay $5,000 to heat your home through the cold months, bills in the $2,000 range are what many Warmboard homeowners are experiencing. But remember, your mileage may vary.

For High Performance Radiant Heat...Conductivity, Not Mass, is King

Often, when people are talking about radiant floor heating, the term “Thermal Mass” comes up. Thermal mass in this context is a term that refers to the ability of a high mass radiant floor assembly to store heat. The concept originally made sense in the design of passive solar homes back in the 60’s and 70’s.

Because passive solar could rarely supply all of the heat needed for a home, these slabs were often poured with tubing embedded in them so that hot water could heat the slab when there was insufficient heat from the sun stored in them. In other words, they were combination radiant floor heating systems and passive solar systems. Because these systems were prevalent during the infancy of radiant and solar, the terms thermal mass and radiant heat became linked in people’s minds.

But the same thermal mass that is so essential to a passive solar home causes one of the more common complaints with radiant heat - that is is slow. In high mass systems, because heat loads change upward or downward more rapidly than a high mass slab can respond, it is not uncommon for the inhabitants of a high mass radiant heated home to be too cold in the morning and too hot in the afternoon. Or if they are returning to a cold home after an absence, they might have to wait a day or more for their home to come back up to a comfortable temperature.

It's the job of a radiant system to deliver heat not store it. A radiant floor system to causes heat to flow into the conditioned space at close to the rate that heat is flowing out. The property of a material that allows heat to flow through it is called conductivity. Concrete is an inherently mediocre conductor whereas aluminum is 240 times as conductive as concrete!

The ideal radiant system is able to adjust its heat output upward or downward, in real time, as is needed to create a constant temperature environment. Surprisingly, the quality most often touted as the advantage of radiant systems based on high mass slabs, namely, that they don’t vary much in their output, is precisely their Achilles heel. The slab itself may maintain a constant temperature, but the conditioned space they serve doesn’t maintain a constant temperature because heat loss varies throughout the day.

Boiled down to its essence, a high mass system is really a control system, not unlike a flywheel. This means that sometimes it will accidentally get it right and sometimes it will, by equally random accident, get it wrong. Like a stopped clock, it's guaranteed to be right at least twice a day.

If we are not getting our heat from an uncontrolled source such as the sun, but instead, from a controllable fuel source it makes a lot more sense to control the heat supplied, by use of an intelligent control system... a thermostat. Such a smart control system can merely turn on a pump in response to a call from thermostat, which can direct that heat to a highly conductive low mass radiant panel, which can deliver that heat in real time as is needed.

This is especially true for modern, active solar systems. These systems do not store heat in an uncontrolled slab but instead store the Sun's heat in a mass of water held in a well insulated tank. A pound of water will store 3 times as much heat as a pound of concrete. So not only are concrete slabs mediocre conductors, they are a mediocre means of storing heat as well. In a well engineered active solar system, heated water can be pumped as needed to a highly conductive low mass panel, controlled in real time by a simple thermostat.

High mass systems are being relegated to the dustbins of history. Low mass, fast response systems are the future of radiant simply because they are better engineered relative to the laws of thermodynamics. Highly conductive radiant panel assemblies allow the use of lower water temperatures than high mass slabs, which saves energy and money. Fast responding low mass systems also save energy by avoiding the waste of under-shooting or over-shooting the desired interior temperature, and by allowing night setback and vacation setback. But most importantly, low mass systems are better able to maintain the desired interior temperature on a constant basis. Constant temperature at the desired set point, the essence of heating comfort after all, is the primary mission of a modern, energy efficient, well controlled, radiant floor heating system.