There is a lot of confusion about delivering enough BTU vs. reaching a heating target in the industry. The two, although connected, are not the same. Even professionals whose job is to install heating systems often struggle reconciling this fact.

When engineers and us designers calculate heat loss for a structure or a driveway, we first figure what the Winter Design Temperature, the temperature on the coldest day of the year is. The other 99% of the year, the temperature must be above this.

The second number determined is the Target Temperature for the dwelling or slab, usually 71°F for residential dwellings and 35°F for heated driveways.

The third number they calculate is the Heat Transfer Rate at Winter Design Temperature​ expressed in BTU/h. This number represents the rate at which we must deliver heat to reach and maintain the Target Temperature. The higher the difference between the the outdoor and Target Temperature, the higher the Heat Transfer Rate is. The speed of this heat transfer is slowed by insulation in walls, ceilings, and under the slabs of course. The higher R-value of the insulation, the slower the Heat Transfer Rate.

The fourth number calculated is the Supply Water Temperature. NOTE: This is NOT the same as the Target Temperature!!!

As a matter of fact this temperature must be higher than the Target Temperature. The higher the Supply Water Temperature, the faster the target medium (floors, air, slab etc...) will reach its Target Temperature. Why don't we just let it rip and heat the water as high as we can?

• High water temperatures can damage floor coverings and concrete.
• Too fast heating can also damage concrete slabs.
• The higher the water temperature the lower a condensing boiler's or heat pump's efficiency.
• Easy to overshoot the target temperature.

For these reasons the following are the recommended maximum Supply Water Temperatures:

• Radiant high mass (overpour and concrete) floor heating: 120°F
• Radiant low mass (staple up, transfer plate thin install) : 140°F
• Snow melt slabs: 150°F

So far all this is pretty easy to understand. One would assume that as long as we deliver enough BTUs we are golden. Nothing could be further from the truth and here lies the confusion.

BTU delivered in hydronic systems is calculated by looking at the temperature difference between the supply and return liquid (∆T) and how fast this liquid is circulated (GPM).

The simplified formula for water is BTU = 500 x GPM x ∆T x 1, for 40% Propylene Glycol it changes to BTU = 500 x GPM x ∆T x 0.895

GPM = US Gallon per Minute
∆T​ (Delta T) is the temperature difference between the supply and return water in °F.
0.895 is the specifi cheat for 40% Propylene Glycol

Note that there is absolutely NOTHING in this formula about TEMPERATURE!!!

We want to melt snow on your 1,000 ft² driveway. We calculate 160 MBTU/h load. Right now it is 20°F and snowing.
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Using the above formula of BTU = 500 x GPM x ∆T x 0.895​ we calculate we need a pump that can push the Glycol at the rate of 14.3 GPM to the slab to have a 25°F ∆T​

160,000 BTU = 500 x GPM x 25°F x 0.895

On the boiler we program the setpoint to be 120°F thinking 120 is more than 46 and we want the boiler to run very efficiently, therefore at a low temperature. We have a pump that pushes enough glycol to reach the required flow, we have the ∆T therefore we must have the BTU, yet 12 hours later the snow is still not melting.

We know we are delivering the BTU so what went wrong? The rate of cooling on the surface of the slab is higher than the rate and temperature sent to it from below. Heat is nothing more than the speed at which molecules vibrate. The higher the heat the faster the vibration. In this case molecules inside the concrete must vibrate pretty fast for molecules on the surface to vibrate fast enough to transfer that motion to the snow above and cause it to melt.

According to Tekmar® to melt snow on a driveway in 20°F weather, the slab has to be 46°F for the surface to reach the desired 35°F


If we have 120°F water (40% Glycol/water solution) exiting the boiler, by the time it enters the slab, especially if there are long and/or uninsulated supply lines between it and the manifold(s), that water maybe only 110°F or less. It loses 25°F in the slab and then at 85°F heads back to the boiler. The temperature difference between the exiting water and the slab target is only 40°F. The temperature of the surface never rises above freezing point.

To avoid burning copious amounts of gas for nothing we need to raise the supply temperature, that speeds up the heat transfer to the surface. We send 150°F water out and with the same 25°F ∆T at 14.3 GPM and 160,000 BTU/h we now see the snow melt and the driveway eventually dry.

To press this point, even if we deliver 55°F Glycol to the slab at 12.8 GPM and 25°F ∆T taking it back at 30°F to reheat we still would have delivered 160,000 BTUs.

To summarize, having enough BTU/h is only one aspect of snow melting or heating. We must also be able to deliver high enough temperatures to reach the desired effect. Pay attention to insulating supply and return lines, tighten up the ∆T by raising the GPM and make sure you are using full flow manifolds, large enough pipes and tubing, strong enough pumps.

Coming up next: "Heat Pumps: Realities and Misconceptions."