March 2020

The Faster the Water Flows, the More BTUs


Tell me if you have ever heard someone say something like this: I think the room is under-heating because the water is moving too fast through the baseboard. If I use a smaller pump or at least throttle down on the pump’s flow rate, the baseboard will start heating better.

I have heard this statement numerous times over the years from technicians that were having problems with their customer’s heating system. I guess you could reason that with the water moving so fast, it doesn’t have enough time while inside the baseboard to “give up” its heat.

When cold air blows, it moves heat away from our bodies much faster than normal, causing the wind chill factor. Photo by Juho Kuva/Getty Images.

 

 

 

 

 

 

 

 

 

 

 

 

So how do British Thermal Units (BTUs) in hot water give off their heat? There are three methods that govern heat transfer: thermal radiant, conduction and convection. Without getting too technical, the mode of heat transfer that we want to focus on with a baseboard system is convection.

Most people are aware of the convective nature of the air surrounding a baseboard. The hotter, lighter air wants to rise up and float into the room while the colder, heavier air wants to drop down towards the floor and move along towards the baseboard to replace the hotter air that has just floated up towards the ceiling. In the process, the hotter air is giving up its BTUs to the cooler surroundings.

However, before this can even happen, there is another convective occurrence that must take place—the hot water (fluid) flowing through the tubing has to give off its heat to the tubing wall. Therefore, before the baseboard can emit heat into the space, the heated stream of water must transfer its heat to the baseboard’s inner pipe wall by convection.

For convection to occur, there are three factors to consider:

  1. Surface contact area
  2. Temperature difference (between the water and the inside wall of the tubing) and
  3. The convection coefficient (which is calculated based upon the properties of the fluid, the surface area’s shape and the velocity of the fluid)

Instead of spending too much time with the math formulas, use your mind’s eye to visualize the following: as the stream of water is flowing through the baseboard, the outer edge of this stream is in direct contact with the tube’s inner wall. This “rubbing” against the wall creates drag, which means the water that is touching the inner wall of the tubing is moving slower than the “core” or inner stream. Because of this, the temperature of this outer layer of water becomes cooler than the inner stream. In fact, this drop in temperature impacts the rate of heat transfer—it slows it down.

Remember, one of the factors that affects convection is temperature difference. A good visual is to think of this outer layer as an insulator that impacts the rate of heat transfer from the hotter inner stream of water to the tubing’s inner wall. This is especially true when the speed of the water approaches laminar flow instead of turbulent flow. So, in effect, the faster the water flows through the tubing, the outer boundary layer or insulator becomes thinner, thus increasing the rate of heat transfer from the hotter inner “core” water to the tubing’s inner wall.

 

 

 

You can confirm this by taking a look at any baseboard manufacturer’s literature and checking out its capacity charts. They will typically publish their BTU output per linear foot based upon two flow rates: 1gpm (gallon per minute) and 4gpm. The BTU output is always higher in the 4gpm column.

Wind chill factor

Here is another way to consider this concept of more speed (faster flow rate) equaling more heat transfer (higher output). Consider the size of a hot water coil used in an air handler and the amount of BTUs it can provide. Now think about how much fin-tube baseboard you would need to install to provide the equivalent amount of BTUs.

The difference is that the speed at which the fan blows the air across the coil is much faster than the air that flows naturally across a baseboard. Everyone experiences this phenomenon each winter. The weatherman refers to it as the “wind chill” factor. He will tell you the actual temperature outside, but because of the wind chill, it will feel “X” degrees cooler. Why? The cold air is moving across our bodies much faster. This takes heat away from us much faster, so it feels colder than it really is.

A trick of the trade passed on to fellow technicians over the years is to turn up the Aquastat setting on the boiler to increase the water temperature. The reason technicians will do this is—if they have a room or a zone that isn’t quite heating up to the thermostat’s setting—by increasing the water temperature, they can increase the amount of BTUs per linear foot available from the baseboard (of course, this will only work if the boiler is big enough to offset the home’s actual heat loss).

Does faster water = more heat?

Knowing that a baseboard can provide more heat with hotter water, follow this next example to see why moving the water faster and not slowing it down will provide more heat.

Let’s use average design conditions when sizing for the amount of baseboard you would need to offset a room’s heat loss. Typically, we design a hot water heating system for a residential home at a 20°F temperature drop. That means the water would enter the radiation at 180°F and exit 20°F cooler at 160°F. The average water temperature in the radiation would then be considered 170°F. You would then check the BTU/h rating of the baseboard at this average water temperature and determine how many feet of baseboard the room needs to offset the heat loss.

What would happen if we increased the flow rate so that the water only took a 10°F temperature drop? If it entered the radiation at 180°F and came out at 170°F, then average water temperature would be 175°F. At this higher average water temperature, the baseboard would be capable of providing more BTU/h output. Taking it one step further, if the system only took a 5°F temperature drop, the average water temperature would be 177.5°F in the radiation, yielding even more BTU/h output.

You can now see that water cannot be moving too fast through your baseboard to prevent the BTUs from jumping off where needed. Of course, to achieve these tighter temperature drops while delivering the right amount of BTU/h, the flow rate has to increase accordingly.

Here comes the trade-off—with higher flow rates through a given pipe size, the pressure drop increases quite dramatically. The result can be a need for very large, high-headed circulators that cost more and use more electricity. Also, the higher flow rates increase the velocity of the water, and at some point, the velocity will create noise issues.

The point is not to design a system around these high flow rate/small temperature drops, but rather recognize that the next time someone tells you that they think the water is moving too fast and that is why the room is under-heating, you’ll know that is not the cause of the problem.

If you have any questions or comments, e-mail me at [email protected] or call me at FIA 1-800-423-7187 or follow me on Twitter at @Ask_Gcarey. ICM

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