RE: [EnergyPlus_Support] Modeling Heat Tape

We design window frame heat trace systems for museums and other high humidity environments and we have field monitored the performance of the systems as well. Note that the heat trace will only work for the frame and edge of glass area, there is no effect on center of glass. We have to use airflow to change cent of glass heat loss, and we can calculate some temperature and velocity values with WINDOW. You cannot do that directly with EPlus because there is no accounting for more complex air flow patterns at windows, but if I thought about it enough I could probably alter the glass properties to simulate some effects of air flow rather than assumed still air in the calculations.

Typically we use THERM to calculate the maximum wattage on frame members needed for design conditions at each component, it is often different for each portion of the frame as we also simulate the connections to the surrounding construction. Since we are usually dealing with fixed interior dewpoints, we can also calculate a critical exterior temperature for turning the system on. We then check the system when on full watts at the critical temperature to make sure the frame does not over heat.

There are constant watt and variable watt heat trace cables. There is little benefit to variable watt cable since the inside frame temperature is typically at the temperature of the low watt output of the cable, consequently there is no variable watt output if the critical temperature is to be maintained.

The big variable is control. The simple control is just to turn on the system at some critical outdoor temperature. This is also the most wasteful unless the windows are always in the shade. I prefer to zone the windows by exposure and monitor the frame temperature directly. This scheme allows solar heating of the frame to turn off the power even when the outdoor temperature is below the critical temperature for condensation.

The power use should be easy to calculate for the simple control, just hours below critical outdoor temperature times total power (window frames do not have significant thermal mass so they react rather quickly. For a complex control, you need some complex EMS calculation. You need to monitor interior surface temperature of the window frame components to see when it drops below the critical condensation temperature. If it is below the critical temperature, you need to calculate the heat loss to maintain critical temperature at current outdoor temperature. This is the power that the heat trace will put into the window frame to maintain the surface temperature.

Your calculation gets even more complex if the dewpoint is a moving target, but you will also need a much more complex controller.

I have a paper in Proceedings of the Third International Building Physics Conference, Concordia University, Montreal, Canada, 27-31 August 2006 with more detail on the design and monitoring of a system we developed for a museum.

Ned Lyon, P.E. (MA, WV)
Staff Consultant

SIMPSON GUMPERTZ & HEGER
781.907.9000 main
781.907.9350 direct
781.907.9009 fax
www.sgh.com

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of jeannieboef@xxxxxxxxx
Sent: Wednesday, March 06, 2013 3:56 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Cc: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Modeling Heat Tape

Internalsource, doesn't work with windows, but good idea. EMS program might be able to assign a fixed temp to a surface? If it can, then the functioning logic can be programmed as well as the electric consumption captured.

Sent from my iPhone

On 06.03.2013, at 03:04, Jason Kirkpatrick <jason.alan.kirkpatrick@xxxxxxxxx> wrote:

No idea if it will work with a window, but you might want to try

Construction:InternalSource

"In some cases such as radiant systems, a construction will actually have resistance wires or hydronic tubing embedded within the construction. Heat is then either added or removed from this building element to provide heating or cooling to the zone in question. In the case of building-integrated photovoltaics, the energy removed in the form of electricity will form a sink."

On Tue, Mar 5, 2013 at 5:45 PM, burin3 <burin3@xxxxxxxxx> wrote:

Can anyone offer some help with modeling heat tape? I am modeling the potential for condensation on a single pane window in an unconditioned zone in a humid climate. One proposal is to use electric resistance heat tape to maintain the glass and frame above dewpoint. I'm unsure how to set up this kind of "system" in EPlus. Any help would be greatly appreciated.

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