Real-time energy load calculations, Part 3

I PROMISED AT the end of last month's column (pg. 32) to tell you what to do with all the digital information you will have obtained from monitoring one of these systems in which the heat source has to be replaced. Once you have retrieved your Hobo recorders (Onset Computer Corp., www.onsetcomp.com, 800/564-4377) from the field, you will download the information from the Hobo and save it with the

I PROMISED AT the end of last month's column (pg. 32) to tell you what to do with all the digital information you will have obtained from monitoring one of these systems in which the heat source has to be replaced. Once you have retrieved your Hobo recorders (Onset Computer Corp., www.onsetcomp.com, 800/564-4377) from the field, you will download the information from the Hobo and save it with the BoxCar software program (BoxCar Pro for Windows, also from Onset).

You can then export the data into a spreadsheet program such as MS Excel and Lotus 123. This will allow you to manipulate the raw data and perform a mathematical analysis of your field findings on an hourly basis. You will be able to generate a graph of the hourly temperature readings, which can be used to show the consumer what was going on during your data retrieval process.

Once you've converted the raw data into useful information, you can now begin an hour-by-hour analysis of the operating characteristics of the home's heating system as a whole.

The event recorder will give you the information on the run time of the gas valve. You've already determined what the hourly output of the system is, and you can take that information and divide it by 60 to determine what its capacity is on a per minute basis. The total number of minutes of operation of the gas valve, multiplied by the caloric output per minute of the heating system, will give you a net hourly Btu delivered factor. The only thing missing at this point is the degree-day exposure factor, which you should have also downloaded from that Hobo and converted to MS Excel or Lotus 123. By looking at the system on an hourly basis, you can determine the degree hour exposure.

For example, let's say that the event recorder had recorded 15 minutes of operation for the hour of 3 a.m. until 4 a.m. Based on clocking the boiler, you determined that the boiler had a net output thermal capacity of 200,000 Btuh. This would equate to 3,333 Btu per minute (200,000 divided by 60 minutes). The outside temperature during the 3 to 4 a.m. period held steady at 20°F. During this period of time, the boiler delivered 50,000 Btu. This equates to 1,111 Btu/degree Fahrenheit difference between the indoor (65°F) and the outdoor temperature (20°F).

Each human puts out about 500 Btuh.

If the normal design temperature for the home is 0°F, then the "real-time" design load could be extrapolated to be 72,215 Btuh (1,111 times 65 = 72,215).

This is where it gets a little tricky. Essentially all that you have learned is how many Btu the "heat source" has contributed to the building's demand. In addition to the obvious fact that there was no evidence of solar gain during the "test case" hour, most probably some consumption of electricity was occurring within the house that should be taken into consideration as it pertains to sizing the boiler. Refrigerators, light bulbs, washing machines and the like will contribute heat to the envelope. Most of their electrical consumption can be considered as thermal gain.

I would suggest that an informal survey be done of the electric consumption on the subject property. Make sure that no major electrical loads are occurring outside the home such as an electric pond heater or a yard light. If there are significant external loads, make sure you have a good idea of what their consumption factor is and note it.

I would recommend that the electrical meter be read during the 24-hour period. If all electrical consumption is occurring inside the home, then that consumption should be converted to thermal energy, and its hourly contribution added to the base load sizing of the replacement appliance. If there are significant external loads, they should be credited against the total electrical consumption. These are internal gains that may not occur during another time frame, and the heat source size compensated accordingly.

In addition to the electrical gains, you should account for human gains. Each human puts out about 500 Btuh. Add this to the base demand, because there may not be as many people at home during design conditions.

Another factor that should be taken into consideration is the thermal mass of the dwelling. If it is conventional stick frame, with minimal sheetrock finishes, its thermal capacity is substantially less than, say, an older allbrick home with masonry load-bearing walls on the inside. Unfortunately, there's not a scientific rule of thumb that can be used to say how many Btu were contributed by the fly-wheeling thermal mass effect, so you must use some common-sense judgment here. One good thing about mass is that it will still be there when design conditions show up, so any adjustment in the base sizing are probably negligible.

Another adjustment would be for solar gain. If you are using daytime hours as a part of your statistical evaluation, it is recommended that a credit be given for the fenestration (solar gain through windows) factor. This information is readily available from the AHSRAE Fundamentals of Heating Handbook. It shows the hourly gain per square foot of glass for east, south and west facing windows of varying glazing components, and season of application and your latitude.

For this reason alone, the use of a time frame for statistical evaluation when there's been no solar gain for many hours eliminates the need to perform this part of the exercise. You are also pretty much assured that internal gains from electrical appliances will be minimal as well.

Tune in next month as we continue to evaluate this method of heating system sizing that takes real-time, real-life situations into consideration. Until then, Happy Fine Tuned Hydronicing!

Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.

TAGS: Hydronics