Determining the Heat Load of Medical Equipment and Systems

When planning the installation of new equipment or systems, it is necessary to calculate the heat loading so that adequate cooling can be provided. The contractor or hospital HVAC personnel base the design of an HVAC system on the room size and the heat generated by personnel and equipment that will be located in the room. As a BMET you may be asked to provide this information.

The basic idea is relatively simple: each Watt of electricity used generates 3.42 BTUs per hour of heat (for the wattage that is not used to generate other kinds of energy). Equipment generally has the Watts or VA (Volt-Amps) rating listed on a sticker near the power cord. If you find a VA rating instead of Watts use the VA rating in place of watts in the formula (it may give a higher BTU number than actual, but this is OK, you just don't want to be too low) For a typical ultrasound unit rated at 1500 VA, the BTU/hr generated is: 1500 x 3.42 = 5130.

A few notes to bear in mind:

1. Don't rely on the equipment manufacturer's BTU rating, they often give a higher BTU than actually generated to account for personnel expected to be present. For example, Hewlett Packard rates their model 1500 ultrasound at 10,000 BTU/hr while its VA rating is 1500 (giving an actual heat load of 5130 BTU/hr), the additional BTUs are meant to compensate for personnel, and if you are unaware of this you will spend too much on HVAC equipment. Their model 2500 Ultrasound, also rated at 1500 VA of power, they only rated at 7000 BTU/hr.

2. Use the average continuous power in your calculation. While an Ultrasound unit draws a fairly constant amount of power, an X-Ray system's power consumption is highly intermittent. A typical X-Ray room connected to a 480 volt line at 100 amps can draw up to 48,000 watts of power (which calculates to over 160,000 BTU/hr). However, the peak power is drawn only during an X-ray exposure, lasting a fraction of a second. You will need to have the manufacturer provide information on average power consumption, which would be considerably lower.

3. Some equipment also generates energy other than heat. A treadmill, for example, also generates mechanical energy to turn the walking belt. In this case, it would be helpful to get the manufacturer's BTU/hr rating, but be sure to cross check it against your own calculations, also find out if their rating compensates for patients and personnel. Keep in mind that a patient on a treadmill will generate a lot more heat than the staff, who are not exercising.

4. When absolute accuracy is not be possible, it's best to estimate BTU/hr heat load on the high side, since excessive heat can reduce the service life of equipment, and make patients and staff uncomfortable, resulting in a negative impression of your facility.

Some scenarios:

1. A typical ultrasound exam room will have an ultrasound scanner, which generates about 5000 BTU/Hr of heat, and two people, a patient and a sonographer.

2. A cardiology exam room may have an ultrasound at 5000 BTU/hr, a treadmill system at 8000 BTU/hr, two technicians, a physician and an exercising patient, all generating heat at the same time.

3. A lab may have a chemistry analyzer at 5000 BTU/hr, a hematology analyzer at 4000 BTU/hr, numerous smaller pieces of equipment rated at 200 to 2000 BTU/hr and twenty technicians, all working at the same time. This generates a lot of heat.