HVAC Boiler Capacity

Boiler capacity is a direct product of ACTUAL heat loss. You must calculate the heat loss by determining the desired space temperature when occupied (or minimum temperature when unoccupied) then calculating thermal transmission factors for each different type of separating medium between occupied space and exterior space- walls, doors, windows, ceiling/roofs and perimeter of floors. You can find this information in several locations for free (most major HVAC equipment manufacturers have some level of guide materials). After you calculate the total transmission load, subtract the heat values of ALL heat producing materials that are located inside the envelope- lighting wattage, wattage of all electrical devices like printers, copiers, computers and monitors, electric motors and any other active devices. Convert these values to thermal values based on 3412 BTU per kW (or similar SI terms) and subtract from the calculated heat loss values.

Then, determine how physically "tight" the building construction is- current new construction standards and less than 5 stories tall will be defined as "tight", mid-60's to mid 80's or elevation between 5 and 10 floors will be "average" and any earlier construction (or high level of people entering and leaving the building - greater than one per minute - or a building elevation of over 10 floors will be classified as "loose". Tight construction will have 1/2 air change per hour, average construction will have 1 air change per hour and loose will be 1-1/2 air changes per hour. Calculate the volume of air that will be exchanged every hour and multiply it by 0.075 (sea level density per cubic foot) times 0.24 (specific heat per pound) times the peak temperature difference between interior space and exterior design temperature and add that value to the NET heat loss calculated earlier.

Multiply the resulting heat loss value by 1.25 to allow for exterior temps below design condition and for possible errors in calculation of transmission factors.

The result calculated above will be the required rated output of the boiler. Select the next highest standard size boiler capacity.

Heating coils are USUALLY sized based on the calculated heat loss for an area divided by the quantity (20F to 40F temperature differential between supply air and the room design temperature, multiplied by 1.08 per CFM (0.075 density X 0.24 specific heat X 60 minutes per hour). For example- a 50,000 BTU per hour heat loss would require => (50,000 / (1.08 x 40F differential)) or 1157 (say 1160) CFM of air. Based on a heating coil design air velocity of 700 Feet per minute (a good value for low pressure drop and reasonable coil size), the heating coil will have an areal of 1.66 sq. ft. Coil layouts for reasonable construction costs are width = 2 to 3 times height. Height is usually whole numbers that are multiples of 1-1/2 inches. This will result in a coil of about 20 x 12 inches (240 sq. in. = 1.67 sq. ft. for 700 FPM) or 28 x 9 (252 sq. in. or 1.75 sq. ft. for 663 FPM).

For lowest cost of pumps and piping, the use of a 40F temperature differential and a supply temperature of 180F will provide reasonable heating effect at lowest cost. Assuming a total building load of 1.4 million BTUs per hour, the pump would be sized based on heat loss per hour divided by 40F temp differential X 8.33 pounds per gallon X 60 minutes per hour, or 70 Gallons per Minute (GPM). The space coil would be fed with 50,000 BTUH / (40 x 8.33 x 60) = 2.5 GPM.

Pipe sizes should be selected based on 1 to 1.5 PSIG (2.5 to 3.5 feet water column) per 100 lineal feet of pipe (which will actually result in 1.5 to 2.5 PSIG (4 to 6 feet of water) per 100 lineal feet whien fittings, etc. losses are added. For larger pipes (> 4 inches, the water velocity should not exceed 8 FPS to minimize sound transmission and turbulence at elbows.

These are reasonable guidelines (with supporting calculations) for a "simple" hot water heating system design. If you are not real sure about your ability to perform these calculations, if would be best if you hire an engineer and use these guidelines to "keep them honest". The biggest issue is the use of internal heat gains to offset calculated heat losses- MOST engineers do not do this and your boiler (and all related prodcts) will be grossly oversized- resulting in MUCH HIGHER installation costs AND dramatically higher operating costs due to over-sized heating equipment.