Normal cool water supplytemperatures in the variety of 45 - 55 degrees Farenheit. This is the correct range of chill water supply temperatures for comfort cooling. Normal hot water supply temperature is in the range of 100 - 150 degrees Farenheit.

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One cfm every square foot of floor area is the average air quantity compelled for a room or entire building.
Rules that thumb are abbreviated quick cuts, time savers, and also approximations. Every is a rapid estimation of what many systems should do most of the time, however remember that none of these numbers room infallible.

The rules space not actors in concrete. They carry out not constantly apply come every system, nor do they occupational in every circumstance. That is meant that the technician making use of them is a expert who have the right to use reason and also exercise caution in your application.

Rules the thumb normally assume details normal design and also operating criteria, which room not always present.

The rules space not to be provided for system design purposes. Rather, they room the result of carefully studying good system designs. Technicians and engineers, however, can advantage from using them as tools for estimating and also solving to work problems.

below are some usual hvacr rules of thumb.


Air quantities (cfm): 400 cfm per ton of cooling is essential for common comfort applications, 500 cfm per ton the cooling for heat pump and also high sensible heat applications, and also 350 cfm every ton that cooling because that high latent warmth applications.

These are the approximate quantities of air that have to be relocating through evaporators for three usual applications. Say, for example, you have actually a lull cooling applications at a savings and loan office that has actually a 10-ton system. Therefore, you main point 10 loads by 400 cfm every ton to expose the complete system airflow requirement of 4,000 cfm under typical operating conditions.

One cfm is essential per square foot (1 cfm/sq ft) of floor area.

This is the mean air quantity forced for a room or whole building. This number is based upon an averaged heat load calculation for comfort cooling. There is an assumption of one 8-ft ceiling, no unusual home window areas, and also average insulation. This preeminence of ignorance provides about 7.5 air changes per hour.

This dominance is a quick way to almost right the cooling load for a room or building and may be advantageous in estimating room waiting quantities. Because that example, to estimate the variety of tons forced to cool a 1,600-sq-ft home, main point 1,600 by 1 cfm/sq ft to acquire 1,600 cfm that air. Utilizing the 400-cfm/ton rule, divide 1,600 cfm by 400 cfm/ton to gain 4 tons of required cooling.

6 to 10 air changes are necessary per hour.

This is the average number of times in each hour that the air in the building or room (assuming 7- come 8-ft ceilings) is removed and also replaced by circulating the air.

Air velocities (fpm):

700 to 750 fpm are needed for disposable filters; 250 fpm because that HEPA filters; and also 500 fpm for digital air cleaners.

These room the encourage air speeds for filters. Speeds greater than these will certainly decrease filtering efficiency.

550 to 600 fpm maximum and 400 fpm minimum are needed for evaporators; 1,000 fpm because that condensers; and also 700 fpm for warm water coils.

These are the encourage velocities because that coils. Speeds faster than this may cause condensation to it is in blown off of the evaporator fins and down the duct. Speeds slower than 400 fpm may reason the evaporator to freeze over.

Terminal velocities room the velocities at the finish of the run, such as registers or diffusers, wherein the wait enters a room.

Bypass air need to be in ~ a rate of 0.1 to 0.35, or 10% come 35%.

This is the median percentage of air the bypasses or stops working to come into contact with the evaporator as it passes v the evaporator. The higher the air speed in fpm, the greater the bypass factor. High bypass determinants are normally discovered on warmth pumps in the heating cycle, and on high sensible heat applications like computer room systems. Short bypass factors are discovered on high latent warmth applications, such as those uncovered in restaurants (especially cooking areas) and bowling alleys.


Building air push is normally in between 0.03 come 0.05 in. Of revolution pressure. Infiltration waiting leaks into a structure from the outside, together as v doors and also cracks about windows. This infiltrating waiting is unfiltered and also untreated.

To stop infiltration, a structure is retained at a slightly positive air pressure. This is accomplished by adjusting the exterior air dampers so they are open up slightly much more than the exhaust wait dampers. Structure air pressure measurements should it is in taken at any time an convey is made.


The thermoststic growth valve controls evaporator superheat, which way it controls the lot of fluid refrigerant boiling in the evaporator.


over there is one speech (1 hp) for each ton that cooling. The takes a l-hp compressor to provide 1 ton of comfort cooling. Therefore, a 25-ton system has a 25-hp compressor motor.Note:
This uses to comfort cooling only. A lower-suction-pressure applications such together refrigeration will have actually a greater hp/ton ratio. The lower the suction pressure, the greater the hp/ton required.

Thermostatic development valve superheat need to be 8° come 12°F.

The valve controls evaporator superheat, which way it controls the quantity of liquid refrigerant boil in the evaporator. The lower the superheat, the much more liquid in the evaporator.

However, superheat the is also low can cause the valve come lose manage of the superheat altogether. The valve just controls evaporator superheat. If the valve is properly controlling superheat, execute not readjust it to readjust evaporator temperature or pressure.The valve is no an evaporator temperature or push control, so carry out not effort an adjustment without measuring the superheat. The 8° to 12° superheat is typical for air air conditioning systems. Low-temperature systems frequently use different valves and lower superheat settings.

Remember, superheat is a temperature differential, not a single temperature measurement.

Normal device superheat is 20° to 30°. Mechanism superheat is not the same as thermostatic growth valve superheat. Device superheat consists of the temperature differential indigenous the point in the evaporator whereby all the refrigerant has changed to a gas come the suction heat — about 6 in. Indigenous the compressor business valve.

Normally, charged and operating equipment will have a system superheat the 20° to 30°. Mechanism superheat higher than 30° may show that the short side the the system is starved for refrigerant. Mechanism superheat much less than 20° may indicate that the short side is overcharged.

Again, mechanism superheat is a temperature differential, no a single temperature measurement. The valve superheat is part of the device superheat.

High-side subcooling is 5° come 15°.

Refrigerant in the condenser changes from a gas come a liquid and also then starts to subcool. Subcooling takes ar in the bottom the the condenser and in the liquid line. The lot of subcooling occurring in the condenser is 5° come 15°. The is never higher than 15°.

Since just liquid subcools, the lot of subcooling is one indication the the lot of fluid in the high side of the system. Overcharged systems have higher-than-normal quantities of subcooling. Undercharged systems have low subcooling.

Comparing high-side subcooling and also low-side mechanism superheat will usually solve most refrigeration cycle difficulties of overcharge, undercharge, and restrictions.

common evaporator waiting temperature fall is 18° come 20°.

Measuring the temperature drop, or distinction of the air as it moves with the evaporator, is one technique of approximating exactly airflow. Assuming 400 cfm per ton the cooling, once the air flow is exactly there will be one 18° to 20° fall in waiting temperature.

Abnormally low air flow will continue to be in call with the evaporator longer, and will it is in chilled come a lower temperature and also greater temperature difference.

regular condenser waiting temperature rise is no to exceed 30°.

Lack of air end the condenser outcomes in high head pressure, reduced system capacity, and increased power consumption. Waiting is heated together it passes through the condenser.

Low air across a condenser is shown by an wait temperature rise better than 30°. The smaller amount of air over the condenser must absorb the same amount the heat; therefore, the temperature rise is greater.


because that chilled and condenser water, the expected temperature drop or rise is usually 8° to 10°. As with air, water has a temperature fall or rise as that is cooled or heated. Together a chiller cools water, the variety of gallons per minute (gpm) circulated determines the number of degrees of drop.

The exact same is true for a water-cooled condenser. An 8° temperature change is proportional to about 3.9 gpm circulated per ton. A 10° adjust is proportional to about 2.4 gpm circulated per ton.

For example, if a 25-ton system has a chilled water temperature differential that 8°, then 25 loads times 3.9 gpm/ton provides a total estimate the the chilled water circulation of 97.5 gpm.

normal chilled water it is provided temperature is in the range of 45° to 55°. This is the correct variety of chilled-water it is provided temperatures because that comfort cooling.

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Normal warm water supply temperature is in the variety of 100° to 150°. This is the correct range of hot water it is provided temperatures for warm water comfort heater or reheat. Equipment that require warm water temperatures end 150° usually have actually a low flow rate problem.

Christopherson is an hvac instructor at san Jose City College, san Jose, CA.