Change of Season

Change of Season
Change of Season

David Robb
HVAC Oracle

2004-02-01

It is the time of year when the ground begins to thaw, the days get longer and warmer but evenings are still quite chilly.

This can be a challenge for the mechanical system in the building to maintain comfort. When both the heating and cooling systems are required. Tenants and office workers often complain about being too warm or too cold. Large swings in the temperature inside the building are a common cause, but they may still complain of being too warm or cold. In the morning, the heating system will run and can easily over heat the space. By the middle of the afternoon you the air conditioning needs to come on. This problem is often tolerated because of the perception that there is little that can be done about it. Sometimes that is the case, but there are steps you can take to improve the situation.
Temperature swings as little as 3?F in an occupied space can cause complaints. People quickly adapt to their environment. Even if all the equipment is working properly, the set points on a standard thermostat will cause large swings. The difference between the cut out temperature of the heating and the cut out temperature of the cooling has to be a minimum of 3?F to prevent short cycling of the equipment.

A basic forced air roof top packaged unit consists of a single stage gas or electric heating plant and a single stage mechanical cooling unit. Single stage equipment do not have any flexibility to it's operation, it is either on or off. In the spring and fall, when heating loads are light. the heating system will be oversized. Pumping out 3 times the amount of heat as required or more. When the heating comes on it can quickly over shoot the thermostat setpoint even if the heating system is properly sized. Even a hot water boiler system will do the same , unless it is equipped with an out door reset control.

The heating is designed for the coldest average temperature, not the mild temperature of the spring and fall. Take for example, Fort Collins, Colorado, where their winter design temperature is -5?F. Only 2?% of the days on an historical average will the temperature fall below this point. This is used for the outdoor design temperature to size the heating equipment. Let's say the building's heating loss requires 100,000 BTU's per hour to keep the area at a comfortable 74?F when the outdoor temperature is -5?F. This gives us a temperature difference is 79?F. When the outdoor temperature is only 40?F, the TD is only 34?F. A lot less energy is needed to heat to 74?F, about 35,000 BTU's per hour for the same building. But a single stage heating system only works a one speed and stills pumps out the 100,000 BTU. This quickly heats your building, faster then a standard thermostat will react. If your heating equipment is oversized to begin with, the problems become much more noticeable.

One way to improve the situation is to use a high quality thermostat. A good thermostat such as Honeywell's T7300 series Commercial Thermostats uses an adaptive logic, which anticipates the furnace cycle, to minimize the temperature swings. This thermostat looks at the recent history of the setpoint verse the space temperature. Delaying when the heat will kick in and shortening the length of the running time. Multiple room sensors can also help with temperature control. Spreading the sensors through out over the space provides better temperature control through averaging.

Temperature swings are also caused by a lack of cooling. Air conditioning units use a mechanical refrigeration system design to work when it is warm outside and in fact should be design for the hottest days of the year. Refrigeration systems require require specialized component to run when temperatures are below 65?F. If the compressor runs when the temperature is too low, it can cause the evaporator coil to and possible damage the compressor. A simple freeze stat connected to the evaporator coil can prevent the coil from freezing. When the coil freezes, it also blocks off the air flow and the office can heat up quickly.

A control called the can also cause problems. The low ambient keeps the unit off until the outdoor temperature warms up above 65?F. The controller can be slow to react to the change in the air temperature which can keep the mechanical cooling off too long. The low ambient controller is normally mounted inside the equipment, it will sense the temperature of the equipment casing more then the actual air temperature. A large mass like a roof top unit can take long time to warm up. This will delay the control from switching over and bringing on the compressor. Snap disc controls, which are often used for low ambient can only be mounted inside the equipment. Some have the flat sensing area of the snap disc pointed through an opening in the control panel which helps improve the switching, but they can still be slow to react. A low ambient controller that has remote sensing bulb such as a A19ABC-24 or a A419 electronic temperature control are a much more reliable at controlling low ambient cut in. The remote sensing bulb can be insulated from the equipment with a piece of insulation such as Styrofoam, allowing the bulb to sense the air temperature and not the equipment casing temperature. The sensing bulb should shielded from direct sunlight and away from any exhaust opening as well to prevent it from switching on too soon.

HVAC units that have economizers is the best ways to cool a building in the Spring. These unit drawn in outdoor air for cooling, but they do have limitations. They work best when the outdoor air temperature is below 55?F and will work up to ~65?F if the air is fairly dry. Is the humidity high, they will do more to heat the building the cool it. If you have and area that has a lot of Southern expose glass, or other heavy cooling loads you will require more cooling then you will get from a typical economizer. In this case you can modify the mechanical cooling you can get the compressor run at lower temperatures. A head pressure control, such as an ICM325, can be used to vary the speed off the condenser fan if you have a single condenser fan or you can cycle off a fan if there is more than one condenser fan. A temperature or pressure control can be used to cycle off on of these condenser fans through the contactor until the last one is running. Using cycling control on a single condenser fan application to start and stop the fan will cause more problems and should be avoided. Reducing the air volume through the condenser will help to keep the head pressure where is should be and lower the minimum running temperature. The unit will run as if it is warmer outside. When it is cold, the pressure is too low in the condenser and the refrigerant can not push through metering device and into the evaporator.

These are only a few of the possible problems that can be created by the mild outdoor weather and with a few simple modifications you can improve your indoor weather. And if these tips don?t help you can always use and old fashion energy saver, put on a sweater or even open a window.