THE NEED FOR MAKE

Exhausting contaminants, undesirable odors, excessive heat and general ventilation requirements, often cause “air starvation” within a building. When exhaust exceeds available replacement air, a negative pressure occurs. This demand for outside air, when uncontrolled, causes air to be drawn in through doors, windows, cracks and other building openings. Back drafts occur in natural flues, ventilator and stacks. Exterior doors can be hard to open. Unwanted contaminants and moisture are drawn into the building resulting in drafts and poor temperature control causing guest discomfort and building moisture problems.

To avoid air starvation, controlled make-up air to replace exhausted air is essential. Make-up air provides clean, filtered, dehumidified, tempered air to maintain comfortable and healthy living conditions. Often it is required by law. By controlling with make-up air, unwanted moisture and contaminants are removed prior to entering the living space and drafts are kept to minimum.

It is a misconception that the addition of make-up air increases energy costs. Quite the opposite is correct. When air is exhausted, an equal amount must be replaced. Operating savings are actually obtained by providing controlled make-up air, as the air is conditioned prior to entering the building by equipment designed for the purpose. Otherwise this air will infiltrate into the building and having already been sensed by the occupants, must then be conditioned by building equipment at less efficient conditions.

Properly designed rooftop heating and air conditioning equipment must be designed specifically for 100% outside air for make-up air purposes. Standard rooftop heating and air conditioning equipment is not adequate for this purpose without considerable field modifications and this is not recommended for many reasons, among which are long term reliability and single source responsibility requirements. Units provided for make-up air should include as a minimum, the following factory installed items:

Control setup and sequence of operation as follows: Set compressor lock-out thermostat to close first stage at 60°F and second stage at 80°F outside temperature. (If 4 stage cooling, set lock-out at approximately 60°F, 70°F, 80°F, and 90°F.) Set discharge override ductstat located in discharge plenum to close first stage at 55°F. Set wall mounted thermostats at 75°F for heating and cooling. System switch in AUTO position and fan switch in ON position. Set dehumidistat at 50%. Set low limit temperature safety at 35°F.

If wall mounted thermostat calls for cooling, (both) compressor(s) will run and heating locked out whenever outside air is above setpoint. Hot gas bypass valve will maintain evaporator coil above icing condition, regardless of enthalpy of outside air. Compressor shut off thermostat(s) will lock out second stage compressor below approximately 80°F and first stage compressor below approximately 60°F outside air. As temperature continues to drop, wall thermostat will energize heating stages (*) as required. If room thermostat is satisfied, 2 stage heating discharge ductstat will override wall stat whenever unit leaving air temperature falls below 55°F. If leaving air temperature falls below 35°F, unit will shut down on low limit safety.

If the space cooling and heating functions are satisfied, the space dehumidistat will activate reheat by bringing on the first and second stage compressors with the first stage compressor reheat valve energized. Should the space temperature rise above the cooling setpoint, the Y1 call will de-energize the reheat and return the unit back to normal cooling and heating functions.

(*) If with 100% to 30% modulating gas heating, room thermostat provides heating enable circuit and discharge air sensor in unit is set at 70°F (field adjustable).