Heating hot water in a home is about 15% of your utility bill.
Cooling your house in the summer can account for half of your utility bill and heating is about 36% of it.
Because of the hot climate we live in, heat recovery units operate with air conditioning and heat pumps by harvesting excess heat which would otherwise be lost; thereby improving overall energy efficiency and yielding useful hot water heating. In addition to free hot water it also delivers longer compressor life and increasing efficiency by as much as 3 SEER. Rate of return on investment is three years on residential and one year or less on commercial units.
Where Does the Air Come From?
- Windows / Doors and their framings
- Plumbing, electrical cables, telephones and other such openings into the structure
- Air Ducts and equipment in spaces outside their pressure boundary (area within the envelope)
- Fireplaces, seal plates, etc.
The Energy code requires all these openings to be caulked, have a gasket or be sealed.
Return Duct Design
The Sizing is Always too Little
Studies done by the Department of Energy have shown that return air systems (ducts/chases) are undersized the majority of the time.
Manual D states "The return air system MUST establish a low resistance return air path between every room and the return side of the blower cabinet. If these paths are not established, the air flow through some or all of the supply air outlets will be affected."
Part of the static pressure in every duct system is created by the return air system.
- Manufacturer specifications generally require a .5 static pressure
- Studies have shown on a national average, the static pressure in most residential structures is 1.3
The effects of inadequate return air paths are as follows:
- Closing doors will pressurize a room resulting in inadequate air flow to that room
- A reduction in air flow to a given room will cause an increase in airflow to the remaining rooms
- NET RESULT - the system is thrown out of balance; some rooms will be positive and some rooms will be negative
- Under these conditions, the structure will experience increase infiltration air as some of the structure will be subject to negative pressures
Pressurization as a Result of Air Leakage
Duct leakage on the return side can case the structure to have an excessive positive pressure, this however is accomplished by drawing hot/cold air from outside the envelope and will lead to many problems not the least of which is loss of capacity. This will have a serious effect on the system's ability to control humidity within the structure.
Ventilation Air is Necessary to Accomplish
- Positive pressurization of the structure with respect to outside
- Indoor Air Quality Issues
Adding controlled ventilation in hot-humid climates seems to be a contradiction. However, supplying outside air increased the latent load. The key is to provide the amount of air that is needed and not 1 CFM more. Next address the resulting latent load.
The problem, however, can be a little more difficult if the effect of duct leakage is considered. The truth is that most structures suffer from EXCESSIVE air changes as a result of duct leakage when the air handle operates as a result of supply duct leakage. The key is to stop the uncontrolled ventilation by sealing all ducts properly then add controlled ventilation. This approach will typically lower the latent load.
Recent studies have shown that duct leakage can cause an induced air change of 150 to 200 CFM or more. In stark contrast controlled ventilation of approximately 60 CFM would reduce the infiltration load to 90 to 140 CFM resulting in a lower latent load.
Remember that it is only necessary to provide enough outside air to place the structure in a slight positive pressure with respect to the outside. Research shows a 1 to 2 Pascal positive pressure with respect to the outside is sufficient.
The proper approach to ventilation in a hot-humid climate is to size and install the system correctly. This will help in minimizing the need for outside air.
A quality installation should be achieved if ALL of the following are accomplished:
- Completion of an accurate room by room load calculation
- Design temperatures as outlined in ACCA Manual J should be strictly observed
- Over sizing should never exceed 15% of the calculated load
- Completion of a duct design utilizing ACCA Manual D
- Selection of equipment utilizing ACCA Manual S - Note that particular attention should be given to Sensible Heat Ratios
- All installations of equipment should follow code requirements for access and service space
- Flexible ducts should be installed being mindful of the following
- Compression rates are the minimum possible
- Acute turns in the duct should utilize metal ells
- Splices should be accomplished utilizing metal duct, air-locked and sealed with Panduit straps
- Matched systems should be installed following accepted guidelines from:
- Verify that air flow across Heat Exchangers and Cooling Coils is within specifications
- Verify that line-sets are sized according to manufacturer specifications
- Ensure that the refrigerant charge meets manufacturer specifications utilizing the Superheat / Sub cooling methods
- Verify that all electrical feeds to the equipment are in accordance with the manufacturers specifications
- For fuel fired appliance, ensure that Venting Systems are sized and installed correctly
- Ensure fuel fired appliances have sufficient combustion air available in accordance with manufacturers specifications and being mindful of the air tightness in the structure that it is being installed in
- Duct leakage for ducts outside the envelope should not exceed 5-6% of the total airflow
- Duct leakage for ducts inside the envelope should not exceed 10% of the total airflow
- Individual room airflows should be within 15-50% of the calculated amount
The necessity for completing heat gain/loss calculations and completion of an accurate duct design cannot be overemphasized. In the end, this is the only method to ensure that the system to be installed will function in concert with the structure.
Once this has been accomplished, then the need for a professional installation of all the components that are required for a complete HVAC system within a given structure cannot be stated strongly enough. Anything less can lead to complications that depending on the structure may not be able to be corrected at a cost that the structure owner could manage.
Any one of the following or some combination thereof could cause the cooling/heating system to be incapable of maintaining temperature set-point and or maintaining control of humidity levels within the structure.
- Incorrect Duct Design
- Excessive Static Pressure
- Greater than 5% duct leakage on the supply side
- Greater than 5% duct leakage on the return side
- Lack of return air direct paths (isolation of zones)
- Lack of fresh air ventilation
- Improper exhaust systems
- Incorrect equipment selection
- Incorrect line set sizing
- Incorrect charging procedures