Manufactured Housing HVAC System Efficiency [Part 1]

Not two concepts that until about 3 to 5 years ago were necessarily used in the same sentence. The United States economy, as well as dependence on foreign oil has brought efficiency of HVAC systems to the forefront of everyone’s attention – including owner’s of manufactured structures. Subscribe to the MHHAC Blog to follow this multi-part edition article on techniques and technologies used to increase the efficiency of your system.

Size Really Does Matter – But, Bigger Doesn’t Mean Better

The first part of this series is the single most pivotal issue when determining and correcting deficiencies in any HVAC system – what is it? Proper sizing. Too big or too small is bad for any system – manufactured housing or otherwise. Assuming you know nothing or very little about the HVAC industry – what can you do to determine the system you have has been properly sized?

Measure – the square footage (width x length) of all the rooms that are being, or that will be cooled. Record these measurements individually. Measure Living rooms and kitchens or other rooms as individual rooms even if they are common to one another – no walls or partitions.

Convert – each room’s square footage into BTU’s (British Thermal Units) – 100 to 150 square foot rooms are 5000 BTU’s, 150 to 250 square feet will require 6000 BTU’s, rooms 250 to 350 square feet will require 7000 BTU’s. Add 4000 BTU’s to the kitchen and 1,000 BTU’s to bathroom(s).

Add – all the BTU calculations you have recorded together, this will give you your total demand. Every 12,000 BTU’s is equivalent to 1 ton.

NOTE (1) :

A professional can more accurately and completely calculate the load of your specific structure. However, this method will provide a solid idea of the structures needs. The online worksheet located here allows you to determine needs much in the same fashion as a professional however, requires multiple and extensive interior and exterior measurements to complete.

NOTE (2) :

1. Make any adjustments for the following circumstances:
   • If the room is heavily shaded, reduce capacity by 10 percent.
   • If the room is very sunny, increase capacity by 10 percent.
   • If more than two people regularly occupy the room, add 600 BTUs for each additional person.
   • If the unit is used in a kitchen, increase capacity by 4,000 BTUs.
   • Consider where you install the unit. If you are mounting an air conditioner near the corner of a room, look for a unit that can send the airflow in the right direction.

Crossover Ducting

Double wide mobile homes have a large crossover duct that transfers heated or cooled air from one side to the other. It is one of those items that may be installed by the factory, the dealer or the setup crew. Improperly done it can be the source of many problems and there is lots of room to shift blame. Any time you notice the house is not heating or cooling the same on both halves there is probably a problem with the crossover duct.

The crossover duct is another one of those mystery items hidden beneath the house where poor workmanship may take a long time to appear. The duct is not supposed to rest on the ground, it should be well insulated, and it should be connected at both ends with a metal or plastic band and sealed with metalized tape. If you have not checked on yours for a couple of years it would be good to look at it. Duct tape just doesn’t hold forever and you can lose a lot of heat or cool through a leaking, poorly insulated crossover duct.

Mobile/Manufactured Home HVAC System – Or Site Built System – Is There Really a Difference

Short answer – absolutely yes. Longer version:

One of the single most critical considerations that any technician needs to understand relevant to diagnostics of issues with mobile and manufactured heating and cooling systems is the design difference between these systems and site built systems.

Smaller – Sealed Combustion Systems

Forced air downflow furnaces are the most commonly installed heating systems found in manufactured homes, as a matter of necessity more than anything – with few exceptions being allowed for modular and double wide structures, there are no crawls or attics allocated for equipment installation. Manufactured housing furnaces typically require the technician to disassemble the burner area of the equipment to access HSI’s (hot surface ignitors, thermocouples, etc).

• Adding Air Conditioning

• Adding a new air conditioner to a mobile home can be a challenge. Some mobile homes may lack ductwork and floor or ceiling vents, making a window unit or self-contained air conditioner the only choice. Other homes feature ducts running to the furnace, in which case cooling coils can be installed inside the furnace cabinet itself. Newer mobile homes that come with a full air conditioning unit installed simply require replacement of the unit outside the home, unless the thermostat or ductwork is damaged.

• Crossover Ducts

  • Many mobile homes feature crossover ducts, which are large flexible ducts that run under the house. These ducts transfer warm and cool area to level out the overall temperature in the house. Due to limited attic or ceiling space, most installers run these ducts in the crawl space. Unfortunately, this leaves them susceptible to damage from water and rodents. If your mobile home’s heating or cooling vents are not putting out the amount of air that they used to, check the crossover ducts for leaks or disconnected sections.

Manufactured Housing Floor Vents/Registers

In older manufactured structures the typical sizing of these vents were 4″x8″ – today’s newer models will typically be fitted with 4″x10″ floor vents. So – why the difference? Firstly, the industry is more mature and subsequently more technologically driven – Secondly, and perhaps just as importantly – a formula that takes into consideration the principle that air pressure is dependent upon the size of available discharge face (surface area), or vent size (specifically the opening of the vent). HVAC systems applied to older homes were not as powerful as those applied in newer homes today. (See our article on static pressure to learn more about this design principle). Just like placing your thumb over the outlet of a garden hose increases water flow pressure – so too – smaller vent face results in increased air flow pressure.

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