Air Exchangers

HRV/ERV -Heat/Energy Recovery Ventilator
How does it work?

An HRV will draw air from the outside and pass it through a heat exchanger where it exchanges heat with clean air being exhausted from the building.  An ERV will do the same thing but tries to exchange latent heat energy in the form of moisture in the air as well as sensible heat energy which is measured by temperature only.   If it is 0° outside and 22° inside.  The 0° outside air is heated by the 22° air being exhausted.   

What is the Efficiency? 

The amount of energy saved by an HRV varies according to the conditions.  Using the example above and assuming the intake air and exhaust air are the same volume,  lets say 150 CFM and the 0° outdoor air is heated to 11° by the 22° air being exhausted, it would be 50% efficient for sensible heat.   If it is an ERV  , you would look at the entropy of the exhaust air VS the entropy incoming fresh air.   In our example, this HRV would save about 1 KWH of energy and would typically use 0.05 KWH.  Depending on the region and HRV can save around $500 year in energy.   In a larger building that may need 1000 CFM of outdoor air, it can reduce annual cost by the thousands.     

Why do we need air exchange? 

Today's building codes require the building envelop to be sealed tightly.  This saves energy by reducing heating & cooling loads but can result in a lack of fresh air for occupants.  People release a lot of carbon dioxide which builds up without proper ventilation.  A busy office can release a lot of CO2.  Anything thing over 1000 PPM CO2 can have big affects. Volatile Organic Compounds (VOC) are off gas from things like printer ink, new furniture or carpet.  Cleaning products and dishwashers release various gas.      Without air exchange these gasses will buildup over time.  Indoor pollution can be far worse then outdoor pollution.    

Where is the heat exchanger?

Air heat exchangers are normally be found in the mechanical room , on roof top units or hanging from the ceiling near the furnace or above a suspended ceiling.   Homes with a forced air HVAC system will connect the HX fresh air to the return duct of the air handler for distribution or in metal ducts as an independent system.  For homes with hot water heating or electric base boards should have a stand alone system and will provide just enough air meet minimum requirements.     

What Maintenance is required?

An air to air heat exchanger consist of a series of small passage ways that can become plugged with dust and dirt if not maintained.  There are air filter on the air intake and exhaust upstream of the heat exchanger which must be cleaned regularly.   Small HRV's under 250 CFM will often have foam filters that need to be washed frequently.  The filter on the intake normally clogs first while the exhaust air which is fairly clean will take longer to plug up.   This results in a negative pressure inside the building which is to be avoided.   Try to clean these foam filters every month, every 3 month if a disposable cardboard filter is used.   If necessary, the heat exchange core can usually be removed and cleaned out with high pressure air.  Check the owner manual for detailed procedures.  Air exchangers can form frost in the winter and need to be defrosted.  Each unit is equipment with a drain to an open drain location or a bucket.  It should never be tied directly to a sanitary line.   

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