8/17/2015

Why Ventilation Should Be Separate From Heating and Cooling Systems

We often get asked if our HRVs and ERVs can be coupled with a building’s heating and cooling system.  It makes sense, right?  If there is already a system of ducts in place for the furnace and or air conditioning system, why have a second parallel system?  While it may be less expensive to combine the two systems together and it may save a little space, the project is better served by having independent systems that each focus on their task.  This blog entry will explain the reasoning why.  Note that for simplicity in writing I will use “HRV” to represent either an HRV or ERV system and “air handler” to represent a furnace, air conditioning system, heat pump etc – whatever forced-air system is meeting the heating and cooling needs of the building.

Exhaust Locations

Forced-air heating and cooling systems are usually designed with a handful of return grilles that are centrally located to bring the ambient room air back to the air handler.  Conversely, a whole house HRV system performs best when the exhaust locations are in the wet and/or odorous rooms: bathrooms, kitchen1, laundry room, possibly mudroom or basement2.  We want to pull this damp and odorous air out of the house to mitigate potential moisture problems and indoor air quality concerns.  With a system designed in this way, the need for independent bathroom fans and their associated ductwork and penetrations to the outside is eliminated.  And an HRV continuously venting from a bathroom will move on the order of 10 times more air through the bathroom over the course of a day than an intermittent bath fan, resulting in a drier room, drier towels, less mildew, etc.  It goes without saying that the air handler returns should not be located in the bathrooms in an attempt to tie the systems together as these systems recirculate air instead of exhausting it.  You do not want to pump bathroom air throughout the house!

Supply Locations

Forced-air heating and cooling systems have the job of delivering heated or cooled air to the house to meet the heating and cooling loads of the spaces the system serves.  Thus the (typically higher) airflow rates and the supply grille locations are designed and balanced to that end.  However, these heating and cooling loads do not necessarily correspond with where we want ventilation air to be delivered.  A good example of this is the relationship between the open living areas and the bedrooms.  In most modern designs an open concept for the living room, dining room, and kitchen are preferred.  These rooms are usually the largest space in the house, have lots of exterior surface area, and often have numerous and large windows for daylighting and views.  These design features often create the largest single heating and cooling load in the house and thus require the highest percentage of the supply air to meet that load.

Bedrooms are much smaller, have less outside surface area, generally more modest windows and result in a much smaller percentage of the heating and cooling loads and thus much less air delivered.  So if the HRV is connected to the air handler to share the supply ductwork, the ventilation air will be supplied in the same percentages – most to the open living area and much smaller amounts to the bedrooms.

This heating & cooling distribution strategy is in contrast to what we prefer for the ventilation air – that most (if not all) of it is supplied to the bedrooms and other closed-off rooms.  The bedrooms are where people tend to spend the majority of their time at home and these doors are usually closed all night.  Without adequate ventilation air to bedrooms, CO2 levels can rise to high levels that do not promote restful sleep.  Open living areas are generally well set up to be “overflow areas”, where the air supplied to the bedrooms cascades through en route to the exhaust location in the kitchen (and often bathroom).  With this strategy, the bedrooms and overflow areas are adequately ventilated without having supply grilles to those spaces.

Ductwork Sizing

A stand-alone balanced ventilation system is designed for the airflows needed for continuous ventilation.  This means that the ducting system is sized for the range of airflow rates that the HRV will produce and balanced for the distribution desired.  A forced-air heating and cooling system typically require a much higher volume of air to carry the heating or cooling energy than would be required for ventilating.  Air handlers also operate intermittently – only when the thermostat calls for heating or cooling.  This mode of operation will trigger frequent cycling on and off in the height of winter or summer and potentially be off for weeks at a time during the shoulder seasons.   The ductwork that is designed and balanced for the air handler will end up being oversized for the amount of air the HRV is supplying for ventilation.

What this means is that when the air handler is inactive, the small amount of air that the HRV moves just crawls through the oversized ducts at low pressure.  Once it reaches the first few grilles (which were balanced for higher heating and cooling airflows), most of the ventilation air comes out of these grilles, not having enough pressure to reach the more distant locations.  So the ductwork distribution layout that was already not ideal for ventilation goals becomes even worse if not all of the grilles have air reaching them.

System Balancing

Another effect of the air handler intermittent cycling is that the HRV supply fan experiences two very different conditions.  HRVs are usually connected to air handlers by connecting the HRV supply duct to the return plenum of the air handler.  When the air handler is on, then the HRV air is supplied into a low-pressure suction environment (the back-side of the air handler fan) which makes it very easy to push air into.  When the air handler is off, then all of the components of that system become additional pressure drops for the ventilation air making it a harder environment for supplying air.

What this means is that if the HRV system is balanced with the air handler off, then when that system turns on then it unbalances the HRV by increasing the supply air volume and the heat recovery efficiency drops.  The same effect would occur if the HRV were balanced while the air handler was on:  it would be unbalanced when the air handler was turned off.  This imbalance can also pressurize or depressurize the building which encourages infiltration – never a good outcome.

In Summary

The above reasons describe why it is better to decouple the HRV ventilation system from the forced-air heating and cooling system.  The two systems have different goals and their preferred exhaust and supply locations are often different.  The duct system for the air handler is oversized for ventilation and can exacerbate poor distribution of the ventilation supply air.  The on-off cycles of the air handler can unbalance the HRV, reduce its efficiency and alter the pressure balance of the building.

The renowned New York City mechanical designer, Henry Gifford, puts it very well with this analogy (paraphrased):  joining the ventilation system with the heating and cooling system is like having a combination car-boat.  It isn’t a very good boat and it isn’t a very good car but it sort of does both functions in a mediocre way.  If you want a good boat, buy a boat.  If you want a good car, buy a car.  They can both sit in the same garage but use each for what it is designed for in its own way.

1 Kitchen H/ERV exhaust locations cannot be connected to the range hood or located too close to the range as grease-laden air can foul the heat exchanger.  A minimum of 8’ away from the range is recommended.

In locations with radon gas concerns, it is recommended to have the basement either pressure neutral or positively pressurized to minimize radon gas infiltration.  It is not recommended to only exhaust from this area.