HRV, ERV and Exhaust Only

When you renovate a green brownstone in Brooklyn you create a very tight house that does not breather like the leaky old brownstones. This means you need to arrange another way for the brownstone to breathe. Another consideration is that Brooklyn is hot in the summer and cold in the winter.

One way is to open the window. This isn’t very energy efficient in the dead of winter or the peak of summer though.

This is where Heat Recovery Ventilation (HRV) and Energy Recovery Ventilation (ERV) comes in. HRV passes the incoming air over the outgoing air and recovers the heat or cold. ERV also controls the humidity of the air. These can be expensive to install but do great at recovering heat.

Another option that some prefer is the exhaust only system. Below is are some comments from an installer that I have paraphrased.

As a Brooklyn green contractor we need to have answers about these systems because people need us to install ventilation in their tight green brownstones. I don’t know if one if better than the other because of the large variations in each house.

But I tend to like the ERV system and that is what we will probably be using in the Brooklyn green show house. Exhaust only is cheap to run but it does not take into consideration the cost to heat the cold air that comes inside the house during the winter or the hot air during the summer.

There are different ways to activate these systems in a brownstone.

When we turn on the shower, steam room or jacuzzi tub the “normal” quantity of moisture in the air increases and needs to be exhausted. If a fan connected to the outdoors is in placed and is turned on, then moisture will be exhausted. If the fan is activated by an on/off switch then interaction is needed to ensure the fan is operated when the moisture level increases above “normal” and deactivated when moisture level is returned to “normal”.

A replacement for someone standing next to the switch with a moisture meter could be a timer which runs for a preset time period, hoping it runs long enough, though not to long. Another option is a humidistat which is more accurate than the timer for run time of exhaust fan.

There are other considerations. If there is more than one bathroom how many fans are required? How many holes through the shell are required? What is the wattage of the fans?

Europeans have developed an effective strategy where they use one fan with one exhaust port through the building shell. Each bathroom/half bath/kitchen has a grille which opens and closes based on an elastic technology which has had a deviation rate of less than 2% over the past 25+ years.

The grille is designed to allow a continuous baseline ventilation rate of 8 cfm in the “closed” position. When the relative humidity raises above 30% the grille damper opens proportionately until it reaches a 50 cfm rate at 70% RH.

No electricity is used to open and close the humidity sensor grille. Grilles are connected to 4″ ducts which are connected to a single 6″ trunk line that connects to an 150 cfm fan that exhaust though a single 6″ exhaust grille. The single exhaust fan consumes 38 W of energy and has a vane configuration that will not increase energy consumption if the static pressure increases due to the inlet grilles operating in their closed position.

In a typical senario four grilles might be installed in a house. One grille in the master bathroom, one in a secondary bathroom, one in a half bath, and one in the kitchen – opposite side of the room from the stove to avoid grease. If all are closed then the house could see a baseline ventilation rate of 32 cfm. If moisture level was to increase in any one room the ventilation rate in that room would increase only at the rate required to remove the moisture and then return to the background ventilation rate.

Depending on the tightness of the house fresh air inlets may be required. In very humid environments the baseline ventilation rate may be slightly higher since the grilles will operate at a higher ventilation rate (dampers begin to open above 30%RH).

HRV/ERV operate based on timers or humidistats located in a central point in the building, use two fans (one in and one out), require two holes in the shell (one in and one out), require cleaning maintenance on the exchanger and have a high installation cost.

An exhaust only system has decentralized humidity controlled dampers, one fan and one hole through the shell, requires half the duct work of an HRV/ERV system at a lower cost.

Operating cost of an exhaust only system could be less than $34/year (38W x 8760 hr/year x $0.10/kWh).

Operable windows work if we open and close them, though not always an effective way of removing moisture year round. Exhaust only systems can be effective and affordable if their design is considered carefully.

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2 comments on “HRV, ERV and Exhaust Only
  1. Gene Keyser says:

    Not sure if this is the right place for this – pls correct location as you see fit. James Walsh is a pretty good guy that does systems integration – basically he can get machines and computers talking with one another by using sensors, wire, and programming. He’s quite green minded.

    James Walsh – (646) 385-0500

  2. Gene Keyser says:

    I like this post. Brings up questions – how big would fresh air inlets need to be in a pretty tight house? Would one put a similar grill on the inlet? Perhaps link the grills? I know a great guy who does some sophisticated systems integration. He’s pretty green minded. Can post his contact info if you think it’s a good idea.

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