Monday, June 13, 2016

HVAC 3 - Ventilation

Installing the ventilation system caused a several week delay.  For reasons I will discuss below, I insisted on installing a Renewaire ERV to provide ventilation.  Since the HVAC contractor doesn't handle Renewaire, he asked me to procure the unit.  Well before the unit was required, I found an online retailer that advertised carrying Renewaire and asked about delivery time.  They replied that a unit would arrive about a week after the order was placed.  I placed the order a couple weeks before it would be needed.  After a couple weeks, I contacted the retailer to ask why it had not yet arrived.  They said they had forwarded the order to Renewaire who would be shipping it directly.  After another week, I called Renewaire.  Renewaire had never received an order and said they had problems in the past with that retailer.  I cancelled the order and ordered it through another retailer that was recommended by Renewaire.  Just as the unit arrived, the weather turned hot.  The HVAC contractor spent the next week responding to service calls from people whose air conditioners weren't working.  Finally, three weeks after finishing the rest of the HVAC rough-in, the contractor was able to return and rough-in the ventilation system.

Rough electrical work was delayed until the ductwork for the ERV was completed so that the ductwork would not need to be routed around wiring.  I am glad they were done in that order because it was difficult to find a route for the ERV ductwork even without the wiring.  I downloaded the ERV installation instructions online and asked the HVAC contractor to do the ERV ductwork at the time of the other rough-in using these instructions.  That would have prevented the delay from cascading and delaying the entire project.  However, he was not willing to do that.

Why do we need a ventilation system?

Building science professionals bristle at the old saying that houses need to breathe.  However, they recognize the need for adequate ventilation and the need to avoid excessive ventilation.  Occupant activities within a house, such a breathing, cooking, etc. produce various types of pollutants.  Ventilation exchanges the polluted indoor air for less polluted outdoor air.  During much of the year, the incoming outdoor air must be conditioned which increases heating and cooling usage, so over-ventilating is a problem.  Traditionally, houses had enough random leaks to provide adequate ventilation.  Even with a leaky house, some force must push air through the holes.  In winter, a force called stack effect tends to pull air in through low holes and out through high holes.  In summer, the stack effect reverses.  Also, wind causes pressure differences around the house that pull air in through some holes and out through others.  Unfortunately, cold, hot, or windy weather does not necessarily occur at the times when ventilation is needed.  For a typical new construction house, the result is excessive ventilation sometimes and insufficient ventilation at other times.  For a leaky house, like many older homes, the result is slightly excessive ventilation sometimes and way too much at other times.  A mantra among building science professionals is "build tight and ventilate right."  The goal is to control the amount of ventilation, control which indoor air is expelled (since it is not equally polluted), and control where the incoming outdoor air is drawn from (since it is not equally fresh).

Why an ERV?

There are various methods of providing forced ventilation in houses.  Most houses have fans, such as bath fans or range hoods, to expel air during periodic activities that cause localized pollution.  One method to ensure adequate ventilation, called exhaust only ventilation, is to run a bath fan on a timer so that it runs a fraction of every hour.  Outdoor air then flows in through whatever holes exist in the enclosure.  This method provides control of the amount of ventilation and controls which air is expelled, but does not control what air comes in.  Another method, called supply ventilation, is based on having the furnace fan draw in some outside air through a dedicated duct.  This provides control of where the fresh  air comes from.  With a damper and appropriate controls, it also provides control of the amount of ventilation.  Air leaves through whatever holes exist.  This is the type of system that the HVAC contractor proposed although he did not plan to install the damper and controls.

The third type of ventilation system is a balanced ventilation system.  One fan brings air in through a dedicated duct while another fan expels the same quantity of air through another dedicated duct.  This provides control of the quantity, the source of the fresh air, and the source of the expelled air.  Additionally, there is an opportunity to run the incoming and outgoing airstreams through a heat exchanger to precondition the incoming air.  This reduces the heating and cooling energy use.  Systems that exchange only heat are called Heat Recovery Ventilators, or HRVs.  Some systems also transfer humidity between the airstreams.  These are called Enthalpy Recovery Ventilators or ERVs.  Since few people know what Enthalpy is, some vendors call them Energy Recovery Ventilators instead.

Why a Renewaire EV130?

This picture from the Renewaire website shows what is inside an EV130.  The tilted rectangular part on the left is the crossflow heat exchanger.  A single motor on the right drives two fans, one for the incoming airstream and one for the outgoing airstream.

There are many manufacturers of HRVs and ERVs.  They use a few different methods of transferring heat and, for ERVs, moisture between the airstreams.  They range widely in price and in heat and moisture transfer effectiveness.  One issue that comes up in cold climates is a tendency of outgoing warm moist air to form frost as it looses heat to the incoming air.  Manufacturers deal with this in various ways.  I chose a Renewaire ERV for the following reasons:

  • The balance between price and efficiency fits my goals.
  • Renewaire has been making ERVs for a long time.
  • It prevents frost by tranferring enough moisture out of the outgoing airstream relative to how much heat is transferred out of the outgoing airstream, so no special defrost modes are required.
  • Renewaire supports using the ERV to replace bathroom fans.
Experts disagree on exactly how to calculate the required amount of ventilation.  Several formulas are available.  The highest quantity using any of these formulas for our house is about 125 cfm, so I want a unit that will provide at least that much.  When replacing bathroom fans, Renewaire recommends at least 50 cfm per bathroom.  Since we have four bathrooms, I initially selected the EV200.  However, when I changed retailers, I was told that the EV130 would ship about a week sooner than the EV200, so I changed plans and decided to replace only some of the bath fans.

Installation

Renewaire supports several different ducting arrangements.  I elected to draw air from three of the four bathrooms and supply fresh air to the return air ductwork.  (The bathroom without a shower doesn't need a full 50 cfm).  A control next to the thermostat sets the percentage of each hour that the ERV will run.  A control in each of the bathrooms forces the ERV to run in circumstances in which a bathroom fan would be operated.
The ERV is mounted on the ceiling in the shop.  The insulated flex duct to the left connects to the outside air intake.  The duct to the right connects to the outside air exhaust.  The two that run between ceiling joists connect to the bathrooms and furnace return respectively.  (Insulated duct is not necessary for these last two, but that is what the installer used.)
Most of the ductwork from the bathrooms to the ERV is 6" rigid round or oval duct.
The air outlet and inlet are under the balcony in the back of the house.  They must be separated by at least 10'.  The other outlet near the ERV exhaust is the dryer vent.  
The bathrooms switches are wired with 24V wiring, so the electrical box has a divider to separate it from the 110V light switch.

3 comments:

  1. I personally have encountered several occasions where I've run into problems with the warmer, outgoing air producing frost along the system. We've been thinking of updating ours, since this winter is predicted to be harsh, and we live further north. The prevention method of transferring sufficient moisture from the outgoing stream of air relative to the heat definitely sounds like it would be a much more helpful and efficient means than what we're dealing with now. Thanks for the insight!

    Ambrose @ Brown & Reaves Services, Inc.

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  2. People forget that a ventilation system does need to be updated or swapped out from time to time, just like anything else does. Those formulas that calculate the amount of ventilation needed always make my head spin, and while I'm definitely no expert, I don't think they're all accurate. Last time we had to update our ventilation, I left it all up to the experts.

    Tommy Hopkins @ Accutemp Cooling and Heating

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  3. As an HVAC professional, it never ceases to amaze me how many homeowners, contractors, and even architects think they can save money by not investing in proper ventilation system. The truth of the matter is that a proper system pays for itself in the long run, especially when you consider the structural damage that's caused by moisture in the long run. Depending on where you live then an ERV is a necessity.

    Dennis Cannon @ Laird And Son

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