Heat Recovery Ventilators

Posted October 3, 2008

In modern airtight homes, stagnant air and high humidity can become problems particularly in the winter when the windows and doors are closed. In older houses, the natural air leaks would cause the air in the house to be exchanged with air from outside, however, newer homes don’t have that natural air exchange.  In order to bring in fresh air, a device called a heat recovery ventilator (HRV) is used.  The HRV will bring in fresh air from outside and heat it up using the warm stale air from inside.

The HRV will have two fans.  One will draw in fresh air from outside and the other will push out stale inside air.  The fans push the air through a chamber made of series of air passages in which the direction of airflow alternates.  These air passages are separated by a conductive membrane, aluminum in the better units, that allow the heat from the warm air to transfer the colder air. In a house with forced air heating, the warm air is drawn from the return air duct and fresh air supplied a few feet downstream.  A better system draws the stale air from the kitchen and bathrooms, which are the main source of pollutants and humidity, using a separate duct system.  In a house with hot water heat, a full ducting system is used with the warm air  drawn from the bathrooms and kitchen and the fresh air is returned in the bedrooms and other living areas.  In the winter, since the fresh air is dryer, it will drop the humidity in the house down so there is little or no condensation on the windows

When looking at an HRV there are two main factors to consider in terms of efficiency.  The first is the sensible efficiency, which is the amount of heat moved from the warm air to the cold air and can vary from less than 40% to more than 80%.  A higher number is better.  The other factor is the amount of electricity the unit uses.  The units can vary from under 40 watts to more than 200 watts for similar sized units.  This is mostly due to the type of motors used in the units.  You want the unit that uses the lowest watts.  A listing of most HRVs is available from www.hvi.org

The HRVs are sized according to the size of the house, with a larger house needing a unit that can move more air (higher Cubic Feet per Minute orCFM ).  This can be determined by taking the square footage of the house and multiplying it by the ceiling height, giving the volume of the house.  This is then multiplied by the desired air changes per hour (usually 0.3 -0.5) and then dividing by 60 to get CFM.

The first winter we were living in our house, we had not yet installed the HRV and had condensation problems so bad that we had to have towels at the bottom of all the windows.  Since we have installed the HRV, we have had next to no problems with condensation, except for the coldest days, and even then it is minor.  I selected the Lifebreath HRV as I found they had the lowest power usage and one of the best sensible efficiencies.

Posted under Devices

Tankless Hot Water Heaters

Posted September 27, 2008

A tankless hot water heater will save you 10-20 percent on your heating bill and give you a virtually endless supply of hot water.  This is possible because the tankless hot water heaters only heat the water when it is needed.  A normal tank based hot water heater will keep a set amount of water at the requested temperature at all times whether it is being used or not, so when you are not using hot water, say at night, the tank is still keeping the water hot.  Since heat always travels from hot to cold, the tank will radiate some heat into the air surrounding it, causing what are called standby losses.  Since the tankless hot water heaters only heat water when it is needed the standby losses are eliminated.

The tankless hot water heaters also use less fuel to heat the water.  A conventional gas fired hot water heater will have an efficiency of about 60%, meaning that 60% of the heat from burning the gas will be used to heat the water, the rest of the heat is lost up the chimney.  For a tankless hot water heater the efficiency is usually about 80-85%, meaning at least 20% more of the heat generated goes into the water and is not lost up the chimney.

On the downside, since the tankless hot water heater has no stored hot water, when you turn on the tap, it will take about 10 seconds longer for the system to kick in and get up to temperature than a conventional hot water heater.  This can result in more water use as you wait for the water to heat up.  If you are on a marginal well this should be a consideration.  On the other hand the water that is used while waiting for it to heat up can be captured in a bucket and used for other things such as watering the garden.  The other disadvantage is that if you have hard water, there is more maintenance involved.  The tankless hot water heater works by having a the water flow through a series of small pipes that go back and forth over a burner.  In a hard water area these small pipes will build up scale and if left untreated will eventually plug up the heater.  To avoid this you have to flush the system with a weak acid solution to dissolve the scale.  There are kits available that have a pump and an acid solution and when installed, you need to put in some extra valves and connectors to allow you to connect to the unit.  I flush my unit twice a year, once in the spring and once in the fall.

Another advantage of the tankless hot water heaters is the size.  The unit I have (Takagi TKD20) is only about 20.5″x14″x8.5″ and hangs on the wall out of the way.  In my house the same unit is used for both domestic hot water and for generating hot water for theinfloor heating.  This saves a lot of room in the utility room, as there is not tank and no furnace.  The unit I have can generate about 7 gallons of hot water per minute, so with low flow fixtures, you could have two people using hot water at the same time and still not run out.

Posted under Devices