November 22, 2018

This week’s article is the first section of Occupant Loads. This set of articles will walk you through how to optimize the equipment and systems that homeowners use day-to-day, like hot water, lighting, and appliances.

The first part of Occupant loads deals with domestic hot water, also known as DHW. We’ll start with equipment types this week, and move along to how to reduce DHW loads at the design phase of a new build or renovation project.  

Here’s the evaluation form, so you can rate yourself on your existing understanding of domestic hot water equipment. The rating is from 0 (I know nothing about this topic) to 4 (I’m an expert in this topic and can handle complex tasks on the daily). You don’t have to share this with anyone else, so rate yourself honestly!

If you find Occupant Loads a challenge, read on. There are links to lots of free exercises and workbooks in this article.  

If you’ve got this part of the competency down, tune in next week for the second part of High Performance Housing, Domestic Hot Water, Energy Efficiency Measures.

Domestic Hot Water Equipment Options

In residential installations, water heated for uses other than space heating is called domestic hot water, often abbreviated to DHW. Typical domestic uses of hot water include cooking, laundry, and bathing.

  • In the kitchen, hot water is needed at the kitchen sink and at the dishwasher, if there is one installed. Sometimes there is more than one sink in the kitchen.

  • Laundry can require a washing machine and a utility sink with a hot water feed.

  • Bathrooms require hot water feeds to shower and tubs, as well as to lavatory sinks.

In North American homes, water heating can represent up to 25% percent of the total energy use, more energy than all other household appliances combined. The actual percentage depends on the house type, number of inhabitants, and the lifestyle of those who live there.

Standard DHW Equipment & Fuel Types

There are three types of standard equipment that provide domestic hot water for most North American homes: storage tank heaters, instantaneous heaters (also known as on-demand or tankless heaters), and indirect tank heaters. Innovative DHW systems that are available on the market now include solar thermal, heat pumps, and combinations systems. All water heaters can use a variety of energy sources to provide hot water. How much energy is used for hot water production is affected by how efficiently the equipment converts the energy source to heat.

Energy is supplied to the water heater from different sources, depending on what is available in the area the house is located.

Natural gas, oil, and propane are commonly used for heating water. Oil and propane are commonly used directly, from a storage tank on site. Natural gas is typically fed to a house from a pressurized utility supply.

Electricity comes from a wide variety of sources, including fossil fuels. It is 100% efficient at the house, meaning all of the energy that comes through the heating elements is used to heat the water.

Renewable energy options like solar energy or geothermal energy can be designed as as stand alone DHW systems. They can also be used in combination with backup systems, or to augment existing DHW systems.

How DHW equipment impacts energy use

All types of DHW systems experience three types of losses that affect energy efficiency:

  • Firing Losses

  • Standby Losses

  • Distribution Losses

Firing losses occur when you convert a primary fuel source (natural gas, electricity, oil, propane, wood and so on) to heat. The two most ways heat is provided to water are electricity and natural gas.

Standby losses occur by keeping water at the delivery temperature in a tank, or by keeping your water heater ready to provide heat at all times.

Distribution losses happen after the hot water leaves the tank. The bigger the distribution system, the bigger the losses. When the water heater is all the way on one side of the house and an end use is at the other, more hot water gets stranded in the pipes than when the two are close together.

Storage Tank Water Heaters

Storage tank water heaters are the most common type of water heater for homes. In these systems, heated water is held in a tank, and a quantity of hot water is available at any time. When a hot water tap is turned on, hot water flows from the top of storage tank, and unheated water flows into the bottom of the tank to replace it. The tank always stays full of water. A thermostat turns on the burner or electric element to maintain the water temperature in the tank.

If the storage tank is inside the conditioned space of the house, in a closet inside the house for instance, the temperature of the air surrounding the tank is closer to the water temperature inside the tank, so the standing losses are lower. If the tank is outside the conditioned space, say in the garage, or an outdoor closet, or in an unheated basement, where the air temperature is much lower than the water temperature, standing losses will be on the high end of the range.

Another factor in standing losses is the climate. The bigger the difference between the hot water heater and the indoor temperature, the higher the amount of heat loss. A tank located in a garage in Florida will have a lower annual standing loss than one that is located in a cold climate.A tank located in an unfinished basement in Saskatoon SK has significant heat loss through a good portion of the year, pushing it's standing losses to the high end of the scale.

Efficiencies and Standby Losses

Electric and gas hot water tanks both have standby losses, losing heat through the sides, top, and bottom. Gas-fired units typically have a higher standby loss than electric units of the same capacity because they have more surface area to lose heat from. They lose heat via the flue pipe that is located in the middle of the tank. A 4 inch diameter vent that is 48 inches high has an additional 605 square inches of surface area exposed to cooler air temperatures. Most gas water heaters have another standby loss in the pilot light.

Electric water heaters are 100% efficient at the tank, they require no venting, but cannot operate during a power failure. Typically electric water heaters use more primary energy (energy contained in the raw fuels to produce electricity, including generation and transmission losses) than natural gas.

Older gas-fired heaters are inefficient compared to newer units, because they use naturally aspirated burners, continuous pilot lights and have constant energy losses up the chimney. They also can spill combustion gases if the house depressurizes. Here is a list of energy efficient features found in new water heaters:

  • Electronic Ignition, eliminates a continuous pilot light

  • Improved or extended heat exchange to increase the heat transfer to the water

  • Sealed and power-vented combustion

  • Increased tank insulation

  • Factory-installed heat traps

  • Bottom inlet for the cold water

Best Practices for Energy efficient installation of Storage Tank Water Heaters:

  1. Specify tank with lowest Energy Factor (EF)

  2. Specify tank with minimum R-24 insulation

  3. Locate the tank in a heated space (especially in cold climate)

  4. Locate tank close to plumbing fixtures to reduce pipe lengths

  5. Insulate the tank from the floor by setting the tank on top of a piece of 2" thick Type IV rigid board insulation

  6. Insulate all accessible hot water pipes, especially the first 3 feet of pipe from the water heater

  7. To avoid condensation, insulate the cold-water inlet pipes for the first 3 feet

If you are renovating but not replacing the hot water tank,

  1. Check to see if tank has insulation with an R-value of at least 24

  2. Drain the tank

  3. Check and replace the anode

  4. Insulate the tank with a blanket

  5. Insulate all accessible hot water pipes, especially the first 3 feet of pipe from the water heater

  6. To avoid condensation, insulate the cold-water inlet pipes for the first 3 feet

Many older storage tank water heaters are not well insulated. Insulation can reduce standby heat losses by 25%–45% and save 7%–16% in water heating costs. Payback on an insulating blanket for a water tank is less than a year.

Make sure to check the warranty to see if insulating it will void the warranty.

Depending on whether your hot water heater runs on gas or electric will affect how you need to wrap it.

Venting Systems

Gas appliances require a venting system. There are four common types of venting systems: naturally aspirated, a power vent, a direct vent, or a condensing unit. How the unit is vented depends on the type of equipment that is installed.

A natural vent, also called a Type B vent is the standard vertical design for a conventional mid- or high-efficiency furnace or boiler. Combustion gasses from conventional equipment are very hot and easily rise through the vent. Backdrafting is more common with this type of vent than other choices.

A direct vent incorporates two pipes of different sizes in one unit. The outer pipe draws air for combustion from outside, and the smaller, inner pipe vents the exhaust fumes. A direct vent can be installed on a sidewall, or through the roof. No chimney is needed regardless of whether horizontal or vertical installation is used.

A sidewall power vent, or induced draft fan, rates highly for both efficiency and safety. A fan at the end of an exhaust pipe draws combustion gases and expels them outside. The fan works in conjunction with the furnace, usually turning on before the furnace starts. Sidewall vent systems can replace old chimneys.

A condensing unit is among the highest efficiency equipment. It extracts heat from the water vapour in the combustion gases. The water vapour can contain over 10 percent of the total heat energy available from gas, about 6 percent from oil.

Condensing units have an extra heat exchange section where combustion gases are cooled to a point at which the water vapour condenses. The condensate is piped to a floor drain or condensate pump and the residual cooled gases are vented out of the side of the house through an approved plastic pipe.

Instantaneous Water Heaters

Instantaneous water heaters have a fuel source that heats flowing water, so it doesn’t need a storage tank. They can also be called tankless, on-demand, or point-of-use water heaters. The water is heated only when needed, increasing efficiency by eliminating standby losses.A tankless water heater is usually more energy-efficient than a storage tank water heater.

Electric on-demand water heaters are not as efficient as those fueled with gas, but they can be matched with a solar thermal system or other renewable energy source as auxiliary or back up heat.

Gas-fired instantaneous heaters are generally mounted on an outside wall so that flue gases can be vented more easily. If the instantaneous unit can supply the whole house, it can be a better option for energy efficiency. Gas-fired tankless water heaters have EFs that range from 0.64 to 0.98, with an average of 0.85 while gas-fired storage tank water heaters have EFs that range from 0.53 to 0.70, with an average of 0.62 (based on September 2011 NRCan on-line listings of the rated energy performance of gas-fired water heaters).

Indirect Tank Heaters

Indirect tank heaters are part of a boiler system, using the home’s space heating system to heat the water. Essentially, an indirect water heater is a well-insulated storage tank that holds a coiled heat exchanger. The ‘indirect’ part of the name refers to the fact that the tank doesn’t produce it’s own heat. The tank uses the boiler to circulate hot water through a heat exchanger in the tank.

A closed-loop water pipe connects the boiler to the indirect water heater. The boiler-fired water never mixes with the water in the storage tank, it circulates through the heat exchanger, which heats the water for household use.

The energy stored by the water tank allows the boiler to turn off and on less often, which saves energy. An indirect water heater, if used with a high-efficiency boiler and well-insulated tank, can be the least expensive means of providing hot water, particularly if the heat source boiler is set to "cold start." Seasonal efficiency of this type of system can be impacted by the climate zone - the larger the boiler, the worse the performance in the summer, as the boiler is oversized for the DHW demand only.  

Innovative Systems

Solar Thermal

The Sun's energy can also be used to heat water in solar domestic hot water systems. Generally these are not used on their own, and instead are selected to provide about 60% of the annual hot water requirements of the home in a cold Northern climate like Canada. These systems make use of solar collectors, a circulating pump, as well as storage tanks. These are usually used to preheat water, using a conventional heater after.

Heat Pump Water Heaters

Heat pump water heaters (HPWHs) use electricity to take heat from the air and move it to the water instead of converting electricity directly to heat. Air from the room containing the heater has the heat removed from it and transferred to a tank of water. HPWHs also remove humidity which can become uncomfortable. In the summer, however, the removal of heat from the home from these systems can be beneficial. In the winter they can increase the need for the use of a space heater.

Combination Systems

High performance houses, with excellent insulation, low air leakage, or located in a mild climate may have a dramatically lower heating load. They could have a single appliance thats does double duty as a space heater and a water heater. This is called a combination system. A combination system can be boiler-based or use a water heater. Combination systems are not currently regulated under Canada’s Energy Efficiency Regulations.

This type of system usually consists of a fan coil installed with ductwork, and a small pump that circulates water from the water heater through the fan coil. Heated water can also be circulated through in-floor piping to provide floor and radiant heating.

Some combination systems use boilers in place of water heaters.

These systems require a different calculation for energy efficiency. For example, a system that only has an energy factor (EF) rating for the water heater will not perform to the EF rating. The energy performance picture is incomplete, as the unit will perform differently when it is in space heating mode.

Combination systems can be rated as a system using the Canadian Standards Association (CSA) standard known as CSA P.9. The test method used in CSA P.9 gives a “thermal performance factor” for both space heating and water heating.

A combination system should be designed by a contractor or designer who can do the job properly. Look for certifications from organizations such as The Heating, Refrigeration and Air Conditioning Institute of Canada (HRAI).

As a builder or renovator, it is out of your scope to control occupant use of hot water, but there are several ways to improve the way that hot water is delivered, reducing the hot water energy load through efficient distribution system design.

Resources:

This infographic from Energy.gov (US) is meant for homeowners but is a great visual map of what kinds of equipment are associated with different fuel types

Another article from Energy.gov on selecting a new water heater that is linked to a series of articles on sizing DHW equipment and types of heaters. Scroll to the bottom of the page to click on the links to the other articles.

Info from Natural Resources Canada on ENERGY STAR® certified water heaters.

Downloadable pdf: Water Heater Guide from Natural Resources Canada




Learn more about becoming an energy advisor.

1.902.821.3090

Schedule Service

Sign up Today!

What our customers are saying

  • If you are planning a house build or renovation, understanding Building Science is important to avoid repetitive heating costs and possible water damage later. Shawna and her team know their stuff.

    K.W., General Manager, Halifax, NS
  • Studies over the last 20 years have shown a worrying energy performance gap – where buildings are consistently found to use more heating energy than designers had predicted and Energy Performance Certificates indicate.

    Oliver Drerup, Former Head of Canada Mortgage and Housing Corporation - International
  • I am pleased to hear that you will be undertaking a new initiative to further trades training in residential energy efficiency. CMHC supports the building of industry awareness and capacity to deliver more sustainable technologies and practices in the housing sector.

    Duncan Hill, P. Eng., Acting Director, Sustainable Housing and Communities Policy and Research Policy, Research and Planning, Canada Mortgage and Housing Corporation
  • Over the last 20+ years I have been speaking to and training thousands of new home builders and renovators across North America.

    Gord Cooke, Air Solutions
  • All I can say is … wow!  You have nailed it! I just watched the 5 minute overview of what all your training videos will attempt to accomplish, and am very impressed.  I am so proud that I know you both, this is incredibly good.

    Gregory A. Pedrick, C.E.M., Project Manager, New York State Energy Efficiency Agency
  • This training is easy use and in a perfect format.

    Dan Drummond, Training Director, Canada Wood China
  • It is very well centred on the learner's experience. Visually-based rather than text-based delivery make the modules straightforward and non-intimidating. I like the way the expected learning outcomes are clearly presented and the information is clearly applicable to practical work.

    Michelle Harding, Professional Builders Institute of BC
  • This is primo education that goes way beyond training.

    Chris Rosemont, BASF