After space heating, water heating is typically the next biggest energy sink in buildings. When dealing with passive houses, space heating is drastically reduced making water heating a proportionally larger chunk of the smaller energy pie. Despite this significant resource intensity, very little fore thought it put into the design of the domestic hot water heating system and it's distribution. These inefficient hot water delivery systems take minutes to get hot water to it's point of use, resulting in a large amount of energy and water being wasted down the drain.
In this article we will dive into efficient design best practices, different types of distribution piping and the some of the best equipment to provide domestic hot water for your Passive House Building.
Efficient Design Best Practices
Less is truly more, when it comes to domestic water heating designs. Once we know the total number of washrooms, showers, kitchens, dishwashers, laundry rooms that a building requires we can get to work optimizing the layout of the building with respect to domestic hot water. Our main two goals is to minimize water waste and heat loss, luckily, the two go hand in hand.
Remember heat losses increase with the diameter of the line, the distance the water must travel, and inversely with the speed at which the water moves. A large volume of slow-moving water sheds more heat as it travels to its destination than a smaller amount of water moving quickly.
Time to tap.
This first step to consider is time to tap. How long does it take to get hot water to your tap? Designing to keep this number as small as possible is one way to ensure the least amount of energy and water is being wasted. This can be achieved by accommodating all hot water fixtures as close as possible to the hot water tank (i.e. mechanical room). Centralize all plumbing fixtures by grouping them next to each other on the same level and stacking in the same area when going up a level, as reasonably allowed.
The next step is to determine which type of distribution system you will install. Trunk-and-branch is what has been installed, in most homes, in the past. While this system is easy to install, it also wastes the most amount of water.
Traditional systems consist of large-diameter (usually 3/4 in.) trunk lines to distribute water throughout a house with smaller branch lines (1/2 in. and 3/8 in.) teeing off to feed individual fixtures. Trunk-and-branch systems have several disadvantages, notably a large number of fittings, which are costlier, slower to install, and more likely to leak than a single run of pipe. While wasting a lot of water down the drain before hot water reaches the tap, due to the large volumes of water left sitting in the large 3/4 in. piping between the tank and the tap.
Manifold systems use the least amount of hot water and use the most amount of piping. This system has significantly less fittings and is typically one direct, continuous run of PEX piping from the manifold to each individual fixture. Tubing size can be lowered to 3/8" for all fixtures except the bathtub which will use a 1/2" tube. An added benefit to this system is the ability to isolate certain fixtures without taking down the entire water system in the building. Adding service cavities throughout the building for tubing can speed up installation time and reduce the amount of drilling required.
While there are other hybrid variations of the above 2 mentioned distribution types but they do little to save on water and heat loss so they won't be discussed here.
Best mechanical equipment
The invention of hybrid air source heat pump hot water heaters (HPHW) has drastically moved the needle in terms of lowering energy consumption when compared to the standard electric resistance hot water heaters.
These HPHW have a combination heat pump plus a back-up electric resistance heater to provide hot water for the building. The heat pump works by pulling heat out of the air in the room it is installed (non ducted option). This can provide free cooling in the summer to your home but unfortunately also works to cool your home in winter. So in winter, you will be paying to heat your air that is then being used to heat your hot water. Solution: Add the optional intake and exhaust ducting to source air from outside. The heat pump will work until the temps drop outside it's operating range and then the backup electric resistance takes over providing hot water for the building.
It should also be noted that that this equipment under a non ducted configuration also has minimum air volumes requirements (typically 1000 ft^3) in order to operate efficiently.
Solution: Ensure your mechanical room is sized adequately to accommodate the specific model you choose. If your mechanical room does happen to be under the min. volume requirements, there are additional work arounds such as adding louvers to you mechanical room door to allow additional air into the space that can still work.
It should also be noted that these HPHW heaters run off a compressor physically combined with the hot water tank that is now installed in your building so there will be an additional noise generated from the compressor running.
Solution: Add additional soundproofing in your mechanical room walls if you feel this is going to be an issue.
Cost is also a factor in this decision and these systems can run typically run anywhere from $2.5-$4k plus installation (based on size of system) with a advertised payback of around 4-5 years based on local electricity rates.
In 2016, there was another product introduced to the North American Market from Japan. ECO2 Systems (previously known as SAN CO2) has developed a split heat pump domestic hot water heater, where the compressor is installed outside and is plumbed to the tank inside. The environmentally friendly CO2 refrigerant also drives energy efficiency up significantly, making this system the most energy efficient system available in the market, today. The design inherently solves some of the common issues found with standard HPHW heaters, but not with out some other cons.
Locating the compressor outside, moves the noise generated out of the mechanical room, prevents the cooling in winter issue and removes any mechanical room volume requirements.
However, you will end up paying for this level of efficiency with these unit's typically starting at $7K plus installation.
Another noted con to this system, is the fact that there are water lines plumbed from the tank inside to the compressor outside. The manufacturer recommends insulating the two lines and combining it with a heat trace wire. However in our cold Canadian climates, without a back up generator on site, outdoor pipes freezing during a power outage can be a concern.
Drain Water Heat Recovery System
Another important energy saving piece of equipment we recommend some new homes try to incorporate is a drain water heat recovery (DWHR) system. This relatively inexpensive ($500 -$1000) system passively recovers heat from drain water coming from the showers and bathtubs in your home and transfers the heat through convection to incoming cold water and acts to pre-heat the cold water entering your tank.
Gomex Engineering works with clients to design an energy efficient domestic hot water heating systems based on their budgets and specific requirements.