I have been interested in energy efficiency since I was very young. Learning about physics and improving systems is just ingrained in my mind. A very popular idea starting about 20 years ago was the concept of using an on-demand tankless water heater rather than heating water to temperature and storing it for later use. The thought process was that there is a lot of heat loss over time which requires the system to run more often to maintain the temperature even though it isn’t being used. By moving to an on-demand method, only the required amount of energy is added to the water when needed.
I am going to explore some ideas around why this is a problem, including some of the frequent realizations that have been observed, as well as extending this idea beyond hot water heating.
The first problem that emerges is that with access to “unlimited” hot water, usage increases and the efficiency losses are eliminated. Depending on habits, energy usage may even increase.
The next problem that arises is one not frequently discussed. Such systems operate rather seldom and then have a surge in energy usage. An on-demand tankless water heater that uses an electric heat element is going to require significantly more current than a traditional tank water heater with a heat element. This Rheem 18kW electric tankless water heater supplies up to 6/GPM (gallons per minute) of hot water, useful for up to 2 showers and 2 sinks, simultaneously, which is similar to what a 40-50 gallons tank can support for the duration of two showers. 18kW operating at 240V is 75A of current, which is massive. Some older homes have only 100A service from the electric utility which doesn’t leave any room to operate much else. Most homes built in the past few decades will have 200A service from the utility, but even still this is a significant portion of the load and operating closer maximum is not recommended, with recommendations being to not exceed 80%, or even 60% of load. This is because each component in the system must be appropriately rated, from the wiring, to the breakers, and the outlets. Each connection must allow be secure which is more important as current draw increases. Upgrading service is expensive to do and incurs ongoing costs from the utility service, not just for usage. Considering that energy conscious homeowners would likely be electrifying more energy usage, such as installing heat pumps to replace gas furnaces and electric vehicles requiring charging, this begins to see horribly impractical. There are systems that can be used to help manage overall draw, but there isn’t much that can be adjusted when such a single component pull such a high load. For a comparison, a home electrical vehicle charger will only be 8kW at 240V, drawing 33A of service, which is less than half. With FUD about the infeasibility of electrifying transport and the strain on the grid, a move to tankless water heaters would be more than doubling that concern by itself. Further, with an electric vehicle, the charging can be downrated to slow charging and draw less current. Any appliance requiring on-demand power cannot effectively be downrated by much.
The Problem with On-Demand Energy
On-demand use of power is a problem. Electric utilities are more frequently providing incentives to reduce peak usage because it costs more to use peaker plants, and it pushes us closer to the limits of what the transmission lines can support. Time of Use (ToU) billing incentivizes customers to reduce peak usage by increasing the price per kWh by around 3x, but giving discounts from normal pricing for other usage. My current non-ToU bill even offers a 20% discount for usage from 11PM-5AM each day. The ToU plan available to me would offer a 3x increase from 3PM-7PM in the summers (June-August) on weekdays, and a 2x increase from 8AM-10AM and 6PM-9PM in the winters (December-February) on the weekdays. All other times are a 20% discount from normal prices except for super off-peak, which is a 50% discount from normal prices from 11PM-5AM, daily. In my area, the base charge is about $0.10/kWh. Peak energy is $0.33/kWh for the summers and $0.25/kWh for the winters, with the off-peak cost being $0.07/kWh and super off-peak being $0.05/kWh. By shifting usage away from peak times, the strain on the grid is reduced and limits the use of the more expensive peaker plants. I would also expect that many showers happen during those peak times.
It isn’t necessary to eliminate peak usage with a ToU billing strategy. Simply reducing peak usage to the inverse of the price premium will be a break even during peak times, leaving discounted prices for all other usage. So limiting peak usage to about 1/3 of usage before switching to ToU will handle the task. In order to do this, many tasks can be shifted to other times by choosing to do so or using schedules. Smart appliances offer these opportunities, as do EV chargers. Schedule as many of these to operate during super off-peak times and significant savings will result. That EV that only costs about 1/3 the operating costs of the ICE vehicle can drop closer to 1/6 the cost of an ICE vehicle.
Energy storage is another means to reduce peak usage from the utility. The obvious option is the use of batteries, but batteries are expensive, despite the additional advantage of offering operating time in the event of power loss (a potentially better solution than a generator that only provides benefit during power loss). Other options do exist. Energy can be stored thermally, like in a hot water tank. Not only do tankless water heats have the several disadvantages that were described, but the lack of tank means that energy cannot be consumed during off-peak times and stored for later use.
A tank hot water heater can be improved, however.
Heat Pump Water Heaters as a Battery
A heat pump water heater will be significantly more efficient than a resistive heating (or electric heat element) based water heater. Many save that resistive heating is very inefficient, but that is actually incorrect. The energy efficiency of a resistive heating water heater is about 93%. This is because the inefficiency of most energy changes results in heat waste. So, making heat is actually extremely efficient. However, heat pumps do not change energy from electricity into heat. Heat pumps transport heat from one space to another, which requires far less energy. This Rheem ProTerra Plug-in Heat Pump Water Heater offers several advantages. For starters, it has an UEF of 3.3 (up to 3.5); this of this as energy efficiency. A hot water heater with a heating element rated as 93% efficient would have an UEF of 0.93. So, the heat pump water heater will use less than 1/3 the energy to result in the same amount of hot water. These water heaters are also smart appliances, so they can be scheduled to operate based on the cost of electricity. So, the water in the tank can be used as a battery. The water can be heated to a higher temp than you might set a traditional water heater just before leaving off-peak time and then set to not run at all during peak times. In addition, because it pulls so little power, it can be installed on a 120V 15A shared circuit rather than a 240V dedicated circuit as a traditional electric water heater would require; this means that it is an easy installation when replacing a gas water heater than would have required running a new electric circuit. Further, newer gas water heaters require forced air venting, which means drawing electric current to vent the exhaust which doesn’t contribute to heating. In fact, drawing in cold outdoor air in the winter reduces the efficiency of the water heater. Instead of shifting all of the gas costs to electric costs, the electricity used to vent is shifted to heating water, which offsets a portion of the electricity that would be consumed to heat the water.
There are some downsides to a heat pump water heater, however. The heat pump water heater is drawing the heat from the surrounding air to put into the tank. In the summers, this is advantageous because it is working like an air conditioner helping to cool the space. In the winters, this is a disadvantage because it is taking the air warmed by the furnace to pump into the tank. It also requires sufficient access to warmed air to operate efficiently; if it is in a small space without changing over the air, it will continue to draw the ever diminishing heat from that air and become less efficient. Both of these drawbacks can be mitigated if it is installed in a basement with sufficient space or passive venting to other space in the home. The basement will maintain a more consistent temperature based on the steady temperature underground. In addition, heat pump water heaters don’t increase the temperature of the water as quickly as a resistive heat element. This can be mitigated by running for a longer duration, which is fine as long as operating off-peak and by getting a larger tank to store more water.
HVAC is On-Demand
We have reached the part where the discussion moves beyond heating water (somewhat). HVAC systems of all sorts generally work in an on-demand manner. Gas furnaces, air conditioners, air source heat pumps, and even geothermal heat pumps have a rather significant amount of “on-demand” use in their nature and all of them can be shifted, to varying degrees. The first way that all of them can be shifted is to use the indoor space in the home as another battery. Whatever the desired temperature is, schedule the system to go further before leaving off-peak times. For many, having a temperature of around 70F year-round would be desirable. Smart thermostats (or programmable thermostats with more schedule windows) can be set to cool the space even lower or heat it even higher just before leaving off-peak. Perhaps reducing/increasing the temperature another 2-4F. This allows for the temperatures to remain more comfortable during peak-times if the system is restricted from running during those times. If the system isn’t restricted, at least it will have less work to do during peak-times, reducing operating costs.
Shifting to more efficient systems will is also advantageous. Geothermal heat pumps are the most efficient of these options, but these are extremely expensive to have installed by a professional, these days. Buying the system itself is easily $10k, and the installation could be that more again, or more. Also, a significant part of the price is the long system of “fingers” or deep well that is used to tap the steady temperatures in the ground; this is where the on-demand nature of even a geothermal heat pump emerges. Even though the ground is working like a battery, the system has to have enough contact with the ground so quickly change the temperature of the liquid from the extreme temperature after leaving the indoor system towards the steady temperature of the ground. However, it is unlikely that anyone would modify these systems.
Air source heat pumps are the next best option. These systems are drop in replacements for air conditioners because air conditioners are air source heat pumps, but they only work in one direction, moving heat from inside the conditioned space to outside the unconditioned space. An air source heat pump has a reversing valve which allows it to also move heat from the unconditioned space outside to the inside conditioned space. The concern with air source heat pumps is that as the outdoor temperatures become more extreme, it is less efficient. It is more difficult to push heat into the outdoor air when on the hottest days of the summer, while it is more difficult to extract heat from the outdoor air in areas with extreme winters (although it can work). In order to do this, convection is used to quickly return gas to the ambient air temperature outdoors.
What if the outdoor coils could have a more steady temperature to work with, like geothermal heat pumps? Many have used underground ducting and vent fans to circulate air in greenhouses throughout the ducting to have a less expensive geothermal setup and extend the growing seasons. Others have considered this same concept to precondition the air that an air source heat pump uses. It requires an extremely long run of ducting which is option plastic corrugated drain piping, which operates like the “fingers” or well pump in a geothermal heat pump design. Corrugated plastic is more of an insulator, so it does prefer a single wall system, whereas for many drainage applications, as premium setup would use double or triple wall to avoid ground freezing. The single wall system must have an extreme amount of surface area because of its low thermal conductivity, and it is using air which is also a poor thermal conductor. This means disrupting a huge amount of space to place the piping. If renting equipment or hiring a professional, this means it will also be expensive. If a smaller area could be disrupted, it would reduce the costs and make the excavation easier to handle if performing DIY. This could be accomplished by having a large thermal mass that is more thermally conductive underground. The mass would act as a battery and slowly return to ambient ground temperature when the heat pump cycles off. Since the material would be more thermally conductive, the length of piping could be much less because less surface area would be required. A large cistern filled with glycol or mineral oil could be used as a thermal mass. The cistern would be better to be thermally conductive, but plastics would have durable and not corrode. Corrugated plastic pipe would still be used to source air, but the air would pass through a coil (easily sourced from a car radiator at a junk yard) and the glycol or mineral oil could be pumped through it. The pipe would not require any additional vent fans if a shroud is placed around the intake of the heat pump coil as the existing fan would circulate the air.
Dustin Dortch 