This page is about sustainable water heating in off-grid and self-sufficient living environments. It shares the extensive research we conducted on tank water heaters versus tankless water heaters versus heat pumps to meet the water heating needs of the earthbag village (Pod 1) and the Duplicable City Center.
This page is divided into the following sections:
This page will continue to evolve until building the earthbag village is complete and we can definitively say what approaches were determined to be most effective. We will then further evolve this page through building the other 6 villages and the experience of others choosing to build teacher/demonstration communities, villages, and cities using our blueprints.
Ron Payne: Mechanical Engineer and HVAC / Thermal Designer
Heating water takes a lot of energy, energy that costs everyone a lot of money, and off-grid homes and communities even more. To sell more units, companies boast their heaters abilities without giving enough data for an “apples to apples” comparison. It takes special consideration and attention to find the data necessary for an accurate comparison and so we’ve created this page to share the results of our extensive research into this topic.
First, it is important to understand what the different types of water heaters are:
The most commonly used water heaters today are tank water heaters heated with gas or electricity. In the case of gas, they use a burner in the bottom to heat the water. In the case of most electric water heaters, they use electric resistance elements to heat the water in the storage tank using two electric resistance elements, which are located at the bottom and top of the storage tank. Each element is controlled by an independent thermostat with the lower element providing recovery from standby losses and the upper element providing heating during periods of large hot water use. Some resistance water heaters contain only a lower element. The size of the tank can vary from as small as 20 gallons to over 300.
Tankless water heaters (also called instantaneous, continuous flow, inline, flash, on-demand, or instant-on water heaters) are gaining in popularity. These high-power water heaters instantly heat water as it flows through the device, and do not retain any water internally except for what is in the heat exchanger coil. Copper heat exchangers are preferred in these units because of their high thermal conductivity and ease of fabrication.
Tankless heaters may be installed throughout a household at more than one point-of-use (POU), far from a central water heater, or larger centralized models may still be used to provide all the hot water requirements for an entire house. The main advantages of tankless water heaters are a plentiful and continuous flow of hot water (as compared to a limited flow of continuously heated hot water from conventional tank water heaters), and potential energy savings under some conditions. The main disadvantage of these systems are their high initial equipment and installation costs.
Heat pump water heaters use an air-source heat pump to transfer thermal energy from the air around the unit into the storage tank. Electric resistance element(s) are typically included to provide backup heating if the heat pump cannot provide sufficient heating capacity.
They can be either tank type or instantaneous. However, in our research it turns out that instantaneous types are usually for larger systems.
Solar powered water heaters are quickly growing in popularity. Their solar collectors are installed outside dwellings, typically on the roof or walls or nearby, and the potable hot water storage tank is usually a pre-existing or new conventional water heater, or a water heater specifically designed for solar thermal.
The most basic solar thermal models are the direct-gain type, in which the potable water is directly sent into the collector. Many such systems are said to use integrated collector storage (ICS), as direct-gain systems typically have storage integrated within the collector. Heating water directly is inherently more efficient than heating it indirectly via heat exchangers, but such systems offer very limited freeze protection (if any) and can also easily heat water to temperatures unsafe for domestic use. ICS systems also suffer from severe heat loss on cold nights and cold, cloudy days.
By contrast, indirect or closed-loop systems do not allow potable water through the panels, but rather pump a heat transfer fluid (either water or a water/antifreeze mix) through the panels. After collecting heat in the panels, the heat transfer fluid flows through a heat exchanger that transfers its heat to the potable hot water. When the panels are cooler than the storage tank, or when the storage tank has already reached its maximum temperature, the controller in this closed-loop systems will stop the circulation pumps.
In a drainback system, the water drains into a storage tank contained in conditioned or semi-conditioned space, protected from freezing temperatures. With antifreeze systems, however, the pump must be run if the panel temperature gets too hot (to prevent degradation of the antifreeze) or too cold (to prevent the water/antifreeze mixture from freezing).
Before jumping into the graphs below, it is important to understand the metrics evaluated and used within the industry and our comparisons:
This is utilized mostly for tank-type heaters as the amount of water they hold.
Usually in gallons, this metric describes the amount of hot water the heater would be able to give over the course of an hour after a prep period. For instantaneous types, this would just be the flow rate (in GPM) times 60 to change it into gallons per hour. Tank-type water heaters will reduce the amount of hot water they are able to give after that hour and the instantaneous will keep the same rate.
Knowing the amount of water we will use in the “shower hour” (all showers used for 1 hour, times the flow rate of those showers for the hour) we can divide that number by the FHR to get the needed water heaters for our purposes.
The energy factor, unlike the Coefficient of Performance (COP), deals with the amount of energy that the unit takes in (kWh) and relates that to the amount of energy the water has in it when it leaves. The most efficient standard water heaters get an energy factor of 0.99 (the theoretical max being 1.00). However, by utilizing the properties of a heat pump, energy is taken from the surrounding air to heat the water yielding a factor of over 2.00. Solar energy factor is used in the same way and means the same thing. In this case, however, it is not counting the energy coming from the collector. This sounds strange, but the overall reason is to compare the performance of the system to others. Therefore, it measures the energy of the exiting water in respect to the electrical energy used by the unit and because the unit is getting more energy from the sun the unit appears to have a greater than 1.00 factor.
This is simply how much one unit will cost.
The cost of buying all the units we need at the Unit Cost.
This is the amount of energy the units will use on the average day. We placed this as a December day because our energy production is sized on energy created during this month. Technically, however, it actually doesn’t matter because there will be just as many people showering in June as December.
The amount of power the unit will instantaneously draw at any given time multiplied by the Units Needed. This is the metric used by the electrical engineers to size our power system for optimization and safety.
We based this cost on our solar costs for our location, a location with about 250 sunny days a year. More specifically, the cost to us for creating 1 kWh/Day in December and multiplying that by the kWh/Day (DEC) metric. This gives us the cost to support the water heaters’ power draw. Right now our cost is $1,723.79 per kWh for a day in December.
This is the combination of the costs of buying the Units Needed and the equipment needed to provide for these units. This is the grand metric and the actual “apples to apples” comparison of what these systems would cost us. It gives the Unit’s Total Cost with respect to the other units and options. For the solar options, it also shows the variability of the cost when solar heat is not available.
This chart is Heat Analysis At 100% Energy Cost. This graph shows the breakdown of the cost for the units into the Total Cost and the Energy Cost assuming that we are paying the calculated rate for electricity.
This graph shows the breakdown of the cost for the units into the Total Cost and the Energy Cost assuming that we are paying HALF the calculated rate for electricity. This would only happen if we found some way to create power 2 times cheaper.
This graph shows us the Unit’s Total Cost as the cost to produce a kWh/d changes. Where unit’s lines cross over each other is the Energy Cost where the two options are equal. Because of different efficiency rates and Total Costs the lines cross each other in many places.
Break-even Analysis of Water Heaters – Click to Enlarge
Here’s a detailed look at how the hot water system could work for One Community. We are continuing to evolve and develop the specifics that we will eventually open source share with complete building plans, materials and labor investment specifics, maintenance details, production and effectiveness data, and more.
Here is what’s happening in the picture above. Ground water is pulled up to one of two different loops, the Solar Loop (Left) and the Hot Water Loop (Right).
Water on the Solar loop is pumped from the ground at a temp of 55° F to a solar collector, if the sun is shining we should be able to heat that water to 65° F with a flow rate of 30 gallons/minute (GPM) even in December sun. If no sun is shining the water can bypass the collector via insulated pipes directly to the Source Side of the Heat Pump. Through the heat pump the water is calculated to lose 9 deg of heat and the now “chilled” water is delivered underground to warm via geothermal heat transfer or used for another purpose.
Water on the Hot Water Loop is pulled out of the ground and past a plate-and-frame heat exchanger where is heated to roughly 70° F from the geothermal 55°. That water is then mixed with some of the bypass tank water via a throttling valve. The semi-warm water is then sent through the Warm Side of the Heat Pump to be heated to at least 80° (more depending on how much bypass water is recirculating from the Warm Tank). The outlet of the Heat Pump feeds directly into the Warm Tank and acts as a buffer for the entire system. Water leaving the tank either goes to the Duplicable City Center or the Earthbag Village (Pod 1) showers.
The Duplicable City Center takes the warm water at anywhere from 80°-100° and heats it in a “Boiler” to 105° at 104 GPM and returns it to the warm tank at 93 deg. The Shower takes the warm water and heats it at the source to 105°. The resultant “grey water” exits in the 90’s at around 30 GPM and is utilized by the Tropical Atrium as a heat source. The Grey Water leaves the Tropical Atrium at roughly 70° and flows through the plate-and-frame heat exchanger to warm the incoming water. The result is 30 GPM of near 55° water for plants or other non-potable uses.
Here is a chart of the benefits of a specific heat pump we evaluated (the Trane EXWE240) when used in combination with a solar water heater or heat exchanger recapturing heat from used shower water or another method:
What you see is a comparison of Trane EXWE240 energy efficiency versus a tankless water heater. Heat comes in with our groundwater boosted with solar water heaters or another method and this is shown on the X axis ranging from 55 to 85 deg F. Electrically heated water is represented as the colored lines (80, 90, 100, 110, & 120). The Y-axis is the coefficient of performance (COP) of the heat pump calculated as a ratio of heating or cooling provided to electrical energy consumed. Basically, it is the multiplier of our heat output with respect to our electrical input.
So if the water coming into the cold side of the Heat Pump is 55 degrees F and the water coming into the warm side is 100, the COP would be 4 and we would be getting 4 times the heated water that a tankless water heater would give us (or 4 times hotter water) for the same amount of electricity.
When conducting an objective “apples to apples” comparison of water heating options in off-grid systems where energy costs are higher because they need to factor in the construction of the energy infrastructure itself, it pays to get the most efficient option. While the tank-type water heater is very cheap and the instantaneous water heaters seem more energy efficient when you consider not having to keep water warm when not in use, the reality is that investing in a heat pump and solar collector make exponentially more sense depending on the cost of your energy infrastructure. The more expensive your energy infrastructure, the faster the maximally efficient (and more expensive) systems pay for themselves. As it happens, we’ll open source share here our experience with building, maintaining, and evolving these systems.
Q: What about Legionella?
We have not decided the most ecological solution to Legionella yet. Chlorine is what is usually used.
Q: Why are all the system comparisons based on electric models?
The only way to effectively compare the two systems is to have them both be electric. Thermodynamically a gas water heater is less efficient though because by burning a fuel some of the energy is lost as light and stack heat. There is no way around this. So to compare the two you would have to take the natural gas energy content per unit and that would be much less efficient than electricity. Also, natural gas, with few exceptions, is not sustainable.
Contact us if you have any others.