It's early morning, and your routine begins. How cold does the bathroom floor feel under bare feet? How long does the shower run before any hot water actually comes out? How much cold and tepid water just went down the drain? These are woes that can irritate many on a daily basis. There are those, however, that never deal with them. Floors and rooms are evenly heated, and piping hot water is delivered quickly, if not instantly: enter the world of recirculation.
Using either pumps or natural convection (in a passive system), recirculation systems advance the flow of hot water from your boiler or water heater through your home and back to the heat source. These systems are most commonly used in radiant floor and baseboard hydronic heating, or to reduce and even eliminate the wait for hot water in fixtures around the home. Clearly, the main benefit here is water conservation. Recirculation saves the many gallons of water that are wasted running faucets for hot water, and hydronic heating circulates the same water over and over again. There's also the comfort and convenience that comes with quick hot water at even the furthest fixture in the home. Add to this that most users of underfloor heating seem to be more than satisfied with it, and you might even begin considering recirculation for your own home. It's not just about saving water, though, and there are a number of important points to consider regarding energy efficiency, convenience and performance.
There are two kinds of recirculation systems: closed or open.
Closed Systems: Hydronic Heating
Cast-iron pumps are designed for closed systems. Closed systems are usually associated with hydronic heating and cooling applications like radiant floors and baseboard systems. These are systems into which fresh water is not introduced, and excess oxygen has been removed with some type of air vent, air eliminator or air purger. Were this not the case, and fresh water was regularly circulated (as in an open system), the iron would quickly rust, eventually polluting the water reaching fixtures and causing failure of the pump. This is why cast-iron pumps should not be used for the potable applications of domestic hot water circulation (open) systems.
In a home-heating set-up, the pump is hooked up to a boiler, and replenishes the hot water in the heating system's tubing while moving the cooler water back into the boiler. Many find radiant floor and baseboard hydronic heating systems to be more comfortable than standard forced-air systems. This is largely due to the fact that the heat in these systems is distributed over a large area of floor and/or wall, essentially creating one big radiator. Heat moves up slowly through the air, warming the body gently and effectively. This is in contrast to most forced-air systems where the bulk of the heat is quickly pushed near the ceiling, resulting in an uncomfortable stratification of room temperature. Some users of these types of hydronic systems also cite improved air quality due to the lack of dust and other particles being forced through their home's air. However, many forced-air systems can be filtered, and with hydronic systems, dust merely settles around the house and still needs to be dealt with.
Radiant floor heating often takes longer to bring a room's temperature up than does forced-air heating, especially if the system has been off for some time. The quality of heat that is eventually produced, however – balanced, evenly distributed, and allergen-free – is usually enough to make up for the temporary inconvenience. With these systems, special attention must be paid to the flooring material they're under. Wood and laminate floors in particular can be damaged from water temperatures that are too high. Radiant-floor heating is usually not recommended for carpeted floors, as the carpet insulation makes it difficult for heat to radiate up through the floor.
Hydronic heating is not immune to freezing and ruptures. In cold climates, propylene glycol is often mixed with the heating system's water as an anti-freeze. This is important for homes with piping that could potentially be exposed to freezing temperatures, and homes that may be vacant for a time. Many circulating pumps can safely handle a 50/50 mix of water and propylene glycol.
Hydronic heating systems can be quite complex, and design and installation can be challenging. Because of this, we highly recommend that those considering such a system have the design and installation carried out by a professional well-versed in hydronic heating.
Open Systems: Hot Water Circulation
An open system is connected to your main water supply by way of the water heater. Fresh water is regularly introduced as water is used at fixtures. Pumps used in these systems are made of either bronze or stainless steel - materials that will not rust. A dedicated return line to the water heater, or the home's existing cold water line, can be used to return cooled water for reheating. These systems are primarily for use in domestic hot water applications, like providing "instant" hot water to all of your home's fixtures. Instead of waiting minutes for hot water to reach the shower at the far end of the house, it can be delivered instantly, or in several seconds. The cold water that would have normally gone down the drain is recycled back to the heater. These systems are most often found in hospitals, hotels, and large buildings, but have found their way into more homes thanks to greater efficiency and lower prices.
Example of Traditional Recirculating Water System
Open systems with a dedicated return line are usually found in older homes or incorporated into new homes during the construction phase. Installation of a return line in an existing structure is difficult and often cost-prohibitive. Return lines are ideal, however, since they are independent of the supply lines and will not mix warm water with cold. When a return line is not possible, the cold water line can be used to transport water back to the heater using a pump installed at the water heater and a special bypass valve installed under the fixture farthest from the heater. Though convenient and easy to install (and often the only option for a retrofit), such systems can create a slight reversal of the original problem: to get truly cold water, the faucet may need to run a bit. Bypass valves close at a set temperature, so any water below that threshold will flow into the cold water line: if a valve is set to close at 103°F, some 102°F water will mix with the cold water supply. While this isn't quite the annoyance that waiting for hot water is, it is something to keep in mind.
As with any mechanical device, it is possible for a bypass valve to fail. Rare though it may be, it is worth keeping in mind: were this to happen, it's possible that water directly from the heater – at up to 140° - will flow through the cold water line, leaving nothing but extremely hot water at all of your fixtures. As a safety measure, you might consider adding a tempering valve to the hot water line, before the bypass valve. In the event of a bypass valve failure, this guarantees that the water going into the cold line is at a reasonable temperature (that is to say, non-scalding). Adding a check valve between the cold line and the tempering valve will ensure that no additional hot water flows into the cold line.
Also worth noting is that when cold water is used at a fixture, where the special bypass valve is installed, the pressure in that line will drop. If the bypass valve is open at the time, warm water will flow through. This will in turn move water out of the heater, which is replaced by water from the main supply and heated. So even when only cold water is used, the water heater may turn on, wasting energy.
If you have a tankless water heater, extra research must be done if a recirculation system is being considered. Recirculating pumps generally have a low flow-rate, and may not have the draw necessary to activate some tankless heaters, and a small storage tank may need to be incorporated into the system. Worse yet, installation of some pumps may cause damage or void a tankless water heater's warranty.
Pumps used in water re-circulation systems can run continuously, or be activated by a timer, thermostat, control panel, or motion sensor. What type of control is used depends on the energy savings and convenience desired. A pump that's always running guarantees instant hot water at every fixture, but operating the pump continually and routinely heating water will use energy even when unnecessary and can shorten the life of the pump.
Those aiming for efficiency have a few options. Timers allow the pump to run only at set times, when demand for hot water is at its highest. This is often in the mornings and evenings for showers and dishes. The rest of the time, demand is greatly reduced (or non-existent if no one is home during the day). This lessens the load on the pump and water heater, but can be inconvenient if hot water is needed outside the set times: the pump will need to be turned on manually, or the faucet will need to run. Remote switches are available for many pumps, allowing the pump to be activated from anywhere in the home.
A thermostat can be used in addition to a timer, or by itself. The thermostat shuts off the pump when the high temperature is reached inside the hot water line, and turns it back on when the temperature drops below a certain point. When used with a timer, the thermostat will only operate the pump during the timer's on cycle, making the system even more efficient. In systems with a dedicated return line, the thermostat is usually installed on that line rather than the hot water line to ensure that the pump does not shut off prematurely.
For the greatest efficiency in pump-driven hot water recirculation, one must be willing to give up on the dream of instant hot water and settle for waiting a few seconds using an on-demand system. On-demand systems rely on the user to activate the pump at the fixture when hot water is needed. This can be done using a button or switch control on the wall, or with motion detectors that turn the pump on when somebody enters the room. Because hot water is only pumped into the lines when activated, it will take a short amount time for it to reach the fixture you're at.
By operating only when there is a demand for hot water, energy use is greatly reduced. In fact, on-demand systems are the only type of home hot water recirculation accepted by the EPA's WaterSense program, and many states and counties are even offering rebates on new installations.
Whatever the options or controls, these systems do save water. How much they save is difficult to determine, and varies with every household. You could get an idea of how much water is wasted by taking a day to track it. Instead of letting the cold water go down the drain, run your faucets into a bucket and use the water for pets, plants, toilet flushing, etc. You'll have an idea of your typical water waste, and what you do with that information is a judgment call: in many places, water is cheaper than energy, and adding the cost of a pump, installation, electricity, and potentially increased water heater use may negate any money-saving benefit initially thought possible. The energy use of modern recirculating pumps is minimal, with some using as little as 65 watts. However, please also consider that the main point of concern regarding energy use in any hot water heating system is heat-loss from the hot water pipes and the subsequent reheating of that water. Insulating the hot water lines is crucial to prevent as much heat-loss as possible.
As always, there is a trade-off between efficiency and convenience. Those more concerned with instant hot water and reduced water-waste may find more benefit in a continuous pump driving their recirculation system. Though energy bills may go up, the comfort and luxury of never having to wait for hot water may be worth it to some. Others may be willing to accept a brief wait – especially if they would otherwise have an excessive one – for their hot shower. And the better-insulated the hot water line, the shorter that wait will be.
Recirculation systems are not the only way to hasten the delivery of hot water, nor are they the only way to save water. Depending on the situation, they may end up being too expensive, inefficient or impractical. If you're waiting too long for your hot water, are tired of wasting it, or are concerned about wasting energy through heat loss, there are a number of simple and inexpensive things you can do before taking the plunge into recirculation. If your water lines aren't insulated, remedy that immediately. Try low-tech approaches, like capturing cool water in a bucket while you wait for hot; you're likely to find all kinds of uses for this would-be waste water.
New homes may enjoy the same benefits as a recirculating system using only design features, like placing fixtures as close to the water heater as possible. If you're having a home built, consult with thethe architect and contractor to find out what can be done to minimize water waste and decrease hot water delivery times. If a recirculation system is chosen, be sure to install a dedicated return line while it's still easy. Passive recirculation (or a "gravity loop") is often cited as an option in some circumstances, but is not something we recommend. Instead of a pump, these systems utilize convection and gravity to maintain a steady flow of hot water to your fixtures. In these systems, the water heater must be at a lower level than the home's fixtures (usually in a basement). Passive systems continually push water back into the heater, causing it to work around the clock. This wastes energy and could result in unnecessary wear and tear on the heater.
We all know that a lot of water is wasted every day waiting for it to get hot. Recirculation systems save that water, and provide the added boon of delivering near-instant hot water to every fixture in the home. We've explored some of the caveats, benefits and alternatives, giving you the information you need to make the best decision for your home. Always remember: however you decide to go about it, conserving water is vital, and your efforts to do so reward not only your household and wallet, but your community as well!
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