Hydronic Heating Products Great Bend KS
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Albert , KS
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Shawnee Mission, KS
Shawnee Mission, KS
Double-Duty Water Heaters
Hydronics the practice of using hot water to distribute heat through baseboard convectors, freestanding radiators or in-floor radiant coils is enjoying a revolution. As more people discover the comfort and reliability of hydronic heating, particularly radiant floor heating, the pressure increases to find more economical installation methods. The temptation to cut corners and compromise consumer safety, knowingly or not, has resulted in the option of cross-connecting the potable-water system and hydronic loops, with a single domestic water heater as the energy source.
The concept of using standard water heaters for combination, or dual-use, potable/hydronic applications is not new, but it has become a controversial and divisive subject between those who promote this practice and heating professionals, master plumbers, and some codes officials.
The conflict centers on the potability of the system's water quite simply, is it safe for people to use? Two primary concerns are the buildup of harmful bacteria and the potential for scalding. We'll explore these issues and review the different types of dual-use water heaters.
The Bacteria Problem
Water heaters are delivered from the factory with a temperature setting of 120° F. There was a time when the factory setting was 140°, but scalding cases and federally mandated efficiency ratings led to the lower factory setting of 120°.
When the move to reduce water-heater temperatures began, however, no one envisioned or even understood the bacterial implications of storing water at temperatures below 133°. Today, we know that lower water-heater temperatures enable bacteria like Legionella to survive and thrive.
Legionella first came to the public's attention in 1976, when 211 people became ill while attending an American Legion convention in Philadelphia. Thirty-four of them died from what was thought to be a previously unknown type of bacterial pneumonia. After this outbreak and the identification of the bacterial strain, the Centers for Disease Control and Prevention determined that the bacteria can colonize in hot-water tanks, and that the disease can be transmitted through drinking water.
To survive and grow, Legionella bacteria need four basic conditions that are commonly found in our residential water systems:
z Water temperatures between 55° and 133° F, with 68° to 122° being the most favorable, while 98.6° is the ideal temperature for rampant reproduction
- A pH between 5.0 and 8.5
- Biofilm (sediment)
The pH of nearly all potable-hot-water systems falls in the most desirable range for bacterial growth. Biofilms are the slime and layer of junk found in virtually all water piping. Sediment, another source of food and shelter for bacteria, occurs in tanks. While these conditions are difficult to control, we can control two other conditions: temperature and stagnation.
Although free-roaming Legionella bacteria begin to die at temperatures above 122° F, they don't really suffer until the temperature rises above 133° and remains there for at least 20 minutes. However, tests have proven that bacteria can survive for even longer periods of time in much hotter water by hiding in the biofilms.
If you think chlorine is keeping your water safe from Legionella bacteria, think again. Chlorine levels in typical potable-water systems are 10,000 times lower than the levels needed to suppress these bacteria cultures. High chlorine concentration flushes of infected systems do not eradicate the bacteria, which typically return within a few weeks. In addition, elevated levels of chlorine in potable water create carcinogens. Therefore, the use of chlorine in a water system is not a viable solution.
So stagnation is the only condition missing from the typical household hot-water system. Stagnation occurs when a cross-connected hydronic system is at rest, even for a brief time.
The Scalding Problem
The delivery temperature of household water is another important consideration. Even at a 120?° delivery temperature, infants and the elderly, in particular, can still receive third-degree burns.
The delivery temperature of hot water is also affected by a phenomenon known as stacking, which can occur in any water heater. Think of stacking as layers of hotcakes, with the hottest on top. If there is a low flow of water, such as a dripping faucet, the burner cycles can add successive layers of heated water, which begin stacking in the upper portion of the tank. A sudden draw from a faucet will send that scalding torrent of water down the line.
The current regulation governing residential water heaters of 75 gallons or less (ANSI Z21.10.1) allows for stacking temperatures of up to 190°! At those temperatures, third-degree burns can occur very quickly. Each year, more than 100,000 cases of scalding are treated by doctors in the United States, and several hundred deaths occur. (To prevent stacking, install an American Society of Safety Engineers certified 1017 mixing valve on the outlet that serves the potable-hot-water system.)
Four Typical Systems
Now let's look at several basic system types that use water heaters for hydronic energy. We'll begin with dual-use open systems, because these are most often pitched as inexpensive, highly efficient systems.
Dual-use open cross-connected potable/hydronic system This system is simply a water heater doing double duty. There are no components that make it especially energy efficient.
As an open system, there is no barrier between the hydronic and potable water. Therefore, bacterial contamination of drinking water is a possibility, as are stacking and scalding. There are no safeguards to prevent this from occurring, as there are with most of the other systems described in this article.
You should avoid dual-use open cross-connected systems. The potential dangers are simply too great. If you do decide to use this system, you can expect to pay about $500 for the necessary components and tubing. Bear in mind that all components of open systems must be rated for potable use. However, before you make a final decision, check with your local code and water-company authorities, since this type of system may be banned in your area.
Dual-use open cross-connected potable/hydronic system with a twist In this implementation, incoming water runs through the hydronic system for all hot-water usage. This results in three theoretical benefits: no stagnation, reduced energy consumption and some free air conditioning. However, these claims are not always what they seem to be.
Stagnation can still occur when you're on vacation, for instance. This system may require slightly less energy to heat incoming water, but only if the water enters the home at a lower temperature than the air surrounding the tubing.
Bear in mind that the incoming water has a normal travel time of less than 30 seconds before it arrives in the tank hardly enough time to provide any substantial change to its temperature. Once stagnant again, the new water will either warm or cool until itÃ?s the same temperature as the surrounding air.
As far as free cooling is concerned, the tubing resides in a well-insulated box, and your system's performance is based on driving energy up through the subflooring and whatever flooring you've installed (carpet, linoleum, tile, hardwood, etc.). You would have to flush large volumes of water through the system to achieve any appreciable reduction in living-space air temperatures.
Even if you find an efficient way to flush cold water through the system, the same condensation you've seen on a cool drink in summer or cold-water piping in your basement will likely occur in that darkened void where your floor-circuit tubing resides. This is a potential mold-breeding scenario, where you're providing food, moisture and perfect temperatures.
Again, check with your local code and water company authorities, since this type of system may not be legal in your area.
Dedicated closed-loop hydronic system This type of system, which is designed to isolate the hydronic loop, incorporates several important components:
- A backflow preventer, which stops hydronic water from entering the potable-water system
- A pressure-reducing valve set to deliver at 12 psi
- An expansion tank that accommodates water that expands as it is heated (this is a sealed system)
- An additional relief valve set for 30 psi
- A combination temperature and pressure gauge.
Why two relief valves? The answer is that you're still dealing with a storage tank, where water under pressure can become superheated above 212°. Superheated water is extremely dangerous, as its energy is released in the form of steam if it reaches its saturation temperature or if pressure drops suddenly. As steam, it expands 1,700 times in volume. The relief valve ensures a discharge if the internal temperature reaches 210°, thereby avoiding any possibility of superheating the water. The 30-pound relief valve is required because this is an isolated hydronic loop.
Since the potable-water system is isolated, none of the hydronic components is required to be rated for potable use. The water heater must be labeled Not For Potable Use to comply with some jurisdictional codes, which is based on the concern that some unsuspecting consumer might reuse it for domestic hot-water production.
Again, check with local code authorities to determine if this type of system can be installed legally in your area. Additionally, not all water-heater models are rated for standalone hydronic use. You'll need to be in compliance with the manufacturer's instructions for any warranty or liability issues.
Dual-use closed-loop potable/ hydronic system Far and away, this is the best system available if you're planning on using your water heater to perform dual use for potable water and hydronic heating. This is a perfect marriage of the open and dedicated systems detailed above.
It features a single addition: a flat-plate stainless-steel heat exchanger. This device, which isolates the hydronic heating circuit, is designed to enable the efficient transfer of heat from one fluid to another without the two intermixing. Models are available that transfer anywhere from 20,000 to 240,000 BTUs per hour, and are priced from about $125. The model you use in your system should match the heat loss for the total area being heated.
If a tiny hole is drilled through the swing-check gate on the bronze flow-check, a nice trickle of gravity circulation will be set up, which eliminates any stagnation without relying on any electro-mechanical devices to maintain potability. The ASSE scald-guard thermostatic mixing valve permits you to raise the manufacturer's storage-tank temperature setting to 133° or above, to maximize the sterilization of the potable water.
Again, be sure to check with local code authorities to determine if you can legally install this type of system in your area. The cost for this system is around $1,200, depending on the price of the flat-plate heat exchanger, which makes this an economical, sensible way to protect your family's health.
David Yates is a master plumber and backflow-prevention specialist who owns F.W. Behler Inc. in York, Pa.