Tesmar Application Technology:

Question: Should a water heater be used as a stand-alone heat plant?

The storage type water heater has long been used as a stand-alone heat plant (Direct System) for radiant heating systems. They have, in fact, performed very well in every case that I know.  There are problems, however, both administrative and technical, which need to be considered when choosing this option.

Direct System Administrative objections: 

The mechanical code and mechanical code officials have jurisdiction over the approval process for the heat plant used in most administrative processes. They, typically rely on the “Boiler Vessel Code” as their “bible”.  The Boiler vessel code defines a boiler as any vessel used to provide heat directly to the distribution system.  This code is quite specific in requiring the boiler vessel to meet strict American Society of Mechanical Engineers (ASME) standards of construction that are necessary for a true boiler that will provide very high temperatures to the system and create potentially dangerous temperature pressure conditions.  Most, if not all, storage type water heaters do not meet these requirements, nor do they need to. Storage type water heaters meet their own specific requirements that are separate from the Boiler Vessel Code, such as the American Gas Association (AGA), the Canadian Gas Association (CGA) and, for electric units, Underwriters Laboratories (UL).

Direct System Technical Objection:

These systems have been used successfully for many years but do have some technical issues that should be considered.

The first problem that one encounters with the direct system is that of glycol protection.  Most storage type water heaters marry a large burner to a large tank and a small burner to a small tank simply because, for the purpose of heating domestic water, the assumption is that a large demand will require a large burner.  Therefore, in order to get a significant sized burner for radiant heating, the amount of glycol necessary to protect the system can also be significant.  A 65 gallon water heater may require up to 30 or more additional gallons of anti-freeze which, at $7 per gallon or more means $250 or more dollars going into the system just for anti-freeze.  That money could be better spent on a better system.

The next problem with the direct system is the amount of expansion that one must accommodate within the closed system.  Storage water heaters are typically filled with fresh water and allowed to back up into the fresh water supply line when they increase in pressure due to expansion.  Also they equalize pressure whenever new water is added because the pressure is vented at the tap.  Once a storage water heater is installed as a closed system, there is no venting of excess pressure at the taps and nowhere to back up into the pressure system.  This means that all expansion must be accommodated in a system expansion tank.  It is a good idea to keep the system pressure somewhere below 30 psig to keep the stresses on heating components low. One way to insure that pressure doesn’t become excessive is to install a secondary pressure relief valve somewhere in the system.  (EXTREMELY IMPORTANT - NEVER REPLACE THE WATER HEATER TEMPERATURE AND PRESSURE (T & P) RELIEF VALVE WITH ANYTHING OTHER THAN AN APPROVED TEMPERATURE PRESSURE RELIEF VALVE.  NEVER MOVE THE T & P FROM ITS APPROVED LOCATION ON THE TANK. THIS DEVICE IS TESTED AND LISTED WITH THE T&P VALVE AS A CRITICAL COMPONENT OF THE SAFETY PROTECTED SYSTEM.)

Finally, a most significant criticism of the direct system is that it is seasonal and people tend to want to turn them off in order to conserve energy. Storage type water heaters are designed to run continuously throughout their service life.  The continuous heat that is applied to the flame chamber and flue surfaces keep the surfaces dry of condensation and free from corrosion.  Turning off the water heater in the summer leaves these surfaces vulnerable to corrosion and early failure. 

Open Direct System Administrative Objections

One way to get closer towards the approval is to get the process shifted over to the plumbing code and the plumbing officials’ jurisdiction and away from the mechanical code.  The plumbing code accepts the use of approved storage type water heaters for heating domestic water, and most jurisdictions approve the use of water heaters providing hot water to a space heating system if it also provides domestic hot water.  This includes supplying water directly to the heating system (Open Direct System).  These types of systems were originally approved for small fan coils that were placed in the ductwork and provided some heated forced air heat to the structure.

Open Direct System Technical Objections:

The problems with using the Open System Direct approach for much larger radiant floor heating are significant.

First off, providing domestic water to the heat distribution system requires that the water be fresh water and eliminates the possibility of protecting the system with glycol anti-freeze solution.  Very large radiant systems are particularly vulnerable to freeze damage because the tubing is routed very near the outside walls.  Freeze damage can be very costly, if not impossible to repair.

Secondly, this approach presents some very serious health risks.  The potable water must remain potable.  That means that every component must be not only compatible with safe drinking water, but it must be approved and listed for that purpose.  The tubing must be certified for potable water.  The National Sanitation Foundation (NSF) International, when listing PEX tubing for potable water use, assumed that no more than 200 feet would be used in the average single family residence and developed its requirements around that assumption.  These requirements are very stringent and many pipe manufactures had a difficult time meeting them.  Increasing the amount of tubing by ten times or more would require the pipe be ten or more times cleaner.  The pipe is good, but not that good.  Also, most manufacturers of manifolds and other devices do not list these products for potable water use.

More importantly, the process of leaving the water stagnant in the system during the non-heating months presents serious risk of developing water-borne pathogens such as legionella, and a serious health risk.  In fact, even during the heating season, the temperatures associated with radiant heating are perfect for growing such organisms. There is little risk of illness from drinking the water because the digestive system will destroy most of these bacteria, but, if inhaled, as in the shower, the organisms will thrive in the lungs and cause, for example, “legionnaires’ Disease’ which, of course, can be fatal. In my opinion, this method should never be approved on the basis of this argument, alone.

Finally, this method exposes all components to the additional stresses of very high fresh water system pressure and, perhaps some degree of water hammer.  Any leakage will be supplemented with an unlimited supply of water and could result in significant water damage to the structure if gone unnoticed.

Combined Indirect System:

Generally the best method of utilizing a storage type water heater is in a combined indirect system.  In this application a storage type water heater supplies domestic hot water on a continuous basis as it was designed to do, directly from the tank. It also provides heat energy to the space heating system through the use of a double walled heat exchanger. The double walled heat exchanger transfers the heat through a protected system that eliminates the possibility of cross contamination of fluids by providing a leak path to the exterior.  The double walled heat exchanger can be internal or external. Internal heat exchangers are very costly additions to the storage water heater and, when the water heater eventually fails, must be replaced with the water heater.  External double walled heat exchangers are generally of higher performance and less cost but require an additional circulator. 

Advantages of Combined Indirect System:

The advantages of a combined indirect system based on a storage type water heater and an external heat exchanger are very significant.  They include:

Limited volume in the heating system:  The external heat exchanger contains approximately one gallon of volume. This alone saves up to $250 on the installed cost of a glycol protected system. Lower volume also eliminates the need and cost of an oversized expansion tank.

This system provides both domestic hot water and space heating from a single storage water heater, eliminating the cost of an additional water heater (or stand alone boiler) as well as the cost and complexity of an additional flue, gas and power connections.  Power Vented units can easily cost $1000 or more.

Eliminating the need for an additional water heater provides more floor space.  A typical water heater and its service area easily uses up 4 square feet or more of floor space from floor to ceiling height. Even a moderately priced building costs upward of $125 per square foot.  At those costs, gaining back that space is valued at $500 or more.

Performance and durability of the water heater are enhanced by the combined indirect system. Storage water heaters typically fail as a result of corrosion, either internal or external at the flue or flame chamber.  Most water heaters in typical applications will last ten years or more and better units have a ten year warranty on their tanks.

It is important to understand the failure mechanisms of a storage type water heater. Most tanks fail as a result of sediment build up in the bottoms.  Sediment builds up when it falls out of suspension to the bottom of the tank. This generally occurs when the water is stagnant.  As sediment builds up on the bottom of the tank it reduces the ability to transfer heat from the fireside of the flame chamber to the water in the tank. This results in additional stresses in the glass lining of the tank which is cooled by the process of transferring heat to the water.  The glass lining is designed to protect the tank from corrosion.  When the glass lining fails, the mild steel tank is left vulnerable to corrosion.  Normally, mild amounts of corrosion are kept in check by a sacrificial anode rod in the top of the tank. Unfortunately, this anode rod cannot protect something it cannot “see” directly.  The sediment somewhat blocks the ability of the anode rod to protect the exposed steel under the cracked glass lining.  The corrosion process continues until the tank fails. 

Another less significant failure mechanism is due to very high stacking temperatures at the top of the tank that occur when the water heater is operated near the top of its temperature settings.  Stacking temperatures also occur when the water is relatively stagnant and natural convection brings the hottest temperature to the top.  In the combined indirect system, the water is constantly circulated between the storage water heater and the heat exchanger.  This circulation keeps sediment from falling out of suspension and keeps stacking temperatures from occurring within the tank, thus prolonging the life of the storage water heater.

Another inherent advantage of the combined indirect system is improved performance.  Most water heaters transfer energy when the water in the unit is relatively stagnant. Stagnant water comes into contact with the heated flame chamber and flue surfaces, accepts the heat and rises within the vessel through natural convection.  This process is somewhat inefficient as the movement is very slow. In the combined indirect system, the water is constantly moving against the heated surfaces in a forced convection.  Forced convection increases heat transfer by a factor of from 5 to 35 times that of natural convection.  The process is somewhat like when the cooling fan comes on at an automobile radiator and the temperature gauge drops dramatically.  The cooling fan is driving forced convection through the radiator, which is a water to air heat exchanger. The forced convection dramatically increases the heat transfer.

The continuous movement of water through the storage water heater in a combined indirect system increases the storage capacity of the water heater by increasing the average overall temperature and keeping it there.

Conclusion: Storage type water heaters are very effective heat sources for radiant panel heating systems.  Many people analyze them as simple and robust and, through appreciation for their simplicity, look to use them in the simplest manners through direct or open direct applications.  These systems will work well if they are installed properly, but are not the best methods of utilizing storage water heaters.  The combined indirect system is far more efficient and robust than the direct or open direct systems.  When you consider all of the additional costs of those systems, and the potential difficulties in getting them to operate correctly, the combined indirect system is really a bargain.


Tom Tesmar