For many years, there has been a practice in the field of hydronics of taking a code-compliant, dual-use, tank-style water heater and using it for both domestic hot water (DHW) and space heating by circulating the hot water through radiant floor heating circuits. From the outset, as a master plumber charged with protecting the health of the nation, I have been opposed to this design. And quite frankly, I, along with other well-known industry experts, have gone before code development bodies to express our dismay and concern with the practice, to no avail.
In most nationally recognized codes, it is still acceptable to use a code-compliant appliance in this manner. One code includes a requirement for a timer to exercise pumps and thereby avoid stagnation. The number of “zone pump” systems is getting very rare these days; most system designs have one high-efficiency pump to move water, and flow is controlled by way of zone valves. To my knowledge, there are no controllers available on the market today that are designed to open a zone valve, with no call for heat, and start the circulator in order to move hot water around a given circuit in an effort to “flush” the system out. Even if such a control were available, how much sense does it make to heat up a radiant floor circuit in the middle of the summer?
Another provision of code required that the plumbing near the tank be set up such that when there was a draw on the DHW system, incoming cold water was forced through the radiant circuits, again in an effort to “flush” the alleged stagnant water out of the system. In certain conditions, this strategy will cause the radiant floor heating system to turn into a radiant floor cooling system, generating complaints about cold floors during the winter months. There is also the potential for uncontrolled production of condensation, as well as potential health issues associated with the growth of mold.
None of the above methods really does anything to address the possibility of bacterial growth (yes, Legionella and Pontiac Fever) in the piping distribution systems, and all methods create human discomfort conditions that cause these methods to be abandoned. In fact, the cold water flushing method perversely insures that the bacteria are being given plenty of fresh food and oxygen to support its proliferation.
Why do people want to use a water heater for both DHW heat and space heating with a single contiguous fluid? Because they are trying to cut corners and save money on the installation at the risk of exposing end users to the real possibility of contracting Legionnaires’ disease, which has about a 10 percent mortality rate.
All of the codes with which I am familiar that allow this practice require that sizing of the DHW tanks take into consideration the compounded loads of space heating and DHW demand. This will result in a minimum doubling of the DHW storage and BTU/hour capacity, resulting in significant continuous standby losses, all for the sake of reducing the installed cost and avoiding the use of a code-compliant heat source (boiler).
In many cases, the inspector is a mechanical code inspector, who is not charged with maintaining the potability of the water.
As the headline of this column implies, this is an inadvertent violation of the plumbing code. Every code with which I am familiar states that any condition that would cause the potable water in a system to become water of questionable character, which could potentially contaminate the potable water distribution system, must be avoided. As water sits in a radiant floor heating system during the non-space heating seasons, conditions are ideal for growth of bacteria that are naturally present in the water.
The reason single-tank systems are not addressed during installation by code inspection officials is that in most cases the inspector who is looking at the physical plant configuration is not a plumbing inspector, and if they are a plumbing inspector, they may not be familiar enough with what is being done to call the practice into question. In many cases, the inspector is a mechanical code inspector, who is not charged with maintaining the potability of the water, but instead is looking at properly sized relief valves and other critical life/health mechanical safety considerations.
Someone who really wants to “cheap out” and use an approved water heater for this application will have to incorporate an approved heat exchanger to transfer the thermal energy from the potable water heating system to the closed loop portion of the space heating system. This will require the addition of more expensive third-party certified pumps, heat exchangers, and other critical components in order to protect the health of the building occupants. Instead of going through all of these gyrations, it would be better for the consumer to install a conventional boiler-based hydronic heating system, with the functional ability to produce DHW through a code-compliant indirect DHW storage tank. It will last many years longer than the throwaway tank-style water heater.
If a person decides to use a code-compliant tank-style water heater as a space heating appliance in a “stand alone” application, I have no issues with that, other than the fact that the water heater is going to be subjected to many more operating hours than it was designed for as a DHW heater, and will therefore have a shorter life expectancy. Also, there are continuous energy standby losses associated with this configuration, because a reasonable, code-accepted method of turning the storage tank temperature down when the space heating demand is low or non-existent has not been developed.
We have a drawing at the Radiant Professionals Alliance website that shows the correct configuration of components if you absolutely, positively must use a DHW heater for both space heating and DHW production. Remember that with the new “no-lead” rules, all components that are in contact with the potable water must comply with specific NSF standards. The drawing and additional useful information may be viewed at http://bit.ly/2hWLZRR. Start the New Year out right by joining this very worthwhile organization — it will cost you less per day than you are spending on Starbucks coffee. Visit http://bit.ly/2h72j4e for reasons why you should join.
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