another excellent reason why not to use open loop systems. The cost savings VS the risk are just not even close to there.
The proper system is not much more.
Dan

I have never used one of these forums before so don't know if I shoud be just posting a new question or if it is ok to add it to this thread. In any case I found this thread initaily by searching for info on simple hydronic systems and had initialy decided that a very simple open loop system was the one for me mainly because of cost. After reading this very interesting debate I am now leaning toward a closed system depending on the cost difference. In my case I have a home that has a very small basement apartment that needs some suplamental heating. To keep the renters happy we need to keep the entire house heated to a higher temperature than we like. We don't have enough spare electrical capacity to use electrical baseboard. I was thinking that I could run a very simple system with possibly one rad from our existing oil fired water heater just to provide the renters with a heat source that they could control. It would add just enough extra heat for them and allow us to lower the main thermostat. The water heater is virtualy right beside the apartment in the basement so it seemed like a perfect option. I was thinking that I could put something together for under $1000 for everything. One of the problems that I am having is finding info on the actual design of the system. I assume I would want a circ pump, something to turn it on/off controled by thermostat, a rad and piping. If we talk closed loop it sounds like I need much more. Any suggestions.

Sharply contrasting radiant floor "open" heating systems, hot water heater/fan coil heating systems are approved (and quite popular for condo type homes in southern climes) by many plumbing/heating codes and are quite safe.

I design many small systems with water heaters set to 140°F mixed down to 105 for DHW and driven by a bronze circulator to a fan coil/toe kick. This can also (though check approvals) be used to drive copper finned tube radiation.

Keep in mind, the more uncommon the application, the more important professional design becomes.

Looking through all the complaints of open system - the open system doesn't look a risk in my situation. Fin radiators for less than hundred feet of piping. Could also make my own heat exchanger but at this short of run doesn't seem to be an issue. Plus - the whole disease issue is mute. Most people have there water heater at 120 degrees already so they are already exposed. Like most building projects there pros/cons and then cost.

This post is quite old. I wonder how much of it is still relevant.

I still have the same open system. Working great.

I had an open loop design plan, although even WA code allows for it, my permit reviewer was not familiar and basically making it very difficult to get approved, as they are 'not familiar with that'. In spite of using products designed - (they ultimately required the water heater to be approved for space heating applications, and stated that it could not be used for BOTH space heating and domestic heating concurrently.... even though its approved for both.... wtf).

Anyhow, my plan for an open system was to convert to a closed after the water tank went out, so I went with a small electric boiler closed system as a substitute.

That aside, working in the health care industry, Legionella really was a low concern. Given the design of the system as well as the fact that Legionella is actually more pervasive than people realize.... the Legionella reasoning/scare really wasnt an issue.

Evidently it kills about 4000 people per year in the US. So about 10x more than electrocution. I'd say that all hot water systems should be paying more attention to this risk.

https://www.awt.org/pub/035C2942-03BE-3BFF-08C3-4C686FB7395C

An high risk example would be a seldom used section of shower water pipe that runs close to a hot air duct or hydronic heating pipe (ie, is kept warm).

Here is more,
its there, most often misdiagnosed as pneumonia


Legionnaires Disease | FAQ

How common is Legionnaires disease?
Legionnaires’ is not rare. It is perceived as rare only because most cases are never detected, and not all detected cases are reported to public health authorities. Because under-diagnosis and under-reporting make incidence of the disease difficult to estimate, figures have varied widely. The (U.S.) Centers for Disease Control and Prevention (CDC), Atlanta, has estimated that the disease infects 10,000 to 15,000 persons annually in the United States, but others have estimated as many as 100,000 annual U.S. cases.
Another reason that Legionnaires’ is falsely perceived as rare is that when cases are detected, the public rarely hears about them. Most cases—at least 65 to 80 percent in the United States and the United Kingdom —occur sporadically (one or two at a time). Thus, only a small percentage of cases occur as part of the multicase outbreaks that sometimes make the news. Cases of the disease are seldom publicized even when lawsuits are involved, because most Legionnaires’ lawsuits are settled quickly and under terms of confidentiality.
A case of Legionnaires’ disease will go undetected unless special laboratory tests are performed. Unfortunately, most U.S. hospitals still have not made these tests routinely available. It is reasonable to assume that undetected cases of Legionnaires’ are occurring because experience has shown that increased suspicion of the disease among physicians, when combined with increased patient testing, leads to more diagnoses. Some hospitals have recognized cases of Legionnaires’ disease only after increased testing of patients with pneumonia. Likewise, in hospitals where only one to three cases of Legionnaires’ were identified over several months, numerous additional cases were recognized after surveillance was intensified.
Studies of community-acquired pneumonia (cases acquired outside hospitals) have also indicated that increased surveillance leads to more diagnoses. A large-scale study in Ohio (U.S.A.) suggested that only 3 percent of sporadic cases of Legionnaires’ disease were correctly diagnosed. By comparison, in studies in which diagnostic tests have been consistently used, Legionella has been recognized among the top three or four microbial causes of community-acquired pneumonia.
Because the symptoms of Legionnaires’ are similar to those of other types of pneumonia, undetected cases of Legionnaires’ disease end up being classified merely as pneumonia with no apparent cause. Based on CDC estimates, this means that 8 to 39 pneumonia deaths occur each week in the United States without anyone knowing that the cause was Legionella. What’s worse is that many of these deaths could be prevented because, unlike most pneumonias, the source (e.g., a hot-water system) of Legionnaires’ cases can be identified. But if Legionella is not recognized as the cause, no investigation ensues to pinpoint and disinfect the source, so the same source remains a threat.


http://hcinfo.com/about/legionnaires-disease/how-common-is-legionnaires-disease/

Fun Fact,
Bradford White puts a water heater in a box every 15 seconds 24/7
bend your head around the scope of tank water storage with numbers like that,
Dan

I am not afraid of bugs, though I do keep my water heater at 140°F, just in case. I am acutely averse to waste however and like to use the proper chemistry in my closed systems separating the, always more aggressive, potable water from expensive hydronic components, many of which are not potable safe.

I use many tank-type water heaters, both fired and indirect for dual space/potable heating and find most are not rated for this purpose but never mix low temperature radiation directly with potable water. As I stated earlier, I have no problem using a fan coil such a First Co with it's built-in stainless pump for small applications e.g. condo or basement.

Hotel hot tubs are a leading legionella distribution/infection point. Hot tub temps are in the legionella growth sweet spot temperature zone, and high traffic hot tubs have all sorts of contamination vectors for introducing it. Diligent maintenance of the chemistry of the hot tub is required to keep legionella blooms from taking off.

In most states where open systems are allowed there is usually a minimum duty cycle requirement for the pump to prevent stagnation, even when there is no call for heat. I'm not sure how much science there is behind the specifics of those requirements though.