Open or Closed System

John_37

I’ve been reading up on RFH trying to decide if it is a good choice to heat my home. On another site I read one person say that “you will get sick” with an open system. Is my understanding correct that an open system supplies both RFH and DHW. This statement seems a bit drastic. I’m mean, if everyone got sick who had an open system than they probably wouldn’t still be on the market. How serious of a threat is this? Do people get sick with a boiler that supplies water for both heat and domestic use?

Also, from an installation and maintenance perspective is an open or closed system better? I realize some of this may come down to a preference of one over the other and that’s fine. I would be willing to listen to any and all arguments as I try to make up my mind.

I’ve been told that some cities do not allow open systems. I’m meeting with my building department next week, so this may be a moot point if an open system is not allowed where I live.

Thanks,

Greg

Dan Peel

closed is best

"Things" can grow in standing, room temperature water (some say things WILL grow). Heat load has much to do with the appliance you should choose to heat the water. A heat exchanger to seperate your two fluids will always be the wise choice. Just because you CAN design a combo system in some areas - do you really want the results? Enjoy....Dan

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Dave Yates (PAH)

Open vs Closed

No worries when using a boiler and an indirect tank or tankless coil because the two fluids (drinking water or potable and hydronic) are separated via a coil or heat exchanger.

The concern with open systems wherein there is no physical barrier between the potable and hydronic water is two-fold.

For starters, an open system will be an amplifier for Legionella bacteria. The bacteria are present in all potable water systems, typically in fairly low numbers - but they look for a few things in order to enjoy a favorable breeding environment. Specifically: water temps between 55 & 133 F; pH of 5.0 to 8.5; biofilms; & stagnation. All of which are present in open systems.

Most people don't get sick when exposed to low doses of Legionella bacteria & the CDC (Centers for Disease Control) estimate that roughly 95% of us have been exposed. They determined this by performing random blood sampling.

Chronic smokers, alcoholics, aids patients, the elderly and anyone with an immune system that's compromised are at a much higher risk.

The CDC estimates that upwards of 10,000 people die each year from CAP (Community Acquired Pneumonia caused by Legionella bacteria). Nosocomial (hospital) acquired Legionella-based pneumonia gets a lot of attention due to there typically being many stricken vs the one at a time in the CAP cases.

Four CAP cases were documented this past year. 2 in PA and 2 in VA & all four were traced to their home's potable hot water system.

So, if you're really determined to play roulette with your health - go ahead and add the open system amplifier. Otherwise, do it right and separate the two fluids.

Does your building department know best? Put this into the Google search engine & decide for yourself "Potable hot water + Legionella bacteria".

The second part of the two-fold concern deals with scalding and what's required to make our potable hot water systems bacteria free.

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John_37

Thanks

That does make sense. I recall my health department training about food storage temps. They preach below 40 F for stroage and cook until it is above 160 F (if I remember correctly).

Dave Yates (PAH)

temperature is the cure

Water heaters used to come set for 140 F delivery, but scalding and lawsuits brought about the change to settings of 120 F. The mantra has been (for decades) that 120 F is a safe delivery temperature. It is not. 3rd degree burns continue to be a reality with more than 100,000 people being treated each year - 25% of which are infants and young children. More than a few die.

Legionella thrive at those temperatures, so turning down the storage temperature had an unforseen consequence - it turned our water heaters into amplifiers.

At 140 F, it takes 20 minutes of contact time to pasteurize the water (kill off enough of the Legionella to keep their numbers in check). Danfoss conducted a study and found the little buggers survive hot water flushes of 180 F by hiding in the outer fringes of biofilms!

Even if you raise the temp and install an open system, the open system will have the potential to continually re-seed the water heater AND potable hot water distribution system. If someone is in the shower and a dose of Legionella laden water is injected from an open system kicking on, the bugs will have anm opportunity to pass on through without the needed contact time.

Curent codes governing potable water heaters allow for a stacking of high temperatures to take place within residential models. The upper limit is 190 F!!! Stacking occurs in all storage tanks and is triggered by short runs - a faucet dripping or perhaps a small load radiant zone. The aquastat is triggered and initiates the call for heat. By the time it "sees" the desired temp, the upper portion has overheated water.

There is a cure that addresses both issues.

CIPH took a small step in the right direction by mandating storage temps of 140 F with a scald guard device required at the tank's outlet to flatline the delivery temp. Once again, however, that outlet temp of 120 F is not a safe bather temperature. Unfortunately, that regulation fell short because it doesn't address the remainder of the potable hot water system - the distribution network where Legionella thrive and the points of use where a safe bather temperature can be set using certified ASSE 1016 scald guard faucets. At 120 F it only takes a few minutes for 3rd degree burns. At 110 F, those same burn times become hours. The human threshold for pain is 106 F - if that human can feel it or know how to react for getting out of harm's way. Infants burn four times faster than do healthy adults. As we age, we lose sensitivity to temperatures that can quickly cause injury.

Our nation's code bodies are bogged down by politics as they debate these issues in committee.

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