Legionella and solar water heating

Bob, great to have you here!

My first question: Are you concerned about the possibility of legionella occurring in solar water heaters? When I heard about this concept, I began thinking about changing my IPSWaH to being just an electric water heater assist.

What's your take on this?

I'd like to see an answer to this question as well.

I know with a normal electric hot water heater, you're not supposed to turn it down below a certain temperature because of the risk of legionella. (I have mine turned down as much as is safe, to make sure that my kids (~3.5yo and ~5yo) don't accidently scald themselves, not because I'm trying to save money.)

Alex and Fred,

I've been designing and installing solar how water systems in VT for almost 9 years now. I think I can answer your question, at least in terms of what can be done to minimize bacterial problems. If you set up the solar tank as a pre-heating tank for your existing hot water tank, then whenever you turn on the hot water tap, fresh water is added and old water is taken away from both the solar tank and the back-up heating tank. So with a pre-heating arrangement, cold water from the well or town, enters the solar tank first. The hot water outlet of the solar tank is plumbed into the cold water inlet of the electric hot water tank, (or indirectly heated from oil or nat. gas hot water tank). Water flows through both whenever anyone turns on the hot water tap, so I doubt bacteria would have much chance to colonize and contaminate.

The temp that the plumbing codes say a tank should be kept at is 140 deg F . Because this could scald you, you should have a mixing valve on the output of the water heater to mix a little cold water with the hot coming out. These cost about $100.

A tank thats only heated by solar might go for long periods at temps which are below 140, other times it might get up to its top temp (I set that at 175). So on a routine basis the solar tank is sterilized by high temp, but even if it weren't it wouldn't pose a problem in my opinion because as long as people are using hot water, then the water in the tank is refreshed daily. If people aren't using hot water then the solar system should get it above 140 on a periodic basis, unless it were poorly installed or designed. The systems I generally install have 2 flat plate collectors and 80 gallons of storage. With one day of sun they usually can go from 50 deg to 140. It depends on the circumstances of the site, but thats typical.

In tanks which have a coil in the bottom, and an electric element half way up, you can have the same tank heated with solar and electric. The one drawback to this is that you only effectively have half the tank to store captured solar energy because the top half is already heated with the electric element. Solar can heat the top up to a hotter temp even if its already at 140, but the collectors will lose efficiency and energy is lost. You can put the electric element on a timer, so that it only comes on at about 4pm and goes off at 10:00 in case the sun hadn't already heated it up because it was a cloudy day. Or you can get a larger tank. For a 2 flat plate system (generally) a 100 gal tank is needed if you want to heat it with electric and solar.

hope this helps
Karl Rosengrant

Greetings,

Karl is absolutely correct. There will always be a back-up water heater in a domestic solar water heatinging system that will be set high enough to keep the hot water safe.

Bob

Bob Ramlow wrote:Greetings,

Karl is absolutely correct. There will always be a back-up water heater in a domestic solar water heatinging system that will be set high enough to keep the hot water safe.

Bob

Not 'always'. I'm in an off-grid situation and don't have the option of electric backup. My backup is a pot on the stove or over the fire and that won't help the solar system.

So under what conditions would Legionella be a problem and how can I design the system to avoid it?

If you are going to drink the hot water or use it to wash/rinse dishes, then you have to heat it to at least 140 degrees F. one way or another. If you are only using it for bathing, then it makes no difference.

Bob

Here's some discussion amoung heating contractors about Legionaires. Specific info on temperatures and durations as well as some far out mechanical solutions.

http://www.heatinghelp.com/forum-thread/128079/Legionnaires-Disease-and-Hot-Water-Tank-Temp

I remember reading that Legionaires most dangerous vector is the "hot" shower because we breath a lot of the water mist and thus any bugs get straight into our lungs - the very worst place for them.

Rufus

UV light will kill most if not all microbes. So if you have a UV light module at the hot water tank output then it will get rid of the microbes. You can also put on at the input too just to get rid of any incoming microbes too.

Rufus, Good information about the shower! Thanks for posting.

A small on-demand gas water heater will back-up a solar water heating system and take care of getting the water hot enough.

Bob

Who makes a good on-demand water heater these days compatible with solar? My understanding is Bosch discontinued the solar backup standby version.

Any heater that will modulate the flame to 0 will work. I think there are several out there. I would check-out the Takagi, but I think others work that way too.

"Instant" hot water heaters have their own peculiarities, mainly temperature fluctuations. Some contractors are now promoting a small (10gal) hot holding tank with constant circulation through the heater to reduce the temperature fluctuations. IIRC at full rated flow they perform as expected but at reduced flow you can get some oscillation with the larger heaters. The standard domestic heated storage tank is actually very hard to beat for most household applications. There are tweaks to prevent heat losses and capture more of the heat before it goes up the flue (gas heaters) but they really are one of industry's better shots on target.

Rufus

Why not isolate the legionaire or any contaminant possibility from potable water? I put the potable water under pressure inside a coil of 3/4 pex immersed in the preheating tank, no connections inside, The water on the outside of the coils, in the tank, has no pressure, and this circulating water is sealed from air, and passes repeatedly through the collector (I use black rubber hose coiled over highly selective solar aluminum) under a double glazed surface. No freeze problem, and high temperature gain results. For the potable pressure water 3/4 pex coils work better than 1/2 because of resistance and pressure drop to the hot water faucet.

In line heaters have temperature fluctuations? So do all cows eat grass? No way; it depends. Some eat silage or alfalfa. I have have an instant-flow micro-adjusting water heater with no tank, built to make some 20 adjustments per second to maintain temperature when incoming water temperature or flow rate changes. Chronomite is one company that makes them. These ultra-small units install easily at point of use, and work well if a pre-heater tank varies in temperature. My tanks are easy to maintain around 80F with energy sources easy to control. I hope my systems last longer than my last investment in a 30 gal. tank hot water heater...

here is a nice sight with back up heating capacity.

Notice the household hot water is not sitting in the tank, it moves through a hot tank of water to be heated by it. I wondered how well that works I would like to build a small model of this to be sure.

http://fourmileisland.com/HotWater.htm

I am building something similar, based on this site http://www.builditsolar.com/Experimental/OffShelfDHW/OffShelfSolarDHWV2.htm

I did get a 500 gallon stainless steel tank for scrap price. I have the water loop in my woodstove. I am trying to find heat exchanger materials for a better price, but that is probably going to be my biggest expense if I go with the unglazed collectors.

Laura and R Scott: My first tank is a 300 gal steel propane tank discarded (and the gas roasted/burned out on a huge bonfire) it's still in use. It's really heavy, and needs a barrier to the air. My 2nd and 3rd tank systems are simple black poly. The hot water from my RMH firebox loop, and the water from my solar collector loop, are almost never above 140F when they arrive at the tank. These storage tanks are not sealed for pressure, they have none except gravity. When the tank approaches 90, a pex pump comes on to heat the floor with that contaminated water. The loops inside are pressurized, with uncontaminated potable water inside to draw out the heat to go to the inline heaters. With pex pipe coils and garden hose collectors, maintenance is not expensive. I have not yet shut the valves to replace any rubber hoses, might about every 5 years if I keep water flowing or shade them in hot weather so I don't let them boil.

David Graber wrote:The hot water from my RMH firebox loop, and the water from my solar collector loop

Do you have photos of your setup? Where did you put the RMH firebox loop, to pull the most heat from it, without affecting the RMH's operation (and risking a steam rupture)? I understand that the loops are open to the air in the tanks, so I gather that if the water is kept circulating, you don't have to worry about steam forming in the piping in the actual RMH?

Thanks,
Fred

Laura

This is not my line of work, but I believe the system you describe (as far as I understand it) will work fine, though in "normal" installations it also has an insulated DMH storage tank. IIRC this system is called "reverse indirect" and the water in the heating tank normally circulates through a boiler along with water that heats the radiators, floor, etc. If the water coming out of the exchanger coil in the heating tank (going to domestic use) is really hot and no DMH storage tank is used to control temperatures, a "tempering" valve is installed on the pipe to the plumbing fixtures to mix in cold water and control the temperature sent to sinks and showers.

DMH systems like this have been discussed extensively at www.heatinghelp.com and they have a solar section also IIRC; serious questions have received good replies in the past so if you wish specific comment after reading their prior posts it might be worth joining and getting some direct feedback. Most of the participants in "heatinghelp" are high end hydronic and steam heating contractors 20 or more years in the business. If you can connect with them there is huge bang for your time-buck. As a rule their heating systems use a lot electronics, but even for the off-grid crowd they are good place to gather concepts, gotchas, materials info and safety precautions.

One safety consideration I remember from my study a couple years ago is contamination of the DMH with boiler water (heating fluid) in the heating tank when your coils spring a leak. For this reason it's important to ensure the DMH pressure is always well above the heating fluid pressure. Usually this is not a problem but it's one of those details you really want to put in place. There are many others to be gleaned from Those That Know.

Rufus

"Do you have photos of your setup? Where did you put the RMH firebox loop, to pull the most heat from it, without affecting the RMH's operation (and risking a steam rupture)? I understand that the loops are open to the air in the tanks, so I gather that if the water is kept circulating, you don't have to worry about steam forming in the piping in the actual RMH?"

Loops are closed to the tanks; I try to keep the airhead in the tanks as small as convenient. There is no valve on the overflow; it's open to a drain. I could/should have designed it with a waterseal "U" to virtually eliminate recharging the airhead with O2, I eliminated check valves, so water can now circulate either way (and does so mysteriously) through the RMH loop. The third loop I didn't mention is driven by a pex pump to bring water from the top of the tank (where temp is highest) into the floor via three pex zones. The pex pump is driven via a simple 110v house Tstat that only has power when a stack Tcouple switch on my RMH on at 120F closes. there is no pump to drive the RMH loop. It works fine that way except when a leak in the outside solar loop from too much heat on the hose connections drains the tank so low that the firbox loop starts reversing like a swing and banging from little steam explosions (remember the old hotel steam heaters?)

Some photos are in our website www.greenwoodfarmmt.org. My firebox loop is S steel 3/4" sched 80 pipe custom welded for high pressure, with my design having six weld elbos, it cost over $200. It's not a BAD investment considering the possibility of failing check valves or obstructions, since a steam explosion is possible given the high temps in RMH firebox. That's my first installation. My third installation is still open to question. I'm working on a barrell lid for my 3rd RMH that will place a 1/2 copper coil just under the lid so the coil contacts the vent flow from the RMH. I hope these gases are not as hot as those in the firebox itself. Should I put a cap of 3/16 steel with a 1 1/2" gap above the top of my burner pipe? That would protect the copper from direct vent flow at this secondary burning location? Not sure it's necessary. But with copper, I need to make sure water is ALWAYS inside those coils.

I found this article at the legionella.org website:

http://legionella.org/about-the-disease/articles-on-legionnaires-disease/legionnaries-disease-contracted-from-patient-homes-the-coming-of-the-third-plague/

The authors are actually most concerned about immunocompromised individuals getting legionella from drinking, not from showering. OK, to be specific, they think the risky thing is legionella in the drinking water, which is then aspirated a little bit. (In other words, some water goes down the wrong "pipe.")

Several more references about sources of legionella contributing to illness here: http://legionella.org/publications/sources/ and you can also find data about transmission, treatment, etc at that website.

I'm working my way through all the podcasts, and I keep hearing about legionella making all these people sick. There are some good blood tests for legionella, it could be that they aren't given to people who actually have legionella, I don't know.