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