My plumber said he’s been installing a lot of setups with (2) 40 gal. heaters in series.
His rationale is:
40 gals are so cheap,
he runs one on vacation setting and the other super hot, and somehow maintains there’s an enrgy savings there over a traditional 80 gal,
if one ever gives out you still would have hot water.
I remember reading something in JLC (I think) about recent concerns over water heaters kept too low and breeding a bacteria, but he thinks the second tank would eliminate this posssibility.
Have any of you run across this recently?
Replies
Yes, I've seen it done. No I haven't seen it done with one lukewarm and one superhot. They were both set to normal temp and worked very well. No energy savings, because 2 heaters have more surface area than an 80-gal so lose more heat. Just cheaper to buy.
Don't know about bacteria, but superhot in one tank is a scalding hazard.
DG/Builder
I do a lot of multiple tank systems, but about 95% of them are in paralell.
Reason is more for volume output verses how much hot water storage.
A lower temp water heater can breed bacteria in an unpure water condition, which a second tank set less than 150 degrees will not kill.
Unless your system sits idle for very long periods you really don't have much to worry about.
Yes you will have hot water if one tank fails, but on a series system if one fails you will have 40 gallons of storage that is not heating. No problem if it's the first tank, but if the second tank fails you need to have a circ pump on the system to maintain temp when in idle conditions.
Liberals thank those that beg for peace. Conservatives honor those that fought for freedoms. BDW
Would not be an energy savings, probably waste energy because, like the other poster said, you've substantially increased your surface area.
Bacteria are becoming recognized as a problem. Canada, I believe, has codes specifying minimum temperatures for water heaters. It's around 140°F, I believe. Anyone who knows can correct me. Having two water heaters won't protect against this because when you have heavy usage you just draw the contaminated water through the other water heater so fast the unit can't heat it up to a level that will disinfect it.
As for one giving out, if it's the hotter one you guarantee you are getting contaminated water. Besides, if the heater gives out you can install another the same day or the next. Even a few days without hot water is no big deal.
I don't know about the Canadian code, but Legionella is more apt to grow in hot water systems where the temperature is less than 140. Water temp is only one risk factor for Legionella growth, of course.
A quick google searh is showing recommendations in Canada (Quebec) of 140 F (60 C).
That high of a temp is a potential scald hazard, though especially for kids.
So the solution mis run it hot BUT install tempering valves.
http://www.hydroquebec.com/advice_hot_water/index.html
If bacteria growth is a concern on vacation setting... shouldn't I be concerned with bad bugs growing in my cold water?
Not really.
A few things
1. you need bad water to begin with
2. remember highschool science & petre dishes
3. you need some oxygen in the mix.
If I was leaving a cabin for a long time & shut off the WH I would give it a good flush when I got back to the cabin, same with the cold water too."can you cross your legs were short a nail" Pontius Pilate
No. The optimum temperature for the growth of the organisms in question is over 100°F.
>>he runs one on vacation setting and the other super hot,
He might be interested to know that plumbers have been held liable for damamges for scald injuries when they leave a water heater set too high.
Maybe it makes sense, maybe not, but that is the reality.
Absolutely, 140 degrees is a scald hazard. Not just kids, either. Elderly folks are more apt to be scalded because they don't sense the high temp as quickly. As you suggest, I wouldn't set the heater temp that high with out tempering valves.
Temp
Exposure Time For a Third Degree Burns (The most serious)
160
Less than 1 second
155
1 second
148
2 seconds
140
5 seconds
133
15 seconds
127
1 minute
124
3 minutes
120
5 Minutes
If I am not mistaken, Vacation setting just inhibits the main burner, but keeps the pilot going.
I don't see anything to be gained except for the extra fees to the plumber.
And 80 gal is not that common.
Much more common would be 50 gal and if it is a high recovery rate then it can produce even more HW.
Edited 8/5/2006 11:42 am by BillHartmann
85 gal. doesn't seem uncommon, at least in gas. I had one installed at our house.
Another reason to use parallel rather than series is that it balances the load. In a series arrangment one heater is working hard (the first one) while the second one only has to work if the first is exhausted.
If you go with parallel, try to equalize the outlet pipe lengths so that water is drawn equally from the tanks.
Scott.
Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.”
There really is no good reason to put heaters in series. Put them in parallel and forget about it.
"With every mistake we must surely be learning"
40 gal electric water-heaters with set-back thermostats in front of 60 gal gas units are what I see a lot in homes with multiple teenagers. The electric unit goes on early in the morning and ensures that there's enough for everyone in the family to get a shower, goes off at 8 or 9, and then may come back on for dinner or bed (it can be manually triggered for special occassions).
Phill Giles
The Unionville Woodwright
Oddly enough, I've seen a few such systems installed. The main difference from what you describe is that the second heater usually wasn't identical to the first one.
The second heater was usually called a "booster" heater, and was intended to deliver a lot of heat, fast.
Your heat losses are related to, among other things, the temperature difference to the inside of the tank, and the outside. This is an exponential relationship; double the temp diff, and you have four times the loss.
There is also the matter of mixing the hot and cold water. I think, given a choice, I'd rather raise the temp in stages, rather than in one big step.
I suppose I can imagine two situations where this "series" arrangement would really shine: "point of use" and "loop" systems.
"Point of use" being where the second heater is close to the final user.
"Loop" being where water is constantly circulated, so as to reduce the wait for hot water to arrive at the faucet.
Which system you choose probably depends on what problem you're trying to solve. As I see it... everything being equal... "parallel" gives you more hot water available, while "series" gives you a quicker recovery time.