I recently purchased a new house with 2 water heaters. They are connected in parallel meaning that the the cold taps of both water heaters are connected to the supply and the hot outputs are connected together to go to the rest of the house.
I’m wondering which is the better connection method: the parallel one as described above or the heaters connected serially where one feeds the other? To me, the serial method would seem to be better. In the serial case, one could turn off the first heater in the series to save energy. If more hot water is needed for visitors or something, one could fire the other back up. In my parallel setup, I cannot turn one of the heaters off or half the water going up into the hot water pipes will be cold from the off tank. What are other people’s thoughts?
One note. The house also has a passive hot water circulation system which is a hot water pipe loop to the farthest bathroom. I would assume that in a serial setup the tank which the loop return is connected to would be the secondary or hotter tank. BTW, this passive system really works. We have nearly instant hot water. Not sure yet what it is doing to my gas bill though :(.
Thanks,
jim
Replies
I have done a few of these and always set them up in series. I like that method because you can set the first tank lower in tempature and use it as a pre heater. Your second heater will supply tons of water if the new mix water is already say 90 degrees. I also agree that the passive return would go to the second unit. But I am sure it would work going to the first unit.
One thought I always plumb the water heaters so with a couple of ball valves I can bypass either one. That way if one fails you are not screwed for hotwater. DanT
I'm a homeowner here, but really like this idea for a new house we're designing for ourselves.
It's a two-story on a lake that will be our primary residence, and we expect guests every weekend during the summer and infrequently the rest of the year. The ground floor will be used as our main living space and the second floor will have the guest rooms and guest bath.
I can see us having two water heaters in series, with the "feeder" heater upstairs set at a lower temperature and the main, larger heater downstairs. That way, a water heater that would normally not see much use (the upstairs heater) has a regular flow of water through it, which I think would be better for it (if I'm wrong, someone tell me), and it in turn cuts down on the work the main heater has to do. And during the summer, we can turn the temp up on that secondary heater for the guests and keep it turned down the rest of the year.
Does that seem feasible and a good application for water heaters in series?
Leigh
I guess that would work, but why put them so far apart? Why not just put them both in the basement (or downstairs) and run a passive loop to keep hot water quickly available throughout the house?
I do mainly repairs and remodels so I don't get the oppurtunity to design from scratch. My work is usually retro fit to fix a problem. In all the series systems I have installed they were in small apartment buildings that had hot water shortages and this was a sound method of repair. DanT
Now, you see, that kind of thoughtful reply is why I spend so much time here on Breaktime!
I guess my thought would be that by putting the second hot water heater upstairs, the source would be close to the application when it was needed, and other times it would just feed the main water heater (which would be almost directly below it), giving us a higher volume of hot water all the time.
If we ran a passive loop up there, would we spend a lot of energy circulating water to a location it wouldn't often be used? Or would it make that much of a difference?
Leigh
"If we ran a passive loop up there, would we spend a lot of energy circulating water to a location it wouldn't often be used? Or would it make that much of a difference?"
You will use more energy with a recirc system because heat loss through the pipes is going to be much greater than heat loss through the water heater jacket. This is due to the far greater surface area(to volume) for the pipes. Insulated properly, however, the net losses through the pipes may not amount to much overall, and you will waste less water.
You can get a pumped recirc system with a timer and a thermostat to save energy, though it costs more to install than a passive system, and uses some (not much) electricity. Tradeoffs.http://www.grundfos.com/web/HOMEus.NSF/Webopslag/37829454ADF8A525C1256C3D003D8AD8
Edited 9/22/2004 2:50 pm ET by csnow
"I can see us having two water heaters in series, with the "feeder" heater upstairs set at a lower temperature and the main, larger heater downstairs. That way, a water heater that would normally not see much use (the upstairs heater) has a regular flow of water through it, which I think would be better for it (if I'm wrong, someone tell me), and it in turn cuts down on the work the main heater has to do. And during the summer, we can turn the temp up on that secondary heater for the guests and keep it turned down the rest of the year.
Does that seem feasible and a good application for water heaters in series?"
I do not think the cost/benefit ratio of such a setup is very good. The cost and complexity is greater, while the net energy savings (if any) would be minimal.
A single larger water heater is not going to use much more energy than a single small water heater. Standby losses may be very slightly greater with a big tank, but not nearly enough to fund the labor and materials for the 2 tank setup. If you put some of that savings into a single more efficient water heater, or one with a better jacket, you would come out way ahead on the deal.
Worse, installed in series, you would still have standby jacket losses in the 2nd water heater, even if it is not on. After running the hot water, the second tank would have 'stranded' hot water in it. It could cost you MORE energy.
With a larger single tank, you can still turn up the temperature when you have company to gain more reserve capacity.
Thanks for your response. Those are good points--particularly the cost/complexity issue you raise.
I will keep thinking on this one.
Leigh
I don't agree with the theory of a single tank and boosting the tempature when company arrives. Anything above 125 degrees is in the scald zone. If you don't worry about the liability issues you surely should worry about your friends or loved ones getting burnt.
A passive loop as stated above has only a check valve for a moving part so the issue of energy use is limited only to the water that is being recirculated back to the tank to be reheated. In my mind a small price to pay for nearly instant hot water. Also this system saves money in the water savings of not running the water for lengthy periods to get the water hot.
Another simple system for instant hot water and increased capacity is to install a small electric unit in the upstairs in a closet. Maybe 8 gallons or so. It will give again nearly instant hot water and increase the capacity of water also. Easy to hide and can be wired to a switch to turn off when not in use. DanT
"I don't agree with the theory of a single tank and boosting the tempature when company arrives. Anything above 125 degrees is in the scald zone. If you don't worry about the liability issues you surely should worry about your friends or loved ones getting burnt."
That's why they make anti-scald valves. If you want to set tank at 125 degrees, you're going to need a larger tank than you would have before the whole anti-scald movement took hold.
You are correct that an anti scald is a possible solution. They along with mixing valves are not a favorite in my area as even the city water here has high ph and lime build up so installing either is a self created maintenance nightmare. Your results in your area may be better. If so, great. DanT
Just wanted to throw a quick note out there about valves and water heaters. If you have a valve on the supply and exit side of the water heater and turn them off but forget to turn the water heater itself off you run the risk of having a water heater explode or launch itself thru your roof. Sometimes those pressure relief valves don't work.
Probably series is better, except in the odd case where you want to minimize flow restriction.
In your case I'd just put shutoff valves on both units so you could disconnect one when you didn't need it. A lot simpler than replumbing for series. If you do this, and use only one heater, it would probably make sense to leave the "off" heater's valve open just a crack to prevent stagnant water problems. (Note that a ball valve wouldn't be the best choice for a shutoff valve in this case, as it's apt to erode when used in "open a crack" mode. Best would be a ball with a separate small globe paralleling it, but that gets awfully complicated.)
I looked in my AOSmith water heater manual and they very strongly suggest using the parallel approach rather than the serial approach. The arguement is that the first heater (the pre-heater) will work much harder and will condense a lot more, hence it's life will be shortened considerably.
Bill
I have two installed in series for the reasons stated above. I believe the post stating that the unit used for most of the heating will fail earliest. I don't see this as a problem, especially if you have the parallel valve arrangement to allow changing out separately while still using your other heater.
Out of curiosity, is there an educter type fitting in your passive recirc loop? My first plumber (later fired for other problems) installed my passive loop but installed it completely level and told me it would work fine when I asked him about it. It didn't work at all so I installed a thermostatically controlled recirc pump, that works pretty well. I'm just wondering how it was supposed to be done.
I don't know what an educter type fitting is. This appears to be a simple loop heading off to the farthest bathroom on the first floor. It is running level along the first floor floor joints. It is 3/4" pipe out and 1/2" pipe back. The return connects to the drain valve at the base of one of the water heaters. It does not have a pump and is said to work only by convection unless a tap is open. It also has a valve to turn off the recirculation. It works surprisingly well. With the passive recirc on, the water is hot in 3-5 seconds. With the valve closed, it takes 30 secs to get hot water in the shower.
I like this idea for our kitchen, which is the farthest application from the main water heater (it will be in the laundry room and next to the master bath--located by 2 out of three major uses isn't bad, I figure).
Anyone know what this typically adds to the monthly water-heating costs? I'm not looking for a specific dollar amount--I know this would vary by setup and by region--I'm just trying to get a feeling of the percentage. For instance, does it doubles it? Add just a little bit?
If you have this, or have installed it, would you do it again in a heartbeat?
Leigh
I have a kitchen like yours with a long run from the HWH. Gravity circulation loops only work with a change in elevation, and I had none. My solution was to add a little electric tank for the sink only, works great, instant hot water.
"Out of curiosity, is there an educter type fitting in your passive recirc loop? My first plumber (later fired for other problems) installed my passive loop but installed it completely level and told me it would work fine when I asked him about it. It didn't work at all so I installed a thermostatically controlled recirc pump, that works pretty well. I'm just wondering how it was supposed to be done."
I do not know what an 'educter" is.
The conventional gravity recirc system has only a check valve for moving parts, which is there to force the water to flow the proper way when there is a call for hot water. The piping does not need to be pitched towards the tank, but the tank does need to be lower than the topmost part of the gravity loop.
Set them up in series..
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Jan - I helped a client whose 4 room B&B ran out of hot water way too often for their guest's comfort. The 90 y.o. house had a regular sized non-commercial gas-fired HWT in the basement. The flue was already at full capy as the chimney (lined) was venting both this HWT and a substantial boiler dedicated to hot-water radiators.
The electrical panel, on the other hand, has capy in reserve...
So I series-plumbed a new residential (60gal) electric HWT, complete with labelled valves (and a corresponding instruction sheet stapled to the wall). In case of breakdown, the owners could operate either heater separately, in safety. As others here have suggested you do, I set it up so the gas heater pre-heated incoming water, while the electric heater increased and maintained. The B&B has yet to run out of water, and that was 6 years ago.
Never done one of those gravity return loops, but would love to do so if the opportunity arises. Something for close to nothing. Plumbing stuff and the laws of thermodynamics is pretty neat, in'it.