Tankless Booster for Tank? - Help

[Renovating]

Does anyone have experience or advice about using a tankless water heater to boost a traditional tanked water heater? We are thinking about installing a small- to mid-size electric thermostatic tankless water heater downstream of an electric 85-gallon traditional tank water heater. We are trying to maximize the availability of hot water for numerous low-flow showers in quick succession when we have guests in our north Florida vacation condo. Our thinking is that the tanked heater would do most of the work, with the tankless only kicking in, as needed, to pick up the slack as the hot water from the tanked heater is depleted.

The condo is in a 30-year-old mid-rise building. Natural gas is not available and our existing 150-amp box only has the capacity to handle another 50 to 60 amps in addition to the existing 30-amp circuit for the tanked water heater. Increasing our electrical capacity is not feasible due to the distance and the way the service cable was routed. For that reason, using a larger whole-house tankless, or multiple point-of-use tankless heaters is out of the question.

Will this set-up work? Will the tankless heater reduce or restrict the flow or pressure of the hot water in any way? Is there anything else we should be thinking about? Thanks for any help.

The water heaters we are considering are the following (or similar models):

TANKLESS:
Eemax “SERIES TWO” Model EX144TC (Twin Module) With Single Thermostat
Hot or cold water feed
Turns on in stages based on hot water demand- (showers, etc.).
Min. flow, 0.7 GPM.
Adjustable, Precise Temperature Setting +/– 0.5°F
Anti Scald Protection
Ideally suited for any booster type application or as solar backup
Volts kW Amps Recommended Wire Size
240V 15kW 64 (2x32)A 8AWG per module
Temperature Rise (degrees Fahrenheit)
65 @ 1.5
51 @ 2.0
39 @ 2.5
34 @ 3.0

TANKED:
Rheem/Ruud – Marathon – 85 Gallon Electric – Model MR85245
First Hour Rating – 90
Recovery in GPH @ 90 degree rise - 21

[master plumber mark]

Do you have room for another water heater next to the 85 gallon unit???

In my opinion, only, putting a 50 gallon electric in series with your 85 wouild be a better course of action because it would probably cost less and be far more reliable over time...

also when you dont have company, youi canalways turn it down of off it you set it up right....

the tankless units are nothing but trouble and usually costs quite a lot more than a normal heater.... just my opinion.

read these links about them

http://www.askthebuilder.com/451_Tan...ng_Facts.shtml

http://www.pmengineer.com/CDA/Articl...141364,00.html


http://www.waterheaterrescue.com/pag.../tankless.html

[Renovating]

Thanks for the quick response! Unfortunately, space is tight and there is not enough room for a second traditional tank. This is our second home, not our primary residence. We are there a couple of times a month and mostly it is just the two of us. However, when we have 2 to 4 guests (mostly in the summer) the existing 50 gallon tank runs out of hot water pretty quickly and takes too long to recover sufficiently when everybody wants to take showers in a relatively short period of time.

We have done a lot of online research on electric tankless water heaters (in fact, had already seen the three informative sites whose links you provided, thanks) and are aware that they definitely have limitations, particularly the smaller ones. However, given our condo's limitations and what we are trying to accomplish, installing an 85 gallon tank plus small- to mid-sized tankless in series seemed to be our best and possibly only option. We know the initial cost will be relatively high and we are not looking for savings on our utility bills. We're just wondering whether such a combined installation will actually work adequately for what we want it to do, or if we would be going to a lot of extra trouble and expense for nothing. We're also concerned whether it would cause any problems, such as with water flow or pressure, that we should be aware of.

We would be grateful for any help or advice you can provide. Thanks.

[mrjetskey]

I had one of the tankless electric hot water heaters about 12 years ago.<when it worked right it was great> but there are 3 things to consider,
1) it had 4 5500 watt elements,pulling as much as 2 electric hot water heaters
2)it had double heavy gauge wires going to it ,I have 200 amp service So when 2 showers were on it pulled a total of 22,000 watts,one shower 2 elements on 2 showers all 4 came on.
3) They have a lot of problems burning elements out if there is any minerals in your water.I recomend a water softner.also I had to replace the circuit board once.
4)Finally got tired of it not being reliable and replaced it with a tank type heater.
For your solution I would recomend a second small hotwater heater perhaps 20 gallon under cabinet .But if your home is all electric and only 150 panel you might be cutting it close if you had central heat and oven on at same time.Perhaps your best solution is to find a 75 gallon single and turn off when no one is there.

[hj]

Typically, as the flow increases through a tankless unit the pressure loss becomes very high which reduces the volume and unless your shower valves are pressure balanced the shower temperature will become erratic. If you do have pressure balanced valves, then the flow could be severely diminished as the hot volume drops and the cold is reduced to match it.

[master plumber mark]

Do you now have an 50 gallon electric water heater or do you have an 80 gallon wter heater?????? I am no tsure at this point what you are presently stuck fighting with.

If you only have a 50 gallon electric water heater,



just try putting in an 80 gallon electric water heater....if you got the room.

you can even get a 100 gallon residential unit from Rheem ...if you got the room

Then for extra life, install heavy duty steel elements or "sand hog" type elements.

an 80 gallon heater has lots more bang for the buck--lots more than a 50......

try that first before going crazy and installing that tankless type.....



also, you can turn the elements up to a higher setting---

lets say try 135 and you will definitely not run out with an 80.

if you got pressure balanced tub and shower faucets , it shoudent be any cause for alarm with too high a hot water setting.


actually it would be far better in the long run to install pressure balanced shower faucets and then the 80 gallon on 135 than to fool with that stupid tankless unit.

[Renovating]

Thanks for the input.

The tankless models we are looking at use electronic thermostatic controls that turn on and off incrementally or in stages based on hot water demand. They don't use mechanical flow restrictors.

As I understand it, in our proposed application, when the tanked heater starts to run out of hot water and the temperature drops, the tankless heater would kick in enough to maintain the desired temperature. If showering contined long enough that the tanked heater was totally depleted and suppling only cold water to the tankless, it is possible that the tankless could not provide the necessary temperature rise per GPM being demanded. At that point, the output from the tankless would become cooler, but the flow would not be restricted, which is the same thing that happens when you deplete a tanked heater. The purpose of the tankless is to postpone the inevitable and based on the specs, it looks like it could probably handle a single low-flow shower by itself, meaning individual, sequential showers could continue indefinitely.

The problem with pressure balancing valves might be a little trickier. It wouldn't be a problem if the tankless by itself can supply enough hot water for a single low-flow shower. Even without the tankless in the mix, wouldn't the pressure balancing mechanism create the same flow restriction problem with a tanked heater that is being depleted?

[Renovating]

We have an existing 50 gallon tank in the condo now, but plan to replace it with an 85. We wanted to add the tankless to extend the number of showers that could be taken within a short period of time.

Our experience with the 80 gallon tank in our primary residence is that it can handle about 3 good showers back-to-back before the hot water begins to taper off, and it takes a while to recover. At the condo, we sometimes need to accommodate 6 or more showers in short order when everyone comes in from the beach, or gets ready to go out or go to bed.

I As I said, most of the time, it is just the two of us there. We liked the idea of having the tankless available when we needed it but not using it unless we had to. Or, on the other hand, if it was just going to be the two of us for the weekend, using just the tankless and not cranking up the 85 gallon tank at all. We have low-flow shower heads, we wash most of our clothes in cold water, the Bosch dishwasher we are putting in has its own built-in flow-through water heater. We mainly use hot water for showers and the sinks in the bath room and kitchen and rarely run two big hot water users at the same time.

[master plumber mark]

try the 80 gallon electric water heater, or the 100 galllon. first before doing the tankless appproach....

you can always spend more than what the hundred gallon unit will cost you for a tankless unit later down the road if you absolutely must get youirself into trouble.

somethinig else you can do , just get shower heads that have a low flow on them that wont waste so much water. that will certanly help too.

turning up the units a littel bit and installing the low flow shower heads will probably be more than enough for a large amount of poeple.....

[SteveW]

I think Mark is right on the money. I had considered replacing my gas WH with tankless but rethought the whole thing after reading some of Mark's posts (as well as reading in Consumer Reports that tankless units do not really save much energy).

It's just not a "mature" technology yet -- maybe in 5-10 years they will have worked out enough of the bugs and we'll all be buying them...

If you put in an 85-gal unit, and have the temp set high enough (say 135), and use 2.5 gpm shower heads, you'd have at least 45 minutes of shower time, by my calculations.

[hj]

The heater does not have to have a "flow restrictor". The coil in the heater has to have small enough pipes so that the maximum amount of water is exposed to the heat. The pressure loss will occur in the coil as the volume of water increases. Most manufacturers show about a 40 psi pressure loss at the maximum flow rate.

[master plumber mark]

dont forget to calcualte the drop in pressure after the coils start to lime up with calcium.......
of course then all ya got to do is install a water softener to slow down that problem.

then all ya got to do every year or two is to tear apart that coil in the tankless heater and somehow run vinegar through it to "de lime " it

i dont know how long that takes, and how mechanicallically inclined you might be to tackle that job either

that really dont sound too hard to do does it???

I have never done it , and it sort of scares me, (liability issues and all)

but they say in the infroamtion from the tankless people that its a very easy thing to do.....probably a lady in a nice white dress could do it...

and you should always believe everything you read, right??


Especially from the people pushing tankless water heaters and what they claim will work.

[hj]

And save you a boatload of money while doing it.

[jadnashua]

All of the tankless systems I've seen (that may not be many) rely on some restriction to attain the prescribed heat gain across the heat exchanger. Think moving your hand across a candle flame. Move it fast enough, even though the flame remains a constant temp, your hand hardly knows it went through. Do it slow enough, and you'll burn your hand. There are two ways to transfer the heat, slow it down, or increase the amount of heat supplied either by lengthening the heat exchanger or raising the temp of same. To provide high flow AND sufficient heat transfer, you need MAJOR amount of heat source. If electric, then think about expanding your service panel, if gas, make sure you have enough makeup air, or use a closed combustion system, as a typical basement just may not have enough volume to support actually turning the thing on without creating an unheathly and dangerous vacuum (i.e., burning more air in the house than can be supplied by leaks or vents in the house). By their nature, all tankless systems restrict the flow in one manner or another. Check the temp gain/volume. Then, note your normal incoming water temp. At say 1 gpm, you might get 100 degrees rise, at 2 gpm, maybe 80, and at 3maybe 70, or less. The more water you need, the less heat can be transferred, and the less rise you'll get. My local utility company is offering a $300 rebate to install a new tankless system. If I haven't been living with one for awile, I'd be tempted. I'm going to replace mine in the next few months with an indirect heater. Good luck in your decision.

If you have a hydronic heating system, have you considered using an indirect heater for your hot water supply? They have (typically) much faster recovery rates than either standard gas or electric. A small 40 gallon SuperStor is rated at something like 150 gallons in the first hour. That's 2.5 gpm continuous for an hour - a typical showerhead's worth. With the 40-gallon tank, you have a much lower standby loss, too. And, the tanks are typically stainless steel, and you may never have to replace it in the relative future - your boiler would probably die first.

My unprofessional opinion.

[mrjetskey]

My tankless electric had a total of a 1 1/2 gallon tank split into four connected compartments,with each compartment containing one 5500 watt element.there was no restriction to the electric the water flowed into section one was heated then into 2 etc.each element would kick in if it was needed,so with a low flow only 2 elements on higher flow all 4 on.with the water flowing over each element before it left the heater.When it was working it was awsome!!!We had it in our lake home so lots of showers at same time.But it wasnt quite perfected and extremely hard to get parts for.I want to say it was called an aurora.But that might have been my wifes car.It was almost right,it didnt save money as the electric meter really hummed when all 4 elements were lit up.But it just missed on reliability!!