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Old 10-14-2008, 07:30 PM   #1
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Tankless or tank, but not both


Let's say I have an electric tankless and an electric preheat tank water heater. I don't, but let's say I do I am concerned about the massive amperage draw of the tankless in combination with the tank, so I would like to wire them such that the tank element is switched off when the tankless is running; similar to the way the upper and lower elements can't be on at the same time.

Sound reasonable?

This would be part of a solar set up, just in case what I described makes little sense...

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Old 10-14-2008, 09:01 PM   #2
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Tankless or tank, but not both


This can indeed be done. It would involve having a relay and contactor. The tank power would come into a contactor rated for the load. The contactor coil, in turn, would get it's power through the normally closed contact of the relay. The relay would be wired into the heater circuit of the tankless. When the tankless kicks on, it also kicks the relay on, disconnecting power to the contactor, which opens the tank water heater circuit.

If you need more specifics, or a diagram, let us know. Also, I imagine such a setup would void your tankless' warranty.

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Old 10-14-2008, 10:32 PM   #3
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Tankless or tank, but not both


Would such a relay work with 110 or 220 volt AC power?
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Old 10-14-2008, 10:42 PM   #4
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Tankless or tank, but not both


Quote:
Originally Posted by gp_wa View Post
Would such a relay work with 110 or 220 volt AC power?
Depends on the relay. If you have a 120 V relay, then you'd need a neutral present in the supply for the tankless. But with a little surfing, you could probably find a 240 V relay that would be ok.
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Old 10-15-2008, 11:11 AM   #5
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Tankless or tank, but not both


If I was going to preheat water, I would use the solar method and eliminate the tank. No way would I have both.
If you are considering a tankless system. Think twice about the initial cost of installation and how long it would take to recoup the cost.

Tankless water heaters are much more efficient if they are positioned at the point of use. Such as a kitchen sink/dishwasher or far away bathrooms ect...
I would not use whole house tankless systems.
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Old 10-15-2008, 11:36 AM   #6
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Tankless or tank, but not both


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Originally Posted by J. V. View Post
If I was going to preheat water, I would use the solar method and eliminate the tank. No way would I have both.
That's how solar pre-heat works, with a tank.

If you have both (or even two tanks), you can take advantage of solar even when there isn't enough sun light to get the water up to set point.


Quote:
Originally Posted by J. V. View Post
If you are considering a tankless system. Think twice about the initial cost of installation and how long it would take to recoup the cost.

Tankless water heaters are much more efficient if they are positioned at the point of use. Such as a kitchen sink/dishwasher or far away bathrooms ect...
I would not use whole house tankless systems.
I would agree on principle, but that's potentially a lot of tankless heaters, which equates to a very high initial cost and potentially very high momentary power draw.
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Old 10-15-2008, 12:48 PM   #7
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Tankless or tank, but not both


Let's say that you have a solar hot water system, not that you do but let's just say you do.

And the goal here is to eventually heat your hot water for free from the sun.

What you need is a heat exchanger, not a tanked water heater. The problem is that most solar tanks have heaters built into them. This doesn't mean that they have to be hooked up.

You are on the right track with your thinking but let me steer you in a slightly different direction.

If it is going to be a sunny day and you want to heat some water but the water tank is already heated from the electricity then there is no water to heat. So the sun just sits there heating your water in the panels and not doing anything else.

The sun goes down and you come home to take a nice hot bath. You turn on the water which comes out of the tank. Since the sun is not up the electricity goes on to heat the water in the tank.

So far your panels have done nothing.

BUT,,,,,,

Lets say that you have a heat exchanger that is NOT hooked up to the electric system. Now the sun comes up. It heats the water in the panels and the system kicks on and pumps the water into the heat exchanger. It heats up 120 gallons of water to 140 degrees.

Now you come home.

You go to fill up the bath and the water comes out of the tank and is not replenished because it does not have a current power source since the sun has set. Even though your bath is full there is little hot water left and it is down to 80 degrees.

But you anticipated this and have a backup system. It is a tankless water heater that will heat your water when you want to take a bath and there hasn't been any sun for 3 days (remember you live in Seattle).

You still have hot water.

When the take is full of hot water it still leaves the tank and goes through the tankless. But being that it is already hot the tankless does not kick on. Even if the water is at 80 degrees it will not take as much energy as warming water that is 55 degrees. You still have some savings.

If you really want to see some money in the bank go with two 120 gallon heat exchangers.

With this setup you really have a savings on hot water versus the first set up that has very little savings if any considering the cost of the setup.

This is how my vacation home is set up. Only I have almost a 1000 gallons of hot water. There is little chance of me every running out of hot water.

And if there is extra hot water being produced and it is winter I run that hot water through the house and heat that also.

Does this make any sense or am I getting senile in my old age????
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Last edited by Marvin Gardens; 10-15-2008 at 12:54 PM.
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Old 10-15-2008, 01:15 PM   #8
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Tankless or tank, but not both


I read the original post as using just photovoltaic panels, because of the concern over Amp draw... If I'm wrong, let me know gp_wa.. Just trying to clarify.
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Old 10-15-2008, 01:33 PM   #9
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Tankless or tank, but not both


Makes perfect sense and it sounds as if you really know what you are doing and saving money. I interpreted the OP saying he was using a regular water heater for pre-heat and a tankless system at the same time.
As far as the "point of use tankless". The reason I like these are due to instant hot water, thus saving some in regard to volume of water consumed. We are in serious drought conditions here in SC. and water is much more scarce than electricity it seems.
I know very little about solar power and solar heating as you can see. But it is something we all should look at. Energy independence is of great importance. And you are providing an excellent example....John
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Old 10-15-2008, 01:40 PM   #10
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Tankless or tank, but not both


Don't hook up the electric heaters of the tank.
Just use the hydronic solar panels to heat the water.

The elements of a tank, and the elements of a tankless, use the same amount of electric to heat the same amount of water to the same temp.
So you get no gain having both elements function.
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Old 10-15-2008, 01:47 PM   #11
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Tankless or tank, but not both


Marvin, you described almost exactly what I have in mind. Except for the part about 1000 gallons of hot water.

The difference is, because my house is all electric, I want a smallish tankless (under 100 amps) that will not keep up with heavy demand if it's fed with ground temp water. Even when solar is not working (at night, on dark cloudy days, etc.) I need to maintain warm water in the tank. I just don't want the tank and the tankless going full blast at the same time, which might occur during the tail end of filling the bath tub.
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Old 10-15-2008, 01:48 PM   #12
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Tankless or tank, but not both


Quote:
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I read the original post as using just photovoltaic panels, because of the concern over Amp draw... If I'm wrong, let me know gp_wa.. Just trying to clarify.
No. No photovoltaics in this scenario.
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Old 10-15-2008, 02:03 PM   #13
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Tankless or tank, but not both


Quote:
Originally Posted by gp_wa View Post
Marvin, you described almost exactly what I have in mind. Except for the part about 1000 gallons of hot water.

The difference is, because my house is all electric, I want a smallish tankless (under 100 amps) that will not keep up with heavy demand if it's fed with ground temp water. .

You would need a commercial water heater, to get one that uses 100 amps.

Standard resi unt is 4500 watts. or less then 19 amps at 240 volts.

The idea of a tankless. Is to save money on teh standby loss of a tank system.
Using a tank system with heaters and a tankless, defeats the purpose of the tankless.
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Old 10-15-2008, 02:10 PM   #14
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Tankless or tank, but not both


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You would need a commercial water heater, to get one that uses 100 amps.
I was talking about the tankless... I'd probly run a 5500 watt element in the tank, that's just the way I roll

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The idea of a tankless. Is to save money on teh standby loss of a tank system.
Using a tank system with heaters and a tankless, defeats the purpose of the tankless.
Not if you use solar most of the time to maintain tank temperature. If the energy you heat the tank with is free, who gives a hoot about standby losses? I'm pretty sure I can maintain at least 90* in the tank with solar almost all year round here in WA.
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Old 10-15-2008, 02:18 PM   #15
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Tankless or tank, but not both


Thats why I said tank heater with elecments.
A 5500 watt element, would give you roughly 1 gallon a minute at a 35F temp rise.(125-90 tank temp)

So you may want a higher rated element, for those days that the sun isn't out enough to maintain a tank temp of 90.

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