Tankless in series with Traditional Water Heater?

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I have a 50Gal hot water heater under my house serving 2 bathrooms and four people. I am looking for a safe way to prolong the availability of hot water when two or three people take a shower in a row. I was thinking about putting an inline water heater before the 50gal tank heater. I don't want to buy a big inline heater, as it's just going to be for stretching out the hot water available during a 40 minute period. The problem I see is the max flow through the inline unit will be too small. Do they make inline units that will flow at arbitrary rates, and just heat the water less when flowing fast? This would be ideal for me. Otherwise, could I split the flow (in parallel) going into the tank water heater: half through the inline unit, and 2) half through some restrictor valve to even the bias?
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You could get a tankless to do the whole job, a Takagi or Rinnai then you can realy waste water and let everyone take 40 minute showers, since cost is not an issue for you.
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Go tankless.
cm

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cm wrote:

AND HAVE TROUBLES! Low flow no hot water, power failure no hot water, need to upgrade power or gas lines and perhaps still no be happy:(
Another option is a tempering valve.
you set the existing hot water tank to HOT and install a tempering valve at the outlet.
it mixes hot with cold water so you cant get scalded. acts like a larger hot water tank.
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a tankless before your hot water tank will help, low flow doesnt matter since you only need the extra capacity during high flow periods.
at low flow times the existing hot water tank can easily keep up with demands:)
I ended up buying a high BTU tank. its 50 gallons at 75,000 BTU. This solved my running out of hot water trouble.
I always shower before laundry but then we have 2 washing machines, so demands can get high........
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why would max demand be low flow?
max demand is when the tankless will be of most help?
are you talking electric or gas?
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My concern is that my current tank water heater will let through, lets just say for argument sake, 10gpm, and by putting an inline in front of it, the inline will only let through, say, 3gpm, thus my whole hot water system will be throttled back to 3gpm. I want to know if small inlines are limited to a certain flow, or if the flow rating is just a number that guarentees full heating if the flow is not higher.
I don't want to buy a full size inline water heater since i have a new regular water heater, and I only want to buy a small inline to put in front of it,to save money on equipment costs, and to boost the hot water system a little.
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snipped-for-privacy@gmail.com wrote:

------------------------------------------------------------------------------------------- The way I understand it -- and I might be wrong so I'm open to comment -- is that an tankless unit has a rated flowrate that indicates its maximum efficiency. IOW, it might be rated at 3gpm, but will actually allow 10gpm to pass through it -- just not as hot.
I THINK I know what I'm trying to say here -- hope it makes sense to someone else.
Mark
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Mark, that is exactly what I want to know. if you are right, then I should be able to put a small, single room, in-line unit before my hot water heater, and during peak hot water usage, still have enough hot water for everyone to take showers. I wonder how much more in electricity this type of set up will run me?
Mark Sparge wrote:

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snipped-for-privacy@gmail.com wrote:

I'd probably be tempted to go a bit larger than the minimum, just because of the increased flow-rate capability. Any amount you can raise the temperature into the big tank will be a savings -- though there may be some point of diminishing returns. You will probably raise your electric bill some with the tankless, but certainly less than if you put another tank-type in line with the existing heater.
Marrk
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snipped-for-privacy@aol.com wrote:

I have a tankless unit that makes it own power to start the flame, it needs no power what so ever from external sources, there is a wheel that spins as water passes by it producing the spark needed to start the flame. As long as the city is still pushing water I'll have endless hot water. I bought the Bosch Aqua Star. Great product in my opinion.
Rich
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You might look into a greywater heat exchanger, or check out
http://www.sunfrost.com/efficient_shower.html
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

The whole point of putting a tankless in series with a tank type heater escapes me. Once the 50 gals of hot water in the tank is gone, for the tankless to be of any significant benefit, it's going to have to have enough capacity to pretty much heat the incoming water up to full temp. This means a gas fired unit of sufficient capacity. Once you have that, what's the point to having the tank heater too? It just defeats one of the main features of the tankless, which is no standby loss of heat from the tank.
And forget the greywater heat exchanger, as you're never going to recover enough heat from tepid water going down the drain to heat incoming fresh cold water anywhere near to the point of it being usable. Just more pie in the sky ideas, without regard to the real world.
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snipped-for-privacy@optonline.net wrote:

You sure about that? The DOE seems to think it works.
I just did a quick google and one of them (GFX) claims over 60% heat extraction from drainwater. If you've got hot water from a shower going down the drain that could make a difference even if you assume that they're exaggerating.
Apparently it can preheat the water coming into the water heater by 20-30 degrees.
Chris
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"The whole point of putting a tankless in series with a tank type heater escapes me. Once the 50 gals of hot water in the tank is gone, for the tankless to be of any significant benefit, it's going to have to have enough capacity to pretty much heat the incoming water up to full temp. This means a gas fired unit of sufficient capacity. Once you have that, what's the point to having the tank heater too? It just defeats one of the main features of the tankless, which is no standby loss of heat from the tank. "
I agree with the above station. I also think the OP is being a big baby. If they want to have hot water under low flow conditions, presumably they are thinking of a situation where they are, for example, washing their hands, then maybe they should try using a separate inline water heater at the sink itself.
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Well a small single faucet 200 buck unit probably wouldnt help at all:(
but any raise in incoming water temp will yield more hot water...
witha new hot water heater adding a tempering valve and setting btank to max will help some
the OP should of purvhased a high BTU tank for greater recovery.
if theres space adding a second regular tank in series should fix his problem./
frankly i HATE running out of hot water!
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snipped-for-privacy@aol.com wrote:

OK, incoming water is 45 deg. Outgoing is 65 deg. Want to take a shower with that? The point is, for a flow rate sufficient to supply additional hot water realtime to 2 bathrooms after the tank is exhausted, you need a big tankless. Anything short of that isn;t going to do it, and once you have that, you don't need the tank. In fact, the tank water heater defeats one main purpose of the tankless, which is to save energy by eliminating standby loss.

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snipped-for-privacy@optonline.net wrote:

if your heater is in say a basement the standby losses go to help heat your home in the winter......
so for mant the supposed savings of a tankless are a dream, espically when you consider the initial cost of a tankless and its installation.
I doubt a small tankless will help the OP, but its his $$$
I really dont understand his concrn about low flow, a 50 gallon tank type can provide low flow forever.
if low flow is say washing hands.
high flow is showering or laundry
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Try a few real numbers, vs faith :-)
Nick
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After convincing people to take shorter showers, the first thing to do is make sure that your showerheads are LOW flow units. Get ones with a shut-off on it to turn the water off while soaping up, and teach people to use it. If your shower valves allow it, don't turn the water all the way on to shower. If you still have a problem, turn up the water heater temp. If you are concerned about scalding, a tempering valve on the output would help. Next step would be to upsize the water heater, or replace it with a faster recovery unit. Any of these steps should be cheaper than a big enough inline unit to do any good.
Bob
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snipped-for-privacy@gmail.com wrote:

tank, then output that to the tankless. Set the HW tank temp down very low (start at somewhere below 90F, adjust to suit as you get experience with it) so the Tankless has to manage a smaller temperature rise. Then you'd pretty much have unlimited hot water. Here's what i did. I got an old HW tank, stripped off the shell and painted the tank flat black with rustoleum [several coats]. It sits on a small concrete pad i made outside my garage on south side. Then I plumbed it in series with the HW tank, the incoming cold goes to the black tank, sun heats it a bit (its outside in the sun) and the mildly warm water feeds into the cold inlet on the HW tank in the garage. This reduces the work load of the water heater and lowers my bill. The outside tank has a temperature relief as does the inside tank for safety. The inside tank is blanket wrapped to also improve its efficiency. My next addition will be to build a plexiglass box around the outside tank and, being up here in NW US where winters are generally above freezing, that should extend the portion of each year i can use it by quite a bit. Eric
Of course its bypassed and drained in the winter but in summer it makes a significant contribution to heating my water.
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