Tankless water heater with storage tank

Danny Scully

Looking for a piping diagram for a tankless water heater with storage tank. Most diagrams show street water being fed into the tank as well, which I think is counterproductive. Any insight would be greatly appreciated.

njtommy

Like this?

njtommy

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Leon82

njtommy said:

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This is the way mine is setup. (second picture) The anode in the ariston will only last a year though.

njtommy

@Leon82 how big of a tank are you using?

Danny Scully

So, just to compare, this diagram doesn't seem as optimal, right?

Leon82

The 4 gallon, I do have a small expansion tank on mine also.

Leon82

Hatt is correct there is usually a maximum inlet temp. I know Bosch is 120. Also using the tank less directly in a recirc loop may void the warranty.

Danny Scully

What I'm suggesting is that, isn't it counterproductive to feed the bottom of the tank with cold water? Won't this drop the temp in the storage tank quicker? Every time there is a draw for hot water, cold water will be entering the bottom of the tank. Where as, it could be piped in such a way that this would be avoided.

Jack

Using the Takagi diagram as a reference, I supply cold to the tankless where Takagi shows their hot return to the tank. As shown the tankless is always being influenced by the hot return, so depending upon building draw, flow rates, return water, etc the tankless goes nuts. It is reacting to the varying flows/temps correctly, but it si always searching and in my experience leads to early problems with the flow control inlet valve.

I don't like to pump off the exact bottom of the tank to the tankless as whatever crap is in the tank then goes to the inlet filter on the tankless and also causes problems. So I pipe to the tankless where Takagi shows their return.

Tankless are designed to make only the set point temp. If the unit is set for 140 it will make only 140. Why return it to the bottom of the tank. That set-up works with a boiler where you get smaller temp gains and a slower rise. I've returned to the hot side of the tank or other penetration on the top or high side. The water is at set point. If there is a draw it is ready to go. If not it will circulate back into the tank and when the A-stat is satisfied you have a tank of HOT WATER. You need a 20* delta T between tankless set point. Lower and the unit may short cycle some. You need a tempering valve on the hot.

Pipe it this way and the tankless is seeing a consistent supply temp and it will smooth right out and gives the best component life.

bob eck

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Check out their Phoenix light duty water heater 76,000 BTU input with 50, 60 or 80 gallon tank high efficiency PVC vented.
Phoenix gas fired water heaters 100,000 - 130,000 - 160,000 - 199,000 BTU input with 55, 80 or 119 gallon tank high efficiency PVC vented

kcopp

we never found out how big of a tank.... That would change the piping set up...

Rich_49

It is HTProducts.com Bob . This gentleman is apparently not interested in a tank type heater as much as a tankless with about 4 gallons of storage and RECIRC it would seem .

If you go to HTproducts.com look at the Crossover , great price point , firetube HX for DHW and recirc pump is integral .

Danny Scully

The tankless heater/storage tank has already been installed. It was a job I did years ago. Recently, the customer has been running out of hot water when guests are showering in succession. I was just thinking about how I piped it and wondering if I could improve upon it. Sorry about any confusion.

Leon82

Could a layer of cold water sandwich form inside the storage tank?

JohnNY

Although I've only installed this system in restaurants, my experience with this set up is that piping the cold water directly into the tankless heater without a direct connection (no check valve between the cold water tee and tank) to the storage tank certainly ensures all the water stays hot, but creates some flow rate problems at the point of use as the tankless tries to modulate flow to stabilize temperatures. The direct cold water connection to the bottom of the tank maintains consistent pressure and outlet flow rates.
On smaller storage tanks, like 50-gallons, I've had to tinker a bit with maintaining balance between recovery and flow rates at the commercial sinks because no restaurant prep worker wants to know about flow restrictors and aerators. When that happens, I tend to install a master flow restrictor at the tank or tempering valve's outlet so I'm not trying to compete with the flow of multiple wide-open ½"-diameter commercial pot sink faucets. https://www.freshwatersystems.com/p-2019-neo-pure-stainless-steel-flow-restrictor-12-fnpt.aspx
Tanks larger than 50 gallons tend to make me uncomfortable to keep at less than 140°F so I always use a commercial grade tankless heater to maintain a slightly higher temperature on larger storage tanks, or if The Health Department requires a particular restaurant to maintain hotter rinse water at the dishwasher. I'll always find out if the restaurant uses a booster or not.
It gets complicated, doesn't it?

hot_rod

Jack said:

Using the Takagi diagram as a reference, I supply cold to the tankless where Takagi shows their hot return to the tank. As shown the tankless is always being influenced by the hot return, so depending upon building draw, flow rates, return water, etc the tankless goes nuts. It is reacting to the varying flows/temps correctly, but it si always searching and in my experience leads to early problems with the flow control inlet valve.

I don't like to pump off the exact bottom of the tank to the tankless as whatever crap is in the tank then goes to the inlet filter on the tankless and also causes problems. So I pipe to the tankless where Takagi shows their return.

Tankless are designed to make only the set point temp. If the unit is set for 140 it will make only 140. Why return it to the bottom of the tank. That set-up works with a boiler where you get smaller temp gains and a slower rise. I've returned to the hot side of the tank or other penetration on the top or high side. The water is at set point. If there is a draw it is ready to go. If not it will circulate back into the tank and when the A-stat is satisfied you have a tank of HOT WATER. You need a 20* delta T between tankless set point. Lower and the unit may short cycle some. You need a tempering valve on the hot.

Pipe it this way and the tankless is seeing a consistent supply temp and it will smooth right out and gives the best component life.

Good point on the stratification. Some water heater manufacturers offer special "spare" tube nipples to help with layering of the tank.

It looks similar to the plastic dip tubs you see in tanks. They crimp the end shut then either slot or perforate the tube so the flow spreads evenly across the surface when the nipple is used in a side port.

Easy to build one with copper or pex.

Here is a 3" I built for my solar/ wood boiler storage tank.

bob eck

The HTP Crossover looks like a interesting water heater.
I have a question. HTP shows the Crossover being used as a water heater and in a hydro duct coil heating system without a plate heat exchanger. Does installing it this way pass code?? Don't you want to keep portable water separate form the water used in the duct coil system. I know it cost more to use a heat exchanger but that is the way I would install this type of job.

Rich_49

It exercises that circ each day for 3 minutes at a time I believe Bob . This prevents stagnation and possible colonization of bacteria . Yes , this does pass most codes .

Paul48

I don't see how that design by Takagi would work, at all. The cold supply feeds the bottom of the tank...what happens when you open a fixture?

Paul48

Hat.....Look at the picture....Now open a faucet.

JohnNY

The diagram is fine. Think gas-fired water heater with external heat source. The loop of heated water is, practically speaking, parallel to the domestic loop.
You have to pressurize the tank with line pressure while the pump and burner heat and shuttle the energy.

Paul48

Evidently, the circ would be able to reverse the flow of cold against open fixtures?

Paul48

I'm missing something.

Paul48

You open a hot faucet...the cold feeds to the bottom of the storage tank. Let's say you're filling a bathtub. At some point, the storage tank will call for hot water. What is going to reverse that flow and send the cold to the tankless?

Paul48

OK....I couldn't visualize the circ doing that. Thanks

Danny Scully

@Paul48, this is my exact concern. I'm wondering if I fed the water in after the circ it might perform better?

Danny Scully

I think the benefit would be the tank never sees cold water?

JohnNY

The tank has to see cold water in most cases. It's what gives the system line pressure. Running the cold water through the heat exchanger first will give you that highly praised condition of "endless hot water", but it will reduce your flow rate at the point of use.
I've been bitten twice by this. Shame on me. Take my word for it. The diagram is solid for 99% of the applications.

Danny Scully

@JohnNY, this may be the 1%

JohnNY

True. I never speak in absolutes anymore.

Paul48

Remove the circ from the cold side, and add a check valve before the tank. To the right of the check on the hot, run a parallel branch with the circ in it.

https://us.v-cdn.net/5021738/uploads/editor/ti/iwpqv3bgwacb.jpeg

I suppose to get around the flow restriction of the tankless, you could run the tank and tankless at 160 then use a thermostatic mixing valve.