Saw this on an inspection today; the seller said he installed the conventional electric water heater(not hooked up to power) as a storage for hot water generated from the on demand water heater. Never seen this before, and it seems odd to me. Anybody else seen this before?
I’ve never seen or heard of this. I don’t understand why you would need a storage tank of hot water when an appliance is capable of limitless hot water.
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Seems like a recirc pump and it’s configuration would be critical to the system. Extra insulation on the tank would be helpful.
It’s similar in theory to the boiler systems with large storage tanks we used to service/replace back in the day.
Exactly what I was thinking!
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Exactly. And it would be a complete waste of energy, because now you are heating the water once, and then heating it again as it sits in the tank.
You say storage, but I assume you mean that it is a working water heater?
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This is where a water heater turns into a hot water heater.
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What do you get when the “storage” tank cools?
A shower that must use tepid water first?
No bueno.
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Morning, Martin.
Hope to find you well and in good spirits today.
I ran into this water heater setup every now and then. Typically larger homes, homes with many occupants, a fourplex turned in 12 single 1 units for students using the shared kitchens and bathrooms, or large busy families.
A on demand tankless water heater combined with a vessel/tank-based water heater is often called a booster. The benefits, extended water supply use.
Refer to Jeffrey’s posts above.
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Here are some of my thoughts. Do they have any merit? In commercial settings (hotels etc.) the tanks offer a buffer against increased or inconsistent demands. Inconsistent flow rate or volume may also be an issue in this house. I suspect it may also work better for a recirculating pump.
Sorry I take hot water showers. This is illogical unless i would have it heating also or can use the hot water stored in non powered water heater up right then and there. Guy must have huge family using showers at same time maybe?
This water heater is electric and does not have power. Do you think this will still work?
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Your on to something here 
I see what you did there
The efficiency of the on-demand water heater is directly related to the idea that it only runs when hot water is needed. When you are on vacation it never fires up because there is no demand. You are saving energy (or a little gas).
The storage tank systems are either two water heaters in line (with one being a conventional tank unit) or simply the tank recirculated through the on-demand water heater. Neither of these setups save energy and thus defeat the whole purpose of an on-demand system. When you are on vacation either the conventional tank water heater fires to maintain temperature (scenario 1) or the on-demand system fires to maintain the temperature of the recirculated tank of hot water water (scenario 2). You are not saving anything (except hot water)!
The electric if present would reduce the cycling of the main heater. If not present, the water still stratifies: hot to the top of the tank. It will be fine.
The split setup here in theory can be more efficient than a gas tank water heater. The 'lectric heater stores heat well: a gas tank water heater of the old chimney style constantly has convective losses through the central chimney.
This setup, in theory, could be better in a big multifamily or commercial use with high peaks (e.g. everyone showers all at once in the morning, but uses some water all day). Just think of the on demand as sort of modern version of an old school “boiler”.
Perhaps not enough gas pressure was available for a true on demand setup to meet the calculated demand. Note the small diameter of the gas pipe on the tankless: that unit may be incapable of running a mansion sized home.
How many bedrooms?
How many occupants?
How many showers or fixtures?
How much is the BTU/Hr rating of the tankless heater?
What model # is the tankless water heater?
Let’s say the residence leave for the weekend. Come back Monday morning to take a shower. There’s no power to the electric water heater. It’s a 40 gallon pipe filled with water between the point of use and the source of hot water.
So how is this just fine?
I happen to know for a fact, the temperature of the water in that tank will not be 120°F. The owner will need to bleed off all of that water until they get hot water from the tankless water heater.
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Until the heat is dispersed through the colder water below. Heat moves from hotter to colder always.
You need to heat the water tank (conventional WH) or recirculate the water. The OP said the electric WH (tank) is NOT energized, so there is no benefit (from the tank) unless the water is constantly recirculated, in which case there is no benefit (energy savings) from an on-demand WH.
The only benefit is to the plumber who charged triple to install this mess!
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Ah, I was assuming this was “done right” which involves a temperature sensor on the tank. Note in the fuzzy picture a rotary knob which could be such a temperature control, for a circulating system like @jjonas described.
If this is just a through system, then it only works under conditions of constant demand. Even then the Monday morning hot will stratify in the cold tank, and won’t be as bad as 40 gallon pipe.
Take a look at the size of the gas pipe in the fuzzy picture above. That’s not looking like enough BTU/hr to do the job by itself.