Combining a Has Tankless with a 40 Gal. Electric Tank Water Heater

[Dewsoddy]

I am having a new home built and I will have gas tankless hot water heater. I will have a recirculating pump to the master bathroom.

I am considering adding a 40 gallon electric hot water heater to use as a reserve tank. My challenge is that I have found very little info online as to how to do this.

Do I have the water from the street go into the electric tank water heater and heat the water to say 100 degrees and then when the tankless system asks for water it pulls water from the electric tank water heater into the tankless

Or,

Do I have the cold water from the street go into the tankless and heat the water and then store the water in the electric water heater tank and the water is then distributed throughout the house from the electric water heater

Or, do the recirculating pump to the master bath make this all unnecessary. I am concerned about the gas tankless water heater cycling on and off frequently and wearing out sooner than it should.

Any assistance would be appreciated.

 

[Diehard]

Either option you describe would pretty much defeat one of the primary reasons to use a tankless unit. i.e.-To eliminate standby heat loss of a storage tank.

You feel that putting those two system in series will reduce the, "tankless cycling on and off frequently and wearing out sooner than it should". I'm not so sure it would, unless maybe you set the tank water heater to the desired temp, not requiring the tankless to even be necessary. I'm not even sure that if you have the tank heater set at 100 degrees and have the tankless heat it the rest of the way that it would not run just as long, since it's "run time" is based on the presence of flow, not time for temperature rise.
I must point out that I have no experience with these tankless units so I could be wrong, but that's my understanding of it.

Now the method of controlling the of operation of the recirculation pump has a lot to do with the frequency of operation of the tankless unit.
For example, based on timer only(common method), or based on a temperature sensor, or a combination of the timer and temperature. e.g. - A combination of the two would make the recirc pump only run when the return water temperature fell below a set point temperature and only during that period of time when you allow the pump circuit to be in operation.

I understand your primary reason was only to keep the tankless unit from wearing out sooner than it should. But none-the-less, I'll add the following.
To use a 40 gallon electric hot water heater as a "reserve tank" could be deemed as a standby, in which case it could be piped in parallel, and only put in operation when/if the primary system wore out sooner than it should.

 

[FishScreener]

The Green Building Advisor site, actually studied the cost effectiveness of modern water heaters. With the upgraded efficiencies of the modern tank type heaters, they are more cost effective than the tankless heaters. And, the longer life cycle the tank tape has, the better the pay off.

If you really wan t to run a dual system, I would install the tank type first, and feed the hot water from it into the on demand. Theoretically the on demand should only run once the water delivered from the tank, starts to cool down.

 

[Mitchell-DIY-Guy]

I believe FishScreener has it right.

I'm trying to do the same thing and will have to research plumbing schematics. The home from the builder, incredulously, includes an electric water heater (on a lot and in a home that has natural gas...). An upgrade to a natural gas water heater from the builder is crazy-money; they take the retail price of a 50-g model, jack it up, then add an additional usurious charge to drop a gas line to it, and do the PVC venting. The cost is in the thousands.

With JUST a tankless (prices have really come down on those), you run the issue of having to wait for hot water on the 2nd floor, LONGER than you would if you had a tank. If you add a recirculation system, they are generally on a timer and you need to predict when you'll need the hot water. This isn't always possible.

If you add a tankless to a system with an electric, you can just turn off the electricity and use it as a storage tank. You can use a recirculation system (thermosiphon if you have the plumbing head; pump if not). You are basically turning the system into a gas water heater. Some very efficient tankless units are under $600; a 50 gal gas water heater with power vent runs $1,000 or more, so this does makes some sense to explore. I know I will!

 

[Mitchell-DIY-Guy]

Hi Dewsoddy, I'm basically in the same boat as you are...but my one difference is my home will have a basement. With a basement, you can have your water heater in the basement, and you can have a recirculating system based on convection/thermosiphon and in that case you do not need a pump. A pump will add a measure of complexity to the system and introduce additional maintenance and point of possible failure and leaks. A convection system is cheap and easy--a return line (insulate all the hot water lines for best results), a check valve, and a shutoff valve is all you need. This is proven technology, a simple solution based on science and I've had friends who have had this for many years without any issues whatsover. So if your remote bathroom is on a higher level than your hot water source, it's do-able. I personally don't like waiting for hot water; I also don't have predictable use of hot water in the remote bathrooms, so that timer-based systems don't work for me. A tankless makes perfect sense if located near the point of use when no waiting is required. I can't be alone in this otherwise the tankless people wouldn't come up with these "buffer" piping schematics for use of a tankless to generate the hot water, and some kind of tank to store it. I have located about three different schematics so far...all slightly different!!!

Now all that being said, in my research today I discovered a kind of unique water heater made under the Westinghouse name. It's called a floor mount hybrid and is basically (from the best of my ability to understand it) a "single package, pre-plumbed" unit that consists of a stainless steel (no anode required!) 10 gallon storage tank, with a tankless all together. It's small, compact and will provide the endless amount of hot water of a tankless. The integral tank acts as the buffer system, so you can circulate out of the tank to your remote bathroom, either with a pump or the convection system I mentioned. Here's a link to this:

http://westinghousewaterheating.com/residential-gas-floor-hybrid.html

One may ask, why not just a tankless itself? Well, you'll note that tankless in full-house size with requisite flow rates are rated at 130,000-199,000 BTU depending on model. This unit is 76,000-99,000 depending on model. That's a lot less gas when running--and you can plumb with 1/2" gas line. Some of those larger tankless will require you to upsize the gas line coming into your home. I don't think you'd need to do that with this.

I know professionals don't like to use new things, often enough because of uncertainty of how well unknown things perform, and the costly problem of call backs; I get that and that makes perfect sense. However if any pros out there have any experience with this Westinghouse hybrid, I'd be interested in hearing about it.

 

[Hamberg]

I (already) have a similar setup that seems to be working fine. We had an addition put on several years ago and installed a "whole-house" Rheem tankless heater to service that section (1 bath, kitchen sink & dishwasher - overkill for what it was plumbed to).

The original (tankless) failed after about 4 years (my guess is from limited use in that section, very hard water and calcium buildup) and when I installed the replacement (which believe it or not was covered under warranty) unit I plumbed it directly into the original system.

So, the tankless now feeds the "cold" side of our gas water heater - effectively supplying hot water, to the water heater, and in my (feeble) mind taking the load off of that unit.

The one recommendation I have, is to make sure you "valve-off" the segments in case you need to isolate either one.

(my 2 cents for what it's worth :O))