Redundant dual fuel with two tankless heaters

[B. John Henley]

What is the best way to configure a redundant system utilizing one electrical tankless heater (probably 20-29kW 80-120 amp), and one natural gas heater (2009 Noritz 200,000 BTU).

My goal is to preferentially use electric heater for 50-100% of my hot water needs (in order to fully utilize electric energy credits from a grid linked solar system). But at the same time, I would like to maintain the existing full flow capacity and reliability of a 2009 Noritz 200,000 BTU tankless system.

Some thoughts I had include the heater setup in series with the electric heater first (say set to 140 degrees F), and if higher demand is needed the gas heater (say set to 120 degrees) would kick on when sensing an inadequate temperature. There is a downstream mixing valve with cold supply so water at the faucet is "constant". This option would seem to have frequent short burst on/off cycles for the gas heater maybe leaving to extra wear or carbon build up? What other problems do you see with this setup?

Other option which I cannot fully wrap my head around, involves a parallel configuration with downstream mixing valve favoring flow from the electric heater until it cannot keep up with demand then the gas heater supplements.

Third option involves a 30-50 gal electric heater tank (rather than tankless electric) in series just upstream from the gas heater. This one seems less favorable due to space, less efficient with the electrical energy, etc.

The existing house breaker box (200 amp) should be good for the 29kW tankless electric system.

The usual higher cost for electric heating is not an issue given the extra electric energy credits from a solar array (I cannot sell the energy back to the grid in my state) but the grid acts similar to a huge battery (from a cost perspective).

Thank you for your advice

 

[Reach4]

Some thoughts I had include the heater setup in series with the electric heater first (say set to 140 degrees F), and if higher demand is needed the gas heater (say set to 120 degrees) would kick on when sensing an inadequate temperature.

Would't the gas heater always see an inadequate temperature, or were you thinking of remoting a sensor to later in the path?

 

[Sponsor]

 

[Terry]

My 199,000 BTU gas tankless water heater in the Seattle area is costing me $8.00 a month for gas. I'm not sure what it cost for the fan electricity.

 

[Reach4]

My 199,000 BTU gas tankless water heater in the Seattle area is costing me $8.00 a month for gas. I'm not sure what it cost for the fan electricity.

Is that exclusive of the Monthly Customer Charge etc?

 

[B. John Henley]

Would't the gas heater always see an inadequate temperature, or were you thinking of remoting a sensor to later in the path?

Well, that's part of the reason I am asking. I don't understand how the remote sensor will work.

With option A, see illustration, as long as the flow rate is minimal (<4 gpm, only 1 shower and/or faucet going on at a time), then the Electric Heater will handle all the demand, therefore the gas heater will see 140 degree temp (most of the time) and not need to contribute. Please let me know if I'm missing something here. Its rare that we use more than 1 shower at a time, but if we end up needing a higher flow, the gas heater will make up for any inadequacy.

 

[Reach4]

Well, that's part of the reason I am asking. I don't understand how the remote sensor will work.

The remote sensor? It sounds like you have one in your plan. I would think a remote sensor on a tankless could cause water temperatures to oscillate. When the heater runs, it takes a second maybe for the temperature rise to be felt at a remote sensor. Then heat shuts down. A second later the sensor calls for heat again. And so on.


I don't have a solution for you . It's much easier to poke a hole in something than to provide a solution.

One thing I was thinking about... and I expect it has a hole in it. Suppose you had another mixing valve adjusted to 130. You hook the electric WH to the cold input. You hook the propane WH to the hot input. Only when the output temperature drops, does the propane WH water get used. That mixing valve output supplies the hot input for your normal mixing valve. The hole might be that the mixing valve is not an on-off thing. It may take propane heated water at low levels, which does not work so well. So I think this idea would fail to work as hoped.

 

[Jadnashua]

On a typical tankless system when ganged, the outlets are in parallel. I suppose that you could set the gas tankless to a lower temperature, and if the electric one couldn't keep up, it would kick in, but I think the flow sensor would force it on, regardless, but then throttle its operation.

First thing I'd research is if either unit is designed to be ganged, and if so, what settings are available. The gas unit may be able to be set as the secondary already, and never turn on until the water temperature dropped.

Lacking that, I think you're going to end up inventing and experimenting, and may not succeed.

 

[B. John Henley]

I don't have a solution for you . It's much easier to poke a hole in something than to provide a solution.

One thing I was thinking about... and I expect it has a hole in it. Suppose you had another mixing valve adjusted to 130. You hook the electric WH to the cold input. You hook the propane WH to the hot input. Only when the output temperature drops, does the propane WH water get used. That mixing valve output supplies the hot input for your normal mixing valve. The hole might be that the mixing valve is not an on-off thing. It may take propane heated water at low levels, which does not work so well. So I think this idea would fail to work as hoped.

Thank you Reach4,
Does this drawing (option B) depict what your final paragraph was describing.
This is definitely a better option. I think this will work, with the degree of success depending on how precise the mixing valve can work. With a perfect mixing valve, this should work very well. Will likely have to tweak the 1st mixing valve settings to achieve my goal.

 

[Jadnashua]

Without some priority logic, at a low flow, neither unit might turn on.

 

[B. John Henley]

On a typical tankless system when ganged, the outlets are in parallel. I suppose that you could set the gas tankless to a lower temperature, and if the electric one couldn't keep up, it would kick in, but I think the flow sensor would force it on, regardless, but then throttle its operation.

First thing I'd research is if either unit is designed to be ganged, and if so, what settings are available. The gas unit may be able to be set as the secondary already, and never turn on until the water temperature dropped.

Lacking that, I think you're going to end up inventing and experimenting, and may not succeed.

Thanks Jim,

From what I can find in quick search through owner and install manual, and google search, the Noritz can only be set up in parallel. And when in parallel it uses a proprietary "quick connect cable".

I can see now where my initial illustration would probably subject the gas heater to flow activation just at low heat output. Thank you for pointing that out. Please see my 2nd illustration above and let me know what you think.

 

[B. John Henley]

Thank you Reach4,
Does this drawing (option B) depict what your final paragraph was describing.
This is definitely a better option. I think this will work, with the degree of success depending on how precise the mixing valve can work. With a perfect mixing valve, this should work very well. Will likely have to tweak the 1st mixing valve settings to achieve my goal. View attachment 47747

Theoretically the mixing valve will provide priority to the electric heater. No?

 

[B. John Henley]

Without some priority logic, at a low flow, neither unit might turn on.

I think I see what you mean. The mixing valve may hang out in the neutral position and allow 0.4 gpm from both inlets and therefore not meet the 0.5gpm min flow rate for either unit.

 

[Reach4]

Theoretically the mixing valve will provide priority to the electric heater. No?

That was the hope, but I fear not the reality. Sure sounds intriguing, however.

Behavior would probably vary depending on which mixer was used, and I know of no way to predict how it would go.

 

[Terry]

Here is a correction on my tankless water heater expense

14.33 Therms 40 gallon tank gas water heater in August billing 2009, gas cost $12.52
2.181 Therms with Navian Tankless water heater in August billing 2018, gas cost $0.80

For a difference of 12.149 Thems

 

[Dana]

A real issue with any tankless ELECTRIC is the abuse it does to the local grid infrastructure & local power quality as they turn on/off. Intermittent large loads like that impose higher capacity requirements to the local grid (particularly the transformer you're drawing from) and wears it out more quickly, and injects voltage & frequency perturbations that can interfere with other equipment hooked up to the same local grid- it's the opposite of "green". Just because you have enough panel capacity to cover it doesn't mean it's a good idea. (A better use for the "extra" capacity would be a level-2 electric vehicle charger.)

An electric tank draws about 1/5 the power, and has fewer on/off switching cycles. A grid-aware/wi-fi enabled (or retrofitted) electric tank can even be a grid-stabilizer, in locations where the utilities or third party distributed resource aggregators use them as a "smart load" to switch in or out for grid voltage & frequency control purposes, or for freeing up peak period grid capacity. A tankless electric can't provide ANY of those grid services. In the PJM grid region where these markets are further along than most homeowners get paid for allowing that use by the utility/aggregator.

A "hybrid" heat pump water heater would use less than half the power of a standard tank, taking your 50% electric water heating with a standard tank to 100% without using up more kwh. The hybrid-mode heating element could be retrofitted for use in grid services programs where offered, but I don't know of any existing models that come pre-fitted with that capability. A heat pump water heater cost roughly the same to install as a gas tankless, often less, depensing on how much gas plumbing is required.

Most gas fired tankless units have an upper bound for incoming water temperatures where they would still be able to regulate the output temperature and not flame-out. The range of modulation of the flame is not infinite- they've proven to be one of the lousiest ways to back up solar thermal water heating, and you would have the same issues here. A condensing tankless won't even condense if you're feeding it 120F water in, and putting 140F water out, a temperature that would have to be tempered down to 120F or lower prior to the how water distribution plumbing. Tempering reduces the flow through the gas tankless, since it's mixing in the water, and during the transition from electric to gas burner behavior becomes erratic, since it hits minimum firing rate limitations of the burner, causing it to flame-out. With any tankless unit it's generally best to set the output to the highest temperature you would actually use, not 120F and certainly not 140F.

There are no good solutions for this, only pretty crummy ones.

The most effective of the crummy solutions involves an electric TANK water heater, which is used as a buffer tank for the tankless, and water is PUMPED through the tankless when it hits the low limit. With an electric tank heater set to 140F the gas burner & pump can be turned on when the temp at the top of the tank drops below, say 120F. When the water in the tank is stratified that gives a reasonable delta-T for the gas tankless to work with. The output temp of the gas tankless can also be set to 140F, and as long as it's given sufficient pumped flow rates that a mere 20F delta-T isn't going to be below it's minimum firing rate it'll work. But it's a klunky expensive kludge of a solution.

Best bang for the buck is to go with a heat pump water heater sized for the biggest tub you have to fill, and no auxilliary "backup" heater. A ~50 gallon tank is usually enough for most families.

If your big water use is taking multiple back-to-back or prolonged shower that would deplete a 50 gallon tank, adding a drainwater heat recovery heat exchanger can add a lot of "apparent capacity" (and REAL efficiency in showering mode), for about the same cost as a tankless electric water heater. The benefit is only gained when the drain & hot water is flowing at the same time- it does nothing for tub-filling capacity, but a big one can nearly double the showering capacity of a tank type water heater.

 

[chahdi]

Hallo

I think it’s a bit late now to share that now, but may be next time you can check this website where you can find a very interesting buying guide on
Electric Tankless Water Heater.
This one.