Can tankless & conventional water heaters join forces?

Question

Although I have properly maintained my nearly 20-year-old 40-gallon (150-L) Kenmore natural gas-fired water heater, I have been considering replacing it with a natural gas-fired tankless water heater unit, a Rinnai 180,000BTU (190 MJ), which I bought a few months ago at a terrific price.

However, due to sudden changes in the household size (basically, grown kids are moving back for an extended stay), I am considering adding the tankless unit such that the Kenmore’s 40-gallon (150-L) water capacity will act as a preheated water supply source for the Rinnai tankless unit, with the assumption that, if I set the Kenmore’s temperature to a low setting, it will not cause the tankless Rinnai to function improperly.

So please check my plan and advise;

1. Location-Los Angeles, California, very mild weather.
2. House’s fixtures; 2.5 bathroom, 2 showers / 3 sinks, kitchen, dishwasher, Laundry.
3. I plan to provide a dedicated gas supply line to the tankless Rinnai unit as follows: 1 inch (2.5 cm) gas line, reduced to ¾ inch (1.9 cm), in the last 12-14-foot (3.7-4.3-m) run to the Rinnai, whereas the installation instructions indicate a gas supply line to be sized at ¾ inches (1.9 cm) for the last 20 ft (6 m) run.
4. Heated water supply exiting from the Kenmore water heater to act as a supply water line, via ¾ inch (1.9 cm) pipe supplying preheated water to the tankless Rinnai, then heated water from the Rinnai for distribution to the house’s fixtures.
5. While the existing Kenmore water heater is garage based, the tankless unit will be installed at the exterior wall, opposite the Kenmore, to minimise the work needed to connect the two and any loss of efficiency.

Your thoughts are appreciated.

Answer 1

Your gas pipe plan seems reasonable -- 1" pipe carries a lot of CFH of gas. We'd have to know the pipe length involved to be sure, but it's likely that covering most of the distance with 1" will keep the pressure loss in check.

Allow me to share some personal experience based on 11 years of living with a Rinnai instant water heater (R75LS). First of all, let me say I've loved it and would buy another.

Let's talk about the elephant in the room first: the GPM rating is peak. Look at the water flow curve in the technical data section of the user manual. With water input about 50° F and a set point of 130° F (temperature rise of 80° F) you can expect about half of its rated/peak GPM.

Don't despair over the flow curve, however. My model, and their others too I presume, have an interesting behavior: it restricts the output flow as needed in order to achieve the output temperature you've selected. If I want 130° water I can open all the hot water taps in the house and I'll get a relative trickle from them all but I'll get the temperature I asked for. The manual doesn't plainly say that the heater behaves in this way; it's just my observation.

In my household we use this flow limiting effect to our advantage. We set the heater to a suitable temperature (106°-108° in winter; 104° or sometimes cooler in summer) before getting into the shower. Then we turn on only the hot water tap. If somebody starts another shower or runs hot water elsewhere (or even flushes the toilet!) then the pressure (flow) drops, but the shower temperature stays constant.

We turn the temperature back up to 120° after the shower. Honestly, sometimes we forget, but the dish washer has a built-in water heater anyway so it's not a problem.

Frequent changing of the set point may sound like a hassle. The available wired remote controls ease that a lot, and I recently learned Rinnai has a WiFi module (US$80 when I last checked) with app and smart assistant support to make it even more convenient.

Like Harper, my suggestion is to go all-in on the tankless -- use the remote to select the temperature you need for the activity at hand and open only the hot water tap.

Case Study

I ran a few experiments with my water heater tonight. A 2 gallon paint bucket and a stop watch were used for metering water; a thermocouple held in the water stream was used to measure its temperature. I ran the bath spigot rather than a shower because it demands more water.

Experiment 1: Heater at 106° filling two bath tubs simultaneously

Water exited the spigots at 101°. Bath 1 filled the bucket in 57 seconds (2.10 GPM) and bath 2 filled the bucket in 53 seconds (2.26 GPM). Total flow 4.36 GPM.

Experiment 2: Heater at 106°, filling bath 1 only

The bucket filled in 34 seconds (3.53 GPM).

Experiment 3: Heater at 120°, filling two bath tubs simultaneously

I adjusted the hot/cold mix at each tub until water exited both spigots at the same 101° as in experiment 1. Bath 1 filled the bucket in 53 seconds (2.26 GPM) and bath 2 filled the bucket in 50 seconds (2.40 GPM). Total flow (hot+cold) 4.66 GPM.

Observations

As expected, the hot water flow around 4 GPM is just over half the "spec" rating of 7.5 GPM. Turning on the second tub decreased the flow at bath 1 by 1.43 GPM (41%). I was surprised to get more total water flow in experiment 3, but in my opinion the difference is minor. I prefer dependable temperature over slightly higher flow.

Answer 2

I see a couple problems there.

First, having a tanked heater operate at less than 140°F (60°C) is problematic. The reason is our science has advanced on the Legionella bacteria which causes Legionnaires Disease, and it turns out a "warm" water heater is a breeding ground for it and other bacteria. 140°F (60°C) is scalding, and that necessitates the modern style of mixing valve that automatically mixes hot and cold (It will not allow stream output to be near 140°F (60°C)).

Second, using a tanked heater to "ease the load on" a tankless isn't much use, because the tankless doesn't need the help. It means the tankless will be barely working for the first 30-ish gallons (110 L-ish) of draw, the tanked heater is now useless and can't help. The tankless will be working very hard. If you did manage to overload this big tankless, everyone would go painfully tepid at once.

The reverse connection makes more sense. The tankless uses its full strength to warm the entering-the-tank water. Even at too much flow for the tankless, it's still giving a 40-60 °F (22–33 °C) boost. Which dramatically improves the tanked heater's recovery time. At flow rates the tankless can keep up with, you never run out of hot (at least warm) water. At rates too much for the tankless, the tanked heater's recovery time (to a showerable 110°F (43°C)) drops from 30 minutes to 5.

However, I am honestly not a fan of either one. I don't like the tanked heater existing, due to its considerable parasitic load. I would just go all-in on the tankless, and school people in "don't run everything at once".

Answer 3

You can run water heaters in series. There's no problem there, typically. Consult the installation manual to be sure.

Your main concern is probably efficiency. While it stands to reason that your new heater is more efficient, this shouldn't be taken for granted. The more efficient unit should be upstream so it carries the heating load. The downstream is then an auxiliary for when the upstream unit can't keep up.

Answer 4

I would use the new tankless heater for the rooms you use most often and the existing tanked heater for guest rooms. It's more plumbing work but it gives you the best of both worlds. You can switch the tanked heater off when you don't have guests, avoid the standing losses, and take advantage of the higher efficiency of the newer tankless unit.