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.

  • 3
    If you want efficiency, just go all tankless like Harper says. If you want enough endless hot water to take two showers while the DWs running, install it up-stream of the tank, as per isherwood. ('enough' : see Greg's answer)
    – Mazura
    Commented Oct 17, 2019 at 23:20
  • 1
    What is the incoming water temp in LA? If it's 70-80 degrees, I doubt you would be able to overwhelm that unit even if you did run everything at once. You can get away with a 30kw unit in South Florida for a whole house, since the incoming water is never very cold. Commented Oct 18, 2019 at 12:42
  • How many gallons of hot water can your tank deliver in the first hour? This new Rheem model claims 68 gallons so as long as you don't have both showers in use back-to-back then everyone can comfortably shower in the same hour. I think a bigger issue is going to be getting enough water pressure for simultaneous showers so people could be showering longer to get the shampoo out of their hair. I grew up in a family of 6 and 40 gallons sufficed for us.
    – MonkeyZeus
    Commented Oct 18, 2019 at 12:55
  • 3
    I'd say wait until the conventional unit dies and install the tankless. You're all adults aren't you? Try to avoid running the washing machine and doing dishes while two people are showering.
    – MonkeyZeus
    Commented Oct 18, 2019 at 13:01
  • 1
    Well maintained or not, that's getting old. We just bought a house and Insurance companies always wanted to know how old the water heater was to approve/deny or tell you to replace within 30 days or lose coverage. Checking a few Google results, life expectancy ranges 8-15 years.
    – dave k
    Commented Oct 19, 2019 at 3:20

4 Answers 4


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.


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.

  • 2
    How's taking two showers at the same time? the pressure (flow) drops - that's a big downside for me.
    – Mazura
    Commented Oct 17, 2019 at 23:21
  • 1
    I'll agree: a drop in flow is a downside. We avoid simultaneous showers. However, taking the instant water heater as a given, consider the alternative. If the heater is set to 120°+ and is mixed with cold at the shower valve, and another user comes on, the temperature will drop a lot! The two users will duel, each adjusting their mix to use mostly hot water (which is being flow-limited by the heater) and only a little cold. The outcome is roughly the same: a reduced-flow shower, but with frustration and angst while the mix is manually adjusted.
    – Greg Hill
    Commented Oct 17, 2019 at 23:46
  • Maybe another make of heaters would go tepid in an excess demand situation as mentioned by Harper. If that were the case, the two shower users would still go through a process of manually adjusting their mix and flow until their demand fit within the performance curve of the water heater.
    – Greg Hill
    Commented Oct 17, 2019 at 23:48
  • 2
    @GregHill Or get a thermostatic mixer valve in both showers. You set the desired output temperature, and then open the (single) tap. The valve then adjusts the amount of hot and cold it takes to achieve the required temperature. These are common in Europe (I have had them in UK, Germany, and Switzerland) but apparently rare in the US. The result is no need to fiddle with the water heater, and no need to adjust anything when someone else starts a shower (and not much need to adjust anything when you get in the shower to start with). Commented Oct 18, 2019 at 9:00
  • @JimmyJames A tank, yes -- but I didn't write about setting a tank to low temperatures. From a Legionella etc perspective it makes no difference what temperature the tankless heater is set to. The interior of the heater and all the plumbing will cool to ambient indoor temps in less than an hour after any use. If the heater were used in a recirculating setup where it maintained a moderate temperature, then I'd agree with the low temperature concern.
    – Greg Hill
    Commented Oct 19, 2019 at 23:07

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".

  • 1
    Can you point to more information backing this up? I'm not aware of such a "no-go" status, or anyone who operates residential (single family home) tanked heaters at 60°C or higher. Most units I see still have warnings printed on them not to use high temps due to risk of scalding (which is present unless all taps, not just showers/baths, including kitchen and bathroom sinks, etc have mixing valves). Commented Oct 18, 2019 at 14:47
  • 1
    @R.. Harper is 100% right about this. See here for ASHRAE guidance on water temp: cdn2.hubspot.net/hubfs/376446/offers/bel-ASHRAE-188-Booklet.pdf and here: cdc.gov/legionella/downloads/toolkit.pdf
    – JimmyJames
    Commented Oct 18, 2019 at 19:23
  • 1
    @JimmyJames: The first document clearly states that it's not a regulatory requirement (at least at the time it was written), so it doesn't seem to be a regulatory distinction vs a technical one. Rather it seems like it's a matter of complex intersecting factors that increase risk, and greatly increasing unknowns once you get into a large installation, especially with nasty things like cooling towers that facilitate growth that could then spread to other places. Commented Oct 18, 2019 at 21:16
  • 1
    I'm not opposed to the idea that higher temperatures should be used everywhere, but there's likely a significant tradeoff for energy consumption (60-22=38, 49-22=27, 38/27 = 41% higher rate of heat loss from tank), and it doesn't seem like this is a widely accepted recommendation for small dwellings at this time. Commented Oct 18, 2019 at 21:21
  • 1
    Just because there are not regulations about dwellings doesn't mean it's not a problem. It's similar to how a lot of localities don't (or didn't) require smoke detectors in private residences but do in rented apartments. You should have smoke detectors regardless. The same person who recommended I do this used is also involved in energy guidance and used to tell me to keep the tank at a low temp for the reasons you mention but flipped based on research. I'm erring on the side of caution.
    – JimmyJames
    Commented Oct 18, 2019 at 21:36

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.

  • Considering the old heater is 20 years old and both units are decent, the likelihood of his new unit being more efficient is somewhere around 99%.
    – Mast
    Commented Oct 19, 2019 at 8:24

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.

  • Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Commented Oct 19, 2019 at 3:09
  • I like your reasoning here but I think 'extended stay' here implies a relatively long period of time, perhaps years.
    – JimmyJames
    Commented Oct 21, 2019 at 13:58

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