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Long version (skip below for tl;dr)

I recently purchased a new house. (first owner) 3 floors, unfinished basement where the hot water heater is located. Approx 3300 sq ft.

When looking through the house, I was shown a tankless water heater. Red flag immediately goes up. Whatever. I later discovered it's a GAS tankless water heater. I didn't even know that was a thing, but seems like it's the worst of both worlds. My worst fears were confirmed: It takes OVER 2 minutes to get hot water anywhere in the house.

The tank is on max heat (120°F) We have PEX throughout the house, and it doesn't seem like there are any inefficient lines, so my assumption is it must be the water heater itself. Of course a tankless heater takes longer to get the hot water than a standard one, but 2 minutes seems quite excessive.

The "unlimited" hot water is nice once it gets hot, but there must be a better solution.

TL;DR

Gas powered tankless water heater takes too long to heat up, looking for a solution that still allows us to have the benefits of not running out of hot water but also provides hot water in a reasonably short amount of time.

My idea:

Get a small tank (I'm thinking 4 gal is big enough, but the smallest that is commonly available should be sufficient) gas powered water heater that I can use to either A) feed into the tankless water heater or B) feed FROM the tankless water heater, but either way basically install the 2 inline so that we have both ready-to-use hot water and also limitless hot water.

My question:

Would it cause any problems with either machine to have hot water fed INTO the machine rather than cold water? Is there a better way to handle this?

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    Not sure why'd you'd expect anything BUT a gas whole-home tankless heater. That's very common.
    – mike65535
    Feb 7, 2019 at 21:25
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    @Rockster160, your system is standard and not substandard in any respect.
    – Jim Stewart
    Feb 8, 2019 at 15:46

4 Answers 4

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The problem is probably not the tankless these heat the water extremely fast, the real problem is probably the size and length of pipe to the point of use. A tanked water heater will probably take a similar amount of time to get the water hot to 120 at the point of use, what I have done is installed a small tanked unit after the on-demand with 2 check valves and used a recirculation pump at the furthest location cycling the water through the loop with a timer. One check valve prevents reverse flow through the loop and 1 to protect the tankless. We did insulate the pipes were accessible and this worked once we got the cycle on off time adjusted, I added the check valve between the 2 to protect the tankless with the tankless kind of isolated from the small tanked heater there were no false turn on's on the tankless (recommended by the mfg). This method will increase your electric usage but the small water heater 7 gallons was enough to keep the hot supply on a ~3500 SF ranch home with 3/4" water lines. Recirculation systems are not bad when you have large lines that are long but because all the lines are kept warm it cost more but the good news is it can be done. On smaller homes I have installed 2 point of use electric units 30 amp. But with larger homes the combination of a large tankless and a small tanked system with a recirculation system is much much cheaper.

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    One of our faucets is about 30-40 ft of pipe from the heater (closest hot water) and it still takes 2 minutes to warm up. At our previous house (gas 40gal tank) it took 30 seconds to get from the basement to our master bedroom. (3rd floor, opposite corner of the house) Still used pex, so the only difference there is tank vs tankless.
    – Rockster160
    Feb 7, 2019 at 18:17
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    @Rockster160 are you certain there were no other variables changed? Size of pex? Pressure? Size of manifold? Rated flow of faucet, shower head etc? What happens if you stand near tankless heater and feel the pex for warmth while someone else turns the water on? A properly functioning tankless produces hot water almost instantly, the delay is how much water, that’s gotten cold again, must be displaced out of all the tubing.
    – Tyson
    Feb 7, 2019 at 18:34
  • @Tyson - Pressure is definitely reduced. I haven't measured the size of the PEX piping, but estimate 3/4 for the main lines, which was the same as previous house.
    – Rockster160
    Feb 7, 2019 at 18:39
  • @Rockster160 what you mean is the flow rate is reduced compared to the tank heater in your other house. To get hot water the fastest with a tankless heater, you turn on the hot full flow and then turn it down and add cold to shower. People are not used to that with a tank water heater because they often have the tank set to 140 deg F and this scalding. Are you using a shower only or is this a tub/shower with diverter? Modern shower heads limit flow to save water and so even on full you only get 2 gal/min (or less).
    – Jim Stewart
    Feb 7, 2019 at 19:42
  • If you have a tub with a diverter, run water through the tub spigot with hot open full to get hot water there faster. When hot arrives, switch the diverter to shower, and add cold before stepping in.
    – Jim Stewart
    Feb 7, 2019 at 19:43
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One solution to consider: there are point of use heaters that are effective. You need to have the space available for the heater near the faucet in question, about the size of a loaf of bread. And you need to bring electricity to the site. They work well and can be cost effective.

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  • Considered this, but as the problem is "everywhere" determined we should have a global solution rather than apply this to the 10 different places we use hot water.
    – Rockster160
    Feb 7, 2019 at 18:46
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    @Rockster160, point of use heaters is the only way to get very quick hot water. Any central system will always have to push cold water out of the pipes before hot is available. That said, 120v point of use instant heaters are not good at all. You'd get warm water at least. Higher voltage/amperage means running wire, which is going to be very hard in a 3 story house.
    – JPhi1618
    Feb 7, 2019 at 20:11
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    Rockster160, you should look into a recirculation system. The systems cost. $200-$500 range plus 2 check valves and a small tanked heater. This simple solution will work quite well for most homes by placing the recirculation system at the furthest tap from the heater and the tanked heater close to the existing unit.
    – Ed Beal
    Feb 7, 2019 at 20:15
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    @jim Stewart As the op mentioned they would need many point of use heaters the cost of even 2 would be Similar to a recirculation systems and these have very low maintenance and power draw I think the last one I installed had a power draw of 50 watts or less while it was running. these are much less expensive than multiple point of use heaters and I know they can last well over 10 years with no maintenance at all and the more expensive pumps closer to 20 years.
    – Ed Beal
    Feb 8, 2019 at 14:56
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    The recirculation retrofits use the existing plumbing. The cold water line is used as a return, using a 2 tank system like this will require a couple of changes to add the tanked water heater as the op asked about , but on a single tanked system the pump is located on the furthest tap from the heater it is a simple hookup using the existing plumbing. I have put a bunch of these in on tanked systems , the tankless +tanked is slightly more complex but much less work than multiple point of use heaters because most of these require 240v 30 amp circuits to be added this is the big cost adder.
    – Ed Beal
    Feb 8, 2019 at 16:06
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Putting a tank water heater and a tankless water heater in series would would not shorten the wait time for hot water if the tank were in the basement near the tankless heater. The only way a tank would help would be if it were near the point of use.

There are two main properties of the hot water system which determine the wait time for hot water: (1) the volume flow rate of the hot water source in gal/min and (2) the volume of the piping between the hot water source and the point of use.

Hot water tank heaters have a significantly higher flow rate than tankless heaters and so for a given piping system they will deliver hot water sooner after turn on, but then of course with continuous delivery the tank will be depleted of hot water and you will get none until the tank "recovers".

You don't have an ideal system with a tankless heater in the basement, but personally I would just live with it. Get used to washing hands in the lavatory with cold water.

You could spend a lot of money trying to get a system that delivers hot water at any use point instantly. One way to do it would be small electric mini-tanks at each use point, but you'd have to decide whether to feed each tank from the central hot supply or the cold supply.

For a lavatory where you generally use a small amount of water at each use, you would want to feed from the cold line because small uses waste hot water by leaving abandoned hot water in the pipes.

For a shower you would want to feed from the hot supply line because you use tens of gal at a time.

Use your tankless heater until it wears out and then decide whether to replace it with a tank. By that time you will probably have learned how to live with the tankless heater and you will just get another tankless. The performance of tankless heaters has improved over time and will surely continue to do so. One thing about tankless heaters is that they must have a minimum flow to stay on, so they will not provide just a trickle of hot water.

If you simply cannot accept the current situation you could get a second tankless heater and put it in parallel with the one you have. That way you would get double the flow rate and so half the wait time. Some commercial tankless water heating systems have multiple heaters in parallel.

EDIT If the wait time for hot water in the master bath shower is the main problem, you could get an electric tankless point of use heater there, fed by the main hot water source, but this would not be cheap.

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  • Honestly the shower isn't a big deal. Kind of used to running the water for that before you get in anyway. What's more of a big deal is the habit we've got of, "Oh we need to brush our teeth or do dishes? Turn the tap on, leave, and come back later." Also, despite it "not being a big deal" - using hot water to wash hands after the lavatory is a nice thing. It's a big nice house, not a 40 year old run down shack... It sounds like it's likely an issue with water pressure more than anything else. Appreciate the idea with the parallel heater. Will likely do that if increasing pressure fails.
    – Rockster160
    Feb 7, 2019 at 19:43
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    Seems like a point of use tank (lets say 2 gallons) would give you 2 gallons of hot, followed by your typical 2 minute wait of cold then hot again when the hot arrives from the tankless (main) heater. Fine if you need less than 2 gallons, but doesn't help for longer uses. Or am I missing something? (I have a house with the same issue with long waits)
    – JPhi1618
    Feb 7, 2019 at 20:08
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    Adding a tanked heater with a recirculation loop will provide hot water, it has been a few years but I did this on a large ranch home with 3-1/2 baths. Water heater + recirculation system since there are 2 heaters a check valve is needed on the loop at the heaters some run on timers and some on temp so the water temp will be instant at the set point.
    – Ed Beal
    Feb 7, 2019 at 20:21
  • @Ed Beal your solution sounds very good. Could you describe with a diagram or text where the check valves would be placed, and what the other connections are between the existing tankless heater and the small tank you would add? Presumably the tank would be fed only through the tankless heater, right? What temperature would be tank be set at? Don't some modern plumbing systems have a branched structure without a "main trunk" that goes far into the structure? Would a manifold with spurs be suitable for a recirc system?
    – Jim Stewart
    Feb 9, 2019 at 11:36
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Sanity check.

40ft of 3/4 pipe = 12m

3/4 PEX pipe = 17mm internal diameter

Pipe contains 0.23 l/m so total 2.7l

Considering a standard flow of 20l/min with the faucet fully open on hot, for water to go all the way through that pipe, that's a volume of 2.7l, so it takes 2.7/20*60 = 8.7 seconds.

Even if the pipe is twice as long, that's still far from 2 minutes. If you wait for 2 minutes, then either the tankless heater has a long startup delay, or it restricts the flow, which means hot water takes longer to run through the pipe, or both.

  • Measure flow on hot and cold.

Just use a stopwatch to time how long it takes to fill a known volume container, for example a kitchen pot. It doesn't need to be very accurate.

Normally with a tank heater the hot flow should be a bit less than the cold flow because the heater safety and check valve introduce some loss. But it should definitely not be half. If it's less than half, or much less like on my previous tankless heater, then here's your problem.

To heat 1 liter of water by 1°C you need 4.4kJ. So if water arrives at about 12°C and is heater to 60°C (delta T = 48°C) at a flow of 1l/min (1/60 l/s) that's a heating power of deltaT * flow * 4.4kJ = 3500W. (watts are J/s).

If you know the power of your heater, for example it has a thermal output of 20kW burning gas, then you know it can't have more than 20kW/(l/min)/3.5kW = 5.6 l/min flow.

If all goes well, the pot and stopwatch method should agree with the calculation from heater power, and you'll probably conclude it has a pretty wimpy flow like all tankless heaters, which means water takes forever to go through the pipe.

So, to get hot water faster, it needs to move faster through the pipe, which means it has to be colder.

The solution is super simple and cheap: install a thermostatic valve at the output of the heater. I don't know how it's called in your country, but it's this thing.

There's a hot input (to your heater), a cold input, and a warm output.

If you set it to a reasonable temperature like 40°C, then, when the faucet is fully open, it will input whatever hot water the heater delivers from the hot input, and as much cold water as necessary from the cold input to give the set temperature. This means the output flow is much higher than what your heater can deliver (because cold water is added to the hot water), and thus, it goes much faster through the length of the pipe. In addition, water at the faucet will be limited to 40°C, which is safer.

This a rather cheap fix (one thermo valve and a bit of PEX and fittings, no need to rip pipe out of the walls, etc) so it's the first thing to try.

It should shave off about 30 seconds off your hot water waiting time, the rest being the tankless heater taking forever to startup, as they all do.

To get a shorter time...

Recirculation to the faucet either wastes a lot of energy if it dumps the hot water into the cold pipe, or requires laying new pipe which means making a mess.

Local electric heaters at the faucets can also work, but you lose the cupboard under the sink, and you have to put them everywhere, which is annoying, more maintenance, etc.

Since delay through pipes is no longer a problem once you add a thermo valve, the simplest is to add a tank to your main heater. This doesn't mean you absolutely need to replace the tankless heater with a new heater. Perhaps it can be reconfigured to work with a tank and keep it at the set temperature with a recirculation pump, but you'll have to check the manual for that.

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