No, Tankless water heater doesn't = Inconsistent hot water.

Really bad design and bad installation = inconsistent hot water.

Yes, I suspect that may be the case in my situation so I'm coming here looking for advice and guidance. ;-) I've seen several posts about issues with tankless hot water heaters and some that suggest that delays in hot water are typical. I'm trying to get an understanding of what is typical and to be expected vs. what is not. Also looking for advice on possible solutions to my problem.

Distance from the heater to the point of usage comes into play with ALL types of water heaters. Tankless is not a magic fix to deliver instant heated water through 50' or 75' of pipe between the heater and faucet.

Tankless units also require a specific minimum flow rate to activate and stay activated. This can present an issue when showering, for example, if the setpoint on the heater is too high, causing the hot water flow at the shower to be reduced low enough that the heater shuts off. The fix in that case is reduce the setpoint on the heater so it's closer to the desired shower temperature.

"The kitchen is on the left side of the house and possibly about 50' away...."

That's a problem. Let me guess that it's not insulated either.

" Also, if you're using the hot water and turn it off for a little bit, the water is cold before it gets hot again." - Again, I'm guessing it's not insulated.

A tankless heater will add a few seconds to the time it takes to get hot water.

This isn't about the water heater.

Are these gas or electric tankless?

A few things... It's a new construction house, but we did not have it built so I wasn't around during construction. There are copper pipes, but I don't know whether they are insulated.

The builder will likely be looking for the easiest fix. I'm trying to learn what would've been the proper (or ideal) way to set up the plumbing and hot water heater, if it seems like it wasn't done correctly here. I take it that "done to code" doesn't necessarily mean it's done the right way to get the hot water heaters working properly?

There's a bathroom above the kitchen that doesn't have as lengthy of a delay getting hot water, although there's a delay sometimes.

I'm planning to find a local plumber that services Navien hot water heaters, but I'm here to get a better idea of what I should be investigating and how this could be fixed.

I'm not sure if the tankless heaters are gas or electric, but I will find out and update.

Maybe you can use an under sink water heater and connect it to the sink and DW.

Update: We have two Navien gas tankless hot water heaters. The one on the left is a NPE-240A and the one on the right is a NPE 240S. Looking at the instruction manual, it looks like the 240 A model is designed to have a recirculation mode, but not the S model.

New houses with plumbing in foundation and using tankless recirculating water is great. Hot water in an instant all over the house. I am not sure a separate plumbing job with recirculating pump will do the same (would not have foundation insulation). I have a tankless gas water tank and the hot water (no recirculating pump) takes forever to reach a bathroom 50' away but it does eventually get hot. A lot of water is wasted waiting for it to get warm/hot.

riellebee- Thanks for your update. I asked about power source because I had an issue with an electric tankless, but, since you have gas, my experience isn’t applicable. However, seeing your photo and that those two large units are teed-in to a single gas supply line, I think you should look at the total Btu’s needed and whether the gas line and meter are sized big enough to meet the power demand when the tankless units kick on. Definitely get a professional to do a calculation: the length of pipe run from the meter to the heaters, the diameter of the pipe, the meter size, and whether other gas-fueled devices (oven, furnace, fireplace, etc.) are connected can all compromise tankless heater performance. For starters, call your local utility and ask what size your meter is and what is the maximum amount of Btu’s that can be supplied by it. It’s entirely possible that your installation might need larger diameter gas piping, and/or a larger meter.

Insulating the hot water pipe should reduce the drop in temperature to the more distant points of use. But the time to get hot water to the kitchen is a function of the distance, which isn't going to change with the equipment you have. An under-sink point-of-use water heater would deliver water quickly, but it's frustrating that that seems to be the only solution to a problem created by a poorly designed system.

I have read that the the issue you describe when running hot water in short bursts (getting a bit of cold water every time you restart the flow) is typical of tankless systems. No personal experience, but that's what I've read when researching these systems. It's one of the downsides for me, since I live in a drought-prone area, and often run the water (for sinks, at least) in short bursts for water conservation.

There's a lot going on:

1. Looking at the photo I also think you need to review the gas supply. You have two heaters - my guess is that they are 200k btu each (if you tell us the model number we would know for sure).

I would bet that the gas supply isn't enough to supply 400k BTU/hour (meaning the gas meter all the way to the heaters)

2. The water lines aren't insulated. They will cool off more quickly between uses and also lose heat based on how far away from the heater they are.

3.. Make sure the temp on the water heaters are set to 120 -125 nothing hotter. Tankless isn't like a tank heater - making it hotter doesn't help, it just causes problems.

4. I think part of your problem is a thermostatic valve that isn't working right in your shower.

Hear's the thing: I suspect this is a really poorly thought out system. If the gas supply is right you may be able to fix some of the problems with insulation, and perhaps a recirculating pump.

If the gas supply isn't right you should probably switch heaters.

...or upgrade the gas supply piping/meter, which might be cheaper.

Unless you have recirculating pump (with the appropriate setup), you will not be satisfied. We have a gas tankless (with recirculating pump) and it works great. Hot water is pretty much instant no matter which faucet (5600 sf house). Hot STAYS hot and there is no fluctuation in temperature. When the recirculating pump was accidentally turned off, it literally took 5 minutes minimum to get hot water to the second floor (and 2 minutes on the first). We also have this little contraception (see pic) under two sinks (one on first floor and one on the second) which has something to do with the recirculation function. I’m not a plumber so don’t know what it’s called. Maybe the plumbers out there can explain.

In any case, without a recirculation function, it will take a very long time for your tankless to deliver hot water

We have one single tankless for the entire house .....Rinnai.

Hahahahaahahaahaa

Oh brother....that was a good one!

It’s like an IUD right?

Close, but more like a bidet!

Thanks to all for the helpful comments and suggestions! (And thanks to @Nidnay and @kudzu9 for adding some levity! LOL.)

If I'm understanding the product specs correctly, both the Navien NPE-240A and NPE-240S have gas supply/capacity (?) of 199,900 BTU each. We have a gas range and fireplace. I'm not sure which other appliances are drawing from the gas line, but I'll be looking into this.

The house is approximately 6000 SF and has 5.5 bathrooms. The 2 tankless heaters do not appear to be "cascading," which according to the installation manual is something that can be done for increased hot water demand.

FWIW, the installation guide says that the recirculating mode on the 240A will work with a max length of 100' of 1/2" copper pipe or a max length of 500' of 3/4" copper pipe.

If the pipes are not already insulated, isn't this something that would be pretty invasive to correct? If a new recirculation line is being added, that one could be insulated though.

The water temp on the heaters is currently set to 130 so I'll decrease it to 120 to see whether it makes a difference.

@Jake The Wonderdog, I don't understand your theory about the thermostatic shower valve. Could you say more about that? In the upstairs bathroom above the kitchen, it's also the sinks that sometimes take a long time to get hot water. However the biggest issue we're having is the amount of time it takes to get hot water at the kitchen sink.

Can anyone else chime in on the name of plumbing configuration under the sink that nidnay posted above?

I’ll be interested to hear what you find out about your gas specs. I have a pretty decent size meter which is rated to supply 320,000 Bru. If I had those two tankless units going at the same time they would demand more than my meter could supply, even without anything else on, such as my furnace. Your problem may well have to do with the distance the hot water has to go and the amount of heat that is initially lost to the water pipes, rather than a less-than-adequate gas supply system, but it would also be good to know whether your piping system and meter is properly sized to supply those two heaters so that they operate in an optimal manner.

@riellebee :

I re-read your original post and I was mistaken about the mixing valve.

So it seems that the issue you are most dealing with are long runs of uninsulated pipes. A recirculating line can help with that - but you still need to insulate your lines. You have a re-circulation pump on the one unit. Is it working?

I think you have a gas supply problem as well. It may not be showing up right now, but when both of those kick full, at the same time you will see it. The problem isn't just how much the meter can supply, it's also the size and length of the gas line to the water heaters. It's always difficult to guess at pipe size from a photo, but that looks like two units on a 3/4" pipe.

Here's the thing: I suspect that none of this was thought through well. It's not a single problem, but a group of problems. I think that you need a competent plumber to really look at all of it and suggest solutions.

It may be that getting the re circulation system going (if it's not) and insulating the pipes would solve the problems. It may be that the gas supply is undersized. It may be that the cheapest solution would be to install a standard tank water heater (solves gas supply problem and reduces wait time for hot water). It could be that you could move a tankless heater closer to where the water is being used.
There are various solutions, but someone needs to sit down and really plan it out.

BTW: None of this is guesswork. Math is a thing. You can calculate how long it will take to get hot water based on pipe sizes and distance. There are tables for gas capacity of various sizes and lengths of gas pipe.

What I can tell you is that a tankless gas water heater, properly designed and installed, will add a few seconds of wait time for hot water - other than that, they work well.

Would just like to add that our pipes are NOT insulated and our tankless with recirculation works flawlessly (the tankless is located in a conditioned crawl space).

When we thought we had a problem (due to recirculation being accidentally shut off), the three things that were carefully considered was the length of the pipe run from the gas meter to the tankless, the diameter of the pipe, and the water pressure coming out of our pressure tank (we’re on a well).....proper water pressure must be maintained in order for the tankless to heat properly. All three were fine and within specs but those were the very first things that were checked.

A recirculation pump masks bad plumbing design. Running a recirculation pump without insulating the pipes will work - just as running the furnace with the window open will work.

A recerculating system without insulation will lose heat quickly to the surrounding air. If you live in a warm climate you then have to remove that heat (more air-conditioning). Even without a circulation pump, insulated water lines means that the water takes longer to cool off between uses. You want to insulate all of your hot water lines - with or without a circulating pump.

Gas supply is important for tankless heaters (because they use a lot of BTU's at a time) Water pressure is generally not a big issue.

Note: If the gas supply is a problem, it will show as an error code on the unit. My point here is that if the units aren't throwing an error code, gas supply isn't causing any of the issues you are experiencing right now. If there is a gas supply issue it will show when both units are running at full capacity. I believe that you probably do have a gas supply issue - not because of the symptoms, but because of two units (and then it appears they are both on 3/4" pipe). You don't want to wait until the warranty period is over to discover the gas supply is undersized.

Some math for gas pipe sizing:

2 units x 200,000 btu/h = 400,000 btu/h

Assumed 1100 btu/h per cubic ft of natural gas (can vary by supplier)

= 364 cf of natural gas an hour.

3/4" black iron gas pipe can deliver 360 cf/h at 10' length (no elbows)

1" black iron gas pipe can deliver 374 cf/h at 30' length.

If the pipe feeding those heaters is less than 1" or more than 30' it's undersized - and that's with nothing else on that branch (furnace, for example).

Q: are you on a well or city water?

Thanks again for the additional feedback and comments! This is really helpful and I'm learning a lot!

Some additional updates/answers:

• The gas pipes are 3/4".
• I found out the model of our gas meter and it has a max of 425,000 BTU/hour.
• I haven't noticed an error code on either hot water heater. (If it would only appear at the time that I'm trying to get hot water, I haven't checked that yet.)
• I noticed that the only the pipe from the recirculation outlet on the hot water heater is 1/2". The others appear to be 3/4" pipes.
• We have city water.

Ok, so again, the main issue you are seeing is long runs of uninsulated pipes.

Also, we don't know if the circ pump is working or not on the one heater.

If you don't see a code flashing on the unit, the gas supply isn't causing a problem RIGHT NOW.

With 3/4" iron gas supply feeding two units the length is limited to 10' with no elbows. I'm seeing more than 10' of pipe in the photo. Although the gas supply hasn't presented a problem yet, it will.

Questions:

1.) Do you have any other gas appliances (furnace, stove, fireplace, grill, outdoor fire pit)?

2.) is the 1/2" pipe on the one heater (the recirculation line) warm when you wrap your hand around it?

riellebee-

1. Presumably you mean 425,000 Btu/hr for the meter.

2. Depending on the specifics of the piping and pressure that your utility provides the gas at, the maximum rating may be different. That’s why I said to call your utility and see what they have it rated at.

3. The diameter of the water piping is not an issue; the diameter and length of the gas piping run from the meter to the tankless heaters could be. This determination is something done with engineering reference tables, so you probably need someone with a little engineering skill to help out. (Too often, installers will tell you that your piping is just fine, whether they know for sure, because they want to get the installation done and move on to the next job.)

Here is how the piping runs are evaluated: https://www.edcgov.us/Government/building/documents/Gas%20Pipe%20Sizing%20(Natural).pdf

kudzu9,

Assuming for a moment that there are no other branches off that gas line - the answer is the same - 10' of 3/4" iron pipe would deliver 360 cubic ft/ hour (which is somewhere between 36000-396000 btu/hr which is still a little under the 400,000 that's needed. And there's no way that's only 10' of 3/4" pipe.

riellebee- How long is it from the water heaters to the gas meter (measured the way the gas line runs)? It doesn't have to be exact, within a few feet

“measured the way the gas line runs,” meaning actual feet of piping. For instance, my water heater is about 50’ distance from the meter, but the piping runs from the meter up a wall, twists and turns through the attic, and then drops back down to the heater, so it’s about 90’ of piping. And every elbow that exists in the piping reduces gas delivery. That’s why the specifics matter.

@kudzu9 Yes, thanks - I meant 425,000 BTU/hr. I edited the post above. I called the utility company but haven't spoken to the person who should be able to answer my questions about the meter's capacity. I did get the specific meter though and looked it up online to confirm the max BTU/hr rate.

The reason why I mentioned the diameter of the pipes is that the instruction manual noted that the allowable distance between the faucet and the heater differed depending on pipe diameter.

@Jake The Wonderdog. We have a gas range, fireplace, and 2 HVAC units (each covers different zones in the house). All of the pipes from the water heater were cold to the touch after the hot water had been off all day. After using hot water, the recirculation pipe was warm.

Looking at the floor plan, the hot water heaters are not very far from the gas meter, but they are not on the same side of the house. I'm guessing it's less than 20' away, but I have no idea how much actual piping there is.

Yeah, there's an equivalency like every elbow = 0.5 feet.

But I'm not trying to engineer it... I'm just trying to establish that it's clearly undersized and they need to fix it.

I think a lot of installers don't realize how much gas those heaters really take - and have never done the math or looked up the pipe capacity in the tables. I don't want the owner getting error codes when the furnace and those heaters are both on - and the warranty period is done.

If you look at the table in the link I provided previously, it shows how much gas can be delivered at various distances. For example, using 3/4" pipe, you get:

10': 360 cubic feet per hour

20': 247

30': 199

Without going into all the engineering, what this shows is how fast delivery falls off as piping length increases. A 30' run of pipe can only deliver about 55% of the gas that a 10' length can. Think of it like a garden hose: the longer the length of hose, the weaker the stream of water is that gets delivered.

riellebee

So I'm going to rough-in some numbers:

Range: 15k btu/h

Fireplace: 80k btu/h

Furnace: 75k btu/h (x2) = 150kbtu/h

water heater 200k btu/h (x2) = 400k btu/h

Total potential load: 645k btu/h - again, an estimate. You can check your furnaces if you want to be be sure.

is it likely that you would have enough things on at the same time to exceed your 425k btu/h service? - Yes, it's quite likely.

What happens when you exceed the capacity of the service? The gas pressure drops and the range and the gas logs will just not run at full capacity - no big deal. But the things like the furnace and water heaters will throw error codes showing low gas pressure.

The water heaters are likely to have that problem anyway, because at 30' of 3/4" pipe they are only getting about enough gas to run one of those heaters full tilt (the water heaters are variable input). The meter may be 20' away, but in actual length of pipe it's probably close to 30' including elbows. I can see close to 10' of pipe in the photo.

The fact that the return line for the circulation isn't warm suggests it's not working. Fix that and insulate your pipes and your immediate problem will probably go away. BTW: Insulating the pipes is not expensive or difficult. Foam pipe insulation comes in 4' sections that just snaps around the pipes and uses self adhesive strips to hold it closed. It cuts with scissors. Even if you just did the exposed pipes in the basement you would reduce a lot of heat loss.

My guess is that the builder can't really fix the "over-subscribed" gas service at a reasonable cost. One solution is to use a different water heater. A hybrid tank/tankless would solve the problem. The link goes to an AO Smith Vertex 75 gal water heater 100k btu/h and is 92% efficient and vents with the PVC vent pipe you have. I'm not particularly endorsing that brand - just using it as an example. It would provide you with plenty of hot water (you could run 1 shower continuously) and would reduce your BTU load by 300k BTU/h (gas service problem AND undersized gas pipe to water heater solved). In addition, because it's a tank of hot water - along with about 6' of hot water in the pipe - your wait would be significantly reduced even without a circulation pump.

It sucks for your builder because it would cost them about $3500-4000 to fix it. They could probably realize some salvage value from the tankless heaters - say $1000-1500.

But If they are only out $2-3k total I'd say they should consider themselves lucky.

Thanks @Jake The Wonderdog for the additional info and break down of numbers! This is a bit overwhelming, but I realize that I need to have an idea of what's going on so I can be sure that we're getting a proper fix for our water heater issue.

One update - I spoke to the right person at the gas company and our meter capacity is 500,000 BTUs. I haven't checked usage by other appliances yet. I've been trying to get a hold of a Navien "certified" plumber to help me troubleshoot. One plumber that I spoke to suggested that if the 2nd water heater without a recirculation line is replaced with the model that has a recirculation line, that might help. I plan to ask about insulating the exposed pipes as well.

It is overwhelming. You shouldn't have to engineer your plumbing - I shouldn't have to engineer your plumbing either.

You have multiple issues:

1. long runs of uninsulated hot water lines.

2. a circulation system that appears not to be working properly

3. undersized gas pipes to the heaters

4. potentially undersized service.

Items #1 and 2 are what are causing you the most grief right now.

Items 3 & 4 are potentially expensive and will bite you in the butt at some point in the future.

As far as the plumber's suggestion: Don't guess - don't let other people guess.

Replacing a tankless water heater with another one in the hope that it fixes it isn't a solid plan.

You will need a return line for the second water heater if that's what you intend to do. You should identify what that heater services and determine if it would benefit from such a re-circulation loop.