Hot water in sink faucet but luke warm in the Shower

[Santosh Krishnan]

We have 2 Navien 240 A units cascaded and brand new.

The problem:

The main heater is set to 130° and the cascaded unit synchronizes. When I open the shower, we get near instantaneous hot water that I have managed to measure at 120° or more.

However, the hot water gets to lukewarm within a minute and doesn't get hot after that.

When I check the sink faucet, the water is very hot and stays that way.

To figure this out, we switched the water heater from external to internal recirculation.

We found that it took about 4 minutes to get hot water in the shower and it got very hot and stayed hot.

The sink also had very hot water.

Please note that we experience this at all 4 showers.

When I switch it back to external recirculation, the discrepancy between the shower and the sink reappears.

I don't know if this is an issue with the heater or the shower fixtures.

 

[Bannerman]

Inspect your home's recirculation plumbing. Is there 1 recirculation loop or does it branch off to multiple loops through the home?

You mention all 4 showers are affected. Are they all located nearby to each other or are they scattered in different areas of the home? If spread-out, are the showers located at the end of the run close to where the hot supply line is connected to the return line?

A recirculation loop requires a check-valve (one-way valve) to prevent the cooler water from within the return line from back-flowing to faucets such as your showers. Although a single check-valve may be located in the return line for a single loop before it enters the WH, if there are multiple loops, each loop will require its own check-valve otherwise the return flow from one loop may back flow to another loop. The best location for each check-valve is usually at the farthest distance from the WH where the supply line is connected to the return line.

In changing the NPE-240A 2-way valve to internal circulation, the re-circulation path through the WH's loop return connection will be closed so there will be no further water flow through the home's return loop. With stagnant water in the return loop, hot water flow will be more likely to flow in the correct direction to all faucets including your showers.

 

[Sponsor]

 

[Santosh Krishnan]

Thanks Bannerman.

After thinking about what you asked and examining a VR that the builder gave us pre drywall, here's what I've determined.

1. There is one hot water out from the heaters and going from the basement to the first level

2. On the ceiling of the first level, this hot water out splits into smaller pipes going to sinks

3. The main continues on to the other side of the main level where a similar split occurs for the showers and another bathroom

The showers in point number 3 are the worst affected with the greatest heat imbalance I've recorded.

It would appear per what you've said and what I've dug up that those showers are at the end of the hot water line. Presumably that line returns to the heater but I haven't yet determined that per the VR diagram that's been provided to me.

How would I determine if there's a check valve at the end of the line as you've described?

 

[Bannerman]

The hot supply line splitting into multiple branches is usual, but does not signify that each of those branches are part of the recirculation loop. Any branch forming part of a recirc loop would need to incorporate a return path back to the WH. Often, multiple return paths will combine into one before connecting to the WH's recirculation return connection.

There may have been a single check-valve installed just before the WH connection which will be OK if there is only a single recir loop, but when there is a single C-V for multiple loops, there will be nothing to prevent the return flow from one loop from backflowing into an alternate loop.

Perhaps if your home is equipped with multiple loops, each return line was routed directly to the basement and is hopefully visible before combining into a single line before connecting to the recirculation inlet fitting on one of your NPE-240A units.

The link below is to a photo showing 4 recirculation return lines before they combine into one to connect to the WH. Although each is correctly equipped with its own check-valve, unfortunately, the incorrect type of C-V was utilized for that vertical orientation. Although this poster's recirculation system was passive as it didn't utilize a pump, because those C-Vs would not prevent backflow as intended, that poster experienced a similar situation as you are currently.

Here is the associated thread: https://terrylove.com/forums/index....tion-line-piping-questions.88621/#post-637125

While I mentioned it preferable to locate each C-V where each hot supply line is connected to the return line, a C-V maybe located anywhere in each return line and will still function correctly. Because C-Vs are mechanical devices which may fail, each should remain fully accessible and should not be buried behind drywall. In placing them where the supply line and return lines connect, they will be often placed below the bathroom sink or behind an access panel for the shower faucet where they will remain accessible.

Suggest starting at the WH's recirculation return connection and follow that run backwards to attempt to identify how it is plumbed and to locate C-Vs.

 

[Santosh Krishnan]

Since your last reply, here's what I've done as an update to this discussion.

1. I turned on the recirculation in the two Navien units
2. The original issue is back, but it's interestingly not as bad. The hot water gets hot (hotter than before at the same temp setting), but then it does drop in temperature, sometimes to almost cold
3. It'll get hot again, but it may be in a few seconds or a few minutes, hard to predict

I don't know where the check valve is. Warranty has told me that they've informed the plumber but the plumber always comes back to examine the heater, which we have demonstrated is not a problem.

I'm going to put it back to internal recirculate tomorrow to confirm that when that's set, the water gets hot and stays hot.

Santosh

 

[Jadnashua]

The recirculation may slightly increase the pressure on the hot line, causing the pressure balance spool valve in the shower to react by introducing more cold. What you might need to do is to adjust the high limit stop in the valve to allow more hot to be introduced. This would likely be alleviated if you had chosen a thermostatically controlled shower valve that would adjust the balance on its own.

 

[Bannerman]

Perhaps there is no check-valve(s) installed whatsoever in which case, one or more will need to be added.

 

[Santosh Krishnan]

I have an update to this issue that I want to share.

Recap - I have 2 Navien 240A heaters and the original post details the original issue.

Since then, and with the advice given here, here's what I have done to improve the situation.

• I set the master unit to external recirculation
• I set the slave unit to internal recirculation
• I set the temperature max to 130

Now what happens is that the water at the shower head get really hot (128 or so). However there are some issues pending that I can't figure out. They are:

1. Despite the recirculation being on, the water at the shower head is warm. It doesn't get hot for a few minutes. I can live with this but it's really contrary to the purpose of the external recirculation
2. Strangely, when the washing machine is on, or the dishwasher is on, the hot water at the shower is nonexistent. It's lukewarm. I can't get it to go hot. So I have to schedule my washing machine or dishwasher so that it runs at night when we don't need to shower
3. The heater is set to 130, and the unit with the external recirc has the setting to go up to 9 degrees above 130 on the return pipe before the burner shuts off. This happens correctly, but on occasion, the burner will be on but the water outflow temp is at or under 130. This means the water at the showerhead is really lukewarm. I don't know how to convince it to go above 130 per the setting all the time
4. I notice a kick in the shower water stream. When I am in the shower, every now and then the water will kick once, and not in a continuous fashion. I don't know why this happens

Thanks for your feedback.

 

[Bannerman]

heater is set to 130, and the unit with the external recirc has the setting to go up to 9 degrees above 130 on the return pipe before the burner shuts off.

How would the return temp be hotter than the outgoing water temp?

The water temp setting controls the outgoing temp from the WH. The return temp sensor senses the return temperature, to turn off the pump once the loop has been filled with hot water.

The highest temp detected on the return line should be 130 - heat loss ocuring throughout the run so maybe 124-126℉.

every now and then the water will kick once, and not in a continuous fashion.

Any water use appliances or devices operating at the time? It sounds like a sudden stoppage of water flow such as in an appliance or device that utilize solenoid valves to control water flow.

You haven't mentioned anything about adding a check valve on the recirculation return line.

 

[Fitter30]

Those 4 checks in the vertical are all wrong. Swing checks in vertical the swing can stay open they need to be spring checks .

 

[Santosh Krishnan]

OK so a small note - the temp setting is set to 130. Having read the manuals and looked at the parameters, the way it's supposed to work is that the burner will heat the water to 130 and beyond when the recirculation is set to external. It'll constantly measure the return water temperature and it'll allow the burner to heat the water up to 9 degrees above what's set before it shuts off.

At least that's how I understood it and the readout is consistent with this understanding.

In other words:

1. Setting is set to 130
2. I go into the panel and look at the water outflow - it's approx. 1 GPM or more, depending on the number of showers that are on
3. Water in is at approx. 45 degrees F
4. Water out is at approx. 127 degrees F, but it increases when the burner symbol is on
5. Depending on the draw, it'll sometimes increase rapidly (with a corresponding increase in the burner noise) and it'll go well past 130
6. It'll never go past 139, but it'll sometimes get up to 137

BTW this is only when the setting is for external recirc. The same parameter for internal recirc will test the internal return line and will fire up the burner if the temperature drops 9 degrees below 130. In other words, it'll let the water get cold and then fire up to heat it vs. letting the water get hot and then let it cool down.

As far as the kick, I don't know of any other appliance that's operating. It happens everytime, so it's not intermittent, but it's only one kick, and it'll happen periodically, not continuously.

As far as the check valve, I have not been able to find one, and so it must be behind the drywall. Honestly I don't know what I am looking for in order to detect the check valve and the plumber hasn't been by yet.

I've practically figured out all of this with your help and the plumber has simply regurgitated whatever is in the manual, which I am well qualified to read and process at this point.

 

[Santosh Krishnan]

Those 4 checks in the vertical are all wrong. Swing checks in vertical the swing can stay open they need to be spring checks .

That's not a picture from my setup, I think it was posted to illustrate something at that time.

 

[Bannerman]

Those 4 checks in the vertical are all wrong. Swing checks in vertical the swing can stay open they need to be spring checks .

If you read the narrative, it explains why those 4 checks are wrong.

That photo was posted to show Santosh what to look for in his search for a check-valve on the re-circulation return line.

 

[Bannerman]

the way it's supposed to work is that the burner will heat the water to 130 and beyond when the recirculation is set to external.

Page #?

The situation you describe could lead to lawsuits against Navien as someone could become scalded by hot water that is exceeding the set temperature.

I have not been able to find one, and so it must be behind the drywall. Honestly I don't know what I am looking for

I included photos to show you how CVs appear and what to look for. A CV should not be behind drywall. Have you looked under the kitchen or bathroom sinks?

If you can't locate one it is then likely no check-valve was installed, which is probably why the shower temp is variable.

While there is an internal check valve within the Navien, that check valve is often unreliable. Failure of that check valve is common and most noticeable with internal circulation as the outflowing hot water will not be as hot as the set temp, but can also cause the problem you are experiencing as cold water is able to flow in reverse into the circulation return loop.

As there is one circulation return connection on the Navien, if there are multiple circulation loops within the home, each will need to be equipped with its own check valve.

 

[Fitter30]

Return piping what size is it 1/2 or 3/4. Can you see the flow rate of the return pump?

 

[Jadnashua]

Plumbing code calls for the max water supply temperature for everything except a few items (dishwasher, washing machine are two) to not exceed 120-degrees, so having it 130-139 would not pass code.

How old is the shower head in that shower, and has it been modified to allow more flow? A modern, code compliant head (individually, multiply that if you have more than one) cannot exceed 2.5gpm. It's not uncommon with a tankless system, especially in the winter, to not be able to keep up with demand. But, with two of them, properly setup, that should provide the full outlet temperature for most average homes even in the depths of winter except maybe in the arctic.

It sounds like you have them setup in series? Most of them call for a parallel setup, and they communicate to each other so it won't turn on both until the flow exceeds a certain amount, or it cannot reach the desired outlet temperature.