My house is <2 years old. We have a new Rinnai tankless natural gas water heater set to 125F. Up until now, there have been no issues. I can take a hot shower for as long as I want (to pay for it). The water heater also has an internal reticulation pump that gives us hot water at any outlet in the house in 5 seconds or so. I also have a whole-house sediment filter installed. We also have pretty hard water, so a softener is also installed and appears to be functioning normally.

Now, I turn on the shower in the master bath and I get hot water. I get in and the temperature drops after a minute or two to "tepid". I'm talking "this sucks and I want to get out" level. If I hold out for another couple of minutes, the temperature goes back up the "tolerable". At that point, it maintains that temperature no matter how high I crank the shower dial.

I disassembled the shower valve assembly to check for any buildup or obstructions in the hot water side. It was clear.

I've heard that a drop in water pressure can also cause such a temperature fluctuation in a tankless, but there is no other appliance running at the time, and I've noticed no systemic drop in pressure. We MAY be on the pressure line because it is very noticeable if, for example, the washing machine is filling and I try to shower. I've considered installing a booster pump. HOWEVER, as I stated, nothing else is using water when this temperature issue occurs.

Thoughts on the next step? I plan on checking the filter built into the water heater, but honestly the water here is so damn clean I'm not optimistic I'll find anything. My first whole-house filter still looked brand new after a year.

I believe you are getting warm water initially from what's left in the pipe, getting cool water as the heater warms and then the second arrival of hot water is your actual unit working as designed. May need a small reservoir system to eliminate the problem completely. Is the problem really new, or are you just dealing with much colder water this time of year? Of course, the unit may quit heating if the flow rate is below a certain threshold, but you don't mention varying the flow rate.

If it doesn't do that at any other appliance or fixture, are you sure you don't have an (possibly bad) anti-scalding valve installed? Or perhaps the mixing valve?

The real question is if it's just your shower...

Here’s my take on what’s happening:

Initially, you have warm-but-not-hot water in the hot water pipes from your recirc pump mixing with room temp water that is in the cold water pipes in your house.

Then your cold water gets really cold because you start getting water that was in the pipes in the cold ground outside your house, and the mixed output temp in the shower drops a bunch.

Then the actually-hot water from your hot water heater gets there and the temp comes back up.

If you turn your shower on cold-only, you’ll see that the cold water starts out cool then gets really cold.

Obviously this is mainly a problem in the winter.

quote:

Originally posted by bdylan:
I believe you are getting warm water initially from what's left in the pipe, getting cool water as the heater warms and then the second arrival of hot water is your actual unit working as designed. May need a small reservoir system to eliminate the problem completely. Is the problem really new, or are you just dealing with much colder water this time of year? Of course, the unit may quit heating if the flow rate is below a certain threshold, but you don't mention varying the flow rate.

quote:

Originally posted by maladat:
Here’s my take on what’s happening:

Initially, you have warm-but-not-hot water in the hot water pipes from your recirc pump mixing with room temp water that is in the cold water pipes in your house.

Then your cold water gets really cold because you start getting water that was in the pipes in the cold ground outside your house, and the mixed output temp in the shower drops a bunch.

Then the actually-hot water from your hot water heater gets there and the temp comes back up.

If you turn your shower on cold-only, you’ll see that the cold water starts out cool then gets really cold.

Obviously this is mainly a problem in the winter.

My main question would be what changed? This didn't happen last winter when we had plenty of days below freezing. It just started happening and is very noticeably not in the norm. (I shower nightly)

Also, that wouldn't explain why it won't get back up to HOT, even after showering for another 5 minutes. It plateaus off at a temperature that is noticeably lower than normal, especially with the hot dial maxed out. Also, the house is very well insulated. It has all blown-in insulation with 2x6 exterior walls, blah blah. It got the fancy green certification (for taxes!).

I don't see why the initial temperature of the water would change anything. The tankless is set to a specific temperature, so if the water coming in is colder than normal, it should automatically adjust heating to maintain the selected output temp. I checked the water temp right at the output; it was right on.

quote:

Originally posted by stoic-one:
If it doesn't do that at any other appliance or fixture, are you sure you don't have an (possibly bad) anti-scalding valve installed? Or perhaps the mixing valve?

The real question is if it's just your shower...

Interesting idea. I'll check the shower in the kids' bathroom tonight. I pulled the mixing cartridge; it APPEARED normal. I'll check the product specs to look into the anti-scalding valve. The only modification I made was to remove the flow restrictor from the shower head, and I did that over a year ago.

quote:

Originally posted by flesheatingvirus:

quote:

Originally posted by maladat:
Here’s my take on what’s happening:

Initially, you have warm-but-not-hot water in the hot water pipes from your recirc pump mixing with room temp water that is in the cold water pipes in your house.

Then your cold water gets really cold because you start getting water that was in the pipes in the cold ground outside your house, and the mixed output temp in the shower drops a bunch.

Then the actually-hot water from your hot water heater gets there and the temp comes back up.

If you turn your shower on cold-only, you’ll see that the cold water starts out cool then gets really cold.

Obviously this is mainly a problem in the winter.

My main question would be what changed? This didn't happen last winter when we had plenty of days below freezing. It just started happening and is very noticeably not in the norm. (I shower nightly)

Also, that wouldn't explain why it won't get back up to HOT, even after showering for another 5 minutes. It plateaus off at a temperature that is noticeably lower than normal, especially with the hot dial maxed out. Also, the house is very well insulated. It has all blown-in insulation with 2x6 exterior walls, blah blah. It got the fancy green certification (for taxes!).

I don't see why the initial temperature of the water would change anything. The tankless is set to a specific temperature, so if the water coming in is colder than normal, it should automatically adjust heating to maintain the selected output temp. I checked the water temp right at the output; it was right on.

Sorry, I missed "If I hold out for another couple of minutes, the temperature goes back up the "tolerable". At that point, it maintains that temperature no matter how high I crank the shower dial." in your initial post - I thought it came back up to reasonable temperature.

The way the recirculating pumps typically work, they don't run all the time, so while they keep warm water in the hot water pipes, they don't keep full hot-water-heater-output-temp water in the pipes. Once you start running hot water, it should come fully up to temp in a few minutes (basically however long it would take you to get hot water without a recirculating pump).

Because of what I posted earlier, in the winter, I have always had showers outputs go cool-cold-hot (with no recirculation pump) or warm-cool-hot (with recirculation pump). The initial drop in temperature is when the cold water to the shower goes from the warmer water in the cold pipes in your house to the cold water from the pipes in the ground outside your house, and then the rise in temperature is when the fresh-from-the-water-heater water gets there.

If your water is NEVER getting hot, but the hot water heater output is good, I have seen two things cause this problem.

1. A problem with the mixer cartridge in the shower. The modern ones are not normally a straight mixing valve, they have mechanical thermostat elements inside (sometimes adjustable ones) that attempt to maintain a specific temperature rather than a specific mix. It's possible for the adjustment to come loose and shift, or for something to get stuck so that the hot water side won't open all the way. This is obviously something that can happen abruptly and would only affect the shower.

I have had this happen before. It wouldn't cause the warm-cold-hot part of the pattern (which I think is explained by the variable cold water temp), but can absolutely keep the water from ever getting hot.

Usually the mixing valve cartridge is pretty easy to inspect/replace. You just pop the handle and cover plate off, remove a couple screws, and it slides right out of the valve housing. Obviously make sure to turn the water off first and drain the pipes.

2. I think this is much less likely in your situation but maybe still a possibility. An improperly installed mixing valve, either in that shower or somewhere else in the house, can allow cold water (which is often a little higher pressure than the hot water) to backflow into the hot water pipes and drop the hot water temperature. It would be odd for this to abruptly be a problem, but possible (for example, if it's another shower that has the improperly installed valve, and the other shower has always been turned off at the mixing valve before, but recently got turned off at a diverter valve with the mixing valve left open), and odd for it to only affect that shower, but also maybe possible depending on the plumbing. I think it's unlikely, but I included it because it's the only other thing I can think of.

A few years ago I lived in a brand new apartment building where every damned mixing valve in the building had been installed with problem #2. Depending on how the water pressure was balanced based on the building supply and other water use in the building, sometimes the water was fine, sometimes you got cold water out of both taps, and sometimes you got hot water out of both taps. It took them months to fix it, they had to replumb every shower in the building.

Run the shower and when you get the temp drop check the hot water at the sink. That should tell you if its isolated to the shower.

Double check the temp setting for the water heater. If you have a thermometer verify the temp is correct. Could the unit be over heating and shower anti scald valve is kicking in?

Have you been keeping up with your annual maintenance??

https://youtu.be/woKVYtf2Ihg

Gas or electric instant water heater? If electric, you may have a bad element. With gas one of the burners may be clogged.

And have you descaled the unit. If not that’s the first thing I would do. And see if it returns to normal. If not then I’d explore the element/burner.

quote:

Originally posted by smlsig:
Have you been keeping up with your annual maintenance??

https://youtu.be/woKVYtf2Ihg

Yes. I actually did the vinegar flush and the inlet screen < 6 months ago.

quote:

Originally posted by MikeinNC:
Gas or electric instant water heater? If electric, you may have a bad element. With gas one of the burners may be clogged.

And have you descaled the unit. If not that’s the first thing I would do. And see if it returns to normal. If not then I’d explore the element/burner.

Natural gas. I also checked the water right at the output, and it is at the proper temperature.

I wonder what the installer or manufacturer says….

Check the flow restrictor in your showerhead, probably green in color. Remove it with an
Ice pick or nail set and needle nose pliers.

quote:

Originally posted by MikeinNC:
I wonder what the installer or manufacturer says….

I’d rather not go that route until I’ve determined that the shower is the culprit.

quote:

Originally posted by sunburn:
Check the flow restrictor in your showerhead, probably green in color. Remove it with an
Ice pick or nail set and needle nose pliers.

Already done right after we moved in.

You say it has an internal re-circulation pump, that is one part of a two part recirc system, the other is a mixing valve located at the farthest end of your plumbing runs, usually under a sink is where I put mine. The recirc valve allows the cold water trapped in the hot water line to rejoin the cold water piping and return to the water heater. It's a simple valve and fails sometimes, allowing cold water to mix in with the hot water. Yours could have failed and is allowing a small amount of cold water to seep into the hot water line and cool it down before reaching your shower.
Find the valve and shut off the cold water tap on that valve and see if anything changes. It looks like a little plastic 4 place T. One to hot tap, one to cold tap, one to cold supply and one to hot supply.
Your heater is set to 125 so no matter how much you turn the handle, 125 is the max you will get. Mix it with a little cold and you will get water less than 125 degrees, and no matter how much more hot you ask for, you wont be able to get it.

UPDATE:

I tried the kids' shower at the other end of the house; it was glorious. HOT HOT water, as much as I wanted. At least that narrows it down a bit.

I'll look into possible issues directly related to the master shower such as a bad anti-scald valve or a bad mixing cartridge. I sent an email to the manufacturer describing the issue.

I actually don't recall a mixing valve during construction, and I was obsessively taking photos. After looking everywhere plumbing comes out of the wall, I didn't see any mixing valve. I also went through the construction photos for the plumbing. HOWEVER, there are two lines coming out of the wall that feed the cold inlet on the water heater. I'm guessing one of those is the return loop for recirculation.

This message has been edited. Last edited by: flesheatingvirus,

This is such an interesting topic, please post what happened to get things fixed up and working properly.
.

quote:

Originally posted by OKCGene:
This is such an interesting topic, please post what happened to get things fixed up and working properly.
.

If I ever do, you got it!

quote:

Originally posted by flesheatingvirus:


I actually don't recall a mixing valve during construction

There’s got to be one, they are required. It’s what keeps you from getting scalded if someone flushed a toilet or having your water go ice cold on you if someone starts the dishwasher while you are in the shower. It’s right behind the shower water control knob or lever usually.

quote:

Originally posted by PASig:

quote:

Originally posted by flesheatingvirus:


I actually don't recall a mixing valve during construction

There’s got to be one, they are required. It’s what keeps you from getting scalded if someone flushed a toilet or having your water go ice cold on you if someone starts the dishwasher while you are in the shower. It’s right behind the shower water control knob or lever usually.

I think he meant the valve on the recirculating system that allows the hot water return, not the thermostatic mixer in the shower.

interesting. it should be a mixer/control at that shower or along the way. i doubt its a leak or something.
have you looked at the service stop screws on the mixer? or is there any other mixer/shower along the way to that shower? they make smaller ones for faucets and such that ive seen with service stop/screws. plumbing use to make more sense...

i had my tankless act up years ago, and it ended up being a bad thermometer at the inlet. but that affected the whole house i believe.