NORITZ Tankless Heater--water temp fluctuation

[shall]

NORITZ N069M TANKLESS Natural GAS WATER HEATER

We installed a NORITZ IN March of 2007. We have had water temp fluctuation ever since. We have had several opinions on cause of problem to no satisfaction. First we reran 3/4 line direct to gas supply about 5'. Second we contacted PGE and they resized the meter upward to 630 btu which is almost twice the size needed. I contacted NORITZ and went through the test procedure and all results were well withing operating ranges for supply and demand of water including outlet temperature. The flow through the heater during shower use was registering 1.5 gpm so have plenty of capacity. The temperature rise is from 58 to 120 degrees. A 60 degree spread and the heater should provide 5.5 gpm at that level.

The water pressure at the residence shutoff is about 60 to 70 during the day and does fluctuate a little within that range. At night the pressure has registered as much as 80 to 85 on the "WATTS'" Regulator Test Guage.

The NORITZ people felt that the problem was likely in our plumbing --like the MIXET valve in the shower. If the shower is in use the temp fluctuates slightly. If water is used any other location a dramatic change occurs. Not very pleasant.

I was about ready to remove the NORITZ and install a storage water heater to resolve the problem. Seems like a step backward so am looking for ideas.

The MIXET shower valve is a mechanical setting so don't see how that could be changing hot water flow. The only thing that I see is the input pressure change but no one seems to click on that.

Stan

[jadnashua]

A tankless system may or may not have a modulating burner - some do, some don't. If it does have one, it takes it a little time to adjust (not much, but it does), if it doesn't, then the temperature of the outlet will change, sometimes radically, when the volume through it changes. Think about a pan of water on the stove. Put an 1/8-cup in the pan and let it sit for a minute on a hot pan, it may be boiling. Now, redo it and put in a quart of water. After a minute, it will be hotter, but no way as much.

In the shower, you might be much happier changing the shower control valve to a temperature controlled one. But, even those can take up to a second to follow the and respond to the change in water temperature.

I think what's happening, is any use of hot water elsewhere is not being responded to quick enough by the tankless system, and the temperature of the shower drops because the temperature of the hot water drops.

Some tankless systems are better than others. They work best as single use devices, some are okay for multipoint use.

[fidodie]

are you on well water?

i have a cute two temperature reading thermometer which i have hooked to the input and output of my tankless - and i can see the output shift with input temps - i have a well, so the expansion tank is "colder" than the well water - the well cycles, and i get temperature cycles.

i think it is an azel unit - runs on a battery.
i also have a fluke dual probe thermometer (about $100) which i could do the same with.

both temp gages attach to the pipe with foil tape, so there is a delay, and buffering effect, but it is obvious that input effects output. the fluke is fast sample, the azel is slow.

if you have a unit that is supposed to create constant temp output, and you are feeding the input water temp in their "temp range" and "flow rate", then it is their unit...

[shall]

The NORITZ system has an internal temperature monitoring setup that can be accessed with the control panel. When we went through the check program we could see that the water temp at the input was 58 degree steady. The output also remained constant within a few tenths. So at least the check system sez it is operating oK. Will re-test and watch output temp while changing demand at two or three faucets.

The local water supply is from wells and stored above ground. The delivery system is all underground for quite a distance. Could the pressure fluctuation at the input cause the output temp to vary? Would the addition of a pressure regulator be a reasonable thing to do? Where would a pressure regulator be installed and what does one look like?

Do I just have to accept this water heater as a marginal single use device? NORITZ continues to say that the unit is sized properly and will supply two or more showers or appliances.

Thank you for the thoughts about monitoring the input and output temp simultaneously.

Stan

[jadnashua]

You'll need to try the varying load and see what happens. Note, until you test, you won't know how fast the temperature readouts will respond. What does the spec sheet say for response to a varying load? It's hard to get either an electrical or gas valve to modulate very quickly - there is likely some hysterisis built in which will slow the response.

A thermostatic shower valve typically responds in 1-second or less timeframes, and might be able to track the variations faster. I don't get big ones at home, but used the same valve in a hotel. The temperature stayed the same, but it also compensated by pressure changes by decreasing the flow; IOW, it used both pressure balance and thermostatic controls to ensure a consistant temperature.

Someone turning on another hot water tap might not change the pressure enough, so the pressure balance function wouldn't adjust to keep the balance between hot and cold, but as the device lags in adjusting the hot water at the source, you notice it.

If the sensors at the tankless unit are covered with mineral deposits, they won't respond as quickly as they should. Your system may need a good de-liming.

[gmt]

I assume that you can check on the output temperature at the Noritz. Have you tried flowing water at normal use and when the problem occurs check the output temperature.

I am not sure about Noritz, but on Rinnai's we can get the outgoing temperature from the control pad.

If the output temp is at the desired temp. then the problem is most likely with the plumbing.

Are there any roughed in valves in the house that are not trimmed out? I've found that before, causing a cross connection between hot and cold. And that issue was occasional.

Just a thought.