Tankless water heater temp fluctuations

diego168

Hello everyone. I have an oil boiler for home heating and water heating with recirc loop. When we turn on the hot water it will warm up then in a few min get cold and after around 5min get warm again but after another few minutes water gets cool again and takes longer to warm up. Or it seems that it takes longer cause you all soaped up cold in the shower. The water settings are L150 H190 diff 20. Those are the setting it had since we move in a few months ago. I would say the temp fluctuAtion has been there since we move it.
On researching the forum the initial cool down of water could be “sandwich” Effect but then it should stay warm after it warms up again. Correct ?
I’m wondering if this is normal for this type of water heating or any ideas ? I’m trying to get an understanding of what could be wrong before I call someone for service.
Could it be my settings? Mixing valve?
Side note home heating works normal.

Snowmelt

The cold may be coming in the fixture from some other point, meaning you have a fixture that’s leaking cold water into your hot line

diego168

@Snowmelt do you mean at the shower opening handle. The hot cold mixing valve. ?
I could run the sink see if it happens also there.

Larry Weingarten

Hi, Try the test for cross connection, which is shutting off water to the heater and then opening a hot tap. Water should stop flowing pretty quickly.

Yours, Larry

diego168

I’ll give that a try thank you

HVACNUT

You have a tankless coil in your boiler and a recirc loop tied directly into it? What a waste of $$. How often does the boiler cycle? 24x a day? In the summer?

diego168

> @Larry Weingarten said:
> Hi, Try the test for cross connection, which is shutting off water to the heater and then opening a hot tap. Water should stop flowing pretty quickly.
>
> Yours, Larry

I turned the water supply to the heater and opened the hot water faucet and there was no water coming out so I guess it’s not getting mixed.

diego168

> @HVACNUT I’m new to owning a home and not familiar with this system. I’m reading to understand how this system works. Yes everyone and then I hear it turn on. Here is a pic of what I got.

Snowmelt

I would just turn the e-switch off in summer

diego168

> @Snowmelt said:
> I would just turn the e-switch off in summer
Not sure which one you mean. How would hot water work on demand

Snowmelt

What you have to do is turn all your hot water fixtures on all the way no cold. Then go in the basement and turn the shut off feeding the coil in your boiler. Then go to each fixture and see if it’s dripping.

diego168

> @Snowmelt said:
> What you have to do is turn all your hot water fixtures on all the way no cold. Then go in the basement and turn the shut off feeding the coil in your boiler. Then go to each fixture and see if it’s dripping.

Oh I did that already water did not run when I turned off the supply to the heater.

I meant what do you mean by turn the e switch off in the summer

HVACNUT

Are you sure there's a recirc loop, with it's own circulator?

You have a mixing valve. It should be set to max 120°.

Sometimes the hot water demand exceeds the coils output (3 GPM?). Think about and indirect.

diego168

> @HVACNUT said:
> Are you sure there's a recirc loop, with it's own circulator?
>
> You have a mixing valve. It should be set to max 120°.
>
> Sometimes the hot water demand exceeds the coils output (3 GPM?). Think about and indirect.

This is a pic of what I got. I thought the loop goes to the small tank. Not sure excuse my ignorance I’m trying to understand how this works

HVACNUT

Yeah you're limited on DHW production. Few are happy with a tankless coil.

EBEBRATT-Ed

Your tankless heater is piped correctly. Tankless heaters are not the best but can be workable for about two people. You could have a bad mixing valve

diego168

> @EBEBRATT-Ed said:
> Your tankless heater is piped correctly. Tankless heaters are not the best but can be workable for about two people. You could have a bad mixing valve

Yes I was thinking about that because it wasn’t as bad before. Is there a way to diagnose a bad mom don’t valve

hot_rod

A rebuild, or better yet a new mixing valve may be in order. Use a ASSE 1017 listed 3 way thermostatic mixing valve for best result.

EdTheHeaterMan

The mixing valve you have is an older design. A newer design with the ASSE1017 listing has a different piping configuration. If you go that route, look closely at the installation instructions. The newer, better valve may resolve the issue.

I do not see a DHW (Domestic Hot Water) recirculation pump in your pictures. I DO see a Circulator (the black pump on the right side of the boiler) connected the pipes that go the radiators. This has nothing the do with the DHW.

Now depending on the condition of the DCW (well water, city supplied water, hardness, and many other factors) your tankless water heater may have some build up on the heat exchanger. If that is the case, a simple chemical cleaning may resolve the issue.

Next is the flow rate thru the DHW coil. The coil is rated at a temperature rise @ Gallon Per Minute. (Delta T per GPM). If you are using the water faster than the rated flow than the water will not get hot enough. slow the water down with an energy-saving showerhead to 2 GPM and see if that solves the problem.

The temperature of the boiler water is the determining factor in the temperature rise thru the DHW Coil. if the water in the boiler is cooler then the amount of heat available to transfer to the DHW is less. Increase the Low limit setting on your aquastat from 150°F to 160°F and see if that makes a difference.

So there are several points that I covered.
1. mixing valve defective
2. mineral buildup on the heat exchanger
3. flow rate too high
4. boiler temperature too low

The lowest price option is # 4. Start there then go to 170°F after a few days if 160° is not enough. Don't go any higher than 170°F. You see, the higher the low limit setting is, the more fuel you will use. Also, there is a message on the low limit that says you must set the high limit a minimum of 20° higher than the low limit.

You can also set the differential to 10°. This will cycle the burner more often but the water temperature fluctuation will decrease or be eliminated.

And what would an Edtheheaterman Post be without a diagram

EdTheHeaterMan

Did you get your C wire issue solved? The wires have insulation on them for a reason you know.... and the electricity is color blind. the blue wire is the one you need to cut to disarm the explosive device in any James Bond movie. In Mission Impossible, it's the Green wire. Or did I get that backwards?

>>>>BOOM<<<<

diego168

@EdTheHeaterMan lol. Thanks again Ed your posts are great/ full of information. Yes sir I fixed the wiring.
I’ll try changing settings see how it works.
Valve I’m going to change anyways for this one https://www.supplyhouse.com/Honeywell-Sparco-AM100-US-1LF-1-2-Lead-Free-Union-Sweat-Mixing-Valves-70-145F

How can you clean mineral build up on the heat exchanger ?

EdTheHeaterMan

First, you will need to set up the coil for cleaning. The original installer did not do that. But that is typical. The yellow valves with the orange handles are regular ball valves. they will isolate the tankless DHW Coil from everything else. The blue valves are hose bibs or boiler drain valves on a 1/2c x 1/2c x 1/2f Tee. connect to those valves are short hoses.



Now you connect a pump to one hose and place the other hose in a 5 gal pail. draw the cleaning agent thru the pump, then thru the coil, then back into the pail. Here is a complete kit including the pump, hoses, and one bottle of chemical.

https://www.supplyhouse.com/Rectorseal-68711-Calci-Free-Tankless-Water-Heater-Flush-Kit?gclid=CjwKCAjwxev3BRBBEiwAiB_PWFAzW-GuIXzfb0DsHVyCIdWPp4LurnRVXDvtesBMacUVErxl9D55WhoCCIQQAvD_BwE

You may already own a pump, it just needs to have garden hose connections. A washing machine hose with female connectors on both ends is needed to connect the pump to the DHW coil.

In extreme cases, the industry-standard was a product called Sizzle. sometimes this would clean the copper right thru to the other side. Then you would need to replace the coil. https://www.zoro.com/hercules-lime-scale-rust-remover-bottle-1-qt-20305/i/G6211256/

diego168

@EdTheHeaterMan thanks Ed.

EdTheHeaterMan

No Problem, @diego168. That will be $350.00, Where do I send the bill?

Nevermind, this site don't work that way

diego168

Ok 👍 diego168@hotmail.com

diego168

@EdTheHeaterMan

Hi Ed so I replaced the valve and the water no longer gets cold.
As for the mineral build up how often do you have to service this. I have city water Does that affects things. I’m sure previous owner has never service for mineral build up

EdTheHeaterMan

Some persons never do the maintenance, then they get to a point where they have no hot water or not enough hot water.

At that time someone will suggest a different water heating solution. I have seen many electric water heaters next to oil-fired boilers with abandoned tankless heaters. Usually unhappy with the electric bill.

I would have suggested that you install the service valves at the time of the valve replacement. If that was done then I would do the maintenance now. Then ever 2 to 5 years thereafter. See how much buildup you have in the 5-gallon pail after you clean the coil in 2 years. This will let you know if you can go longer between cleanings.

diego168

@EdTheHeaterMan ok thank you. I’ll get it done