Tap water too hot?

Bob Barrows · Dec 8, 2007

Hi everyone.
I just had a Superstor tankless water heater installed yesterday. It's Sat. so i cannot contact the installer, therefore I came here.

I told the plumber who was installing the deater that the tap water had always seemed to be too hot with the old water heater and that adjusting the thermostat seemed to have no effect. So he told me he used a new thermostat and set it to 125. When I checked it today it still seemed very hot, so I used an instant-read thermometer to check the temperature of water dispensed into a coffee cup: it registered 154 degrees (F)! So, I opened the thermostat and sure enough, it was set to 125.

Is it typical for tap water to register 30 degrees above the setpoint? A possible theory is that the thermostat only controls the lowest temp that the water in the tank is allowed to reach, and that the water is allowed to get as hot as the furnace/boiler will get it to. if that's the case, then there is nothing wrong with the thermostat, is there.

I'm considering turning it down to 100, but, if the above theory is correct, I may be allowing it to get too cool to kill bacteria ...

Am I testing the right thing? Could the water coming from the furnace/boiler be too hot? Any advice?

Bob

Leejosepho · Dec 8, 2007

Bob Barrows said:
I just had a Superstor tankless water heater installed ...
... he used a new thermostat and set it to 125 ...
I used an instant-read thermometer ... it registered 154 degrees (F)!
...
Is it typical for tap water to register 30 degrees above the setpoint?

As you have mentioned, it could be that the heating source continues heating the water while it is "cooling down" after the thermostat cuts out. I would think the thermostat should have some kind of anticipator to address that matter, but I have also heard tankless heaters are either "on" or "off" irregardless of the volume of flow going through them, and that that can cause their temperature outputs to be all over the place.

Bob Barrows said:
I'm considering turning it down to 100, but, if the above theory is correct, I may be allowing it to get too cool to kill bacteria ...

You might experiment a bit, but I doubt you will get results that are truly controllable.

Bob Barrows said:
Am I testing the right thing? Could the water coming from the furnace/boiler be too hot?

Your water for the tankless heater is being preheated?

Sponsor · 2022-01-29T12:20:58-0800

CHH · Dec 8, 2007

Was the Superstor zone added with a control valve or just a circulator?

Assuming the thermostat is working correctly, I'm guessing that "ghost flow" may be a problem.

Bob Barrows · Dec 8, 2007

Thanks for the reply.

leejosepho said:
Your water for the tankless heater is being preheated?

Well, I'm a little confused by this question. Maybe I used the incorrect term for this water heater? The plumber also called it an "indirect water heater". Is that different from a "tankless" heater? Frankly, I was a little puzzled by the term "tankless", since this thing consists of a tank containing coils fed by hot water from the furnace to heat up the tap water inside the tank.
Anyways, I don't think any preheating of the water is occurring.

Bob Barrows · Dec 8, 2007

CHH said:
Was the Superstor zone added with a control valve or just a circulator?

U-u-u-u-m ... errr ... good question?

Assuming the thermostat is working correctly, I'm guessing that "ghost flow" may be a problem.

So are you saying the the 30 degree differential is unusual? I guess I need to call the plumber back Mon., eh?

CHH · Dec 8, 2007

The SuperStor is an indirect water heater. It's basically a big tank with a heat exchanger in it. It looks similar to a stand alone water heater but it doesn't have a gas or electric line attached to it. It does have hot water from the boiler piped to a connection near the bottom of the tank.

A tankless water heater is much different. No big tank, just an inline burner and the plumber wouldn't mention using a new thermostat since temperature sensing is a built-in function.

CHH · Dec 8, 2007

Bob Barrows said:
U-u-u-u-m ... errr ... good question?

Pictures, post pictures

Bob Barrows said:
So are you saying the the 30 degree differential is unusual? I guess I need to call the plumber back Mon., eh?

Yup - the aquastat should control flow to hit the target temperature. If it doesn't then something is wrong. This is kind of important and it could be a safety issue. Give the guy a call today. Leave a message if he doesn't pick up.

Bob Barrows · Dec 8, 2007

OK, sorry for the confusion.

CHH · Dec 8, 2007

Pictures help. An existing indirect tank was replaced with a SuperStor.

A few more close up pictures of the entire supply and return lines between the boiler and SuperStor would clarify the questions I have on the controls. The circulator is apparent but I can't see if there is a zone control valve or a flow control valve in the circuit.

Here's a link to a discussion of ghost flow:

http://www.bellgossett.com/Press/CounterPointJune2002-A.pdf

Bob Barrows · Dec 8, 2007

I've got a call into the plumber, but figured I'd finish this discussion anyways. Here goes with more pictures:

CHH · Dec 8, 2007

I sure don't see a zone valve on the indirect piping. The only thing that might be a flow control is the brass fitting between the indirect and the circulator. Is that just a check valve? I can't tell. Maybe someone else recognizes it.

Do the lines from the boiler to the indirect get hot when a heating zone makes a call for heat and the aquastat isn't calling for heat? If so, its a ghost flow problem.

Bob Barrows · Dec 8, 2007

CHH said:
I sure don't see a zone valve on the indirect piping. The only thing that might be a flow control is the brass fitting between the indirect and the circulator. Is that just a check valve? I can't tell. Maybe someone else recognizes it.

I can't tell but I've been trying to fit your terms to the objects in the picture ... how'd I do?

Do the lines from the boiler to the indirect get hot when a heating zone makes a call for heat and the aquastat isn't calling for heat? If so, its a ghost flow problem.

I don't know if I can answer this since I'm not sure how to tell if the aquastat is calling for heat or not. The plumber called back and sounded as if he was dismissing my concerns ("those thermostats aren't perfect, you know") until I mentioned the term "ghost flow" at which he kinda perked up and said "hmmm, that's a possibility ...". So, he's supposed to come by later today.

CHH · Dec 8, 2007

Your picture labels are right on the mark.

If the aquastat is calling for heat the circulator for the indirect will be running. The circulators are pretty quiet compared to the noise of a boiler firing so you might not hear it. There is a little vibration and the motor housing gets pretty hot when they're running. Of course, anything in contact with 160+ degree water gets pretty hot.

Anyway, it's good that the plumber will look at things. That water temperature is up near scalding so that alone is a hazard. Hope the plumber gets things straightened out.

Jadnashua · Dec 8, 2007

While something is wrong with the setup as it should be about the set temperature, it isn't a bad idea to install a tempering valve on the outlet of the WH. Having the tank with very hot water wastes some energy, but with a tempering valve would make the tank function like it was larger, since you'd be mixing the hot with some cold before it got into the distribution lines.

Bob Barrows · Dec 8, 2007

The plumber came tonight and dismantled, cleaned and reassembled the flow/check valve. I should be able to tell tomorrow whether this made a difference.

BigLou · Dec 8, 2007

My guess would be its the temperature sensor in the superstor. Just today I installed a buderus temp sensor in my superstore, set it to 130 and got 154 degree water out of my tap. ghost flow is not a possibility as the system was designed to prevent that. its simply a design difference between the sensor pocket on a buderus and a superstor

Lou

Alternety · Dec 8, 2007

Poor thermal conductivity between the sensor and well wall could produce what you are seeing.

You might want to check to see if there is good thermal contact between the sensor and the well wall. Perhaps something to wedge it a bit if the sensor is loose in the well and a bit of heat sink compound between the surfaces.

Bob NH · Dec 9, 2007

The best solution is to keep the temperature setting higher and have a tempering valve on the outlet of the indirect heater.

There is an advantage to having the temperature setting higher in an indirect heater because it helps meet high demands. The higher temperature setting will give you a greater supply and recovery because the tempering valve will add cold water to the flow, and the sensor will call for heat more quickly when there is demand.

Jadnashua · Dec 9, 2007

When I was having my new boiler/tank installed this pas Jan, the installer mentioned he had to go back to one where the sensor was not fully inserted into the well. Depending on where the wire comes out, it can be pulled out a little by accident.

CHH · Dec 10, 2007

That's a good point Jad. The thermometer well has a ledge in it that can "hang up" the sensor. Since the ledge is in a ways, if the sensor bulb is short enough then the installer might not notice that things weren't quite right.

I had a few moments of "what's going on here?" when installing my SuperStor but probed with a screwdriver to figure things out.

Tap water too hot?

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