Water Heater Temperature Control FAQs

Q&A: How to Safely Set Hot Water Water Heater Temperature

What Hot Water Temperature is Needed FAQs

Hot Water Temperature: is 100°F enough for dishwashers, hand washing, etc.

My apt hot water registers only 100*-- is this sanitary for dishwashers ,hand washing etc? On 2018-01-30 by Phyllis

by (mod) - 100°F is not hot enough for commercial or restaurant applications

Perhaps for residential use 100 F is ok but as we replied to your question elsewhere, 100 degrees F is not sufficiently hot for commercial use such as in a restaurant.

Commercial dishwashwers as well as residential ones generally include their own heating elements to produce the necessary washing temperature.

Keep in mind that for washing hands, it's not water temperature but washing time (20 seconds or more) and the use of soap that are critical. Certainly you'd not try to sterilize hands by using scalding temperatures.

Watch out: A proper, safe solution to "not enough hot water" can be obtained by increasing the water heater output temperature but also including an anti-scald device either at the point of use or at the water heater itself.

See ANTI_SCALD VALVES & HOT WATER QUANTITY

Hot Water Temperature Too High - Too Hot FAQs

Tip: See WATER HEATER TEMPERATURE ADJUSTMENT CONTROLS

Watch out: hot water temperatures that are too high risk dangerous scalding burns.

See details at ANTI SCALD VALVES & TEMPERATURE CONTROL / MIXING VALVES 

Gas water heater is making boiling water -

Gas hot water unit boiling, and we have no cold water mixing valve to adjust the hot water temperature. No water at all in kitchen On 2017-09-27 by Jason

by (mod) - boiling water heater, nothing at the taps

Jason that sounds dangerous. I would turn off the water heater immediately.

A water heater that is boiling, at sea level is at 212F and WAY above any safe temperature.

Watch out: I would expect the temperature-pressure relief valve to be spilling boiling water if your water heater is really boiling. Since you don't indicate that's happening your heater is UNSAFE and risks a bleve explosion. Turn it off if you haven't done so.

All gas water heaters include a temperature dial or thermostat, usually right on the gas valve, like the one in the photo I've included in this discussion. Turn that to a lower temperature.

Now take a look at GAS HOT WATER TEMPERATURE CONTROL

With the immediate safety problem of addressed, you can search InspectApedia.com for NO WATER PRESSURE to read the diagnostic steps to address the problem of "no water at all in kitchen".

Indirect water heater is too hot - scalding!

My oil burner's indirect-fired hot water heater is Overheating.

The hot water tank thermostat (usually set about 10:00 position) will click on & off at the 3:00 position, after a scalding hot shower, will accurately click around 1:00 position - so I don't think it is a bad thermostat.

Could have screwed something up during oil boiler repairs last spring?

I am GUESSING that the hot-water pump is coming on every time a baseboard zone asks for heat, even though tank thermostat didn't ask for it. If so, DHW would have never overheated during hot summer; wouldn't have noticed during fall & early mild winter; but now that bone-chilling cold is (tripping a different zone thermostat every half-hour) it is hyper heating the DHW tank.

Not certain, but what could cause the DHW pump to come on every time the baseboard circulator pump comes on? What relay could be stuck or could a setting be screwed up? - On 2018-01-02 by Eric

by (mod) - suggestions for diagnosis and repair of too hot indirect water heater

Watch out: hot water temperatures that are too high risk dangerous scalding burns.

See details at ANTI SCALD VALVES & TEMPERATURE CONTROL / MIXING VALVES 

I don't know what water heater temperature settings correspond to your clock face notations, but

Start by checking the actual temperature setting at the indirect water heater's thermostatic control. Set it no higher than 120°F

I've included a photo of a typical indirect water heater, this one is a Burnham model. The yellow text points to the indirect water heater QUASTAT / LIMIT CONTROL - you'll see a small dial showing the temperature setting. You may need to remove the cover to adjust the aquastat dial to a lower setting.

For help setting the control

see INDIRECT HOT WATER HEATER TEMPERATURE CONTROL

Honeywell L8124C Aquastat adjustment trouble - water temperatures are abnormally high over 200°F

I have adjusted the settings on my Honeywell L8124C Aquastat to a high of 160 a low of 120 with a diff of 20.

Yet the water temp soars to 190 and up to 205.

And it sounds like steam noise on inside of boiler.

Tried closing return and draining with a hose to see if it was airbound. But after 100 gallons still a steady stream no bubbles. Could my aquastat be fried? Thank you. (July 28, 2015) Richard

Reply: safety warnings

Richard:

Watch out: this sounds dangerous:

1. there is a risk of scalding burns to people using hot water

2. It sounds as if the limit control is not working properly or is not sensing temperature properly, perhaps your service tech, like mine, does not like to use the thermally-conductive grease that assures good contact between the aquastat's temperature probe and the sides of the well into which it is inserted.

Watch out: This is a ** DANGEROUS** situation.

Before giving up, search InspectAPedia.com for AIR-BOUND HEATING SYSTEMS  and also for AIR DISCHARGE at FAUCETS to read about a couple of other conditions that might be occurring at the same time. But do not ignore that over-temperature warning.

Frankly I'd prefer to shut the system down pending repair.

Richard said:

Dan I think and or am hoping for "airbound" the system was working fine until well pump failed and system had to be replaced.

I am going to drain at bottom of boiler this time for air and take it from there. Thanks for the reply!

And considering the age of system, Buderus might be the ultimate final step....;)

Reply:

I'm not clear why air in the heating system would cause abnormally high water temperature levels.

More likely air in the heating piping will prevent hot water circulation and you'd have cold radiators or baseboards.

While the aquastat sensor probe itself may fail, requiring aquastat control or probe replacement, be sure to also read

AQUASTAT SENSOR PROBE HEAT CONDUCTING COMPOUND - required on some, not all, aquastat temperature sensing probes.

Hot Water Stand-By or Extra Storage Tank FAQs

Use old water heater tank as a stand-by or temperature booster

I am considering using an old water heater tank as a standby tank for hot water in the east end of the house, while using the newly purchased heat pump water heater to actually heat the water

If installed so that the new water heater supplies directly to one bathroom, could the hot water then be piped directly to the 2nd tank to increase the hot water available and decrease the wait time for hot water at the farthest away point from the new water heater?

It is approximately 80 feet from one end of the building to the other, with bathrooms on both ends. On 2017-06-04 by Ray Hawkins

by (mod) - make sensible use of hot water reservoir tanks

Ray,

In my opinion that's not a reasonable possibility.

A lesser question will be the standby heat losses in the remote old water heater tank that is just serving as a hot water reservoir.

I mean to say that if no one uses hot water from that tank for a time then it will cool off and people in that bath will not only not have hot water but it will take much longer for their water to get hot since you are going to require that the remote tank be completely reheated by the primary hot water source.

Second but more significant problem is the heating capacity of the new water heater. Your water heater is rated to heat water to a given temperature rise over at particular interval. The specifications are matched to the size of the water heater.

For example if your new water heater is a 50 gallon unit and you are now using it to try to heat the old plus new water tanks - say 100 gallons- to an acceptably hot temperature, you are doubling the work of the new heater and you're likely to be over tasking it.

I can see an option for making use of the old water heater tank provided it is in good condition than worth the trouble.

That is to use the old tank ahead of the existing new water heater as a temper tank, that is to act as a standing or holding tank to allow incoming cold water to rise to room temperature when no hot water is being run through the system. That reduces the temperature rise work that your new heater has to perform.

In that role the old tank would actually work better if it were not insulated as that would expose it and its water reservoir to the room temperature in which the tank is resting.

Hot Water Not Hot Enough FAQs

Tip: See the detailed advice at WATER HEATER TEMPERATURE TOO COLD or TOO HOT for steps to correct water that just won't get hot at all or won't get hot enough.

"Hot water" is only lukewarm

I have no domestic hot water coming through faucets. it is lukewarm. I changed the mixing valve, but it is not getting hot. On 2017-03-15 by Ed Todd

by (mod) re: what to check if water temperature is only lukewarm

Start by checking water temperature at the water heater itself.

If the water heater temperature is too low I suspect

1. that you have an electric water heater

2. that its upper element is burned out

3. that the mixing or anti-scald valve settings are correct

Tankless Water Heater Doesn't Get Hot Enough

Brand new gas instantaneous hot water unit is turned up to maximum but hot water produced is still only hand hot.

How can this unit be turned up? The temperature of hot water is too cool even to wash wishes and too cool to have a shower. Builder inspected but says temperature set. What and how can I turn up the heat? On 2015-05-11 by Wendy

by (mod) re: new instantaneous gas water heater won't get hot enough

Wendy we offer some diagnostics for instant or tankless water heaters at

TANKLESS WATER HEATERS

Your heater will have a temperature control dial or digital pushbutton setting control that allows changing the target water temperature.

Start by checking the flow rate in gpm through the system and the temperature of the incoming water.

These heaters can do a great job, but I've sometimes spent a lot of time fooling around to get the system to behave, especially when water pressure or flow rate or temperature is more than the heater is rated to handle. Remember that whatever the incoming temperature the heater can only raise that by a fixed maximum.

Try partly closing the incoming cold water to slow it down a bit through the heater to see what happens.

Remember that a tankless water heater is rated to raise the temperature of incoming water a specific number of degrees; if incoming water is very cold or if water is running too fast, either of those conditions could be the trouble.

Keep me posted here ify ou like, but better to post further comments on the Tankless Heater diagnostic pages.

Why hot water temperature varies wildly from week to week

One week the water temp is VERY hot and the next week it is back to normal. Why? On 2017-03-12 by Gary

by (mod) -

Gary

Ruling out normal conditions such as someone using most of the hot water then someone else finding tepid water at another tap - I would guess that the thermostat on your water heater is not working properly and needs replacement. It could be a bad temperature sensor or, less likely, a loose electrical connection or still less likely a failing temperature control.

We don't know how your hot water is being made so I can't be more specific. - Daniel Friedman

Water Heater / Calorifier / Geyser Won't Stop Running FAQs

Our Water Heater Never Turns Off

Hi,
The water heater started working continuously, never stops. I had to unplug it. It is 100 gal. natural gas hydrojet heater.

The water temperature after many hours, form old cold water to 18 hours, is just warm that I can put my hand under.

My guesses are the burner, the thermostat (thinking that it is hot and keeps shutting and the other saying no let's warm it).

Maybe it is the cold water pipe in the tank. ? Thanks. On 2016-03-29 by Gale

by (mod) re: water heater won't stop running

1. confirm that hot water is not running out of a tap or fixture somewhere that you've missed

2. if the burner still does not shut off but the water is not getting hot, then the burner is not operating properly, and could be unsafe (fatal carbon monoxide risk) - shut down the system

3. If the water heater temperature is not reaching the temp set by the heater's thermostat then there is a problem with the burner (or electric calorifer/heater/geyser elements)

4.Or there may be a defective water heater thermostatic control

Watch out: excessive temperatures or pressures at a water heater that won't shut off are very dangerous, risking both scalding burns and a BLEVE EXPLOSION (search InspectApedia for that phrase to see details)

Hot Water Quantity Runs Out Fast FAQs

Tip: Diagnosis and repair for hot water quantity are at HOT WATER QUANTITY IMPROVEMENT. 

Boiler & Tankless Coil Runs Out of Hot Water Almost Immediately

Hello, I have a Riello boiler that is about 8 years old. I dont have a [separate] water heater.

Over the last two months the shower when turning on goes from hot hot hot water but only lasts a few min before it turns to just warm. what is causing this? even when i have it at the highest nozzle setting. On 2015-12-21 by sabreena

by (mod) hot water doesn't last - tankless coil? -

Sabreena I'm not sure what nozzle setting you mean;

But if nothing else has changed and your hot water is provided by a tankless coil at your heating boiler, it sounds as if the boiler temperature is not being maintained high enough.

Ask your heating service tech to check the aquastat control settings and function.

See details at AQUASTAT HI LO DIFF SETTINGS 

Also, there may be a mixing valve at the boiler - an example is the photo at the very top of this page.

Depending on where you live, (e.g. Norway), mixing valves are also often installed at point of use in bathrooms and kitchens.

by Sabreena

Hi Dan, meant that i have the shower nozzle turned to the max hottest setting and it isnt that hot. someone mentioned a mixing valve? is that something that i can manually check myself? the aquastat?

Shower water starts out hot and suddenly goes cold. Anti-scald valve problem?

Shower water starts out hot and suddenly goes cold. Single mixture valve.

Next shower shortly thereafter is fine for hot water. How does anti scald valve figure into such a problem. Thinking about replacing anto scald valve in shower. On 2017-12-27 by Bob

by (mod) -

Bob

I speculate that the anti-scald valve may be sticking, failing to mix hot water. Try having a friend suffer in the shower while you observe the pipe temperatures into and out of the anti-scald valve.

See ANTI SCALD VALVES & TEMPERATURE CONTROL / MIXING VALVES 

Hot Water at Some but Not All Fixtures

Our water is hot at the bath tubs but not at the sink faucets

Hot water runs through bath taps ok cold running through sink hot taps On 2015-03-27 by Jill

Reply by (mod) - diagnose loss of hot water pressure or temperature at some but not all fixtures

If your hot water temperature is hot enough at some fixtures but not others, I would look for

1. Look for an automatic mixing valve or anti-scald valve that is set to a too-cool temperature at the too-cool or too cold fixtures
2. Look for a longer run or uninsulated pipes or pipes running through a cold space to the too-cool or too-cold fixtures.

If your hot water pressure has been lost but you still have some, poor hot water flow at all, some or just one fixture,

see HOT WATER PRESSURE LOSS

 

Therm-O-Disc 59T Water Heater Control FAQs

Boiler is running a lot, controlled by Twin Thermo Disc 59Ts

Hi folks. Hope everyone's warm. I have a question an hope some details may help:

I have a nice cast iron OLSEN OSC-4 burning nat gas with the original TACO pushing through all iron rads via a single zone valve. 1500 ft of home, roughly, with decent insulation and windows, so sized right.

Off to the side of the OLSEN, there's a separate loop to an old AERO A-50 storage tank.

All from 1999 and still running strong.

The AERO tank is fitted with its own TACO circulator which runs the tank water through a single-walled sidecar exchanger nestled into the side of the ext tank skin - no internal loop in this AERO - and so the boiler TACO pushes one way, while the tank TACO pushes the other. Voila, hot water in the storage tank.

Priority for the full kit is DHW. Eg if the AERO still wants DHW heat, we get no space heating.

On the AERO are two Therm-O-Disc 59T stats that run back to a control module bolted to the OLSEN boiler. Installer labelled 59T#1 as "boiler" 59T#2 as "pump". I reckon this means #1 calls for the boiler to fire and circulate while #2 runs the other (dedicated) circ for the heat exchanger. All guesses but it kinda makes sense to me.

Assuming this to be mostly true, what should these be set at? Same preferred temp, near enough? One high, one low?

I get the sense that the boiler is running A LOT these days, being called by the tank trying to heat that tank water, and the boiler bumps up against its own high limit, then cycles again after it cools some.

I've tweaked the 59Ts in the past and suffered from a "lock out" of some kind, then re-tweaked one or both of them until the zone valve opens for the rads and all is good. No clue how I messed it up, no clue how I "fixed" it. Just me twiddling slotted dials.

Yet there must be more logic to it versus grabbing a screwdriver and turning the dials.

What's the best practice I should follow here in terms of setting up the twin 59Ts?

We live in the Golden Armpit/Toronto, with -10*C winters not uncommon.

We like ~130* water at the tap and this system delivers unless all the laundry, showers and kitchen are hauling hot water, then it wobbles for a half hour or so (but this doesn't concern me).

- Sidecar heat exchanger stripped and cleaned a few years ago, all good.
- Tank TACO replaced at the same time, all good.
- High Limit on the OLSEN is 180*:
- WATTS tank replaced a few years ago, working great, all good.
- System rests at 12-14 lb
- System runs at 22-26 lb
- T&P on the OLSEN is clean, no drips
- Flue gas analytics shows she still burns clean.

Any ideas on optimizing those two 59Ts or otherwise fine-tuning this rig?

Thanks and Happy New Year.
JC
Toronto, ON On 2023-01-05 by JC

Reply by InspectApedia Publisher - Your "boiler" is a domestic hot water heater using Thermo Disc 59T temperature controls

@JC,

Update: my initial reply below was confused as not until later in this conversation did it become clear that we were discussing a water heater (for domestic hot water for washing and bathing) not a heating boiler used for heating the building's occupied space.

For a conventional aquastat like the Honeywell described above on this page, I'd use the optimal settings given above on this page - usually.

In Toronto the heating people often set up a boiler so that the circulator runs continuously and the thermostat (the one that calls for heat in the occupied space) will simply turn the boiler on when it wants heat.

The 2nd thermostat for your hot water will run the circ that runs boiler water through the coil to re-heat the indirect hot water tank.

Now about the Thermo-Disc-59T - that's normally used as an electric water heater hot water control - you can see its settings - factory- in the photo.

A Therm-O-Disc 59T 4300 66T 4401 Surface Mount Thermostat Limit 120 to 180 F, 120V like that shown below has its operation given in this nice summary

THERM-O-DISC-59T-66T-CONTROL [PDF]

by Jim H - still have issues with boiler short cycling

Hi, follow-up to the conversation about our boiler running 4 minutes then off for 8 minutes repeatedly. I had an appointment for today but the tech called and asked what the problem was. He says that's the way the boiler is supposed to to work, to avoid overheating, but agrees it shouldn't take hours to come up to temp.

I was told the service manager was going to be coming but now he's not available. He's going to try to get in touch with the service manager and I'm not sure if anyone is still coming to day or not From what understand from this conversation, this is not the way t should work and until several years ago, it was fine.

It could come from 62-68 by the time I set the thermostat for, with the early recovery mode. Now it can't do 64-66 in less than a few hours. Suggestions, ideas?

by InspectApedia Publisher

@Jim

At our step by step diagnosis and repair advice at SHORT CYCLING HEATING BOILER I've exhausted this topic.

I've outlined every situation suggested by your earlier comments, questions, and discussion - thank you.

Do let us know what the service manager ultimately says - as that might help other readers.

by InspectApedia Publisher - why is boiler short cycling

@JC,

Keep in mind that a common issue with any heating or hot water control device that depends on contact with an exterior surface like that of a pipe or tank is that its behaviour can become erratic or flat wrong when any of several factors affect the quality of the contact between its sensor surface and the hot water pipe or tank surface:

• it has to be tight
• the surface of the pipe or tank has to be clean
• the surface of the sensor has to be clean
  • the location of the sensing point might also be relevant
    

For example, if nothing else has changed but your system isn't working as it used-to, I'd consider removing the two sensor controls and cleaning up everything, then re-installing them.

by JC

@InspectApedia Publisher, cheers for your reply.

Nope, no continuous-run TACO circ on the boiler. Not set up that way. When nothing calls for heat, nothing runs: either full on or full off.

I think my Q is more about setting the twin 59Ts on the indirect AERO. If one of those calls on the boiler to circ + fire (main loop), while the other runs the heat exchanger circ (tank loop), should they be set at approx the SAME TEMP? Should the heat exchanger circ stir up the tank a little before the other stat calls the boiler?

** And thanks for the schematic and info on the 59T you attached. Mine are older than that, but the point's the same. What that info told me is these stats "sense the surface temperature of the water heater TANK..." eg not the water inside the tank, but the jug that holds it.

Which may answer that part of my question as to the apparent short cycling: these stats are inside an ACCESS PANEL the cover of which was lost long ago. So they sit open. Makes it easy to adjust sure, but if they are sensing the TANK surface temp they're probably quickly cooling to the ambient air and fooling the boiler into firing more often.

Of course, the WATER in the tank is pretty hot and the stat is satisfied pretty quickly, ergo, short-cycling. **Does that make any sense to you? I'm gonna search for that access cover or jury-rig something to see if the behaviour changes meanwhile.

Thanks for your thoughts again.
JC

by JC

@InspectApedia Publisher, I'll try the low-hanging fruit first and carve some pink foamboard to fit the access cavity in lieu of the lost cover, ... give the 59Ts a warm dark cave to sit in, un-exposed to drafts etc - see if that makes them happier ... cheers, JC

by InspectApedia Editor

@JC,

I appreciate the sense of trying to insulate near temperature sensitive control, but if this is a new problem and the installation hasn't changed and I don't think that that's the issue. Also I'm a little nervous about the fire safety of putting combustible foam insulation around and up close on an electrical device.

by JC

@InspectApedia Editor , understood, yet the stats were surrounded by yellow batt, then a steel cover (until i foolishly lost that cover... ) so someone thought these could be enclosed by design i guess... we're not talking mains voltage or any kind of current here, 24V off the transformer ... and beyond losing that cover, nothing else has changed no, just that it seems to be short cycling;

i didn't even think that these were a kind of hard surface stat and not immersed in some fashion - never crossed my mind ...

by InspectApedia Publisher

@JC,

There is both line voltage (120VAC) and also lower voltage inside the aquastat.

by JC -

@InspectApedia Publisher, thanks but this is all 24Volt running through the 59Ts on 18 ga. wire, stepped down by transformers bolted near the boiler motherboard. (Ignore the AERO rating plate in the picture).

When the stats close across TV-T the mains throw for pumps & the blower happens back in the relay box (R8222C).

by InspectApedia Publisher - why a temperature sensor on a hot water tank would turn on a circulator

@JC,

Thank you for the added detail. It's another example of the difficulty we face in trying to be helpful in discussing heating controls when we have so little information about an actual installation. Even now I don't have a complete, clear picture of your installation.

The phrase

When the stats close across TV-T the mains throw for pumps & the blower happens back in the relay box (R8222C).

leaves me uncertain about what components make up your system and what is being controlled. Why a temperature sensor on a hot water tank would turn on a circulator is clear: to re-heat water in an indirect-fired domestic hot water tank, by turning on a circulator pump that circulates heating boiler through a coil that in turn heats hot water in the hot water tank.

But what "blower" is where and what you mean by "blower happens" and what "relay box" we're discussing is, apologies, unclear. Maybe a sketch of the system and its components would help.