Water heater ???

[nizak]

I’m doing a small commercial job and ran across something that doesn’t make sense.

There’s a 208/240V hot water heater that’s wired at 120V. Tennant is telling me that it’s been in service for several years producing hot water for 2 of the 4 suites that are part of this small complex.

I haven’t followed any of the piping but suspect they must be getting their hot water from one of the other 2 water heaters that are onsite.

Would the element heat at all at 120V ?

 

[kwired]

It would still heat, just at 25% the rate of what it would if supplied with 240 volts.

If demand for hot water is never enough to deplete the hot water in the tank you could go for years and never know it is under powered as recovery time is all that would be impacted.

 

[Barbqranch]

When replacing a floor in our house, I moved the water heater and used a 120 volt cord and plug. Still had more hot water than we ever used.

 

[Dennis Alwon]

I’m doing a small commercial job and ran across something that doesn’t make sense.

There’s a 208/240V hot water heater that’s wired at 120V. Tennant is telling me that it’s been in service for several years producing hot water for 2 of the 4 suites that are part of this small complex.

I haven’t followed any of the piping but suspect they must be getting their hot water from one of the other 2 water heaters that are onsite.

Would the element heat at all at 120V ?

If the water heater is rated 240V at 4500 watts then at 120V the element will be at 1125 watt.

 

[Tulsa Electrician]

Some math to share for others.

It would still heat, just at 25% the rate of what it would if supplied with 240 volts.

If demand for hot water is never enough to deplete the hot water in the tank you could go for years and never know it is under powered as recovery time is all that would be impacted.

(120÷240)^2=.25
.25*100= 25%

If the water heater is rated 240V at 4500 watts then at 120V the element will be at 1125 watt.

4500×(120÷240)^2= 1125

 

[LarryFine]

Or explain why:

Half of the voltage will push only half of the current through a given impedance.

Since power equals volts times amps, one-half times one-half equals one quarter.

 

[Tulsa Electrician]

Or explain why:

Half of the voltage will push only half of the current through a given impedance.

Since power equals volts times amps, one-half times one-half equals one quarter.

1/2= .5
.5 ×.5=.25
(1÷2)^2= .25
4500 x.25= 1125

4500÷240=18.75
1125÷120= 9.375
9.375÷18.75= .5
1125÷4500 =.25

 

[hbiss]

Tennant is telling me that it’s been in service for several years producing hot water for 2 of the 4 suites that are part of this small complex.

If it only supplies hot water for a restroom or break room nobody would ever notice it. It will come up to temperature and as long as the tank is not depleted and you have to wait for it to recover there will be plenty of hot water.

-Hal

 

[MD Automation]

Somewhat related to this thread - while waiting a few weeks ago to have my main breaker replaced I was attempting to "manage" the usage in the house so as to try and not have large loads come on simultaneously. I was curious how often my water heater comes on if there was no (or very little) hot water drawn from it and how long it would stay on to satisfy the thermostat once it started up.

So I hooked up a cheap current sensing switch to a PLC and wrote a simple program to log the time(s) when that switch changed state.

With no water drawn from the tank, it lasts roughly 11 hours before the thermostat kicks on. And it runs for about 8 minutes to get back up to temp. Pretty well insulated it would seem.

It's a regular "nothing special" State 50 gallon water heater. Typical 4500 W elements (top and bottom).

So... if a heater like this was fed from 120VAC, and not much hot water was drawn from it, it would run for as little as ~30 minutes to get from the cut-in to cut-out temps.

As Kwired and hbiss were saying above, depending on the situation and how much draw down there is, I bet it might be easy to never notice this. It's all about the need for recovery.

 

[Little Bill]

Are they putting the neutral where the other 240V leg would go?

 

[kwired]

Somewhat related to this thread - while waiting a few weeks ago to have my main breaker replaced I was attempting to "manage" the usage in the house so as to try and not have large loads come on simultaneously. I was curious how often my water heater comes on if there was no (or very little) hot water drawn from it and how long it would stay on to satisfy the thermostat once it started up.

So I hooked up a cheap current sensing switch to a PLC and wrote a simple program to log the time(s) when that switch changed state.

With no water drawn from the tank, it lasts roughly 11 hours before the thermostat kicks on. And it runs for about 8 minutes to get back up to temp. Pretty well insulated it would seem.

It's a regular "nothing special" State 50 gallon water heater. Typical 4500 W elements (top and bottom).

So... if a heater like this was fed from 120VAC, and not much hot water was drawn from it, it would run for as little as ~30 minutes to get from the cut-in to cut-out temps.

As Kwired and hbiss were saying above, depending on the situation and how much draw down there is, I bet it might be easy to never notice this. It's all about the need for recovery.

I often wondered just how much loss there is, and would guess is not all are same but most new units likely need to meet a certain standard.

4500 watts for 8 minutes is only about .6 kW/hrs of energy used.

If you were talking say a vacation home where it went many days or weeks between actual use then it only is doing this just a little over twice a day. So even with relatively high energy rates this still only is costing you maybe 20-25 cents per day to sit totally idle. And keep in mind if it is in the conditioned space it is costing you a little more because it adds to cooling load, though in a vacation home you may not be cooling at all when not in use, and in heating season it offsetting the heating load and not a total loss. Bottom line, the extra cost for larger service to be able to handle a larger kW on demand unit I really feel is not worth it. Plus maintenance costs of a conventional storage tank unit are usually going to be less over time in comparison as well.

 

[LarryFine]

Are they putting the neutral where the other 240V leg would go?

That's how I took it. How else?

 

[Little Bill]

That's how I took it. How else?

I meant to edit that to say just at the thermostat.

 

[kwired]

I meant to edit that to say just at the thermostat.

Thermostat on those just interrupts one line. Ungrounded line would be preferred but will work either way. High limit does open both lines but is a manual reset and usually only opens when there is some sort of a problem.

 

[MD Automation]

I often wondered just how much loss there is, and would guess is not all are same but most new units likely need to meet a certain standard.

Don't want to steer this thread too far off track - but if you were interested... these are the first 16 or so ON and OFF cycle times after I last powered up the PLC to record them. This represents about 4 days or so of data.

The ON times are in minutes and the OFF times are in hours. It's kind of self explanatory, longer ON times and shorter OFF times are when we were doing dishes or laundry. And the Long OFF times of 10 and 11+ hours is when no water was being drawn. This is a water heater in the original house, there is a second water heater for the main bathroom. So this heater routinely sits for a long time with little to no demand since the kids moved out.

I was surprised it retained heat as well as it does.