The Tank › Element Ratings and Safety/Longevity
- June 28, 2013 at 1:47 pm #19924ceclarkParticipant
I just had my heater serviced yesterday – new elements and thermostats were installed. The plumber showed me that he had installed 3000w elements. After he left, I noted that the decal on the tank specified 4500w elements. Is this something to be concerned about? I’d liked to be armed with information before I contact the plumbing company.
Much thanks to anyone who offer insight!
ChrisJune 29, 2013 at 9:19 am #19925TschmidtParticipant
Wattage rating determines how fast it recovers. The lower wattage element means recovery will be slower. Assuming house wiring is adequate there is no reason to reduce wattage rating.
What was the purpose of replacing those components? They are typically pretty trouble free. What problem were you having?
Did the plumber check the anode rods and if so are they in need of replacement? Those are the only electric water heater components that need preventative maintenance.
/tomJune 29, 2013 at 12:33 pm #19926ceclarkParticipant
The heater (Bradford White) is 10 years old and was producing limited hot water. Buying a new unit isn’t possible for me financially. Both elements were corroded and the lower one was no longer working (I think the plumber said it wasn’t getting any juice). The anode never came up. I checked the top of the unit yesterday after doing some reading here, but don’t see a nut of any sort. Just two water connections and one power connection.
Thanks for your reply!
ChrisJune 29, 2013 at 2:44 pm #19927Randy SchuylerKeymaster
Bradford Whites don’t have that kind. They have an anode that is attached to the nipple in the hot port, and if it’s 10 years old, it probably needs to be replaced.
The anode is longevity. The elements and thermostats are function. You want the heater to function, but you don’t want it to rust out, either, so both things are important.
I suggest you use the Basics link on the left side of the Topics index and get a brief education on water heaters.
Elements and thermostats do fail over time and we see a fair amount of those kinds of problems in The Tank.
Randy SchuylerJune 30, 2013 at 2:46 pm #19931EjParticipant
I would check the end of the element to see if it is rated for dual voltage like many replacement elements are. It would read something like 4500 watts at 240v and 3000 w at 208v. Then the next thing you need to know would be your voltage. If your heater was originally equipped with two 4500 watt elements and now you are heating with 3000 watt element then you recovery time will be slower.July 3, 2013 at 6:57 pm #19944energyexpertParticipant
Heating elements have a maximum voltage rating. For residential water heaters this would be 240 volts.
Power output is calculated by watts = voltage x voltage / resistance.
For the case of a 4500 watt element:
4500 = 240 x 240 / resistance
Working out the math gives a resistance of 12.8 ohms.
208 x 208 / 12.8 = 3380 watts
120 x 120 / 12.8 = 1125 watts
Alternately, Ohm’s Law states for a resistance circuit, the current is proportional to the voltage. If you cut voltage in half, current goes to one half. Power is also volts x amps. So in the case of 120 volts vs 240 volts: volts is half and current is half so power (watts) is one fourth. 1125 watts is one fourth of 4500 watts.
A water heater element “rated” at 240 volts and used at 120 volts will likely never burn out. But it takes 4 times as long to deliver the same energy.
A timer might be placed on a water heater to permit only 6 hours of operation in a 24 hour period. Rewiring the WH to 120 volts results in the same thing, plus you do not have to buy the timer, plus the elements will likely never burn out, plus thermostat life is extended.July 4, 2013 at 7:01 am #19945today1Participant
would you talk a little more about re-wiring for 120v?
I’m thinking it thro and dont, on the surface, see any theoretical difficulties.
However, there are Manufacturers Instructions?
Have you done this to a number of working appliances, or to your own W/H?
Anyone else see any possible difficulties?July 4, 2013 at 11:32 am #19946energyexpertParticipant
A 240 volt appliance is supplied by a double pole breaker. On a residential electrical service, 120 volts per phase with the phases 180 degrees apart, for 240 volt potential. At the electrical panel, remove one of the wires from one phase of the breaker and reland it on the neutral bar. Most homes are wired with non metalic sheathed cable; so you will have a black and white wire in the cable. The white wire should have been taped black but this is not always done. If you can identify the white wire, take it to the neutral bar.
I have a 105 gallon WH at my house. I have had it wired in the past at 120 volt from the panel and the thermostats and elements in series at the WH. This gave 60 volts drop across each element with an element output of 281 watts (562 watts total). In pratice this gives you a water temperature of the top thermostat above the top element, and the water below the top element will be 1/3 of the temperature difference from inlet temperature to the top thermostat setting. EG, If inlet is 50 F and top thermostat is 140F, water below the top element will be 80F. Currently, I am at 120 volts and the thermostats wired in parallel with the top thermostat set at 90F. WH temperature is controlled by the lower thermostat.
I have rewired 3 phase wather heaters from phase to phase to phase to common. A phase to phase configuration means one element is between phases A and B, one element between phases A and C, and one element between phases B and C. On a 208/120 volt service this gives 208 volts across each element. If you rewire from phase to phase to phase to common, you tie one side of the elements together so you have phases A, B, and C going through an element but then coming together rather than going back to the other phase. This will give 120 volts across the elements. Using the equation w = v x v / R, you find each element output is reduced to 1/3 of the nameplate output. A 36 kW WH will now be 12 kW.
Difficulties: A WH may say “240 volts AC only”. An inspector may not permit the WH to be wired at 120 volts. There is nothing electrically dangerous but it is all about government control.
- You must be logged in to reply to this topic.