Gas water heater question

[jtfoxman]

Hi everyone! Having an issue with my gas water heater. Occasionaly at night we will have trouble getting hot water to the shower. If we wait a while we eventually receive it.Thermostat? We have never had this problem. Water is always instantly hot. The water heater is only a few years old and is well insulated. I have it turned down a little bit to right above warm. I went down to the basement to check and I turned it on and it fired right up. It then turned it back down and it shut off. So it is working properly. I am at a loss here and I do not want to call a plumber for something stupid. If it is the thermastat are they easy to replace?

Thanks everyone
Jason

Jtfoxman

[LazyPup]

On a scale of 1 to 10 with ten being a worst case,,i personally would rate changing a gas water heater thermostat at a solid 12.

You stated that you had turned the thermostate down and up again, and in each instance it responded correctly so I would doubt if your problem is in the water heater.

Sight unseen I would guess that you are on a municipal water supply. During the day their is a constant demand on the water main so the water is fairly rapidly in motion. At night the demand drops and the flow rate drops proportionally.

While the subsoil temps at 4 or 5' down remain a constant 50 to 55deg year around, soil is a fairly poor insultor which is evidenced by frost in the soil during winter months. The frost line is the maximum depth at which freezing will occur in the soil at any given location. (In my region the frost depth is 36".

In order to prevent freeaing the codes require that buried water lines must be 6" below the frost line which in my locality would be 42" minimum burial. While that is sufficient to prevent freezing you would have to go nearly twice the average frost depth to reach a point where the earth maintains the constant 50 to 55 deg. The layer between the frost line and the 55deg area will be progressively varying somewhere between freezing (32deg) and the natural ground temp (50 to 55deg.)

During the summer months the surface temp of the earth would be somewhere between the ground temp and the air temp, thus the soil will be between 50 to 55 and 70 to 100deg.

During the summer months supply water temps of 60 or 75 deg are not uncommon, while in winter months 40 to 45 deg is typical.

Coupling the ground temps with the reduced flow would result in the water cooling to a much greater degree at nite, which could account for why this problem occurs at nite.

You may want to turn the tank temp up slightly in winter months so it will anticipate the demand quicker. Usually a 5 or 10 deg. incresse is more than sufficient. (Be careful as the code limits the tank temp to a maximum of 140 deg unless all showers and tubs have anti scald valves. ( Some local codes and most plumbers will set the tank to 125deg)

[Hube]

It will actually not matter what temperature the incoming water is coming into the heater.
The water will be heated to the T stat's setting.
With a variance to the incoming water temperature, colder water coming in will just take slightly longer to reach the T stat setting.

I don't know the answer to the poster's problem, but it certainly has nothing to do with any incoming water temperature.

[LazyPup]

You are correct that it will heat the tank higher, but if you turn the demand temp up it will call for heat sooner as the cold makeup water enters the tank, which means the average temp at the demad will remain higher.

[Hube]

But, the bottom line is the water will still go out of the tank as per thermostat setting.

[kactuskid]

You say occassionally at night you don't get the hot water right away. I don't know how late it is when you try, but try this to see if it works. A few hours before you want to take a shower run some hot water in the sink or even tub for like 3-4 minutes. Then see if the water is hot a few hours later when you go to use it for your shower.

My heater does the same as yours occassionally, and even though I know the theromstate is working, it's like there is this happy lull spot where the water must just sit in the tank and cool down when not in demand for a long period of time, like say during the day, then once water is drained from it a bit, the thermostat will kick in and heat the entire tank back up to setpoint.

If this works for you, then try moving the thermostat dial back and forth alot of times quickly. Then move it to the setpoint desired. See if that helps.

[LazyPup]

The required amount of heat is directly proportional to the temperature of the incoming water.

Irrespective of what size of water heater you have, once the water in the tank has been heated to the set point the tank will not require any addition heat except to make up for the radiational loss of the tank.

When hot water is consumed an equal amount of cold water must enter the tank to replace what was consumed. The only energy required by the tank is the amount of energy required to raise the temperature of that make up volume from the incoming temp to the set temp.

By Example: If the incoming water supply was at 55deg.F and the tank was at the code prescribed set temp of 125deg.F and one gallon of hot water was extracted from the tank, you would have one gallon of water entering the tank at 55 deg.F.

The temperature differential would be 125deg.F minus 55 degF = 70deg.F.

1 British Thermal Unit (BTU) will raise one pound of water one degree of Fahrenheit.

One gallon of fresh water weighs 7.48 pounds.

Thus the total energy required to restore the tank to the 125degF set point would be:

70deg/lb x 7.48lbs = 523.6BTU.

If the incoming water were reduced to 40degF during the winter months the differential would be 85Deg/lb x 7.48lbs = 635.8BTU.

We could then compute the initial temperature drop in the tank by deducting the differential from the latent heat of the tank at set point.

Example: consider a 40gal tank at 125 deg F.

The latent heat of the tank is 40gal x 7.48 lbs/gal = 299.2lbs of water x 125degF =37,400BTU.

If we extract one gallon and replace it with one gallon of cold water at 55degF the latent heat of the tank is now (39 x 7.48 x 125)+ (1 x 7.48 x 55) = 36,465 btu + 411.4btu = 36,053.6btu.

dividing 36,053BTU by 299.2lbs = 120.49 btu/lb

Thus extracting one gallon of water will drop the tank temperature to 120.9DegF.

The ability of the tank to meet the demand would then be dependant upon the rate of recovery BTU input to the tank.

A shower is rated at 2.5gal/min, therefore in order to maintain the shower temp steady the burner would have to come on the instant the shower starts and will have to supply 523.6BTU/gal x 2.5gal = 1309 BTU/min @ 55deg supply temp.

If the supply temp drops to 40degF the differential is now 85degF.
thus you need 85 x 7.48 =635.8BTU/gal x 2.5gal/min = 1589.5BTU/min.

As you can see, in winter the burner would need to supply 280.5BTU/min more energy than what it would require in summer to meet the same demand. Thus a water heater burner should be sized for the worst case scenario which is determined by the average mid winter supply temp.

The overall size of the tank would have a great effect on the percent of dillution and overall initial drop in tank temperature, but the actual amount of energy required would remain the same whether it was a 5 gallon or a 500 gallon water heater.

[Hube]

and if a Summer hot spell continues for a few days,the ground could warm up so much the tank's heat element could be even shut off[]

[vdotmatrix]

I replaced my aging hotwater heater 2 weeks ago after seeing a couple water drops leaking.

It wa actually a snap. Unless you don't like sweating pipes and it is was fun.

I bought a whirlpool 40 gallon from Lowes. and put it in...I could have gone the real easy route using the flexible hose quick connects for water supplies and the gas, but the new just slipped right in there...I did however, put in 3 full bore ball valves above the water heater, one for cold , one for hot and a valve connecting the two.

Why?

Well when I flush and drain the tank out twice a year, I simply close the valves coming and going to the water heater and open the valve between so the water by-passes the water heater and I don't have to shut water off to the entire house.

Oh, and I did use the flexible 18" copper supplies, so the water heater, except for the gas line (that has a union joint anyway) would be a rigid system and i could remove it for any reason a hell of lot easier.......

quote:Originally posted by LazyPup

On a scale of 1 to 10 with ten being a worst case,,i personally would rate changing a gas water heater thermostat at a solid 12.

You stated that you had turned the thermostate down and up again, and in each instance it responded correctly so I would doubt if your problem is in the water heater.

Sight unseen I would guess that you are on a municipal water supply. During the day their is a constant demand on the water main so the water is fairly rapidly in motion. At night the demand drops and the flow rate drops proportionally.

While the subsoil temps at 4 or 5' down remain a constant 50 to 55deg year around, soil is a fairly poor insultor which is evidenced by frost in the soil during winter months. The frost line is the maximum depth at which freezing will occur in the soil at any given location. (In my region the frost depth is 36".

In order to prevent freeaing the codes require that buried water lines must be 6" below the frost line which in my locality would be 42" minimum burial. While that is sufficient to prevent freezing you would have to go nearly twice the average frost depth to reach a point where the earth maintains the constant 50 to 55 deg. The layer between the frost line and the 55deg area will be progressively varying somewhere between freezing (32deg) and the natural ground temp (50 to 55deg.)

During the summer months the surface temp of the earth would be somewhere between the ground temp and the air temp, thus the soil will be between 50 to 55 and 70 to 100deg.

During the summer months supply water temps of 60 or 75 deg are not uncommon, while in winter months 40 to 45 deg is typical.

Coupling the ground temps with the reduced flow would result in the water cooling to a much greater degree at nite, which could account for why this problem occurs at nite.

You may want to turn the tank temp up slightly in winter months so it will anticipate the demand quicker. Usually a 5 or 10 deg. incresse is more than sufficient. (Be careful as the code limits the tank temp to a maximum of 140 deg unless all showers and tubs have anti scald valves. ( Some local codes and most plumbers will set the tank to 125deg)

It's sometimes better to be lucky than smart.

[LazyPup]

vsotmatrix

You really don't need the third by-pass valve because closing the cold valve would shut off the supply and closing the hot valve would prevent the water in the hot side from backflowing.

I personally like the idea of a valve on both the hot and cold so you can totally isolate the tank to change it out, BUT some local codes prohibit a valve on the hot line.

[vdotmatrix]

ya, know what..i think you are right....i put the by-pass valve set-up (lke i know what i am doing) so if i did close off the hot water air doesn't get back in the lines when i am performing maintainence....but your idea makes sense, i got the idea from DIYNETWORK

It's sometimes better to be lucky than smart.