Temperature fluctuation of gas hot water heater / thermostat

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I installed a new Bradford White gas tank water heater URG140T6N model year 2019.
It has a honeywell V2 thermostat, see page 20 in https://s3.amazonaws.com/s3.supplyh...rdWhite-URG140T6N-394-InstallInstructions.pdf
My house is old and the showers have no thermostatic mixing valves.
The output temperature of the water heater fluctuates about +- 8 degrees over time, for example between 100 and 108 degrees. I am unable to set it to a specific temperature, as the temperature changes over time (even when the house and exterior temps are constant). I would like to have it at 105 degrees. The control knob on the honeywell thermostat seems to be very difficult to set accurately.
Just to be clear, it is not just the difference from beginning to end of a burner heating cycle. I measured the output temp a few dozen times over a few days, for example right after a heating cycle i.e. just after the burner stopped heating.
I called Bradford White customer service, who said that +- 10 degrees fluctuation is normal, and recommended adding a tempering valve at the water heater outlet.
But on my old water heater (25 years old) I could set the temp accurately and it stayed constant. No need for external thermostat.
Is it normal that the output temp of a modern water heater fluctuates +- 8-10 degrees?
Why do I have to add an external thermostatic valve when my water heater already has a thermostat? Why can't they put in an internal thermostat that is as accurate as an external tempering valve? Doesn't make sense to me.
I would think that technology got better within 25 years, not worse.
Please help - should I return the heater or replace the valve of the heater, or is it normal and I should add an external thermostat? I like to keep things simple.
Could there be any other potential reason for the temp fluctuation? I assume the tip tube is unlikely broken as the heater is almost new.
 
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Fitter30

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Thermostat has 10 ° spread because if incoming water is really cold usage is only a gallon or two that cold water hits the thermostat before water in tank gets mixed turns stat on than it would short cycle off. Tempering valve is mixing the hot and cold and the sensor adjusts constantly.
 

John Gayewski

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This is more than normal, it's expected.

A watery heater's thermostat is not considered a way to deliver consistent temps to the fixtures. They aren't even considered accurate. Technically a water heaters outlet should have a temperature gauge in it and even then it can be inaccurate.

If you want to deliver a reliable temp tio your fixtures you would need to install a mixing valve on the water heaters outlet with a cold line also.
 
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Thanks for the replies so far. After doing additional experiments, I noticed that when having a shower with very low water flow rates, such that the heater can instantaneously catch up with the energy consumed, the temperature increases during the course of the shower. I conclude that the newly heated water goes into the outgoing pipe of the water heater, before it mixes well with the existing water.
I don't know the internals of the Bradford White heater; but the fact that this was never an issue with my 25 year old heater, tells me that this is not something that has to be like that as dictated by laws of physics, but rather a design issue with the Bradford White heater. I am disappointed that I have to buy and install another gadget (external thermostat) and have another maintenance item, for no reason. 25 year old heater was better than a 2021 brand name model that even sells as a premium product at a premium price - how retarded is that?!
 

John Gayewski

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Thanks for the replies so far. After doing additional experiments, I noticed that when having a shower with very low water flow rates, such that the heater can instantaneously catch up with the energy consumed, the temperature increases during the course of the shower. I conclude that the newly heated water goes into the outgoing pipe of the water heater, before it mixes well with the existing water.
I don't know the internals of the Bradford White heater; but the fact that this was never an issue with my 25 year old heater, tells me that this is not something that has to be like that as dictated by laws of physics, but rather a design issue with the Bradford White heater. I am disappointed that I have to buy and install another gadget (external thermostat) and have another maintenance item, for no reason. 25 year old heater was better than a 2021 brand name model that even sells as a premium product at a premium price - how retarded is that?!
I'm doubting your hypothesis.
 

jadnashua

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Where are you measuring the temperature? If it's in the shower, a couple of things come to mind. Since it's not a thermostatically controlled valve, it's mixing whatever hot is coming in with the cold. Depending on the size of your pipes and how long they are, keep in mind that as you run the cold water, especially in the wintertime, it gets colder, too, so the mix at the shower valve will drop unless you change the setting (which may be limited).

Most modern shower valves have a temperature limiting mechanism, and it's a good idea to consider changing it season-to-season to accommodate the incoming cold water temperature variations.
 
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