Too much hysteresis in water heater thermostat

As long as some hot water is used farily frequently, the heater works fine. But if several hours elapses when no hot water is drawn, then when I do finally turn it on, the temperature varies considerably depending on whether the heater has been ON recently. If I set it so that the minimum temp is always just hot enough, the max temp (if the heater has just shut off) is way too high.
It's clear that the thermostat (Robertshaw) on the heater has a bunch of hysteresis built in - it takes a pretty large movement of the thermostat dial to manually switch the gas between on and off. And I just wondered if anyone here had dealt successfully with this problem by replacing the thermostat (if that's even possible), or in some other way.
As it is now, I leave the heater at a fairly low temp, but if I want to make sure the water is hot, I go out to the garage, turn the thermostat up until the gas switches on, then put the thermostat right back where it was (the gas then usually stays on). By the time it finishes cycling 20 mintes later, the water is hot. Quite often, I find that the gas will switch on when I just touch or barely move the dial, which further indicates to me that the thermostat is "sticky".
I'd just like it to stay within a narrower temperature range over long periods of time when no water is drawn.
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Peabody wrote:

This is a very common problem with gas heaters. As you note, there is a lot of hysteresis and that's by design.
Some are worse than others and most get worse with age. The only repair possible is replacement of the complete gas control. That's not a "huge" job, but the control cost is a large fraction of the cost of a new heater.
Jim
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Speedy Jim wrote:

If you are heating hard water the minerals will insulate the TH's thermo sensing functioning and delay response time. On my 80 gallon electric heated tank the TH's got so they would let the low wattage elements stay on for hours at a time. It started leaking and I replaced it with a 50 gallon tank with high wattage elements and a new water softener! - udarrell - Darrell
http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
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Speedy Jim says...
> This is a very common problem with gas heaters. As you > note, there is a lot of hysteresis and that's by design.
> Some are worse than others and most get worse with age. > The only repair possible is replacement of the complete > gas control. That's not a "huge" job, but the control > cost is a large fraction of the cost of a new heater.
But this IS a new heater. And it's done this from the beginning, as the old one did. Well, as you say, it's by design.
Thanks. I guess I was hoping I could add a negative feedback resistor, or replace a microswitch with a more sensitive one. Oh well.
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Peabody wrote: <SNIP> I guess I was hoping I could add a negative

Nope. They're all mechanical. Levers and springs...
Jim
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