Peabody wrote:

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.


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

Speedy Jim wrote:

Peabody wrote:

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.


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

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-conditio...tent-heat.html

Peabody wrote:
SNIP I guess I was hoping I could add a negative
feedback resistor, or replace a microswitch with a more
sensitive one. Oh well.

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

Jim