Too much hysteresis in water heater thermostat

[Peabody]

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.

[Speedy Jim]

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

[udarrell]

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

[Peabody]

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.

[Speedy Jim]

Peabody wrote: I guess I was hoping I could add a negative

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

Jim

[rdankwort]

replying to Peabody, rdankwort wrote: This is a very common phenomenon in water heaters. In the morning my temp. is just warm; after I do a load of laundry it is piping hot. And in-between for moderate use. Unlike you I don't like to mess with my thermostat setting. It's a crummy honeywell regulator on a Bradford-White tank. The Robertshaw I used to have on my earlier tank at least had visible and large temp. setting marks; the honeywell is a tiny knob with useless setting markers.

Anyway, if I want hot water in the morning I try to get up a bit early & run a water faucet somewhere. I know that'sa sinful waste of water but what would you.

BTW living in a hot desert climate I have a very different summer problem also: between the hot garage temperatures and the pilot my water runs almost dangerously hot.. The pilot alone keeps the temp. up! I once tried just shutting off the pilot but then the water temp.was way too low.

[rdankwort]

replying to Peabody, rdankwort wrote: There is a silver lining to hysteresis too though. It minimizes turn-on & turn-off of the burner and reduces gas cost by running low at night. As someone else says, it's very probably by design.

[Ed Pawlowski]

After 15 years we finally have an answer.

[Ralph Mowery]

I found that to be a problem with a house I moved into. About 10 years later I replaced the water heater with a new one. The new one acts the same way. Luke warm water comes out of the single leaver shower , but if I run some water and then get in the shower the water is very hot at the maximum hot setting of the lever. I do have it set for about 130 or 140 deg. Just me and the wife so no small children to get burnt.

[Chuck U.]

The Honeywell on my natural gas Lochinvar works fine but it's a pilotless ignition model.

[micky]

This doesn't make sense. It should be up to its maximum temp when no one has used any hot water.

And if you take a deep bath or very long shower, it should run out of hot water until it has time to heat it again.

Are you just talking about the water in the pipes between the water heater and the faucet? Of course that cools off over night.

How early? It takes 3 minutes at most to run the hot water until the water in the pipes has gone through it and hot water comes out You can brush your teeth during that time, don't have to get up early at all.

[trader_4]

If it's a common problem, I've had decades of experience and never had noticeably temp swings like that. Granted it's only been with maybe five of them, but they were mostly just the basic pilot light type, nothing fancy or expensive and they maintained temp so I never noticed a difference.

Unfortunately the cost of a new gas valve assembly is probably so much that unless it's new you might as well change the whole thing. And if it was doing this when new, should have documented how much the temp varies and called the manufacturer. They might have sent a new valve assembly. I had a State WH where the thermocouple failed while under warranty. I called them up, they had a new one here in a few days, no charge. Of course a valve assembly costs a lot more, so they may not be so quick to replace, but I'd think if it was really out of range, they would.

[trader_4]

Not after a couple of hours.

Sure, but they didn't say they were pulling a very long shower, just a load of laundry. So, it's enough to trigger the WH to fire up and start heating the water again.

Lost in the wilderness, as usual.

[Ralph Mowery]

The water will be the hottest just after the thermostat cuts off. I don't know how much temperature variation the water heaters have, but seems to be a lot on mine and the one I just replaced. They were both tall and had the dual heating elements.

I did measure the temperature of my electric oven and it surprised me how much it varied. Seems like at either 350 or 400 deg F it would swing at least 50 or more deg low and high.

I worked on instrumentation at a large plant. We had controllers that would hold things with in just a few degrease or less. However the controllers were proportional and not just the 'bang bang' off and on types. I would have thought by now the ovens would have something like that, but I have not seen any in the common house price range.

[trader_4]

I haven't monitored the one here to see how much it swings. Its a ten year old Kitchenaid double wall oven. I do know that once it gets to the set temp of say 400, it will continue to show that no matter what. If I open the door to put something in or check on it, then turn the oven off and right back on again, then it will show like 350 and start working it's way back to 400 again. I guess I would design it the same way, to make it look rock stable.

[Ralph Mowery]

That is close to the way our 5 or so year old LG oven is. Once it reaches the set temperature the number does not change. I am not even sure the numbers really mean anything but are just for looks. I was using a seperate digital thermometer to check mine out.

[Clare Snyder]

First thing I would suspect (if it is a top entry heater) is a broken off or corroded "dip" tube thatis supposed to put the cold water into the bottom of the tank. The hot water is supposed to come off the top. If the cold water goes in the top instead of the bottom, you will get pre-mixed, warm water instead of hot until the turbulence of the water flow gets the hot water to the top. Good heaters use a "turbulator" tube that makes the water swirl in the bottom of the tank as it enters, avoiding stratification issues.

[Clare Snyder]

electric and gas are totally different in operation. If it is an electric heater he likely has one bad element.