Water heater mystery

I’m looking for some help solving a mystery that has been driving my wife (and therefore me) nuts for as long as we’ve lived in this house (almost five years). This is the kind of problem that is impossible to reproduce when the service person is here (and tends to make them look at you like you’re crazy). My wife likes to take baths in the evening, and the availability of hot water for the bath is inconsistent. Sometimes the water never gets above luke warm for as long as she runs it.

Now, my first instinct upon hearing that is to think that something else must have used up all of the hot water right before the bath. However, this happens most often when no hot water (or very little – e.g. hand washing) has been used throughout the day. If we use a bunch of hot water, for example by doing a load of laundry, right before her bath, she always has hot water. If we come home from being out all day and do nothing after getting home, most often she won’t have any hot water.

Seems weird to me. But even weirder is the fact that we actually completely replaced the water heater (we wanted a larger one to go with a bathroom remodel) and the same symptoms continue. So I guess that rules out a problem with the water heater, unless the new one just happens to have the same problem as the old one. Seems unlikely.

So, any ideas what’s up? Is this just normal? I don’t know anything about how water heaters or thermostats work. I was thinking that maybe the temperature which makes the flame kick back on is way lower than the temperature that makes it turn off (e.g. it heats the water to 120 degrees, but doesn’t start re-heating until it gets back down to 90). So over the course of the day, the temperature of the stored water slowly drifts down but never reaches the lower limit (and doing the laundry quickly brings in cold water, making it re-heat the whole thing to the upper limit). But I’d think that such a big gap would be a malfunction, not an intentional feature, so the new heater would have fixed it. And I don’t see any way to adjust that on the thermostat…

My experience has been that after using hot water, the water heater over heats water. For example, I can usually wash my hands in the kitchen sink with only the hot water faucet on. After a bath, the water from only the hot water faucet is so hot as to be painful.

I take it from your post that you’ve got a gas water heater.

Several things could be causing the issue. Since you replaced the water heater and the problem continued, the problem is not with the heater.

First question - do you always have the problem with one fixture, or does the problem seem to happen with every fixture in the house?

Second - Do you have a recirculating pump on the system?

Third - What size (gallons and BTU rating) water heater do you have?

Fourth - Do you have anything else using gas in the house when the problem occurs (i.e. heating system)

We 're experienced similar just after we moved in to our energy efficient house. We had insufficient water pressure due to a small leak in waterline. Fortunately we had not much damage.
But sometimes if you have huge tree around your incoming waterline, it can break it and it may causing your problem.

Yes. Sorry I wasn’t specific.

It happens at least in both bathtubs in the house. The master bath is right on the other side of the wall from the water heater, so it shouldn’t be a very long run of pipe.


50 gallons. It says “Input BTUH 40,000” which I assume is the BTU rating? The brand is Rheem if it matters.

We do have a gas furnace (the only other gas appliance we have). I haven’t noticed either way whether there’s a correlation between the problem and the furnace running, but I’m pretty sure we’ve been seeing it year round (and the furnace is pretty much never running in the summer). Now that we’re in the warm(er) season again, I’ll pay closer attention to be sure.

OK, so it happens in more than one place. That helps narrow it down a bit. As does the no recirc pump.

First thing I would check is the amount of combustion air available to the heater. Is it in a closet? Is there airflow going in & out of the closet? It sounds to me like your issue might be related to a lack of combustion air, which will cause a weak flame and insufficient heating. Of course, when you open the closet door, the flame burns nice & bright, since you just gave it lots of air.

I’ve always attributed this phenomenon to being related to the distance of pipe between the heater and the faucet being used. In a big old house I lived in for a while it took literally 3 to 5 minutes for the hot water to make it to a bathroom sink upstairs. On cold mornings it seemed like forever. The first time that area of the house is used for the day an entire distance of pipe has to be drained of already cooled off water before any new, hot water from the heater makes it out. The longer the distance and lower the water pressure, the longer this takes. Once the pipes have been refilled with hot water that way once it takes a while before it cools off again so subsequent uses of the sink or other devices in that area of the house shortly after will yield hot water much sooner.

The OP says the bath never gets hotter no matter how long its run, so this might not be the explanation for the original problem.

It’s in the garage, so I think the airflow should be ok.

Try bumping up the thermostat on the side of the gas control a bit and see if that helps.
It sounds to me like the thermostat is set at too low a temp to be comfortable for you. So the water seems cold. When the heater has just cycled the water near the top of the tank is warmer than the set temp, and you have nice hot water.

[Lost in Space robot B-9]Danger, Danger! Will Robinson![/LiSRB-9]
Be careful when you use hot water after adjusting the t-stat, you might have made it so hot, you can get a scalding burn from it.

Rick’s advice sounds good to me; can anyone comment on what effect a bad mixing valve might have on such a situation?

Sounds to me like a bad thermostat. Not kicking in on occasion when the water cools slowly over time. If the water heater is fairly new, fixing it isn’t out of the question, but you might as well replace an older one completely. I limped ours off for 6 months or so, just by turning the thermostat setting every morning forcing it to come on in the morning, then turning it back to the original setting.

A bad mixing valve can also cause the problem, however (at least in the states where I’ve worked to date) they are not normally installed in a residential application. They are normally only installed in a building where you need 2 different temperatures of hot water, like a restaurant that needs both 140 degree hot water for dishwashing and 110 degree hot water for domestic use. The only other time you would install one would be if you were trying to boost the capacity of your water heating system by raising the temperature of the water in the storage tank and then cooling it before it hits the fixtures.

If there is a mixing valve in the home, that could very well be the culprit.

replacing a thermostat on an electric heater is very easy, they just clamp on to the side of the tank and can be unclipped and replaced in a couple of minutes. I assume a gas water heater is similar. Obviously, one has to be careful-make SURE, then double check, that all power and gas to the heater is off. Then check again with a meter. I got a replacement thermostat at the local hardware store, so they aren’t hard to find.

Maybe you have a combination problem.

  1. Your baths have anti-scald mixing valves and are set for the water can only mix to a maximum amount of hot water.

  2. Your water heater varies between 90 and 120 degrees.

  3. The anti-scald valve was set for mixing the higher temperature supplied water or even hotter before you moved in.

  4. Your water is a good temperature when the heater has the water at the 120 degree temperature.

  5. Try resetting the mix on the anti-scald valve on the tubs.

  6. You may have to raise the set point at which the water heater starts heating to something like 100 so there is less variance in the hot water supply to the valves.

I can’t help beyond telling you what I just did.

A solution I have used at a place I stayed at with a similar situation is I ran the hot water perhaps 30-40 minutes before I wanted to take my shower. What this did was get cold water in the water heater to trigger it to heat up the water. In a 1/2 hour the water was hot enough to take a enjoyable shower.

If you want to check the mixing valve issue, you can check the water heater by opening the valve at the bottom to let some water out. See if it’s really hot, or just lukewarm. If it’s really hot, your problem is not the heater, but if it’s just warm, it is.

There is one other option that would likely work in this situation.

If running water seems to help the issue, install a recirc pump on the system. There is a fairly simple to install system made by Grundfos (http://www.savewaternow.com/) that will circulate water between your hot & cold water systems to get you a hot shower quickly. The only thing you’ll need is a 120V outlet at your water heater.

Thanks guys.

I don’t think it can be a mixing valve problem, because the main bath we have the problem at has separate hot and cold valves (basically this (different handles and color)).

The thermostat was the first thing I suspected, but what are the odds of the new water heater having a thermostat with exactly the same problem as the old one? Anything is possible, but I’d be a little surprised

I’ll look into recirc pumps.

Thanks again!

Two handles don’t mean it’s not anti-scald. Example

All storage type gas water heaters do this. The temperature is hottest after a fresh heating cycle. The temperature it turns on can’t be the same as the temperature it shuts off at or it would immediately shut off. There is a range of heat associated with the process.