Two Parallel Plumbed Electric Water Heaters Tripping a Lot

Steamback

I have a situation which I find somewhat baffling. At this site there are two electric water heaters plumbed in parallel. They were operating fine for a few years but recently, about once/month, tanks are tripping (reset button on thermostat) a lot. All I can think to do is replace both thermostats and both electric elements. I'm wondering though, could the problem instead be something related to power quality from the Utility Company? The site does also have a solar electric system installed...could the solar inverter be a cause? Has anyone encountered before a situation of repeated water heater tripping even after replacing the thermostat and element? Both water heaters are on independent circuits and not connected electrically, except through the service panel, of course. Any thoughts are appreciated.

Larry Weingarten

Hi, Do you have any way of knowing hot hot the tanks are getting? Something as simple as a split sheath on an element prevents it from fully shutting off, so it could be non-stop heating until the limit trips. I think I'd take a volt-ohm meter and test all elements for leaks to ground... but only if you're good around 240 volts and have power off and one wire disconnected from the element being tested. Hard water, recirc line, poorly connected thermostats... various things could be helping cause this...also, coincidence happens!

Also, you posted this in solar. Is solar preheating the tanks? Could the preheat be hot enough to trip the limits?

Yours, Larry

hot_rod

The temperature snap discs? Does both the temperature control disc and the high temperature limit open?
Seems, like Larry indicated, it has to be a temperature related issue, what is the temperature of the tank or water when they trip?
Those snap disc controls have been around for a long time, they are fairly reliable.

Put a clamp on amp meter on the element wire, maybe it is pulling a very high current and over-heating the control, although the breaker should trip under that condition. What size breakers feeding those dual tanks?

Jamie Hall

If they are actually tripping at exactly the same time... it almost has to be electrical, not temperature. I mean, I believe in coincidences, but that's just too much. Further, it almost has to be something common to both tanks, or a problem in one which feeds back through the common switchgear.

EdTheHeaterMan

Happens about once a month? I think it’s a Lunar thing
Nothing to do with Solar
Or what Larry or Bob or Jamie said.

mattmia2

I would have thought the t-stat was 2 pole but from Larry's comment i gather they are not. If one tank is overheating relatively quickly it may trigger the limit but only once it is significantly overheated since it takes some time for the heat to migrate to the t-stat then either thermosiphoning or a recirculation system moves the overheated water in to both tanks.

JUGHNE

One overload could trip without anyone knowing it. As the other WH was then over worked to keep, it may have over heated and tripped later. With them both tripped one assumes it was a spontaneous event.

Being in parallel does not guarantee they have equal flow, one could be doing most of the work until it overheated. Then the remaining tank overworks. Is it plumbed perfectly for parallel? Pipe lengths and fittings differences will induce more flow in one than the other.

Also without some use/flow the water will stack up from the lower element and upper tank water will overheat.

EBEBRATT-Ed

Could it be as simple as the thermostats are set too high?

PC7060

Voltage drop in service line could cause units to pull high current and and pop the overload. Any problems with utility service?

mattmia2

It is a resistance heater, if the voltage drops the power consumption decreases.

EdTheHeaterMan

I still think it’s Lunar. When is the full moon?

PC7060

mattmia2 said:

It is a resistance heater, if the voltage drops the power consumption decreases.

Dooh! right, not a motor.

hot_rod

I thought of voltage issues also, but the control is basic silver points enabled by the snap disc, not sure that current would have any effect on the thermal disc? Almost need to catch it in the act, quickly measure some things, like temperatures.
Some of the clamp on Amprobes capture high current draw conditions.
But so do circuit breakers

HomerJSmith

Is it exactly 1 month or is it every 28 days? hmmm

HomerJSmith

Replacing the elements and thermostats is cheap. You can buy the kit at Home Depot for peanuts. Well, not literally for peanuts.

If you aren't experiencing any other electrical problems in the building, I would think it isn't electrical. Your service provider's engineering dept might be able to help you.

I assume that you are not getting any seriously hot water out of the taps, so it isn't over temp related.

"There are strange things done in the midnight sun", where water heaters toil, and fail to work, a monthly quirk, that drives men insane.

PC7060

HomerJSmith said:

"There are strange things done in the midnight sun", where water heaters toil, and fail to work, a monthly quirk, that drives men insane.

 

mattmia2

HomerJSmith said:

I assume that you are not getting any seriously hot water out of the taps, so it isn't over temp related.

Unless there is a thermostatic mixing valve.

HomerJSmith

mattmia2, Good point! Yes, you are correct. I guess we should ask.

Steamback

I would like to thank everyone for the thoughts and to apologize for the delay replying.
Below I have provided more information and clarification. The format is informal and I apologize for not crediting people directly.

- Yes the title was not correct. I will edit it. There is no way to know if both trip at same time, and probably are not. Plus, after posting this thread there were a few times where only one tank was found to have tripped.

- I should also add that these tanks are also supplying heat for a radiant system during the heating season. Outdoor temperature does not seem to be the factor that causes the tripping. Meaning, it doesn’t only happen under very high heating load.

- Yes there is a solar thermal system on these two tanks. 6 SunEarth Plate. However, I do not suspect it is the problem. The tripping only happens during the heating season. This has been true last 2 or 3 years. No problems in summer. In fact, I was there yesterday and the 2 tanks were both at 155 and I’m sure they max out all the time in summer and the system enters steamback. Only one person lives there.

- I checked both elements for leak to ground (1 element per tank). Both passed with flying colors.

- Not long after creating this thread last winter, I went ahead and replaced both thermostats and both elements with genuine AO Smith replacement parts. Tripping happened again a few more times before Spring arrived. No tripping has happened since April 2021 (I’m writing this Sept 2021).

- Each tank has it’s own 30 amp breaker on single 4500w elements per tank. These are solar tanks with internal coils, so just 1 element per tank.

- These tanks are indeed plumbed in parallel which could be an issue, but I did my best to make equal length pipe runs on the solar coils and on the output piping for DHW and to the radiant heat exchanger. I have always observed tanks at equal temperature, or very close at least, during summer and in winter when heating.

- Thermostats on both tanks are set to approximately 135.

- Yes I am using mixing valves. The DHW has a mixing valve and the radiant loop to the exchanger has another mixing valve.

Before I sign off I'll add again that this entire system worked great for a few years. Then one heating season there was a trip or two, then next season more, then last season it was every few weeks.

All thoughts welcome. At this point, nobody could be wrong. It's a full blown investigation at this point.

Thanks again everyone.

Jamie Hall

I won't comment on the supplying radiant heat... if this isn't through a heat exchanger separating the radiant heat water from the domestic water, I don't like it one little bit. But it's what you have.

So...

Only happens in the winter? I'm thinking -- and I know this sounds odd -- that your solar panels are, in some odd plumbing way, overheating the tank. I know, you'd think in winter they wouldn't heat as much -- but depending on exactly how the panels are oriented (angle off horizontal) they may actually be capturing more heat in the winter than in the summer.

mattmia2

Maybe there is a failed flow check or something of that sort and the ciruclator for the radiant is moving fluid through the panels when it shouldn't be?

Oh, the solar is electric. Never mind on that then...

Larry Weingarten

Hi @Steamback , frequent small draws of water can cause "stacking" in the tank, where the tank heats incrementally hotter and hotter, exceeding setpoint. Hand washing dishes can do this. A simple approach could be to turn down the thermostat settings to 150 or so to see if that calms it down and still provides enough heat for the radiant.

Yours, Larry

Steamback

I'll add more information and repeat a little.

The radiant is definitely through an exchanger. It's there buried behind the pipes on the right. I wouldn't install a system without one.

Panels are close to 45 degrees, and the solar controller would control overheating anyway.

The thermostats are already set to about 135. Nothing close to 150 or more.

Thanks for the thoughts. Keep them coming. Someone should make a TV show called 'CSI HVAC'. It would be a hit.

I think my next investigative steps are to put a datalogger on each tank to monitor temp, and something like Sense in the main panel to monitor current draw on each tank. Maybe if I know more about power consumption and utility power quality, and exactly which tank trips and when, I may have a few good clues. And honestly, I'm going to take a few minutes to redo the electrical connections at the water heaters. Who knows?

mattmia2

Wait so is it solar thermal or just photovoltaic? If something is not closing or sealing properly and it is also solar thermal then the controls maybe don't matter, the ciruclator can still cause ghost/untended flow through that circuit.

Jamie Hall

45 degrees? Directly south facing? In that case unless the collectors are tracking, the maximum output power will be at noon on the day of the equinoxes, assuming you are in the general latitude of New York, So that's probably not the problem.

However, you say cheerfully that the solar controller will take care of overheat. Don't count on it. Valves do fail...

mattmia2

Jamie Hall said:

45 degrees? Directly south facing? In that case unless the collectors are tracking, the maximum output power will be at noon on the day of the equinoxes, assuming you are in the general latitude of New York, So that's probably not the problem.

However, you say cheerfully that the solar controller will take care of overheat. Don't count on it. Valves do fail...

If something has failed such that it is heating the collectors from the tank, the collector only has to add the difference between the setpoint and the limit, not necessarily anywhere near the full theoretical output of the collector.

hot_rod

Most of the digital solar controllers have over heat protection. This function is to protect the panels from getting excessively hot and destroying the glycol.

Looks like you have a Stecca brand, or a rebranded version, I'm more familiar with the Resol.
Basically the control watches the temperature at the collector, when it reaches the overheat temperature, usually adjustable, it turns on the solar pump and flushes that high temperature from the collector. So this will cause the solar tank to overshoot the setpoint temperature. I have seen tanks get hot enough to trip the 210* temperature relief valve with this function enabled on systems that don't get used a lot, vacation homes for example.

This function is mainly for summer time stagnation protection, but with an oversized array, or undersized tank it's possible to overheat in winter months.
Look through the solar contoller manual to see if yours has this function, either disable it, or adjust temperaturers.

There may also be a tank cooling function to bring down temperature in a tank, but it pees away the solar energy you just payed to collect.

Your best bet is to put some data collection on the system as you mentioned. Many of the solar controllers have some data collection built in. If so it will show the maximum temperatures that all the sensors have recorded. that would help determine if the solar is the cause.

Examples of the Resol and Stecca functions attached.

HomerJSmith

This may help. Understanding how the ECO works may help you correctly diagnose your problem. Your dip tubes in good order? Sometimes, draining a little water from the drain valve, a little waxy substance can be observed, indicating a failed dip tube.

http://waterheatertimer.org/Stacking.html