Why does my tankless water heater fail when the HVAC is running?

Question

Florida, USA. 1963 or 1964 home.

TL;DR: When my central (electric) heat is running, my A.O. Smith tankless (electric) water heater malfunctions.

We purchased this home a few months ago. Over the last six decades it’s had a lot of work done, and quite a bit of electrical work in the two years prior to our purchase. A tree took out the main electrical service in 2021 and the then-owners that the incoming service routed to the other side of the house and I believe at that time the circuit breaker panel was replaced. There is a 100A sub panel as well, that I believe is older than 2021 because it refers to an irrigation well that was removed before 2021.

When we moved in over the summer, some of the LED bulbs would flicker when the central air conditioner was running. They were cheap LEDs. Replacing the bulbs fixed it, but I was reminded after the following issue surfaced.

The home has always had two water heaters. As part of a previous renovation, the previous owners changed the bedroom side water heater to a 28kW electric tankless model, powered by (EDIT) two 60A 240V circuits. It is approximately an 80’ run from the breaker panel to the tankless water heater. It was leaking when we bought the home and we had it replaced with the same model under warranty.

The unit began to malfunction in a strange way a few weeks ago: sometimes it would just start clicking and beeping like crazy. We narrowed it down to this: when the central heat is running, and we call for hot water from the bed/bath side of the house, the tankless water heater goes nuts. I believe it’s relays firing to turn on the heater elements and then resetting rapidly. Water at the tap will cycle from warm to cool. The unit gives an error code E03 indicating “not enough water in cylinder.” But there’s no sputtering at the tap and following the troubleshooting steps doesn’t accomplish anything.

We are dealing with a home warranty (provided by the seller). An electrician came out (the manufacturer suggested I start with the electrical side). He was very pleasant and seemed earnest enough, but not terribly knowledgeable and didn’t seem to know where to start. I suggested he start by checking for voltage drop at the equipment. He tested voltage here:

I believe what he tested was black to red. He found a little over 240V for each pair whether all the equipment was off or running (malfunctioning). I believe he tested on the input side of those connectors if that matters.

He also tested at the indoor air handler and outdoor heat pump and found slightly over 240V at each. I don’t remember if we checked the voltage at the HVAC components while the water heater was running.

He tightened all the terminal at each breaker. He did not do anything with the main service lugs or anything “upstream” of that. He stated the panel was suitable and the previous electrical work all looked fine from what he could see.

He deemed it likely a faulty water heater, but since it heats water under all other conditions that seems unlikely to me? The home warranty company then sent a plumber out, who took one look and said “this is obviously an electrical issue.” Home warranty company has recalled the very nice but slightly clueless electrician. He has asked if I have any ideas of what else he can check before he comes out again.

Obviously I can call another electrician at my own expense but I would really like to avoid having to do that at this stage. I would also like to be able to shower without remembering to turn off the heat.

Edit to answer questions from comments:

• Re: Heating System & Emergency/Auxiliary/Resistive Heat: It’s a Carrier Infinity heat pump system. The failure occurs when normal heat pump heat runs, regardless of resistive auxiliary heat kicking in (according to readout on Carrier smart thermostat, which displays what kind of heat is in use).

• Re: It used to work, and now it doesn’t: The water heater (replaced about 3 months ago due to an internal leak) and its predecessor always worked. But, they were not tested with heat till now. What the previous owners experienced, I cannot say.

• Re: Water from another part of the house: Hot water service is split between a conventional water heater on the kitchen/laundry side of house, and the tankless on the bedroom/bathroom side.

• Re: Just switch to a conventional water heater: The former home of the water heater tank was previously remodeled away. There isn’t an obvious 3x3 location without a significant renovation. I’d be more likely to have gas service installed at this point.

• Re: Electrical service amps: I don’t know. It was “upgraded” according to the (unreliable) previous owners, presumably when they moved the meter to the other side of the home and replaced the mast. Documentation is scarce. No main breaker. Panel with tankless water heater circuits highlighted:

• Re: Startup only: Tested: It definitely is not related to startup only (either way - heater on first or tankless on first, the issue manifests once both are running and persists till one is turned off).

Meter Main:

Answer 1

In the words of Captain Kirk, We need more power!

Update cancelled because OP commented that they have now verified the manual actually calls for 2 x 60A. All other issues of total load capacity still apply, but there is no longer a concern of 4 cables vs. 2 cables.

**IMPORTANT UPDATE**

Thank you to Ben Voigt and Jim Stewart for noticing that OP's highlighted breakers for the water heater are 2 60A double-breakers rather than 4 30A double-breakers. That is a major code violation and a huge safety issue. We need to see what wires are actually connected, but this (a) must be fixed and (b) may be the source of the problems.

There are 3 likely configurations:

• 8 30A (e.g., 10 AWG) wires in 4 cables

In this case, there are 2 hot wires on each breaker connection. In certain fault situations, up to 60A could flow on a 30A-rated wire without causing a breaker trip.

• 4 60A (e.g., 6 AWG) wires in 2 cables

In this case, there is only 1 hot wire on each breaker connection. The wires are protected properly by the breakers, but then joined up to each feed two heater circuits instead of one, allowing for a heater circuit fault to overload up to 60A without tripping a breaker, damaging the heater more.

• 4 30A (e.g., 10 AWG) wires in 2 cables

This is the worst of all. The wires would not be properly protected and would routinely be overloaded at 60A on 30A-rated wires.

All of these are code violations (though some worse than others). Need to carefully remove the front of the breaker panel and get a picture of the wires going into the water heater breakers.

The good news is that there is a Square D HOM 30A quad breaker so if the only issue is the breaker and there are sufficient wires (8 individual 10 AWG or larger wires, or 4 10/2 cables) then this can be fixed by replacing the 2 60A doubles with 2 30A quads.

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A tankless water seems like a regular water heater, just a little different. Instead of heating up water over a long time and letting it sit, it heats up water when you need. Same amount of total heat, maybe even a little bit less because you don't lose heat from hot water sitting around all day.

Great idea. In theory.

The reality is different. 28 kW = 117A. That is more than an entire 100A service, which was common not that long ago. It is more than half of an entire 200A service, and it uses that much when you turn on a faucet. That's twice as much (or more) than typical "maximum rate" home electric vehicle charging, and several times as much as most people need to charge an EV overnight. It is a LOT of power.

What else uses a lot of power in a typical all-electric home?

• Clothes Dryer - typically ~ 24A
• Cooking - anywhere from 20A - 40A, occasionally more
• Heating and Air Conditioning (HVAC)

HVAC, or more specifically the heating part is a big variable. With modern heat pumps, the heating part is usually comparable to the cooling part. However, many systems have emergency heat that is basically a giant toaster. Like the tankless water heating, emergency heat can use a lot of power. Emergency heat generally isn't needed until it gets very cold, but sometimes it actually does get very cold (that's why it exists) and sometimes, due to either thermostat wiring or system configuration or other problems, the emergency heat runs instead of the regular heat pump heat. The house still gets warm and it uses more power - but you don't normally notice that until you get your electric bill. But if you have emergency heat pulling 80A and tankless water heating pulling 117A and a bunch of other things going on at the same time, you can really max out your electrical system, causing all sorts of problems including problems with the tankless water heater.

There are some ways to tell for sure whether this is a problem or not. But unless you have an extreme need for tankless water heating, replace it with a tank. I am sure when your home was built that it had a regular tank water heater. Put another one in. Electric will be easy (it will use just one of the 4 existing circuits). Plumbing will require some work, but as long as you have a 3' x 3' space to put the water heater it should be fine. Since it is electric you don't have to deal with exhaust venting or gas supply. That is guaranteed to solve your peak electric demand problem, which is almost certainly the source of your current (pun intended) water heater problem.

This is not a $0 solution. But it is the right solution.

After that is resolved, post a new question with details about your heating system and maybe we can help some more.

Confirmed based on meter (CL200) and meter main (200A service disconnect) that this is 200A service. 200A is more than enough for a typical all-electric house as long as it does not include tankless water heating. But with tankless water heating @ 120A that leaves only 80A for everything else. It is plausible (but we still don't know actual current draw/rating of the heating system) that the heating system plus ordinary loads (lighting, computers, etc.) raises the entire to load to 200A or beyond. Main breaker is not likely to trip quickly unless current draw is substantially (20% or more) greater than 200A, but voltage drop can be a real issue at 200A if the entire electrical is marginal in any way.

Answer 2

Call the power company and explain the problem. I had a similar call from a homeowner and it ended up being corrosion in an electrical junction(?) box outside of the house, off of the property

Answer 3

I don't know if this is the cause of your problems... But I wonder if a screw driver, some ox guard and 10 minutes to plug in a cable would help.

One thing that has bothered me about this picture from the very start is the bottom right input wire..

Yes I know the electrician claimed that it is all good and well connected... But to me it looks like the bottom wire is not properly screwed in... I can see 1/4 of an inch of copper, and the strands are in a perfect circle.. I have put a lot of stranded wires in terminals... and they always flatten out a little bit when I tighten the terminal wire..

Additionally there is one red-striped wire connected to red... and one connected to black. While it shouldn't matter as this is 240V. I think it would look better if they were connected the same way. red-strip to red wires... black to black, etc..

Answer 4

To try to determine whether you can get the existing equipment to work for you, determine if the fault is occurring only at the startup of the HVAC heat pump.

Lower the thermostat on the HVAC to cause it to shut off, wait a min or two, then verify that the water heater is working. Shut off the hot water tap.

Then set the HVAC thermostat high enough (say 80 F) so that the heat pump will come on and stay on for say 10 minutes. After HVAC stabilizes draw hot water through the shower or the lavatory on full hot. Does the water heater perform properly or does it malfunction?

If it performs properly, then suspect that the HVAC startup transient is messing with the four breakers or the electronics of the water heater. What kind of breakers are these--standard or GFCI or GFCI/AFCI? Maybe the breakers are tripping and resetting automatically. Maybe something can be done about a start-up transient from the HVAC compressor, like a soft start kit on the outside unit.

If the water heater does not perform properly during the time the heat pump HVAC is on, determine if the electric resistance backup is on. The system may be set up so that the electric resistance b/u comes on when certain conditions are met, e.g., temp difference between inside measured and the set point, or outside temp below a certain value, etc. Read the manual on HVAC and find out how to prevent the backup resistance element from powering up.

Erratic operation of tankless WHs due to low flow

If the flow through a tankless is below a certain amount it will not come on. If on and the flow drops below a certain amount, the water heater will turn off. Check to exclude this problem by running the lavatory faucet on full hot.

If the problem was always with the shower, run the lavatory faucet on half or one-third flow while showering. If the erratic operation is no longer present, then the problem was that the shower by itself was not drawing enough hot water to keep the WH on. Simple solution is to lower the output temp of the water heater so that the shower is taken with little or no cold added in. The WH should then stay on. This problem occurs most often with a low-flow shower head.

Temporary procedure with electric tankless WH

When you want to take a shower turn the HVAC thermostat down to some low value so the heat pump does not cycle on while during the shower. Or turn it off at the thermostat. After the shower return the thermostat to its normal setting. This will buy time to try to get to the bottom of the malfunction.

Edit your original question to describe your electric service, main breaker is what size, 200 A, 150 A, . . ?

Answer 5

I’d be more likely to have gas service installed at this point.

That would in fact fix both the tankless (switching from electric to gas) and the auxiliary heat for the heat pump. Note that a heat pump needs an auxiliary heat system when the temperature gets below freezing.

In general, heat pumps used for heating (rather than cooling, where they are a lot more efficient), operate in a range. Above a certain "high balance" point, they use only the heat pump. Mine is configured at 50 degrees Fahrenheit (by the HVAC company, not by me) or about 10 degrees Celsius. Below a certain "low balance" point, they only use the auxiliary heat. Mine is configured at 35 degrees Fahrenheit (about 2 degrees Celsius). Between those points, it may use either the heat pump or the auxiliary heat.

If you try to use the heat pump at too low a temperature, it will freeze and go through a defrost cycle. A defrost cycle is essentially running the air conditioning to shift heat out of the house and into the heat pump. That's just as bad in winter as it sounds, so you want to use your auxiliary heat at that point.

Auxiliary heat in an all-electric system is resistive heat. What this answer described as a "giant toaster". You could also describe it as like the coils of an old electric range. Resistive heat is an energy hog, which is why they prefer to use the heat pump for heating unless it is cold enough to freeze the heat pump.

Auxiliary heat in hybrid gas/electric system is just a gas furnace. Gas is a very effective heating method. Above a certain point, the problem is that it is too effective and puts out too much heat. That's why they combine naturally with a heat pump, which is extremely efficient at 50 degrees Fahrenheit/10 degrees Celsius. And of course, a heat pump can also be used for cooling in the summer.

Same thing with the tankless water heater. Gas is very effective at heating things. A nice solution in a tankless water heater. If you prefer electric, I would suggest an efficient electric heat pump water heater with a tank. That would cool the house to heat the water, particularly helpful in summer.

It would of course be possible to expand your electric service to cover both resistive auxiliary heating and resistive water heating. In my opinion, switching them to natural gas makes more sense, but I prefer gas for all intense heating applications. A 200 amp service would be plenty then, and you wouldn't have to go through the hassle of switching to a 300 or 400 amp service (just the hassle of signing up for gas and replacing appliances).

I'm guessing that you currently have a 200 amp service, but it's possible that you're already at 300 (two water heaters plus HVAC plus an oven might strain 300). If you want to list out all your electric appliances (including the exact models) and provide a picture of your main electric panel, there are people here who are good with load calculations who could verify that.

From an environmental standpoint, gas vs. electric is pretty much a wash. The truth is that on the margins, they use gas to fill in the gaps with renewable electric. They would have to add a lot more renewable generation capacity (and storage capacity) before that would change. So really, you're just shifting from generating electricity with natural gas to generating heat with natural gas. There's line loss both ways. The centralized electricity generation would be more efficient than running your own local generator, but your application is heat.