Wiring for tankless water heater


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Old 05-11-21, 01:57 PM
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Wiring for tankless water heater

I'm planning to replace my 50-gallon electric water heater with a tankless one and I'm trying to figure out the best way to do it. The new one requires 3 40-Amp circuits and it is about 30-35 feet from the main panel (including going up to the attic and dropping back down).

Should I run 3 individual 8-gage wires from 40-Amp breakers in the main panel to the water heater? Or should I run a single bigger wire to a new sub-panel closer to the water heater and then run the (3) 40-amp circuits from there?

If I run a single cable from the main to a new sub-panel, what gage should that be and what size breaker should be on that circuit in the main panel? I'm guessing it would need to be 120 Amps to feed the 3 40's, but please correct me if I'm wrong. Is there anything else I need to consider?

And finally, a couple of people who I've told about this project have advised me to not replace my 50-gallon tank with a tankless because it will kill my electric bill. I don't see how that would be the case since the new one would only be heating water as needed. But am I wrong?
 

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05-11-21, 06:23 PM
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You also might want to check with the power company about the transformer feeding your home. They may have to install a larger one to carry that type of load.
 
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Old 05-11-21, 02:24 PM
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The first thing you need to do is a load calculation to see if you have enough power available. Assuming you do, if you have 6 available spaces I would install three 40 amp breakers and three runs on 8-2 NM-B cable. Assuming only two spaces are available I would then install a 125 amp 120/240 volt circuit to a sub panel and install the three 40 amp circuits from there. To feed the subpanel you could use 1/0-1/0-1/0-2 Aluminum SER cable, but I personally would use 3 - 3 #2 THHN W/1 - #6 THHN Green Ground in 1 1/2" conduit.
 
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Old 05-11-21, 03:25 PM
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This question comes up from time to time. I was an advocate for using a sub panel but many manufacturers are requiring three independent circuits to the main panel.

You need to have at least a 200A service.
As Joe mentioned.... overall load must be considered.

This heater can demand 120A. Your A/C could require 30A. Dryer requires 30A. Electric cooking can be 25A-40A. House heating, if electric, can be high. All running at the same time would be pushing the service.
 
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Old 05-11-21, 06:23 PM
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You also might want to check with the power company about the transformer feeding your home. They may have to install a larger one to carry that type of load.
 
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Old 05-11-21, 10:05 PM
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Thank you all for the feedback.

It's are all electric out here. I am already scheduled to have my service upgraded from 100A to 200A prior to installing the new water heater. I had not even thought about the possibility of needing the electric company to provide a larger feed.

I'm pretty sure my electrician said the new panel will have 40 slots. I'm only using 16 now and the current water heater breaker is taking 2 slots. So I only need 4 more. And since I really don't want to mess with conduit in the walls and attic, so it sounds like 3 separate runs is the better option. It's just a matter of the total potential load in the house as @PJmax mentioned. We have not had any problems with overloading and tripping breakers with the current 100-Amp service. So would it be safe to assume we should be OK when we get upgraded to 200-Amps since the potential net increase for the new water heater is 90A?

I'm in central Ohio and, looking at the manufacturer's guidelines, I am right on the border between the 18kW heater (2 40A circuits) being adequate or needing the 24kW model (3 40A circuits). The marginal difference in price led me to "go big" just in case.

[Thinking "out loud" here] I wonder if I could wire all 3 circuits, but only turn 2 of them on to see if that provides enough hot water and then turn on the 3rd circuit only if needed. Or would that cause a problem if it is not fully powered? I know that's probably a question for the manufacturer.



 
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Old 05-12-21, 02:48 AM
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So would it be safe to assume we should be OK when we get upgraded to 200-Amps since the potential net increase for the new water heater is 90A?
Actually it is not safe to assume anything like this. Instead of "assuming" we are SURE your electrician did a "load calculation" prior to advising you on the size of new larger service you need with the installation of an instant hot water heater that you have considered installing. It is best to ask your electrician. You should already have your answer from your electrician and it will be more of a calculational fact instead of an assumption.

We don't know what your future plans are in the way of needing/using more power for other things. We don't know how close you are to your service limit at this point even.

Giving you 40 spaces may in fact be a 20/40 panel. So your two pole breakers will actually take up 2 full spaces which would be if let's say you used tandems would have given you 4 circuits for each of the two pole breakers. They do make quads but let's not get into that. More important you speak with your electrician at this point. You electrician can give you far better more accurate answers than we can.

 
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Old 05-13-21, 08:17 AM
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It seems like a lot of money to spend for you to go tankless. Does the 50 gallon meet your needs? Are you replacing it because it's bad or do you just want to go tankless. I'd try to figure out what your ROI is for a project like this.
 
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Old 05-15-21, 05:08 PM
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It's are all electric out here. I am already scheduled to have my service upgraded from 100A to 200A prior to installing the new water heater.
All electric and you just had a 100 amp service? I find that hard to believe unless your house was 900 square feet or smaller, how much electric heat did you have?

I had not even thought about the possibility of needing the electric company to provide a larger feed.
Tolyn brought up an excellent point. Most electric utilities size their lines and equipment on calculated load. It is the customer's responsibility to inform the utility of any substantial increases in load. Failure to do so could result in failure of transformers or service drops or at the very least very low voltage. The customer usually is liable for damages to the utility equipment.
 
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Old 05-22-21, 10:08 PM
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All electric and you just had a 100 amp service? I find that hard to believe unless your house was 900 square feet or smaller, how much electric heat did you have?
It's just under 1,200 square feet. We have a heat pump with fuel oil furnace for emergency heat. A capacitor blew in the heat pump and we burned a lot of diesel this past winter!

It seems like a lot of money to spend for you to go tankless. Does the 50 gallon meet your needs? Are you replacing it because it's bad or do you just want to go tankless.
The 50 gallon tank is overkill and is costing me a lot to keep that much water hot at all times. Even with the much higher power requirements of the new unit, considering the relatively small amount of hot water we use, I think [HOPE?] it will save on our electric bill since it will only be using power to heat the water as we are using it instead of all the time.

But I am not doing all of this just for the sake of going tankless. I work from home and have a lot of electronics. We are pushing the limits of the 100-AMP service and have had some minor issues. That's why I am getting it upgraded. As @CasualJoe mentioned, it's hard to believe it hasn't been upgraded before.

At the same time, I also need to replace the well pressure tank and the old one is installed in a bad location, likely due to the size of the house and limited space. Changing the air filter for the heating/cooling system is extremely difficult and having the furnace serviced is practically impossible. It would be a very beneficial to install the new tankless water heater on the wall and put the new pressure tank where the current hot water tank is.
 
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Old 05-24-21, 02:13 PM
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It's are all electric out here.
Not really, you said you have oil heat for emergencies.
 
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Old 05-24-21, 03:45 PM
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Which region are you at?
I recommend against installing electric tankless if you leave in northern region.
Electric tankless will have hard time bringing temperature up if your cold supply water is very cold. I know this from experience. I have installed 27kW tankelss at customer's house few years ago. It never could bring water hot in winter. It was just warm enough to take shower with faucet open to full hot. They had 18kW installed before and that did not work out. So my company installed 27kW and that was barely enough. Could not go back to tanked water heater because when the customer had remodeled his condo water heater room was removed and put 18kW tankless under kitchen cabinet. A big mistake.

Tanked heater is usually better if you don't have gas or oil.
Consider propane if you really want tankless.
 
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Old 05-24-21, 06:00 PM
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The 50 gallon tank is overkill and is costing me a lot to keep that much water hot at all times.
Why not install a 30 gallon then? Tank water heaters do not run all the time, only when they need to to keep the water at a certain temp. I just don't see how a tankless that draws 90 amps when you use hot water saves you that much compared to a tank heater that uses 24 amps now and then.
 
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Old 05-25-21, 08:44 PM
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I do sincerely appreciate all of your inputs!

Not really, you said you have oil heat for emergencies.
OK. So it's not technically ALL electric. But there is no natural gas or propane. The oil is only for heat when it gets too cold for the heat pump to work. If there is a fuel oil burning tankless water heater, I haven't found one.

Which region are you at?
I'm in Central Ohio, just outside of Columbus. We're in the north, but not like Cleveland or Michigan north. I have not measured it myself, but the maps I've seen puts us in the area with ground water around 52-57 degrees.

We just got this house about 6 months ago knew it needed work. I'm getting the electric service upgraded and figured I'd do this at the same time. As I said in a previous post, one significant consideration is space. The well pressure tank is right in front of the furnace, blocking access to it. There's no telling how long it's been since that was last serviced and it's probably some sort of safety hazard too.

So I was wanting to put the new pressure tank where the current hot water tank is. But since you guys all seem to be strongly advising against switching to the tankless, it's looking like I may need a Plan B. This would have been a lot easier if you all had just give me the answers I wanted. LOL
 
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Old 05-26-21, 02:29 AM
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This would have been a lot easier if you all had just give me the answers I wanted.
We are here for answers but we are also here for advice. Many people just don't realize the cost involved with a tankless. Although the thought sounds good the practicality is not worth it most times.
 
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Old 05-26-21, 03:39 AM
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If you are on a well, that is another reason tankless is not a good idea. Most wells have significant amount of mineral and this will contaminate heating element surface very quick. You will loose efficiency, then eventually element will fail. Gas tankless will also suffer from scaling problem on well.
You can remedy this using anti-scale filter and regularly flushing the tankless.
So, when you install tankless, make sure to install anti-scale filter and service valves.
 
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Old 05-26-21, 05:46 AM
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Most wells have significant amount of mineral and this will contaminate heating element surface very quick.
One of my other improvements was to install some serious water filters. So tankless or not, I've got good clean water going to the heater.
 
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Old 05-26-21, 10:28 AM
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I checked the cold water at the kitchen faucet and it was 63 degrees. So that is presumably what would be going into the water heater.

I also reached out on a local community Facebook group to ask if anybody else near me has an electric tankless water heater and several people said they work well as long as regular maintenance is done as recommended by the manufacturer. Nobody has responded to say they have a problem with one.
 
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Old 05-26-21, 03:11 PM
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One of my other improvements was to install some serious water filters. So tankless or not, I've got good clean water going to the heater.
Unless you have whole house reverse osmosis, you should still install anti-scale filter. If you have water softener it will also help. Other standard filters does not filter calcium minerals in the water.
 
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Old 05-27-21, 02:39 PM
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I've got a Spindown filter, water softener with red out salt, and a 3-stage whole house filter that includes a sediment filter, carbon block filter, and an iron/manganese filter.
 
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Old 05-28-21, 02:34 AM
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Rottpaws it seems you have your mind made up to install a tankless. That's fine with me and it is your right to do so.

You posted this thread for answers and advice and we have given you our thoughts and concerns. Personally I would not install it because of the overall cost at the end.

I had a very wealthy repeat customer ask me about a tankless. I advised him not to install it because of the costs. I did a load calculation for him and then went ahead and did a quote in front of him. He would have had to do a complete upgrade of service. He realized it just was not worth the money. The home is only a weekend/vacation get away. So running a regular hot water heater was not viable. Would take too long for it to heat up each time they came to the house from a week or two stretch of not being there. He ended up installing a WiFi controlled hot water heater. He could turn it on hours before he even got to the house so it would be ready when he got there.
 
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Old 05-28-21, 08:17 AM
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He ended up installing a WiFi controlled hot water heater.
I'm generally not a fan of controlling things over the web, but this is an instance where it is the best solution.
 
 

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