Tankless Hot Water Heater

[Rock86]

I recently had a job which included electric tankless hot water heaters. The house is on the bigger side, but they wanted (3) water heaters, each being 18kW by the specsheet. Because they are such a large load, am I correct in using 220.82(B) and taking the first 10kVA of the calculated load plus 40% of the remainder?

I'm not too familiar with these units, but I have had people ask about them. They seem great if you live in warmer climates, but here in NYS, because we get cold, they don't seem like they can handle the colder water temperatures. I would thing a recirculation pump would be a better option for us.

 

[Coppersmith]

Electric tankless water heaters are a terrible idea. When people call me asking about installing one I have to explain I can't add two or three 2-pole 60 amp breakers to a 150 or 200 amp panel. I ask them if they want me to increase their service to 320/400 amps and they routinely say no. If you will be installing multiple units, assume they will all be on simultaneously (because they will be sometimes). Your problems will be multiplied.

Plus in cold climates the water flow rate will have to be severely reduced to get the water hot enough to shower.

Gas units negate the electrical problem, but not the flow rate problem.

 

[Rock86]

Electric tankless water heaters are a terrible idea. When people call me asking about installing one I have to explain I can't add two or three 2-pole 60 amp breakers to a 150 or 200 amp panel. I ask them if they want me to increase their service to 320/400 amps and they routinely say no. If you will be installing multiple units, assume they will all be on simultaneously (because they will be sometimes). Your problems will be multiplied.

Plus in cold climates the water flow rate will have to be severely reduced to get the water hot enough to shower.

Gas units negate the electrical problem, but not the flow rate problem.

Thank you! I had to tell someone the same thing my first time looking at them. "Sure I can do it... but you'll need a full service upgrade and a new panel to fit everything." I guess I could propose to have a second service drop to a separate meter just for the water heaters hahaha.

 

[augie47]

IMO, your method of calculating in post #1 is correct but would be a bit hesitant to go that low.
Using that method still gives you a 115 amp load, I believe, and most residential services are not originally designed for that increase.
An added service disconnect seems like the best approach.

 

[suemarkp]

Most of the energy required is in heating the water. Standby losses are minimal with modern tanks type water heaters. So tankless doesn't really save much energy but allows endless hot water if that is really what is needed. If they don't have gas or propane, I'd recommend to them to put a standard tank type at each of the 3 locations. If they are afraid a 40 or 50 gallon tank won't have enough capacity, put another 50 gallon tank someplace else (with temp set higher) and feed the other 3 tanks from the hot water output of that one. Still won't be endless, but will give a lot of hot water before running out. And at most you'll need four 30A circuits. Tanks with 5500W elements would also heat longer/faster than 4400W ones.

Another option is one tankless at the central location to feed the other 3 tank type water heaters. This could work even better than the first scenario depending on demand (would give one shower user an endless shower, but not two), but you'll need more power to accommodate it. It would also work well for filling a large soaking tub as long as not much hot water is being used elsewhere.

 

[winnie]

Remember: the customer gets anything the want if they can state the request on a large enough check.

You can help them spend their money more efficiently by pointing to alternate methods that achieve their desired end goal, but in the end they need to be satisfied and need to pay you enough that you are satisfied.

If they want endless high flow showers with three bathrooms running at the same time, then they might actually need 3 18kW units. (They throw "parties ' and you don't want the details....) and if you take the time to do the calcs you might even suggest going to higher caoacity. This might force the additional service drop etc.

If they have normal water needs but just want hot water as soon as they turn the tap, then their true goals might be met by an entirely different setup.

I personally have a nice gas tankless unit. Cheap to run, as much hot water as I want, and large enough to shower and wash dishes on a cold day. But I want to add a small electric tankless at the bathroom sink so that I don't need to wait for hot water there.

Ask the customer what they're after, and tell them what it takes to get there. You'll both be happier. (But unless their goals are pretty strange, electric tankless is a horrible idea...in addition to another service drop the poco might need to install a new transformer.....)

Jon