Instant water heater

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Barbqranch

Barbqranch

Senior Member
Location
Arcata, CA
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Plant maintenance electrician Semi-retired
I am to install an electric tankless water heater. The manual says 40A (at 240V) and "Recommended Wire Size 8 ga". It is my understanding these have to be considered continual duty (even though it never will be) so there should be a 125% factor in breaker size. Then how can I use 8 gauge wire?
 
Barbqranch said:
I am to install an electric tankless water heater. The manual says 40A (at 240V) and "Recommended Wire Size 8 ga". It is my understanding these have to be considered continual duty (even though it never will be) so there should be a 125% factor in breaker size. Then how can I use 8 gauge wire?
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Only storage type water heaters less than 120 gallons are required to be considered continuous.
 
One more thing to consider if you had to call it a continuous load is that you are only in trouble if you use NM cable which must be sized per the 60C ampacity column.

Otherwise 8 AWG is good for 50 amps in the 75C column.
 
I have a natural gas tankless water heater, endless showers its 180,000 BTUs (convert that to kW!) But for any tankless, you will need to run the water for a minute to get hot water at your faucets. i am going to install a circ pump.
 
keith gigabyte said:
Recently installed one of those..wow electricity user. The one I had was a 3 element. Each element used 40 amp 240 volt. I tonged it when testing adding Three elements I was at 112 amps
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They take high current when in use, exactly what size of element(s) needed depends on flow rate and needed temperature rise. They still can save on energy use as they don't need to maintain heat in a tank while you are not using any hot water. You are billed for energy (watts over time) not instantaneous current, but you do need heavier equipment to carry the higher operating current which means higher initial installation costs.
 
tom baker said:
I have a natural gas tankless water heater, endless showers its 180,000 BTUs (convert that to kW!) But for any tankless, you will need to run the water for a minute to get hot water at your faucets. i am going to install a circ pump.
Click to expand...
For any water heater you need to purge the line before you get hot water at point of use, distance along with size of line determines how long it will take to purge that line.
 
kwired said:
For any water heater you need to purge the line before you get hot water at point of use, distance along with size of line determines how long it will take to purge that line.
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Yeah, and tankless adds a few more seconds to that, so it can feel like a long time. Also, with a tank you might get a little thermosiphon preheating of the water in the line, depending on the geometry.

Cheers, Wayne
 
tom baker said:
I have a natural gas tankless water heater, endless showers its 180,000 BTUs (convert that to kW!) But for any tankless, you will need to run the water for a minute to get hot water at your faucets. i am going to install a circ pump.
Click to expand...
Put circulating pump on timer so you're not having to keep water in lines hot 24hrs if you only need it for 12 hrs?
 
tom baker said:
I have a natural gas tankless water heater, endless showers its 180,000 BTUs (convert that to kW!) But for any tankless, you will need to run the water for a minute to get hot water at your faucets. i am going to install a circ pump.
Click to expand...
An automatic circulating pump is not a good fit to a tankless system since the tankless cannot deliver a low flow rate or low temperature rise.
A manual circulating pump you run for a minute before turning on the tap will save water while not wasting as much heat as a conventional circulating pump.
 
When we built our home, with fairly long runs to the kitchen and master bath, we fed power from the light switches back to the circulating pumps, so when you turned on the lights you started the water sequence. When the return line from the kitchen hit a certain temperature, the pump stopped. We found for the bathroom, it had to stay on as long as the light was on, otherwise, the shower had a sudden temperature change when the pump stopped. Altogether, a nice system.
 
Barbqranch said:
When we built our home, with fairly long runs to the kitchen and master bath, we fed power from the light switches back to the circulating pumps, so when you turned on the lights you started the water sequence. When the return line from the kitchen hit a certain temperature, the pump stopped. We found for the bathroom, it had to stay on as long as the light was on, otherwise, the shower had a sudden temperature change when the pump stopped. Altogether, a nice system.
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Was that using a tank or a tankless heater?
Where did you connect the return line from the circulator?
 
That was a tank type water heater, and the return line teed into the cold supply after the shut off valve.
 
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