A client of mine wants to know what size electric water heater he can use in place of a 98k btuh gas heater. I used an online calculator and came up with 28722 watts, call it 29 kw. Does that sound right? Also, does this mean he needs 120 amps @ 240 vac?
btu to watts?
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Assuming his current gas-fired water heater is an appropriate size for his needs, just look up the first-hour rating of that model and find a comparable electric tank. Don't be surprised if you need multiple electric units to do the same job...98k btu/hr is a decent size burner.
Most electric tanks have two 5000 watt elements.
Most electric tanks have two 5000 watt elements.
Originally Posted By: eljefino
what's his motivation? Most would rather have gas ($$$).
Agreed. Gas is cheaper and if you have several people taking showers in a short period of time it heats the water quicker than electric.
Agreed. Gas is cheaper and if you have several people taking showers in a short period of time it heats the water quicker than electric.
240amps at 120VAC
120Amps at 240VAC
Quite a power monger that doesn't make economical sense.
Not to mention that it would not be possible in most residential applications (at 120V).
120Amps at 240VAC
Quite a power monger that doesn't make economical sense.
Not to mention that it would not be possible in most residential applications (at 120V).
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Originally Posted By: eljefino
what's his motivation? Most would rather have gas ($$$).
This is a commercial TI job, and there is currently no flue vent installed. It's a multi-story building, and we're on the first floor. I'm looking at power-vent models which would allow us to vent horizontally, but a commercial electric water heater may be the most practical in this situation.
This is a commercial TI job, and there is currently no flue vent installed. It's a multi-story building, and we're on the first floor. I'm looking at power-vent models which would allow us to vent horizontally, but a commercial electric water heater may be the most practical in this situation.
One other thing to note is that the calculation that was made was a direct conversion of the gas appliance. An electric heater would not have the same efficiency, thus would need to be larger, if the total btu output is required in the existing unit.
Originally Posted By: Gotch
One other thing to note is that the calculation that was made was a direct conversion of the gas appliance. An electric heater would not have the same efficiency, thus would need to be larger, if the total btu output is required in the existing unit.
Really? I thought gas appliances were rated by energy input in the gas, not their actual heat output? And electrical heating was usually assumed to be 100% efficient?
Anyways a quick search shows that Rheem makes up to 81kw electric tanks at 480V. Maybe a gas power vented isn't so expensive afterall...
I've got a Rheem 75k btu gas powervented for my house(it heats the house as well) and I don't recall it being that expensive and its exhaust is just 3" black ABS pipe
One other thing to note is that the calculation that was made was a direct conversion of the gas appliance. An electric heater would not have the same efficiency, thus would need to be larger, if the total btu output is required in the existing unit.
Really? I thought gas appliances were rated by energy input in the gas, not their actual heat output? And electrical heating was usually assumed to be 100% efficient?
Anyways a quick search shows that Rheem makes up to 81kw electric tanks at 480V. Maybe a gas power vented isn't so expensive afterall...
I've got a Rheem 75k btu gas powervented for my house(it heats the house as well) and I don't recall it being that expensive and its exhaust is just 3" black ABS pipe
Ok, so does anyone know if the calculation itself is correct?
Originally Posted By: Stelth
A client of mine wants to know what size electric water heater he can use in place of a 98k btuh gas heater. I used an online calculator and came up with 28722 watts, call it 29 kw. Does that sound right? Also, does this mean he needs 120 amps @ 240 vac?
Yes your conversion is correct.
98000 btu/h = 28700 watts
Gas has a high energy density. This is the reason Gas hot water tanks recover quickly. A typical electric tank will recover slower and if you convert to an electric tank you will typically go to a larger tank to get the equivalent supply of hot water.
For example a typical home gas hot water tank is 50 usgal and about 40,000 btu/hr (12,000 watts).
To get a similar supply of hot water a 65 to 75 usgal, 9000 watt electric tank is installed and it will likely still underperform the smaller gas unit.
Also, typical standard gas hot water tanks are very inefficient. They run about 55% efficient. On the other hand electric tanks are 100% efficient other than the heat loss from the tank itself. There are higher efficiency gas units available.
To properly size the electric tank we need to know the size of the gas tank and the type of service. A good plumbing supply store can do this for you.
A client of mine wants to know what size electric water heater he can use in place of a 98k btuh gas heater. I used an online calculator and came up with 28722 watts, call it 29 kw. Does that sound right? Also, does this mean he needs 120 amps @ 240 vac?
Yes your conversion is correct.
98000 btu/h = 28700 watts
Gas has a high energy density. This is the reason Gas hot water tanks recover quickly. A typical electric tank will recover slower and if you convert to an electric tank you will typically go to a larger tank to get the equivalent supply of hot water.
For example a typical home gas hot water tank is 50 usgal and about 40,000 btu/hr (12,000 watts).
To get a similar supply of hot water a 65 to 75 usgal, 9000 watt electric tank is installed and it will likely still underperform the smaller gas unit.
Also, typical standard gas hot water tanks are very inefficient. They run about 55% efficient. On the other hand electric tanks are 100% efficient other than the heat loss from the tank itself. There are higher efficiency gas units available.
To properly size the electric tank we need to know the size of the gas tank and the type of service. A good plumbing supply store can do this for you.
JHZR2
Staff member
Originally Posted By: Gotch
One other thing to note is that the calculation that was made was a direct conversion of the gas appliance. An electric heater would not have the same efficiency, thus would need to be larger, if the total btu output is required in the existing unit.
i think you have that backwards.
Electric heat is roughly 100% efficient, as every electron that goes in produces heat as it is a pure resistive load.
Fired water heaters are at best 62% efficient. Specialty ones like power vent and indirect fired via mod/con boilers can get into the low 90% range.
So a 98k BTU burner on a 62% efficient fired heater may only be putting 60k-ish BTU into the heating work, the rest serving as draft for exhausting. Thus the electric heater need only be sized at roughly 60k BTU.
One other thing to note is that the calculation that was made was a direct conversion of the gas appliance. An electric heater would not have the same efficiency, thus would need to be larger, if the total btu output is required in the existing unit.
i think you have that backwards.
Electric heat is roughly 100% efficient, as every electron that goes in produces heat as it is a pure resistive load.
Fired water heaters are at best 62% efficient. Specialty ones like power vent and indirect fired via mod/con boilers can get into the low 90% range.
So a 98k BTU burner on a 62% efficient fired heater may only be putting 60k-ish BTU into the heating work, the rest serving as draft for exhausting. Thus the electric heater need only be sized at roughly 60k BTU.
Originally Posted By: JHZR2
Electric heat is roughly 100% efficient, as every electron that goes in produces heat as it is a pure resistive load.
Fired water heaters are at best 62% efficient. Specialty ones like power vent and indirect fired via mod/con boilers can get into the low 90% range.
So a 98k BTU burner on a 62% efficient fired heater may only be putting 60k-ish BTU into the heating work, the rest serving as draft for exhausting. Thus the electric heater need only be sized at roughly 60k BTU.
I agree with most of your post. But the gas firing itself is better than 62% efficient. Much of the loss comes after the burner shuts off and room air continues to travel up through the heat exchanger and out the stack thereby cooling the hot water.
Electric heat is roughly 100% efficient, as every electron that goes in produces heat as it is a pure resistive load.
Fired water heaters are at best 62% efficient. Specialty ones like power vent and indirect fired via mod/con boilers can get into the low 90% range.
So a 98k BTU burner on a 62% efficient fired heater may only be putting 60k-ish BTU into the heating work, the rest serving as draft for exhausting. Thus the electric heater need only be sized at roughly 60k BTU.
I agree with most of your post. But the gas firing itself is better than 62% efficient. Much of the loss comes after the burner shuts off and room air continues to travel up through the heat exchanger and out the stack thereby cooling the hot water.
JHZR2
Staff member
Originally Posted By: mva
Originally Posted By: JHZR2
Electric heat is roughly 100% efficient, as every electron that goes in produces heat as it is a pure resistive load.
Fired water heaters are at best 62% efficient. Specialty ones like power vent and indirect fired via mod/con boilers can get into the low 90% range.
So a 98k BTU burner on a 62% efficient fired heater may only be putting 60k-ish BTU into the heating work, the rest serving as draft for exhausting. Thus the electric heater need only be sized at roughly 60k BTU.
I agree with most of your post. But the gas firing itself is better than 62% efficient. Much of the loss comes after the burner shuts off and room air continues to travel up through the heat exchanger and out the stack thereby cooling the hot water.
That is true, as it makes some change in terms of firing rate. Good point.
But it is also an important consideration for TOTAL energy use, as the lack of stack losses is large.
Still, what would we say the actual firing efficiency is? 75%? I doubt it is much better than a standard vented boiler that is what???
Edit: I would assume that a power vent tanked water heater would be pretty close to the ideal firing efficiency for a standard vented non mod-con WH.
Originally Posted By: JHZR2
Electric heat is roughly 100% efficient, as every electron that goes in produces heat as it is a pure resistive load.
Fired water heaters are at best 62% efficient. Specialty ones like power vent and indirect fired via mod/con boilers can get into the low 90% range.
So a 98k BTU burner on a 62% efficient fired heater may only be putting 60k-ish BTU into the heating work, the rest serving as draft for exhausting. Thus the electric heater need only be sized at roughly 60k BTU.
I agree with most of your post. But the gas firing itself is better than 62% efficient. Much of the loss comes after the burner shuts off and room air continues to travel up through the heat exchanger and out the stack thereby cooling the hot water.
That is true, as it makes some change in terms of firing rate. Good point.
But it is also an important consideration for TOTAL energy use, as the lack of stack losses is large.
Still, what would we say the actual firing efficiency is? 75%? I doubt it is much better than a standard vented boiler that is what???
Edit: I would assume that a power vent tanked water heater would be pretty close to the ideal firing efficiency for a standard vented non mod-con WH.
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Originally Posted By: Gotch
One other thing to note is that the calculation that was made was a direct conversion of the gas appliance. An electric heater would not have the same efficiency, thus would need to be larger, if the total btu output is required in the existing unit.
What I was trying to get at was that if you are looking for the same performance changing to electric, it would have to be larger tank/higher wattage due to the slower recovery time.
You may be able to go smaller if the existing gas unit is being under utilized. Has your client run out of hot water in the past?
One other thing to note is that the calculation that was made was a direct conversion of the gas appliance. An electric heater would not have the same efficiency, thus would need to be larger, if the total btu output is required in the existing unit.
What I was trying to get at was that if you are looking for the same performance changing to electric, it would have to be larger tank/higher wattage due to the slower recovery time.
You may be able to go smaller if the existing gas unit is being under utilized. Has your client run out of hot water in the past?
OK. Firstly, thanks for all the advice. There are a gazillion different water heaters available, so before I started going through first-hour ratings and recovery, I at least wanted to know what ballpark I would be looking in.
I found an AO Smith 24 kw electric that looks like it will do the trick. I'm a plumbing contractor, not an engineer, and engineers usually specify water heaters on a commercial job of this size. So I'm going to ask the general contractor to submit this water heater to the engineer and get it approved before we move forward.
I found an AO Smith 24 kw electric that looks like it will do the trick. I'm a plumbing contractor, not an engineer, and engineers usually specify water heaters on a commercial job of this size. So I'm going to ask the general contractor to submit this water heater to the engineer and get it approved before we move forward.
Don't forget that a heat pump can use far less electrical power than a resistance heater, for the same BTU output.
I have a hybrid water heater. It has a heat pump and electric elements. The 2000W electric element has exactly the same "real world" BTU input to the water as the 600W heat pump!
I have a hybrid water heater. It has a heat pump and electric elements. The 2000W electric element has exactly the same "real world" BTU input to the water as the 600W heat pump!
Originally Posted By: Cujet
Don't forget that a heat pump can use far less electrical power than a resistance heater, for the same BTU output.
I have a hybrid water heater. It has a heat pump and electric elements. The 2000W electric element has exactly the same "real world" BTU input to the water as the 600W heat pump!
+++ Good advice for warm or mild climates especially since OP's posting from California.
Don't forget that a heat pump can use far less electrical power than a resistance heater, for the same BTU output.
I have a hybrid water heater. It has a heat pump and electric elements. The 2000W electric element has exactly the same "real world" BTU input to the water as the 600W heat pump!
+++ Good advice for warm or mild climates especially since OP's posting from California.
I briefly considered a hybrid water heater until I realized that my water heater is in a heated space. All the heat sucked out to heat the water would have to be replaced by the main heat pump.
They are the way to go if they are in a garage in a moderate climate.
Ed
They are the way to go if they are in a garage in a moderate climate.
Ed
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