Tankless water heaters

[mgookin]

Looking to add a tankless water heater to our facility (we have no hot water presently). In looking online at what's available, Rheem wants to sell us a 36kW heater to get 4gpm. Maybe they think we're melting ice, but we're in south Florida. I can't imagine potable water ever coming out of the pipe lower than 55F and that would be that 1-2 days per year when we get that arctic clipper and it gets down into the 40's at night. Other manufacturer's say their 1.1kW heater will do 3GPM, so we know we can't rely on manufacturer's data, from any of the manufacturers.

Two questions:
Question 1:

So here's what I came up to determine what we need assuming 100% efficiency if we want 4GPM:

So I'm thinking in the range of 4kW will be fine to get 4GPM.



Question 2:
Now to the voltage.
These heaters come in 120V & 240V single phase and also in 208V 3p.
Does everyone agree I can run a 240V heater at 208V 1p (2 legs of my 208Y) and I just need to size my breakers & conductors using the 208 in my calcs to determine the amperage?
I'm not saying that's what we'll do; I just want to affirm that that option is on the table.

Thanks!

 

[Little Bill]

If I was you I would put the option of getting a gas tankless and taking the electric option off the table!:happyyes:

 

[mgookin]

If I was you I would put the option of getting a gas tankless and taking the electric option off the table!:happyyes:

There's no gas here.
This thing is going to be used almost never.

 

[jumper]

A quick google search on a couple of models show 208V or 240V is acceptable. I know you would check your specific model when you decide.

 

[mgookin]

A quick google search on a couple of models show 208V or 240V is acceptable. I know you would check your specific model when you decide.

That's where the problem lies. The installation manuals on some of themcall for the same size wire & breaker whether you run it 120 or 240 for the same kW.
Some manuals I read have so much broken English they are worse than a Nigerian lottery e-mail.
I'm going to find something that's listed and do my own calcs. It's just resistance heat, so I think even if I end up with one that just says it's for 240 I can run it at 208 safely as long as I size the conductors & breakers for 208.

 

[jumper]

Call Rheem or Rinnai and ask. You cannot be the first person to wonder.

 

[kwired]

There's no gas here.
This thing is going to be used almost never.

Get an LP tank then you will have gas. Still a better option then electric tankless IMO.

That said what is incoming water temp. Here if it is straight from the well it will be near 55 deg F, if it is from municipal system it can vary depending on how long it was able to be tempered from sitting in the system, If you are close to a main distribution line then actual well water temp is more likely to be seen then if at the end of a run that has little usage.

If you run a 240 volt unit at 208 you will get about 25% less watts out of it, and will need to compensate for that if you want to target a specific GPM, but also need to know incoming temp and how much you want to raise the temp.

 

[iwire]

I think even if I end up with one that just says it's for 240 I can run it at 208 safely as long as I size the conductors & breakers for 208.

You know a 240 unit is going to use less amps at 208 so i am not sure what you mean about conductor and breaker sizing.

It also will not put out as much heat so you would need a higher wattage unit to get the same output.

 

[Ingenieur]

1000 btu = 0.293 kwh
1 gpm = 60 gph
Assume 65 deg F rise (55 to 120) = gal x 8.33 x (delta temp) = 540 btu (100% eff)
in 1 hr = 60 x 540 = 32400 btu = 9.7 kwh
4 gpm ~ 38 kwh close to Rheem's rating of 36

the 1 gpm at 1.1 kwh is bs

your calc is off
you are not raising 4 gal in 1 hr but 4 x 60 or 240 gph
so btu / hr = 240 x 540 or 2160 btu/min
38 kwh

 

[wwhitney]

So here's what I came up to determine what we need assuming 100% efficiency if we want 4GPM:

View attachment 16099

I think you have a few errors. I agree that 540 BTUs will raise the temperature of 1 gallon of water from 55F to 120F. So for 1 GPM you will need 540 BTU/min, or 9 BTU/sec.

Google tells me 1 BTU/sec = 1.05 kW. That means for 1 GPM you need a 9.5 kW heater, or for 4 GPM you need a 38 kW heater.

Cheers, Wayne

 

[kwired]

I think you could get by with much less kW if you had a storage tank ahead of the heater, even without any heater in the tank you can have water near room temp entering the heater and will take less kW to raise it to desired outlet temp.

 

[gar]

161119-1203 EST

mgookin:

In the first post you have the wrong units of measure. A BTU is a unit of energy, and so is a kWh.

From one web site I obtained the value of 0.0002931 kWh to raise 1# of water 1 F. Using 8.34 # of water per gallon we get 0.0002931*8.34 = 0.00244 kWh per gallon per degree F.

For a 65 degree rise it takes 0.00244*65 = 0.159 kWh for 1 gallon. Thus, 4 gallons of water raised 65 F takes 0.636 kWh. If this rise is to occur in 1 minute, then it takes a power level of 60*0.636 = 38.2 kW.

If you take a 38 kW heater designed for 240 V and operate it at 208 V, then the output power will drop to 75% of its 240 V specified value. That means for the desired temperature rise that flow rate will have to be limited to 3 GPM.

.

 

[mgookin]

Thank you to the last 3. That's what I was looking for.

 

[Ingenieur]

Tankless are more efficient
how much more depends on usage
capital cost is more but they last longer
http://energy.gov/energysaver/tankless-or-demand-type-water-heaters

 

[junkhound]

I think you have a few errors. I agree that 540 BTUs will raise the temperature of 1 gallon of water from 55F to 120F. So for 1 GPM you will need 540 BTU/min, or 9 BTU/sec.

Google tells me 1 BTU/sec = 1.05 kW. That means for 1 GPM you need a 9.5 kW heater, or for 4 GPM you need a 38 kW heater.

Cheers, Wayne

What is (sadly) amazing is that it took 8 answers to get to your (and gar's) correct answer !

 

[kwired]

What is (sadly) amazing is that it took 8 answers to get to your (and gar's) correct answer !

Electricians typically aren't determining what size of water heater is needed for an application - that is usually the plumber or mechanical designer, us electricians just connect whatever was provided in most instances, and in the case of instant electric water heater often have to tell the owner they will need to double the electric service capacity because of that water heater.

 

[ptonsparky]

Electricians typically aren't determining what size of water heater is needed for an application - that is usually the plumber or mechanical designer, us electricians just connect whatever was provided in most instances, and in the case of instant electric water heater often have to tell the owner they will need to double the electric service capacity because of that water heater.

I told one of my customers that his 150 amp service was not big enough to run one plus add the distance to the transformer and he was looking for problems....He had already installed and successfully operated it for several months before talking to me about it.hmy: So much for my knowledge of anything.

 

[kwired]

I told one of my customers that his 150 amp service was not big enough to run one plus add the distance to the transformer and he was looking for problems....He had already installed and successfully operated it for several months before talking to me about it.hmy: So much for my knowledge of anything.

The non continuous nature of such a load makes it possible to get away with it and have little or no troubles, nature of other loads connected to same supply has an impact as well. However voltage drop can be one issue that you must make some changes to overcome any effects that are noticed.

 

[electrofelon]

There's no gas here.
This thing is going to be used almost never.

I assume because it will be used infrequently is why you are leaning toward tankless? Despite that, I wonder if just getting a small talk unit and setting it low, like 90-100 degrees is worth considering? You would have tank losses, but capital cost would be much less and if set low, losses may be minimal. Just a thought.

I havent been happy with most of the tankless electric ones I have come across. The temp regulation is poor. some (many?) do not modulate the electrons so the output temp depends on the flow rate. Turn the faucet to part way and it gets super hot. Most (all?) have a minimum flow rate that is kinda high. My friend put one in and we had to take off the faucet diffuser to get the thing to kick on. Just be aware of that. Maybe there are better ones out there now.

 

[winnie]

The minimum flow requirement is also a problem (possibly more of a problem) with gas tankless units.

I learned the cold hard (well wet) truth during my first shower supplied by a tankless unit.

-Jon