Insta-Hot and Voltage Drop Solution

[infinity]

We're looking at a 120 volt, 3600 watt, 30 amp insta-hot water heater for a shower. The 30 amp branch circuit is ~200' and the contractor used #10 conductors. As you may guess the water is not hot making this an insta-luke warm water heater. Would it be possible to use a buck/boost transformer at the shower end to get the voltage up to 120 volts? My quick calculation shows a voltage of ~108 volts at the HWH. If so how would you size the buck/boost trans?

 

[drktmplr12]

To address your question, I figure the buck/boost needs to be minimum 3.6 kVA plus a safety margin to support the 3.6 kW heater. What are the sizes available? Will it cost more than replacing the feeder circuit conductors?

I have no experience specifying buck/boost devices and don't have any code references to support this, just a hunch.

I'm not sure if building code in New Jersey requires 2% on feeder and 3% on branch circuits like in Florida. This requirement is found in the energy conservation section of FBC. If so, the voltage drop for the branch is currently 10%, and no buck/boost device will change that. To me, the answer is to tell the contractor to install a properly sized branch circuit to comply with building codes, if applicable.

Why not pull #8's or #6's to improve the VD instead of installing a 3kVA eyesore? Don't forget to upsize the ground!

If the water is luke warm, I would also question if the heater is properly sized for the water velocity/pressure. If the flow and pressure is too high, the water would not have adequate retention time in the heater and the result would be luke warm water. I would ask a mechanically inclined person their opinion. Maybe a pressure or flow regulator to slow the water down a bit?


Hope this helps you figure it out.

 

[mgookin]

We're looking at a 120 volt, 3600 watt, 30 amp insta-hot water heater for a shower. The 30 amp branch circuit is ~200' and the contractor used #10 conductors. As you may guess the water is not hot making this an insta-luke warm water heater. Would it be possible to use a buck/boost transformer at the shower end to get the voltage up to 120 volts? My quick calculation shows a voltage of ~108 volts at the HWH. If so how would you size the buck/boost trans?

We went through this a couple weeks ago. 3600 watts ain't squat. It's only going to raise the temp a couple degrees. Customer can reduce flow (not open valve to full flow) to get hotter water.

Insta-hots don't raise water to a particular temp; they raise it by a particular temp, as a function of kW vs. flow rate.

AFAIK there is no graph in the public domain with these data. One day when I have some idle time I'll crunch it and plot it. Until then the consensus is

for 1 GPM you need a 9.5 kW heater, or for 4 GPM you need a 38 kW heater

and

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.

I don't know what your customer was expecting on that 3600 watt heater or what whoever sold it to them represented, but it's not a heater; it's a luke-warmer!

 

[iwire]

We're looking at a 120 volt, 3600 watt, 30 amp insta-hot water heater for a shower. The 30 amp branch circuit is ~200' and the contractor used #10 conductors. As you may guess the water is not hot making this an insta-luke warm water heater. Would it be possible to use a buck/boost transformer at the shower end to get the voltage up to 120 volts? My quick calculation shows a voltage of ~108 volts at the HWH. If so how would you size the buck/boost trans?

First I would not do a thing until I put my meter on the heating element terminals while it is running to measure the real voltage.

I can't imagine 3,600 watts being anything near enough power to supply a shower. That is like a commercial coffee maker

 

[iwire]

To address your question, I figure the buck/boost needs to be minimum 3.6 kVA plus a safety margin to support the 3.6 kW heater.

No, if he was to use a buck boost the size would be more like 0.25 KW, that is the beauty of a buck boost it only has to be sized for the amount of boost.

 

[wwhitney]

We're looking at a 120 volt, 3600 watt, 30 amp insta-hot water heater for a shower.

That heater is grossly undersized for the application. One kW of resistive power will raise 1 gpm of water 6.83 degrees F. So for a 3.6 kW heater and say 1.8 gpm of water for a shower, you get a temperature rise of 13.6 degrees F. With, say, 60 degree incoming water, you'll get 73.6 degree F outgoing water.

A heater 3 times as large would give you a 40 degree rise and be barely adequate, with 60 degree incoming water.

Would it be possible to use a buck/boost transformer at the shower end to get the voltage up to 120 volts? My quick calculation shows a voltage of ~108 volts at the HWH.

It seems to me that if you did use a buck/boost to increase the voltage at the tankless water heater, then the current on the long run would be higher than 30 amps, which would be a problem, no?

Cheers, Wayne

 

[infinity]

There are two identical showers on opposite ends of a pool, both with ~200' branch circuits. One is fed with #6's the other #10's, the shower with the #6's gets hot enough the one with the #10's doesn't. And of course the one with the #10's has the insta-hot inside of a soffit above the hung ceiling so access is very limited.

Would a buck/boost work? The branch circuit MC cable is run above a finished Sheetrock ceiling so replacing it would be more expensive than installing a BB trans near the shower.

 

[iwire]

There are two identical showers on opposite ends of a pool, both with ~200' branch circuits. One is fed with #6's the other #10's, the shower with the #6's gets hot enough the one with the #10's doesn't. And of course the one with the #10's has the insta-hot inside of a soffit above the hung ceiling so access is very limited.

Would a buck/boost work? The branch circuit MC cable is run above a finished Sheetrock ceiling so replacing it would be more expensive than installing a BB trans near the shower.

Maybe but something is not making sense.

3,000 watts is nothing when heating water for a shower.

Any chance the other heaters are using 277 volt 30 amp circuits?

 

[ptonsparky]

I'm thinking that you should apply the BB at the start of the circuit or Maybe jump it to 480 then back down with another transformer.

Sounds like they can afford whatever it takes.

 

[infinity]

Nope both heater are the same 120 volts, 3600 watts, 30 amps the only difference is the size of the branch circuit wiring so that seems to be the problem but it could be a number of other issues including the heater itself.

As Bob said it would be best to get a voltage reading on the luke warm heater but that will take some doing. For now we're just trying to come up with the easiest and cheapest solution.

 

[Fnewman]

Certainly not saying that raising the voltage at the heater wouldn't be a move in the right direction, but the calculations don't seem to support a satisfactory conclusion. Is it possible that the flows are not the same at both showers? According to the Bradford White web site, a 3.5kw, 120 v heater (29 amps) will produce a heat rise of 48 deg. F., but only at 0.5 GPM.

 

[mgookin]

Do you know where the water enters the building? The shower on the far end could get water from piping that's at ambient room temp (78F) while the one at the cold end could be getting water from outside the building in the cold climate up there. Just a thought.

That's a long pool!

 

[wwhitney]

The only way a 3.6 kw heater works for a shower is if:

(a) it has a small tank, and it supports only military style showers, or
(b) it is fed with hot water that has cooled down a bit from a long run, so it just has to boost the temperature 10-15 degrees.

Cheers, Wayne

 

[infinity]

The only way a 3.6 kw heater works for a shower is if:

(a) it has a small tank, and it supports only military style showers, or
(b) it is fed with hot water that has cooled down a bit from a long run, so it just has to boost the temperature 10-15 degrees.

Cheers, Wayne

There is only one pipe feeding the shower area and it's the building cold water. Adjacent to the shower is the insta-hot which produces the hot water. I would guess that it is possible that one shower has a house cold water temp slightly different than the other but both pipes are run though heated spaces and are insulated. Also the VD calculation seems to suggest that the #10 conductors are just too small to get the unit anywhere near the 3600 watt output. I'll see if I can get a few photo's tomorrow.

 

[JFletcher]

Can it be wired 120/240? Not even sure why anyone would make a 120V 30A heater except for maybe an RV. 10% VD would yield 21% less heating power, but even getting the full 120V at the heater while running (which not even #6 will do; ~%4.8 VD) isnt going to change lukewarm to hot.

Are the showerheads identical? My guess is no. It would be much cheaper to change to a lower flowrate head than doing anything electrical here, save for possibly converting to 240V.

eta: Speakman makes great products. One thing to note is that they come with a removeable internal water restrictor, so even if both heads are the same model, one may have the internal restrictor and the other may not. The only way to tell is to remove them and check, or take a 5 gallon bucket and time the time it takes to fill them. The water pressure at one shower may be higher than the other, so even with identical heads (and restrictors), one would flow more water than the other, with a corresponding drop in outlet temperature.

Reducing flow 20% is essentially the same as boosting power 20%. Some of the nicer, better designed low-flow heads sacrifice nothing over higher flow models, aside from cost. Still, changing a shower head is a 5 minute fix.

 

[cpinetree]

Here is a link to a pamphlet that americanheatus put out with their water heaters (I think they may be out of business now)
https://issuu.com/fdapena/docs/american_heat_booklet_09

On page 4-5, if you zoom in, it has a chart for flow vs water temperature rise as well as typical flow rates for water usage.
(Typical shower 1.5gpm - 2.5gpm)

On some of the following pages are sizes of the heater they recommended to get to a desired temperature.

 

[infinity]

Here's the nameplate:

 

[GoldDigger]

Here's the nameplate:

Broken link to image. Possibly not stored in Google as public?

Sent from my XT1585 using Tapatalk

 

[kwired]

That heater is grossly undersized for the application. One kW of resistive power will raise 1 gpm of water 6.83 degrees F. So for a 3.6 kW heater and say 1.8 gpm of water for a shower, you get a temperature rise of 13.6 degrees F. With, say, 60 degree incoming water, you'll get 73.6 degree F outgoing water.

A heater 3 times as large would give you a 40 degree rise and be barely adequate, with 60 degree incoming water.


It seems to me that if you did use a buck/boost to increase the voltage at the tankless water heater, then the current on the long run would be higher than 30 amps, which would be a problem, no?

Cheers, Wayne

buck boost can still work, but you do need to factor in increased current and the VD effects of that increased current. Is probably best to increase conductor size if possible instead of using the buck boost though, you are still losing energy in the circuit when you have a significant voltage drop.

As to why the other showers have sufficient temperature when it appears there isn't enough heater - check incoming water temp vs desired output temp, as well as flow rates. Maybe the cold shower has a different head or doesn't have a flow restrictor in it?

 

[infinity]

buck boost can still work, but you do need to factor in increased current and the VD effects of that increased current. Is probably best to increase conductor size if possible instead of using the buck boost though, you are still losing energy in the circuit when you have a significant voltage drop.

As to why the other showers have sufficient temperature when it appears there isn't enough heater - check incoming water temp vs desired output temp, as well as flow rates. Maybe the cold shower has a different head or doesn't have a flow restrictor in it?

According to the plumber the two showers are identical with the exception of the output water temp. The plumbers says that it's an electrical problem and that his HWH is working fine (not sure how he knows this). If we assume everything else is the same (plumbing hardware, input cold water temp, HWH size) then the only noticeable difference between the two is the conductor size.

So the real question is if the good HWH is operating at 3600 watts and the bad HWH is operating at about 80% of that would the current results of lukewarm water on the bad one be expected?