Electric Tankless / Instant Hot Water Heaters

Forums › Home Workshop DIY › Electric Tankless / Instant Hot Water Heaters

davecla

61 posts

Master Geek
+1 received by user: 6

#169655 22-Mar-2015 08:02

Any one have any experience, tips around those electric tankless instant hot water heaters?

New kitchen going in a long way from the HWC. There is only the kitchen sink hot tap. Will be used to fill the sink 2, maybe 3 times a day. Seems like a waste to run a hot pipe all that way and those small under bench HWC's seems like a bad idea.

I've never seem a single sink instant heater in the wild, but have read mixed stories.

thanks in advance,
Dave

timmmay

20858 posts

Uber Geek
+1 received by user: 5350

Trusted

Lifetime subscriber

#1264832 22-Mar-2015 08:44

Relevant threads (found with google)

One
Two
Three

Should be easy for just a sink. Go into any plumbing store.

Hammerer

2480 posts

Uber Geek
+1 received by user: 802

Lifetime subscriber

#1264833 22-Mar-2015 08:52

For general infor the NZ government site Energywise doesn't bother to list the electric tankless option because gas is the preferred option here given about 2 to 3 times faster flow rates for the equivalent installed cost. The US government does ahve more info on Tankless or demand type water heaters and sizing them.

They've been discussed before here: Which continuous flow hot water system? and Deciding on a continuous flow hot water system. The latter thread refers to a graph comparing tanked and untanked among other options. They come out the same but tankless has the anytime electricity rate and tanked has the controlled rate so in your case you should get the controlled rate for tankless heating which makes it cheaper.

Having used them I'm not keen because they are usually too slow for me. So how quickly do you want the hot water and how hot do you want it to be? That largely determines the cost of your system.

Neglecting heat losses the power formula for kWh is:
Pt = (4.2 L T ) / 3600
Pt is the power used to heat the water, in kWh
L is the number of liters of water
T is the difference in temperature from what you started with in degrees Celsius

If you want 5l of water heated from 20C to 60C then Pt = (4.2 * 5 * (60 - 20) ) / 3600 = 0.23 kWh

The 3600 seconds is an hour so you can either work from how much power the element has or how quickly you want it heated.
If I want 0.23 kWh in 5 minutes then multiply by 3600/300 = 12 so I would need at least 2.8 kW element but at 80% efficiency that would be 3.5 kW.

davecla

61 posts

Master Geek
+1 received by user: 6

#1264858 22-Mar-2015 10:23

Sorry, perhaps i wasn't super clear.

I want to run a single sink. Not replace the rest of the HWC.

There are small point of use systems available but I've read mixed results about their abilities in NZ (cold water can be too cold to lift to usable temp for example)

Also gas isn't an option for me.

I've also got little faith in what someone in a plumping store will tell me. I'd rather hear first hand if possible.

thx
Dave

Aredwood

3885 posts

Uber Geek
+1 received by user: 1749

#1264993 22-Mar-2015 14:10

Just get a little underbench electric storage cylinder. Those instant electric units are fine to run a single basin. Because you don't need much flow or temperature just to wash your hands. But even a 4kW one struggles to do that. And since you want to run a kitchen sink, You will be wanting hotter water compared to a basin.

Also when comparing electric instant to instant gas. An infinity 24 has an output of 42.1kW And that is already taking into account efficiency losses in the unit. So a instant electric heater with only an 8kW output will be very poor. Yet 8kW is still 33A current draw. So it will still need a big cable to feed it. And it will take up half the capacity of a residential single phase power connection.

Also what is your main reason for wanting a second water heater for just the sink: Don't like waiting for hot water to arrive? Don't like wasting water to get hot water to the sink? Worried about extra power usage due to extra water usage?

richms

29099 posts

Uber Geek
+1 received by user: 10209

Trusted

Lifetime subscriber

#1265009 22-Mar-2015 14:17

If you have mains pressure then the thiner pipes will get hot water to your sink a lot faster than the old 25mm OD metal pipes. Due to a leak had to replace some pipe here and its probably a third of the time to get hot water to the bathroom sink/shower now than before. I just ran the new pipe all the way as the entire run of old copper looked dodgey as, and couldnt really see it well as it was wrapped in cloth of some sort. Used to wait a good 50-60 seconds for the basin mixer because it was so low flow, now its good to go way faster, and less tepid, seems to get hot prettymuch instantly as the hot water comes thru.

I wonder if the smaller stuff like used on fridges and water coolers is any good for hot water, that should hold so little water that it would be really fast.

Richard rich.ms

Niel

3267 posts

Uber Geek
+1 received by user: 80

Trusted

#1265065 22-Mar-2015 15:13

Metal pipes also need to be heated up, plastic does not (or much faster).

We had an instant water heater in South Africa. Draws a lot of power (causes lights to dip, also needs a dedicated circuit to the switchboard) and flow rate was low, but sufficient for doing the dishes. If you will use it more than once a day (or even once a day), I'd get a small storage heater. They have smaller elements so your power consumption is spread out instead of causing voltage dips.

In SA we install water cylinders in the ceiling close to where it is needed and you also benefit from solar gain (higher ambient temp. -> lower heat loss). Don't think they do it it in NZ due to the weight and issues with earth quakes.

You can never have enough Volvos!

Geekzone

Want to support Geekzone and browse the site without the ads? Subscribe to Geekzone now (monthly, annual and lifetime options).

richms

29099 posts

Uber Geek
+1 received by user: 10209

Trusted

Lifetime subscriber

#1265069 22-Mar-2015 15:17

Damn, wrong window for the last reply.

Anyway, Im wondering what efficiancy is gained by moving from metal to smaller plastic. Any stats?

I also dont get how they can say that putting an intermittant 30-40 amp load on a 63 amp supply is acceptable. During winter better hope that the other water heaters not running, and noone is cooking and using a heater before you turn on that instant hot water supply!

Richard rich.ms

Hammerer

2480 posts

Uber Geek
+1 received by user: 802

Lifetime subscriber

#1265085 22-Mar-2015 15:29

davecla: Sorry, perhaps i wasn't super clear.

I want to run a single sink. Not replace the rest of the HWC.

There are small point of use systems available but I've read mixed results about their abilities in NZ (cold water can be too cold to lift to usable temp for example)

Also gas isn't an option for me.

I've also got little faith in what someone in a plumping store will tell me. I'd rather hear first hand if possible.

thx
Dave

I did say that the electric tankless system is too slow for me. That's why gas is so popular. I gave you the calculations for you to estimate the size of system you would need so you could get a real world price that would bring you down to earth. ;)

Timmay and I linked those threads because the same issues apply whether providing water for a single sink of the whole house. I linked to the other sites to give you more background and comparisons because you didn't appear to know much about the issues. In a nutshell, what you are talking about is overly expensive compared with using your existing system or putting in a small tank as other people have commented in this and other threads.

davecla

61 posts

Master Geek
+1 received by user: 6

#1265098 22-Mar-2015 16:00

Ok, you guys have talked me out of it.

I just take a hot pipe to it from HWC and put up with a few seconds for the hot to run through.

(On tank water so low flow rate)

Cheers,
dave

SepticSceptic

2263 posts

Uber Geek
+1 received by user: 779

Trusted

#1276102 2-Apr-2015 14:00

davecla: Ok, you guys have talked me out of it.

I just take a hot pipe to it from HWC and put up with a few seconds for the hot to run through.

(On tank water so low flow rate)

Cheers,
dave

Just because you have a tank supply, doesn't mean you need to put up with a low flow rate, unless it's intentional. A properly specced water pump should equal mains pressure. My water rate for showers etc would put quite a few mains pressure water showers to shame.... - I have to intentionally restrict the shower mixer lever throw distance by putting a 3 inch ringclip around the back of the mixer collar.

The piping from the tank to the pump, and from the pump to the dwelling inlet is 1.75", at least.

jnimmo

1098 posts

Uber Geek
+1 received by user: 255

#1276895 3-Apr-2015 20:44

Not sure if it was mentioned in any of the above other threads, but I saw this one on Kickstarter last year and like the idea
http://www.myheatworks.com/
Can be used as an extender to reduce the time you'll have to wait for hot water, or can just be connected to the cold supply