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#261410 26-Nov-2019 11:15
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Most of our power bill seems to be consumed by our hot water cylinder. I wont name and shame brands but this is a 3 year old cylinder from a big brand - so a recent model.

 

We dont have plumbed gas at our place but I just found out there are electric instant/continuous hot water systems.

 

Something like this;

 

https://www.stiebel-eltron.co.nz/dhe-instantaneous-3-phase-water-heater

 

Has anybody got one of these? Any opinions out there? Pros/Cons? Recommendations?

 

 


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  #2360857 26-Nov-2019 11:19
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It's common for 1/3 or more of your power bill to be from hot water, more if you don't use power for heating or cooling. Is your water heating bill high because it's poorly insulated (seems unlikely) or because you use a lot of hot water?


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  #2360863 26-Nov-2019 11:27
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Do you have 3 phase power?....

 

To get it just to heat hotwater is probably not economic....

 

 

 

 

 

 

 

 


 
 
 
 


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  #2360866 26-Nov-2019 11:29
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I can't speak for electric geysers but in Europe gas ones are common and cylinders rare. The gas ones generally work well but the temperature of the water depends on the rate of flow. If you turn the shower on too far, it may go luke-warm on you. If anyone else uses the hot water at the same time, you may get a cold bath unless you have separate heaters (not common). I wonder (no experience) if electric ones will struggle to maintain good temperatures at an adequate flow rate. This should be verified before committing to anything.

 

 

 

 





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  #2360867 26-Nov-2019 11:31
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timmmay:

 

It's common for 1/3 or more of your power bill to be from hot water, more if you don't use power for heating or cooling. Is your water heating bill high because it's poorly insulated (seems unlikely) or because you use a lot of hot water?

 

 

Hi @timmmmay

 

Like I said the unit is only 3 years old. The house itself is 4 years old - so a relatively new build. When we first moved in I thought the unit must be faulty as our power bills suddenly went much higher than we were used to - our previous hot water was gas heated (but the combined gas+power was much much lower).

 

The original one in the new build developed a leak and was replaced with the same model after a few months (quality lol) but the power bill remained the same.

 

We do see a dip in usage over summer when we aren't using the heat pump but checking usage when no one is home (eg if we are out for the day or overnight) the power use is still high even though the majority of devices are powered off. All of our appliances are modern (ie < 4 years old).

 

The only time we see power usage drop right down is when I turn the cylinder off when we go away for an extended period.

 

So from my calculations the cylinder is using more than 1/3.

 

 


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  #2360872 26-Nov-2019 11:38
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How many people are in the house, and what's their hot water usage like, and where are you in the country..

 

Also what sort of plan are you on?....

 

Your Hot water will typically be on a cheaper rate if you lines company uses ripple control, but this is not universal

 

 

 

 

 

 




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  #2360874 26-Nov-2019 11:41
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wellygary:

 

Do you have 3 phase power?....

 

To get it just to heat hotwater is probably not economic....

 

 

Single phase only


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  #2360913 26-Nov-2019 13:05
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I would be curious to see what the power usage of something like that is, I would think the energy required to instantly heat hot-water like that for a high flow rate can't be insignificant. I figure that is why most instantaneous systems are gas, since it has a high calorific energy.

 

 

 

Not what you asked, but one question worth asking would be how long do you intend to stay in this house?

 

If your concerns are economic only, then you want to be sure that the cost of a new unit is actually offset by running cost savings over the time you intend to stay at that property. If the existing unit is otherwise fine, have you thought about looking into a complementary solar system to lower the amount of heating the cylinder actually is required to do?

 

Alternatively you could go look at going down the heat-pump water cylinder route, but last time I checked the running cost savings didn't ever pay off the premium it costs over other designs. If I was doing a brand new house, I would probably look at a combined water heating and a/c heat-pump design


 
 
 
 


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  #2360920 26-Nov-2019 13:26
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Hot water heated by electric elements doesn't vary much in efficiency from one heater to the next (aside for heat-pump water heaters). Changing to instant electric hot water will only eliminate insulation losses which are likely to be low anyway on a cylinder meeting AS/NZS standards.  

 

Gas hot water is not $$ efficient for low volume users, as the bottle rental/connection fees eclipse consumption charges and the equivalent electricity costs, however if you are a high volume hot water user (such as a large family or generally use lots of hot water) then you will find gas water heating to be cheaper than electricity.

 

To me, it sounds like your family is simply high users of hot water.

 

If you are committed to re-plumbing your hot water system to get the savings you had at your old place, then the logical thing to do is use the formula you had at your old place and install gas heating, not change it up and go for something unproven to you. 

 

It is also possible that you are on an unsuitable electricity plan (for instance perhaps you are on low user plan, paying high unit charges and low daily charges when you would do better on a high user plan, paying low unit charges and higher daily charges). There are ways to economise electrical heating as well - for instance changing to a day & night plan and using a timer on your hot water cylinder to use only cheaper night power.

 

 


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  #2360921 26-Nov-2019 13:29
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Have you still got an old power bill from your old place that we can compare to your new place? I'd be interested in the daily charges.

 

Does your new place have a spa pool or under floor heating?


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  #2360938 26-Nov-2019 14:15
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How much is your power bill? How much per month do you estimate is for hot water, in $ or kwh? How many people in the house, how many showers per day, and are they short / medium / long?




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  #2360941 26-Nov-2019 14:20
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tripper1000:

 

Have you still got an old power bill from your old place that we can compare to your new place? I'd be interested in the daily charges.

 

Does your new place have a spa pool or under floor heating?

 

 

I'll see if I can dig something up. Yes the daily charge has probably gone up - incidentally an email from my current provider this morning is what has me rethinking this issue.

 

No we dont have underfloor heating or a spa pool.

 

I dont think we are high users. There are 4 of us - two adults + two teenagers. However the two teenage boys aren't yet in the 30 minute shower phase and tend to skip showers (I know right).


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  #2360997 26-Nov-2019 15:58
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You've compared the cost of hotwater and total electricity usage in the old and new houses. But AFAIK you haven't told us what is different and what might explain the difference. If you went through the factors then you should be able to explain the difference.

 

Did the water usage change?

 

  • Pressure change? An old low pressure hotwater system, e.g. with pressure provided from a tank in the roof hotwater, might only output 2 liters of water a minute. A new high-pressure system can output 40 l/min. New houses tend to use more hot water.
  • Different shower head? There are low-flow shower heads that are installed to save money. Old shower heads often have low flow. New shower heads tend to be bigger with greater flow.
  • Restricter being removed? Some hot water flows to showers are restricted to save hot water. Flow restricters are not common.

 

 

Did the hot water storage system change?

 

  • Thermostat temperature. This is a big one. Higher temperatures mean greater losses.
  • Piping? Tank location, ie distance to carry the hot water, is a significant factor. Heat losses are proportional to pipe length. Pipe insulation generally reduces those losses. Losses increase with smaller pipe diameter, more bends, pipe type i.e. metal transfers heat faster.
  • Storage tank? Size because larger means more hot water can be used at one time and more heat loss. External insulation generally gives a positive economic return for many cm of insulation. Taller cylinders and baffles reduce heat loss by reducing convection currents in the tank.
  • Leaks? Unlikely but it is not unheard of. Check the overflow pipe from the tank.

 

 

Did the electricity rates increase?

 

 


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  #2361386 27-Nov-2019 10:21
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As Hammerer said, 'check the overflow pipe from the tank'. I just skimmed the thread, so I'm not sure whether you have a standard low pressure cylinder or a mains pressure. If it's the regular low pressure type, there'll be a relief valve outside the house, above the cylinder. These things nearly always drip, and it's amazing just how much hot water is wasted over the course of a day, when there's a drip every second or so.

 

You'll never stop this - it fluctuates along with the local air pressure - but you can back off (anti-clockwise) the adjusting screw on the pressure reducing valve which will be alongside the cylinder. Put a small nail through the hole on the valve stem and you can turn it easily.

 

HTH.

 

Oooops! Just realized I hadn't answered your original question about electric instant\continuous water heating. I looked into this last year, when my own (ancient) low pressure cylinder sprung a hidden leak and my power bills suddenly doubled.

 

Before I replaced it, I researched the instant\continuous system and found much negative commentary about it. Gas instant heat (with full mains pressure) is great - my daughter has just installed into her three story house and it works brilliantly, but it remains to be seen how the running costs work out. Personally, I think an electric mains pressure cylinder might be the best option. There's always plenty of hot water and the running costs don't seem to be any higher than with a low pressure system. And there are no overflow drips :-)

 

 





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  #2361501 27-Nov-2019 13:32
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I didn't answer your original question because it is important to know why your costs are so high. Then you can accurately predict the likely savings, if any, from changing water heater.

 

A three-phase electric heater would be essential for instant water heating.  I've lived with a single-phase water heater and it struggled with heating water for washing the dishes. The flow rate had to be dropped substantially to get 55 C. The positive side was that there was little change of scalding ourselves.

 

Three-phase may cost you more to install, for the fittings, because of the lines company charge, or if you need an increase in the maximum capacity available at your house. Being new, I'd assume that there is a lot of headroom but you should check that early on. If you have a further need for three-phase, like using professional tools in your garage (e.g. welding) then this may help your economic case.

 

In general, "The next most efficient water heating option is probably a heat pump hot water system, followed by wetback water heaters and instant gas."  (https://www.smarterhomes.org.nz/smart-guides/water-and-waste/hot-water-options/) But you should still confirm that your situation will generate those efficiencies.

 

The gas heaters probably have higher energy levels e.g. 40kW versus 20 for 30kW for the electric models. The So it is possible that you might hit the limit of the electric system's performance. The Rinnai range tell you how how many litres of hot water they produce (e.g. 16-26 l ) when they raise the water temperature by 25 C. https://rinnai.co.nz/Product/137/300/water-heating-gas-hot-water-systems-rinnai-infinity-a-series-rinnai-infinity-a-series-external  If you want higher temperatures than 55 C then you will need the more expensive heaters that go up to 85 C. For comparison, a mains pressure shower often uses 10 litres of hot water a minute. I've been in a 40 litre a minute shower which probably uses 20 litres of hot water a minute. That is a massive increase over my old low pressure shower used about 1 litre of hot water a minute - only 2 litres in total - and at least three times my newer low-medium pressure system which only uses about 3 l/min of hot water.

 

An instant gas system is best outside for ventilation and access whereas a storage cylinder will always be best inside the house to reduce heat losses. It may not be possible to get a gas system in the optimum position for efficient distribution of hot water so that would waste more energy.

 

The gas system will be less efficient the colder the environment - I don't think you said where you live - but the difference between Auckland and Taupo / most of the South Island they expect to be about 8 C drop at the maximum flow rate. Most of the locations below Auckland will lose about half that.

 

The electric unit you linked to isn't designed as an outdoor option which is why it say "No safe tray, flue or ventilation required". So it can go in the existing cylinder location which is likely to be a better position with some saving in energy losses.

 

You might need to treat the cold water if it is hard/acidic. If that is the case then your cylinder probably has protection too.

 

You'll most likely have to replace your instant system at 2/3 the life of the cylinder system. But I notice that the Rinnai gas systems have warranties matching the 10 years of the cylinders. Whereas the electric system you linked to is only 5 years. But it pays to check the fine print on these.


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  #2371437 10-Dec-2019 00:12
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As said, the only power bill difference between instant and a cylinder involves heat losses.  The improvement in efficiency for replacing a working cylinder system is unlikely to be cost effective.

 

The instant electric system you've linked to is a three phase system and would require three phase power to your house. Instant electric hot water works well so long as the wattage is high enough which is seldom the case. Lower wattage systems will only provide a trickle of water if they must heat water from cold and are only workable in tropical environments like Brazil where the water is already warm. Higher wattage systems are high drain devices so may not be workable in houses where there isn't enough overhead left after induction cooktops, EV chargers, heat pumps etc.

 

Although new cylinders have insulation the value of insulation differs to the point some newer cylinders are cool to touch while others are warm. If you want to reduce the amount of heat leaking from an electric cylinder you can buy a polyester insulation jacket for $55 plus hot water pipe insulation for a few dollars per metre. Most plumbers in New Zealand don't bother to insulate pipes.

 

If your cylinder is solar/HP ready you could invest in a hot water heat pump or solar thermal panel. 

 

https://www.econergy.co.nz/

 

http://atonengineering.co.nz/thermocell-about/

 

Solar thermal is impractical in high frost areas unless the cylinder has a refrigerant coil and there are good sunlight levels.


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