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  # 1012001 24-Mar-2014 18:38
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I thought so :)




Involuntary autocorrect in operation on mobile device. Apologies in advance.


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  # 1012078 24-Mar-2014 21:28

The COP of a heatpump water heater will never be as good as a heatpump room heater. Reason being that the hot water cylinder must be heated to 60 deg. Therefore a much larger temp difference between outside air and the cylinder - lower COP. Also some heatpump water heaters use the heatpump to only heat the cylinder to 55 and then use an element to heat from 55 to 60 deg. And then there are some that monitor the outside air temp and then change to using an element when the temp gets below a certain point.

I don't think a heatpump water is suited to Dunedin. Better suited to Auckland, Northland or other areas where the temp doesn't go below 5 deg too often. Due to the lower efficiency at lower temps this means that it is not worthwhile connecting them to night rate power. Better to have them on a timer so they only run during the day when it is warmer. I know of a house with the heatpump unit in the ceiling space. And on a timer so it will use the heat from the sun that makes the ceiling space hot during the day.

OP - is your current cylinder on Night rate electricity, and if not can you get night rate where you live? And if you can get night rate, what is the cost per kW/Hr?

As for the cost of bottled LPG, each 45Kg cylinder contains 614kW/Hr. Meaning you can easily calculate a per kW/Hr price for LPG in your area. In Auckland this is 16.4c per kW/Hr. ($101 per bottle)



If you are not already on night rate and intend going on night rate get a cylinder that has min 100L of storage per bedroom in your house. Even better 100L per person in your house.

Typically 60% more electricity is used for water heating during winter than summer. So you don't want a water heating system that is at it's lowest efficiency right when you need it the most.

What do you do currently for space heating? Maybe a woodburner with a wetback could be the best. And yes you can get mains pressure wetback systems.

 
 
 
 


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  # 1012083 24-Mar-2014 21:33
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We had a HWHP in our pre-earthquake house in Christchurch. 

Observations: 

 

  • The unit was expensive, but the cost to heat the water was about half using an element. Hot water is a major part of most home power consumption, so savings mount up pretty quickly.
  • The unit was a bit noisy. Originally we had it on a balcony next to our bedroom. That was too much. We later moved it outside the back of the house, next to the new tank, which was in a very small outdoor cupboard under the eaves.
  • HPHWs only really efficiently heat to about 55 degrees. Most councils think you need to heat to 60 to prevent bacteria multiplying. We found 55 was fine. 
  • As the water wasn't that hot we'd run out pretty quickly - family of four and a 180l tank. But the heat pump had a very fast recovery rate, about half an hour, so this wasn't a problem. 
I've had solar previously (in Sydney), with piped gas backup. If you have piped gas I'd go for that any day. The heat pump is a good compromise in a place with no gas and less than reliable sun, but the solar was very much a 'set and forget' system.



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  # 1012084 24-Mar-2014 21:36
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welcome! hot water cylinder discussion of all things :D




Involuntary autocorrect in operation on mobile device. Apologies in advance.


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  # 1012115 24-Mar-2014 22:41
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I've seen a chart showing bottled gas for a hot water cylinder is the most expensive way of heating water.

Make sure the hot water pipes are lagged as it may be losing alot of heat before it reaches the shower.

What kind of cylinder do you have now and how many people does it serve? How old is it? The old ones weren't insulated very well.

A heat pump doesn't need to be integrated into the hot water cylinder. This is an external unit and made in NZ
http://www.econergy.co.nz/

There is no need for a shower to be dribbling. It's possible to have near mains pressure for hot water without a mains pressure system.

If you get a mains pressure cylinder make sure the plumber doesn't set the valve kPa potentiometer too high at installation time because the amount of hot water pressure can be crazy (coming into shower faster than plughole can cope, tap washers spraying etc).

A solar and heat pump ready mains pressure cylinder costs around $1400 - $1600 for 250 litres.

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  # 1012118 24-Mar-2014 22:57

That chart that claims that bottles gas is the most expensive must be very old. Unless you know of a power company that charges just 16.4c per unit of power (including GST).

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  # 1012169 25-Mar-2014 07:45
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Aredwood: That chart that claims that bottles gas is the most expensive must be very old. Unless you know of a power company that charges just 16.4c per unit of power (including GST).


I wouldn't think gas water heating is 100% efficient though is it? Quite a lot of heat is lost out of the flue, so maybe once that is taken into account it is similar?
Honestly I don't know, but just a thought.

 
 
 
 


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  # 1012171 25-Mar-2014 07:51
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Mattnzl:
Aredwood: That chart that claims that bottles gas is the most expensive must be very old. Unless you know of a power company that charges just 16.4c per unit of power (including GST).


I wouldn't think gas water heating is 100% efficient though is it? Quite a lot of heat is lost out of the flue, so maybe once that is taken into account it is similar?
Honestly I don't know, but just a thought.


Yep, instantaneous hot water systems are about 80% efficient so 16c of gas becomes 19c ish Also as well as the bottle price you have cylinder rental and sometimes delivery fees.

Even mains gas suffers from an overhead issue, I pay about $25 a month for gas, but around $25 to have the gas connected.

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  # 1012202 25-Mar-2014 08:44
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You can get instantaneous electric water heaters, 1 and 3 phase versions. I have no idea how good they are though.

If you put in a decent sized cylinder, and get a dual element with the second element higher up you can use the night rate power to heat the cylinder up which in most cases is all you will need, and if you need a boost switch on the upper element. This is what I do, however its part of a solar system hence 300l cylinder and twin elements.

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  # 1012239 25-Mar-2014 09:26
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We've got a Clage brand electric hot water heater at work, and it seems to run pretty well.

Supplies  the water for a kitchen, plus 1 shower (only used by me & one other guy a day)
Temp is set at 60



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  # 1012312 25-Mar-2014 10:47
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kiwitrc: You can get instantaneous electric water heaters, 1 and 3 phase versions. I have no idea how good they are though.

If you put in a decent sized cylinder, and get a dual element with the second element higher up you can use the night rate power to heat the cylinder up which in most cases is all you will need, and if you need a boost switch on the upper element. This is what I do, however its part of a solar system hence 300l cylinder and twin elements.


If you want instant electric water heating for showering then you'll need a wiring upgrade to cater for it. A typical cylinder element is 2kW whereas continuous flow gas systems can output ten to twenty times that power 20-50kW.

You can work it out yourself because 2kW will heat one liter of water up 1C every two seconds. Here's an example: to heat water for an economical shower using 5 litres of hot water per minute the water temperature is raised from 20C to at least 50C so it will require 10kW heating which is similar to what ovens are wired for.

I agree about getting a cylinder with dual elements even if you don't have solar but here is one issue to be aware of. If your electricity plan is controlled (uses ripple control to switch off hot water heating during peak periods ie usually the daytime) then the electricity supplier can switch off the supply to both water heating elements so you cannot always boost when you need to.

[Edited to clarify that the water flow of 5l in the example is only hot so when mixed with cold water the shower flow will be somewhat higher]

gzt

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  # 1012327 25-Mar-2014 11:14
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Yeah low pressure might not be the real issue. Could be degradation over time or just less than optimum older components. Get one of those good Dunedin plumbers in to give you an overview of the situation. Might be a case of updating a few components at low cost for a vast improvement.

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  # 1012339 25-Mar-2014 11:29
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McIvor plumbing did our place, took a day and a half to install on demand gas heater after our hotwater cyclinder discovered leaking/ pouring ... (see Dunedin master splitter thread ;-) )
Very efficent.

Havn't got the final bill yet but have called them out before for the old cylinder and chat had a quick look at my gas stove as he was gas certified while he was there.

Waiting for the insurnce people + builders to sort out the water damage...

A.

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  # 1012825 25-Mar-2014 23:21
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Had gas put in here in Dunedin a couple of years ago. I also looked at Hot Water Heat pumps at the time. From memory the HWHP was about 7.5K plus installed and the gas was about 3K incl installation.
I love the convenience and high pressure of gas/mains hot water. We had no troubles with anything failing after we went to mains pressure and had to do no extra work/or fit any special valves. (But maybe we were lucky).
I do not think it is any cheaper to run gas than electric, maybe about the same price. But the payback on the HWHP extra upfront costs was definitely not economic. 

I had a conversation the other day with a masters student in renewables who claimed that solar was getting so cheap it was more economic to put in an electric cylinder and generate solar power to cover the costs, than a HWHP or gas. Never done the numbers myself, maybe he is be right.

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  # 1012831 26-Mar-2014 00:22

Handle9:
Mattnzl:
Aredwood: That chart that claims that bottles gas is the most expensive must be very old. Unless you know of a power company that charges just 16.4c per unit of power (including GST).


I wouldn't think gas water heating is 100% efficient though is it? Quite a lot of heat is lost out of the flue, so maybe once that is taken into account it is similar?
Honestly I don't know, but just a thought.


Yep, instantaneous hot water systems are about 80% efficient so 16c of gas becomes 19c ish Also as well as the bottle price you have cylinder rental and sometimes delivery fees.

Even mains gas suffers from an overhead issue, I pay about $25 a month for gas, but around $25 to have the gas connected.




Depends on what water heater you buy. Sure the base models are around 80% efficient. But you can get condensing ones that are 95% efficient

http://www.rinnai.co.nz/product_2_rinnai_infinity_ef24_external.html

As for bottle rental I was last charged $128.80 for a whole year. Which is 35.2c per day. If you are on a low user rate for power. (which you could easily be when not having to use electricity for hot water). Then you are still paying less than $1 per day fixed charges for both. The per bottle cost I quoted previously includes standard delivery. And the rental fee is the same no matter how many or few bottles you use. Will shop around when the current rental term expires. But If I decide to have 4 bottles at my place instead of 2 the rental fee will stay the same. (Im with Elgas). Some companies charge double rental fee for 4 bottles. Although there will be a 1 off fee for a location test cert if I want to have 4 Bottles. (Having more than 100Kg of LPG on site is the trigger).

As for electric hot water. Yes the element is 100% efficient, But the cylinder has heat losses so not 100% efficient overall.


(edited to add)


As for mains gas the lower per kW/hr cost will easily offset the higher fixed cost unless you only use a small amount of gas. (Approx 7c per kW/Hr in Auckland) Shouldn't be a problem if you are using gas for hot water.

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