Our hot water heating selection; a hot water heat pump

Since there will be no cost for heating or cooling our home with a passive solar design the biggest energy consumer in our home will be for hot water. Hot water heating accounts for around 30% of the average household energy bill and over the course of a year, most kiwi households spend more money on heating water than on space heating. This will especially be true for Northland with a mild climate so the type of hot water heating is an important one in keeping our operational costs and emissions down.

Possible options we have are:

  • LPG/gas
  • Electric hot water cylinders
  • Heat pumps
  • Solar water heating

Because of environmental reasons I never really considered gas an option for our home but luckily it turns out this is also one of the most expensive options over the lifecycle to heat your water. Since there is no gas pipeline the running costs of LPG bottles are high so gas is easily removed from the options.

Electric hot water cylinders are the most common method to heat hot water in New Zealand. They are cheap to buy but their running costs are relatively high. See below the graph of EECA (the Energy Efficiency & Conservation Authority) on the average running costs of hot water heating.

running-cost-graph

Before looking into the options of hot water heating I thought my choice would become solar water heating. A few things made me decide against it:

  • You still need a back-up in case there is not enough sun
  • My user experience wasn’t very positive having rented a bach which had a solar water heater. Not very hot water in the morning when I like to take a shower and very very hot water in the afternoon.
  • They are pretty expensive to buy (more than a heat pump) and their running costs are more than a heat pump so this seems to be the favored solution

Hot water heat pumps are more expensive to buy but they are cheap to run, always provide hot water when you need it, can be programmed to run on solar PV during the day or at night with cheaper electricity tariffs.

Heat pumps use the ambient temperature in surrounding air to heat water in the tank. Heat pump water heaters use electricity to move heat from one place to another instead of generating heat directly. Therefore, they can be two to three times more energy efficient than conventional electric resistance water heaters. To move the heat, heat pumps work like a refrigerator in reverse. You have indoor, outdoor or split systems and options to retrofit an existing hot water cylinder.

Although there are many criteria to look at when selecting a hot water heat pump like maintenance, noise, heat up times, capacity, location, my dominant trade-off between the different brands was the COP (Coefficient of Performance) and price ratio and how this would relate to operating costs. Difficulty in comparing data is that different suppliers measure their COP’s at different temperatures so some data is difficult to compare.

Below my table with hot water heat pumps I could find which are sold in New Zealand:

Brand Price COP 7° COP 15° COP 20° Volume [L] Vendor Location
Bosch compress 3000  $3,450 ? 3.3 ? 270 L Hubands Outdoor
Atlantic  $4,542 2.8 ? 4.3 270l,
4.1 200L
270 or 200 Hubands Indoor venting outside
Econergy  $5,990 3.3 3.8 4.1 Hubands Split system
Aquarian 250L indoor  $3,995 ? 2.9 at temp? ? 250 L Hotwater-cylinders indoor, air source ducted ceiling, or outdoor sheltered
Aquarian 275L outdoor  $3,985 2.8 ±3.7-4.0 4.5 275L Energy solutions Hotwater-cylinders Outdoor
Ecospring 190L  $3,279 2.5 3.2 3.4 190 L multiple plumber chains. Indoor/outdoor
Ecospring 300L  $4,495 2.5 3.2 3.4 300L multiple plumber chains. Indoor/outdoor
Parex /stiebel WWK300A  $5,019 2.3 3.2 3.6 300 L Plumbingplus Outdoor
Rheem hdi310  $5,812 ±2.3? ±2.8? 3.1 310 L plumbing shoponline Outdoor
Rheem Mdi325L  $6,465 ±2.1 ? 3 325 L Plumbingplus Outdoor
Rheem MPI 325  $4,720 ±2.3? ±2.8? 3.1 310 L Plumbingshoponline Outdoor
Rinnai Hotflo retrofit  $2,275 ? ? ? na Hotwater-cylinders Outdoor retrofit to hot water cylinder
Aqua fire MB-6kw retrofit  $3,495 ? ? ? na Aquafire retrofit outdoor unit to hot water cylinder

 

As a retrofit option is not really applicable for our situation and we would still require an additional water cylinder and a 190 L tank would be a bit too small with a family of four, the cheapest option is the Bosch Compress 3000 sold by Hubands in Whangarei. It has an above average COP of 3.3 at 15 °C which is the average temperature in our area. Looking at the best COP the Aquarian scores best and still has a reasonable price tag. The question now is how does the higher COP and price play out over the years compared to the Bosch option and a regular hot water cylinder?

Having used the EECA water heating tool on their website (https://www.energywise.govt.nz/at-home/water/) to calculate different water heating options returns an energy consumption of 3,740 kWh for a regular hot water cylinder. With the different COP’s, prices and electricity prices at day and night gives me the following table:

Energy consumption

kWh/year

Invest-ment Cost per year Lifecycle cost 5 year
Daytime Night-time Average electricity price
$0.30 $0.16 $0.23
Electric Hot water cylinder 225L 3,740  $1,800 $1,122 $598 $6,101
HWHP Bosch COP 3.3 average 15°C outdoor 1,133  $3,450 $340 $181 $4,753
HWHP Bosch COP 3.0 estimate at night outdoor 1,247 $3,450 $199 $4,634
HWHP Bosch COP 3.7 estimate indoor 1,011  $3,450 $303 $162 $4,612
HWHP Aquarian COP 3.8 outdoor 15°C 984  $3,985 $295 $157 $5,117

 

Based on this table I draw the following conclusions:

  • The extra investment in a hot water heat pump pays off. Pay-back time compared to a regular electric heater is 2-4 years depending on day or night time use.
  • The difference between location inside or outside is not huge with about $37 per year difference due to lower efficiencies.
  • Although during the day the efficiency is higher with higher daytime temperatures it is cheaper, even with the lower efficiency, to run the heat pump at night with lower prices.
  • However, when we have solar PV generation during the day it is best to schedule the hot water heating during the day to store the generated electricity and avoid exporting to the grid.
  • It takes pretty long, more than 10 years before the higher COP of the Aquarian with the higher price tag starts to pay off.

So unless there is a sale on with one of the other brands we will most likely choose the outdoor Bosch 3000 compress from Hubands.

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