Revisiting hot water heat pumps

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AklBen

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#296123 23-May-2022 15:27

Did a google on the other threads here and they're all about 2-3 years old at least.

Anyone got views on if the market here has moved and payback on these has improved?

A quick scan sees reputable brands now retailing around $3-4k, which is around $1-2k more than a standard hot water cylinder. If your hot water cylinder is nearing replacement and you're keen on potential savings - it might be worth it?

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38 posts

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#2917718 23-May-2022 18:54

I've installed a Reclaim Energy Co2 HWHP, with a 300L Rheem internal tank (recommended alternative to their own tank) in my new build.

It's been running for 2 weeks now, prior to that I was using the 3kw element, overall energy consumption is less however I don't have Kwh meter on the system itself (I'll trial one in due course)

So far it has been working as advertised, seems to recover quick after hot water use, faster than the element did and is quiet.

The outdoor unit looks like it belongs to a split type AC system, so not too unsightly.

I can't comment on pay back yet but tank and HP cost around $7500, compared to other water heating methods this is supposed to be the cheapest to run - not to set up though.

dasimpsonsrule

132 posts

Master Geek

#2917729 23-May-2022 19:47

Funny timing, I have been working on putting together a spreadsheet recently to try to help decide between the different options. The pricing is mostly what I can find online, a trade account with Mico or similar would probably get some good prices.

insane

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#2917741 23-May-2022 22:13

I looked a few months ago, but couldn't get past the difference in costs between units available here vs the US.

At the current prices I'll be sticking with my efficient Rinnai EF24 continuous gas system for a while longer.

Mattnzl

274 posts

Ultimate Geek

#2917803 24-May-2022 09:20

dasimpsonsrule:

Funny timing, I have been working on putting together a spreadsheet recently to try to help decide between the different options. The pricing is mostly what I can find online, a trade account with Mico or similar would probably get some good prices.

Great spreadsheet thanks. Question though: would you be more likely to run the HPHW system during the day when warmer air temp? This would change the cents per kwh.

The big question looks to be: how long will the units last for? If less than 10 years then no benefit.

Would also be interesting to compare standard cylinder with solar PV direct to element heating.

timmmay

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#2917807 24-May-2022 09:48

I don't know how much difference the outside temperature makes to heat pump efficiency. I've search for that info a few times but never found any useful information.

With the rise of plans such as Contact's Good Night or EK MoveMaster you can get your hot water heating more cheaply with a standard cylinder. 3 hours in the evening is enough to heat most cylinders up from dead cold. On MoveMaster we use the free hour and the half price power at night to reduce our water heating cost significantly. Both require a timer on your hot water, which would cost around $500 to supply and fit. This should be weight up against the heat pump hot water cylinder. You can of course combine the two.

Nate001

607 posts

Ultimate Geek

#2917822 24-May-2022 10:33

I think the case for heat pump hot water does not stack if you only use it for hot water at the taps. The real benefit is when you start using hot water for home heating (radiators, underfloor heating) or large volumes like swimming pools. For normal domestic use its hard to justify the costs.

Also consider any maintenance costs, immersion heaters can easily go 20+ years without any TLC whereas I'd be expecting some sort of maintenance for HP after 5-10 years.

timmmay

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#2917974 24-May-2022 14:11

I found some information on how outside temperature affects air temperature on this website.

Based on this it looks like a ten degree difference in temperature changes your COP (coefficient of performance) by approx 0.2 - 0.3. If your COP falls from 3.3 to 3.0 you are getting 3 units of heating or cooling for every kw of power instead of 3.3 units. So, very roughly, 1% of efficiency per degree. Hardly seems worth worrying about.

raytaylor

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#2919204 26-May-2022 22:44

I was looking at putting in some solar panels and setting the electric cylinder to run during the day.
However ended up going on the contact plan and put a timer on the cylinder to run between 9pm and midnight with a short half hour boost at 6.15am

As we mostly use hot water in the evening, it works quite well for us.

My cousin is a plumber and has nothing good to say about the reliability of the heat pump water systems. He reckons he has replaced many of them and installed a standard electric boiler in his new house last year (fed with solar)

Ray Taylor

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SomeoneSomewhere

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#2919209 26-May-2022 23:00

Hot water heat pumps run at quite a high temperature differential - the hot-side temperature needs to be around 70C rather than the 35-45C of typical air-to-air units. This hurts COP but I think means they're a bit less sensitive to outdoor temperature because there's already a large gap.

I have heard comments and seen anecdotally that it's a bit more essential to do the maintenance that you should be doing anyway - namely, replacing the sacrificial anode in the cylinder every few years, if it has one.

An all in one unit is going to be much cheaper and simpler than a split unit because there's no refrigeration work to install them, but you definitely want them outside of the conditioned area so you're not pulling heat from inside your house. But you should probably reduce their exposure to weather, because tank penetrations for thermistors seems to be a weak spot...

raytaylor

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#2919217 26-May-2022 23:45

SomeoneSomewhere:

Hot water heat pumps run at quite a high temperature differential - the hot-side temperature needs to be around 70C rather than the 35-45C of typical air-to-air units. This hurts COP but I think means they're a bit less sensitive to outdoor temperature because there's already a large gap.

I always understood they use the heat pump to heat the first 45-50 degrees then resistive heating to top off the rest.

Could be wrong.

SomeoneSomewhere:

but you definitely want them outside of the conditioned area so you're not pulling heat from inside your house.

I have seen a couple of installations where they are inside an enclosed cupboard on the outside of a garage, and use the garage attic as a hot air source. Am told it makes them very efficient on a sunny day.

Ray Taylor

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AklBen

101 posts

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#2919339 27-May-2022 08:31

raytaylor:

However ended up going on the contact plan and put a timer on the cylinder to run between 9pm and midnight with a short half hour boost at 6.15am

As we mostly use hot water in the evening, it works quite well for us.

Does this mean that your HW cylinder is effectively off/cold during the day until 9pm?

Interested to hear how this works for you with showers, HW usage etc...

Jase2985

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#2919340 27-May-2022 08:39

AklBen:

Does this mean that your HW cylinder is effectively off/cold during the day until 9pm?

Interested to hear how this works for you with showers, HW usage etc...

your definition of cold is not really cold.

using no water at all my 180L hot water cylinder will do a top up heat every 7-8 hours for about 7-8 minutes a time, which is about 400w of energy each time. as long as the cylinder is properly insulated then it wont lose much heat at all.

AklBen

101 posts

Master Geek

#2919343 27-May-2022 08:43

Yeah I don't mean cold-cold. In your explanation your HW cylinder is able to heat up when it needs to. In Ray's its restricted to set times, so wondering what happens if you want to have a bath at 6pm?

Jase2985

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#2919346 27-May-2022 08:52

AklBen:

Yeah I don't mean cold-cold. In your explanation your HW cylinder is able to heat up when it needs to. In Ray's its restricted to set times, so wondering what happens if you want to have a bath at 6pm?

you will have a bath at 6pm, the water might be at 55 degrees instead of 60 degrees. The only difference you will notice will be you have to use a little more hot water to get the same temperature bath.

djtOtago

981 posts

Ultimate Geek

#2919349 27-May-2022 09:11

My 220L Cylinder can be off for 12 hours and still be plenty warm enough for a couple of showers. The shower mixer will be turned a few more degrees towards hot but not much. I'm guessing the water temp in the cylinder has probably only dropped about 10 degrees over the 12 hours. At least that's the way it feels.

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