Revisiting hot water heat pumps

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nickb800

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#2920469 30-May-2022 16:17

The big problem I see is that the warranties don't necessarily cover the payback period.

Our case is probably a bit extreme, but currently using 2,000 KWh per annum heating a 180L mains pressure cylinder for 3 people. Hot water is on ripple control (at least 20 hours heating per day, so we don't run out) at 17c/KWh. Rest of house is on an EK MoveMaster style plan averaging 21c/KWh (or 23c/KWh if you only include 'awake' hours).

Looking at dasimpsonsrule's spreadsheet - it looks like at least a $2000 increment for HPHW over an equivalent simple mains pressure cylinder. Also shows average COP of 3.6.

Payback period for a $2k HPHW upgrade (i.e. assuming you had to replace your cylinder, but chose to spend an extra $2k to make it HPHW) varies from 7-9 years, depending on what power price is assumed. I suspect a HPHW wouldn't like being turned off randomly (being connected to ripple control like an old school cylinder), so you end up comparing an old cylinder running on 17c/KWh ripple control power with a HPHW running on 21c power, or 23c when you factor in that it's less likely to be heating overnight.

Maybe you can program your cylinder to do it's once-per-day legionnaires heating cycle overnight - but then again might be cheaper to do that cycle in the middle of the day when the ambient temperature is higher so the COP is higher.

This paper from BRANZ (albeit 10 years old) measured HWHP in service in Auckland and Wellington, finding COPs were in the 1.5 to 2.0 range, unless an extremely low or high hot water use household. I'm sure technology has improved a bit since then, but heat pumps haven't fundamentally changed in that period, so I'd be skeptical about achieving a 3+ COP in the real world. A 2.0 COP could take the payback period to 18 years, and I think counting on a heat pump to perform for 18 years without failing or servicing is closer to gambling than investing, especially once cost of borrowing money is factored in.

SumnerBoy

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#2920526 30-May-2022 16:54

Yeah I was looking this about 12 months ago and came to the same conclusion as a few others here - the HWHPs are nice and efficient but their maintenance costs tend to ruin any financial upsides. I ended up going with a simple HWC with 2x electric elements and will heat via solar during the day (bottom element) and boost at night as required (middle element).

I asked the guy installing my ventilation system (who also installs heat pumps and HWHPs) and he said he would not recommend them. They need expensive maintenance after 5yrs or so and it just destroys any payback calcs.

YMMV of course.

AklBen

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#2920739 31-May-2022 09:21

Sorry just another question for the folks using timers and only running let's say between 9pm-12am on something like Contact Goodnights...

If you ran a bath at 5pm (and yes it's hot) - then someone went to have a shower at 7pm (and there was a bit of usage here and there elsewhere) is the HWC filling itself with new cold water that means the hot water gets diluted down to the point that it's not hot enough until it starts heating based on your timing settings?

Keen to hear the pitfalls of this approach and any life adjustments required...

SumnerBoy

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#2920740 31-May-2022 09:24

The hot water is drawn off the top of the cylinder. The cold water which replaces it, comes in the bottom. So you can have 15 degree cold water at the bottom of the cylinder, and 60 degree hot water at the top.

In other words, no, if you use half your cylinder, the remaining hot water will rise to the top and be available for use, so you won't get diluted hot water.

AklBen

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#2920741 31-May-2022 09:25

^ thanks

timmmay

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#2920751 31-May-2022 09:51

I don't know how they work, but I can say if the cylinder is hot and we have two baths, a shower afterwards is easily hot enough.

I realised yesterday that I didn't have quite enough hot water heating time on, and it was falling behind, getting a little colder each day. I've increased the heating during off-peak times and a bit extra during peak, I'll keep an eye on it and make sure our free hour is still well used.

nzmatt

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#2920790 31-May-2022 10:56

Wondering what gives the better outcome. HWHP, or standard cylinder and PV up to the same cost? I know a PV would be more reliable and expected to last longer.

Prices For Solar Power Systems In New Zealand | Pricing Guide (mysolarquotes.co.nz) works out $8000/3kW - So 1.5kW of PV + standard cylinder will cost about what a HWHP costs. The PV will generate over 1500kWh per year (Which Solar Power System Size To Choose? (mysolarquotes.co.nz) gives a 3KW install generating 3700kWH/year in Auckland). In the case above - 2,000 KWh per year for hot water, and a COP of two (I do not believe Heat pump technology has improved significantly in the last 10 years, the science behind it has been around for a century), the PV appears to provides better ROI.

There is fine detail like when the power is used and generated, and power plan pricing that makes it impossible to say one or other, but for most people, I think adding PV would be a better investment than going HWHP for hot water (not talking space heating which is a different set of calculations).

If you roof is maxed out on PV, then it might be time to consider reducing power use by installing HWHP, but look to other power reductions that might give better ROI.

Another thing to also consider before going HWHP is what your appliances do. All of mine only have a cold water inlet and heat their own water as needed. No benefit from the HWHP for these unless I replace them at another couple grand. My kitchen is a long way from the rest of the house plumbing, and has its own small cylinder - again, HWHP would not produce cost savings, so for me, PV install would almost certainly be the way to go.

timmmay

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#2920800 31-May-2022 11:13

I would look at the payback period of solar compared with simply using EK MoveMaster / Contact GoodNights and fitting a hot water timer which costs about $500. Payback period is likely less than a year given how much your hot water bill drops.

nickb800

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#2920836 31-May-2022 12:06

timmmay:

I would look at the payback period of solar compared with simply using EK MoveMaster / Contact GoodNights and fitting a hot water timer which costs about $500. Payback period is likely less than a year given how much your hot water bill drops.

This, but also price up ripple control which would be a similar cost. There are some great options out there - my lines company offers night (8 hours overnight), night boost (8 hours overnight plus four in the afternoon) and controlled 20 (20 hours minimum per day, with potential disruption in morning and evening). In my case, controlled 20 was only 0.5c/KWh higher than night rate, so it made little sense to go for night or night boost. My lines company scarcely uses ripple control too, so it's a great deal.

I do a lot of timeshifting to take advantage of peak/offpeak/night tariffs with my current retailer, but having hot water (plus dryer and main living area heater) on ripple control means that I have a wide choice of retailers as virtually all of the retailers offer ripple controlled tariff options. I didn't want to spend a lot of money on a timer that would tie me to a particular retailer - although admittedly the market for timeshifting has matured now with Flick, EK, Powershop and Contact that I'm aware of.

Back to hot water, getting a larger cylinder (300L instead of 180L) and/or upping the thermostat temperature (as the tempering valve regulates temperature to the taps) are cheap/free options to reduce the risk of cold shower when using timed electricity (whether it be ripple controlled or your own timer). Also, upgrading to a stainless steel cylinder is normally a cheap (as in $300) upgrade which extends the life expectancy and warranty (from 5 to 10 years with Rinnai). It's hard to believe that some of the HPHW models (e.g. Ecospring 190 - with 3 year warranty) comes with inferior steel enamel cylinders when you're spending so much on the heat pump bit!

raytaylor

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#2921154 1-Jun-2022 00:21

SumnerBoy:

The hot water is drawn off the top of the cylinder. The cold water which replaces it, comes in the bottom. So you can have 15 degree cold water at the bottom of the cylinder, and 60 degree hot water at the top.

In other words, no, if you use half your cylinder, the remaining hot water will rise to the top and be available for use, so you won't get diluted hot water.

I have never understood how that works with the wide varying temperature being so close but it appears to be true.

Ray Taylor

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raytaylor

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#2921156 1-Jun-2022 00:40

For anyone thats interested - sorry i know this is a bit off topic from OPs original question.

By using a timer for the 9pm-Midnight this is the difference

Before the timer was installed

And After the timer was installed

The heating during the day has pretty much been shifted to the evening. I know the cylinder is reaching temperature due to the drop off. In the 9pm block it used the full 3kw in addition to the heaters etc that also start up. By 10pm the whole house has used less than 3kw therefore the hot water cylinder must have switched off during that time.

Sometimes a late shower will push the 10pm block a bit higher but I havent seen any more than 2kw used in the 11pm block so its never had a day where it hasnt reached the set temperature. Though I do have my timer set to run until 1am just in case.

You can also see the 20 minute boost it has in the 6am block.
We dont get up until 8am so assuming a base load of 0.4-0.5kw, there is approx 1kw used most likely by the water cylinder in the 6am block. 20 minutes at 3kw just so happens to equal 1kw of consumption so its being forced to stop by the timer. That tells me the cylinder is loosing more than 1kw of heat in the 6 hours between midnight and 6am.

Ray Taylor

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fe31nz

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#2921157 1-Jun-2022 00:59

nzmatt:

(I do not believe Heat pump technology has improved significantly in the last 10 years, the science behind it has been around for a century),

Heat pump technology has indeed improved over the last ten years, mostly by using better refrigerants. Our old kitchen HP used an older refrigerant (R410A) and did about 3.8. The new one (a virtually identical model) uses the newer R32 refrigerant and is rated at 4.2, and has decreased our power bill a little bit. It is an incremental change though, nothing dramatic. The R410A refrigerant was significantly better than the previous one used (R22?). And so on - I think there is another new refrigerant starting to be used this year.

Smaller heat pumps also seem to perform significantly better than larger ones - the small bedroom size units are now saying 5.8-6.0 with R32.

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