Author Topic: Heat Pump Hot Water System vs Gas Hot Water  (Read 31608 times)

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Offline Kleinstein

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #25 on: November 06, 2022, 07:05:12 am »
The direct thermal solar collectors are good efficiency, but the required piping and maintenace makes them relatively expensive. With the now relatively low costs for PV panels even wth way with PV and resistive heating the buffer can make more sense. With PV the excess energy can be used, direct solar thermal has usually way to much heat in the summer and still too little in winter.
 

Offline nfmax

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #26 on: November 06, 2022, 07:46:09 am »
I agree with Kleinstein. I’ve had an evacuated tube system for 21 years now. Back then, solar PV wasn’t a realistic option. It works, but supplies too much hot water in summer and too little in winter. Maintenance costs have exceeded fuel savings except for the first 10 years or so when I was using LPG cylinders. I’ve been through two de-airing valves, a pressure gauge, and an expansion vessel.
It was installed from ladders but any roof work would now require scaffolding because of changes in the law.
 

Offline sam_

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #27 on: November 06, 2022, 01:33:48 pm »
I have been watching the recent videos about the heat pumps and battery storage with interest, as they are both things I have been considering for my own house. 

One of things i'd be interested in in how you factor in longevity when evaluating a potential purchase.  When i was looking at heat pump based hot water systems previously, the anecdotal evidence seemed to indicate that they often failed around the 5 year mark. Now whether that's true or note, I noted that you calculated the payback period to 7.6 years, but when I looked up the warranty information for Reclaim they had the heat pump listed as 6 years warranty.

So the nett result is that you would run out of warranty before you had reached payback.  I have personally always subscribed to the theory that you should reach payback before the warranty expires (otherwise you run the risk of never getting to payback), but I am curious how you view this particular aspect/attribute??
 

Offline aargee

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #28 on: November 07, 2022, 02:44:31 am »
Following your journey with interest, Dave.

We have an aging gas HWS (22 years old) and want to replace it at our leisure and not when it decides to unexpectedly throw in the towel.
At the same time, we are looking at replacing the gas cooktop with an induction unit to completely remove gas from the house.
So we will get an electrician in to run the power for both at the same time.
Our solar up here in Qld is a bit different (for us) as we got in on the 50c per kW feed in tariff about 12 years ago, we have a 2kW Solar array, so hot water will be running at night as the preference. Although I think that FIT is about to run out, we can't add solar panels or we'll lose the FIT we are on.
The other option is to add an independent solar array just to feed the HWS, but I'm not sure if that would be allowed.

- Rob.
Not easy, not hard, just need to be incentivised.
 

Offline EEVblogTopic starter

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #29 on: November 07, 2022, 04:25:53 am »
I meant the gas-powered instantaneous hot water. They basically use no electricity at all apart from the igniter (you can even get ones that don't require power at all and use water pressure to drive the spark). I understand the point about using excess energy and storing it in another form (like your hot water battery).

Oh well, I did whole video on instant electric  :-DD
The whole point is using the excess solar and disconnecting the gas, so instang gas was not an option, might have well just kept our existing gas tank in that case.

 

Offline EEVblogTopic starter

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #30 on: November 07, 2022, 04:27:58 am »
The other option is to add an independent solar array just to feed the HWS, but I'm not sure if that would be allowed.

No one can stop you doing that. Could be completely disconnected form the grid.
 

Offline EEVblogTopic starter

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #31 on: November 07, 2022, 04:29:53 am »
The direct thermal solar collectors are good efficiency, but the required piping and maintenace makes them relatively expensive. With the now relatively low costs for PV panels even wth way with PV and resistive heating the buffer can make more sense. With PV the excess energy can be used, direct solar thermal has usually way to much heat in the summer and still too little in winter.

Yes, makes no sense for use to waste roof space just for hot water. Might as well put more panels that can power everthing including hot ware, EV, 4 x aircons, stove, oven, dryer, dishwasher etc.
Makes it a no brainer for us to consolidate everthing to electric.
 

Offline DrGeoff

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #32 on: November 07, 2022, 04:36:46 am »
The direct thermal solar collectors are good efficiency, but the required piping and maintenace makes them relatively expensive. With the now relatively low costs for PV panels even wth way with PV and resistive heating the buffer can make more sense. With PV the excess energy can be used, direct solar thermal has usually way to much heat in the summer and still too little in winter.

Yes, makes no sense for use to waste roof space just for hot water. Might as well put more panels that can power everthing including hot ware, EV, 4 x aircons, stove, oven, dryer, dishwasher etc.
Makes it a no brainer for us to consolidate everthing to electric.

I'm interested in your follow-ups and how it performs. My Rheem Loline solar HWS is now 12 years old and still working (with a couple of anode replacements) although one day it will fail.
Perhaps heat-pump and more solar panels will be more appropriate when it does.
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Offline thm_w

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #33 on: November 07, 2022, 08:49:32 pm »
I have been watching the recent videos about the heat pumps and battery storage with interest, as they are both things I have been considering for my own house. 

One of things i'd be interested in in how you factor in longevity when evaluating a potential purchase.  When i was looking at heat pump based hot water systems previously, the anecdotal evidence seemed to indicate that they often failed around the 5 year mark. Now whether that's true or note, I noted that you calculated the payback period to 7.6 years, but when I looked up the warranty information for Reclaim they had the heat pump listed as 6 years warranty.

So the nett result is that you would run out of warranty before you had reached payback.  I have personally always subscribed to the theory that you should reach payback before the warranty expires (otherwise you run the risk of never getting to payback), but I am curious how you view this particular aspect/attribute??

The tank has 10 or 15 year warranty, although it requires some maintenance. So hopefully just the heat pump portion could be replaced after that time if it failed?
However, it does seem like a low lifespan if thats really typical. Home heat pumps typically last much longer than that.

In NA you can get a Rheem heat pump unit for ~$2000 USD, with 10 year warranty, although the tank is smaller: https://www.rheem.com/group/rheem-hybrid-electric-water-heater-professional-prestige-series-hybrid-electric-water-heater

Reclaim was $5500 AUD ($3500 USD).
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Offline ludzinc

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #34 on: November 07, 2022, 09:25:59 pm »
There are systems that are available for just that:

http://www.kuhnplumbing.com.au/services/sunflux-hot-water-systems

Why?  If you have maxed out your export, you can add this in parallel, for example. Mind you maxing out your export doesn’t make much sense these days but did in the past.
 

Offline EEVblogTopic starter

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #35 on: November 08, 2022, 01:46:19 am »
One of things i'd be interested in in how you factor in longevity when evaluating a potential purchase.  When i was looking at heat pump based hot water systems previously, the anecdotal evidence seemed to indicate that they often failed around the 5 year mark. Now whether that's true or note, I noted that you calculated the payback period to 7.6 years, but when I looked up the warranty information for Reclaim they had the heat pump listed as 6 years warranty.

So the nett result is that you would run out of warranty before you had reached payback.  I have personally always subscribed to the theory that you should reach payback before the warranty expires (otherwise you run the risk of never getting to payback), but I am curious how you view this particular aspect/attribute??

I don't see a difference between a heat pump and my 4 x reverse cycle aircon units, none of which have failed, and one has lasted 17 years now, another at 10 years and 6 years. Reclaim use a quality Japanese heat pump, so don't have much concern about this. Also, the payback period does not include increases in gas prices.
 

Offline EEVblogTopic starter

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #36 on: November 08, 2022, 01:52:45 am »
I'd look carefully at possible legionella infections in the hot water pipes; you really need at least 60 deg. C from kettle to tap in order to make sure legionella can't survive. IOW: there is a reason gas heaters are set to rather high temperatures like 90 degrees.

Measured the outlet of my gas tank overflow and it's 53C
But it could have a tempering valve inside, don't know.
« Last Edit: November 08, 2022, 01:54:19 am by EEVblog »
 

Offline Ed.Kloonk

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #37 on: November 08, 2022, 02:03:42 am »
I'd look carefully at possible legionella infections in the hot water pipes; you really need at least 60 deg. C from kettle to tap in order to make sure legionella can't survive. IOW: there is a reason gas heaters are set to rather high temperatures like 90 degrees.

Measured the outlet of my gas tank overflow and it's 53C
But it could have a tempering valve inside, don't know.

Make sure the plumber puts back that heat trap (those bends) on the hot pipe coming out of the tank. Connecting straight to the pipe going up into the roof is a trap for young players.
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Offline hve

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #38 on: November 09, 2022, 11:14:41 am »
Maybe unrelated...

But this thread made me wonder how feasible hybrid solar panels are today. Potentially a win win:
Improved efficiency for your PV pannels due to cooling and the direct conversion of sunlight to hot water.

https://en.wikipedia.org/wiki/Photovoltaic_thermal_hybrid_solar_collector

Main question is, how well will this peform during the cold season here in Holland... 


 

Offline Marco

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #39 on: November 09, 2022, 12:00:19 pm »
Solar collectors in colder climate need to use vacuum, it's always going to remain expensive while PV gets ever cheaper.

Maybe a plastic collector could be added without exploding cost, but then it's useless in winter.
 

Offline Kleinstein

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #40 on: November 09, 2022, 01:58:34 pm »
One can still get heat from normal flat thermal collectors in winter - if there is direct sun light. The vaccum tubes can start with less intensity, but the power is than still low.  The main problem in winter in central / northern europe is just the lack of sun, not so much the lower outside temperature. For only partial coverage (some 20%) in winter one does not need a high temperature and this help the flat collectors that don't get a high temperature from low light. If there would be large enough a market the vaccum collecor tube could also get quite a bit cheaper. Quite some part of the system costs is from plumming (can vary a lot with houses) and the buffer to hold hot water for a few days.

The big problem with the hybrid collectors is that the extra pluming for the cooling heat collector adds quite a bit to the costs and is possible point of failure.  The point with the solar-thermal collectors is to get heat at some 60-80 C, while cooling a PV collector is more like 20-50 C and thus barely usefull low temperature heat. Especially in winter the temperature would be low  There could be some use together with a heat pump and underground thermal buffer that needs to recover over the summer, still unclear if it is worth the extra effort. So I don't think a combination of PV and solar thermal is useful - maybe with a heat pump, that can use low temperature hea, or maybe with a swimming pool.
 

Offline coppice

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #41 on: November 09, 2022, 03:14:30 pm »
I'd look carefully at possible legionella infections in the hot water pipes; you really need at least 60 deg. C from kettle to tap in order to make sure legionella can't survive. IOW: there is a reason gas heaters are set to rather high temperatures like 90 degrees. Recently my youngest son couldn't go to school because they found legionella and had to flush all the water pipes in the building (which is less than a decade old).
Instant gas water heaters, whether the stand alone type or a feature of a combi boiler, only heat the water to 50 degrees or so. I've never heard of one of those leading to a disease problem.
 

Offline Marco

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #42 on: November 09, 2022, 04:00:41 pm »
Instant gas water heaters, whether the stand alone type or a feature of a combi boiler, only heat the water to 50 degrees or so. I've never heard of one of those leading to a disease problem.
I don't know about practice, but the official guidelines in the UK are 50 degrees or more at the tap, which pretty much means 55 or more at the heater. Here in the Netherlands it's the same, but the guidelines also say >60 at the tap is preferred. Of course at that point you're trading scalding risk against Legionella.
 

Offline Kleinstein

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #43 on: November 09, 2022, 04:53:16 pm »
Legionella usually is only a problem with a relatively large water reyservoir that is kept warm (but not higher than 50 C) for longer time. This makes it a problem especially for heat pumps, as there extra temperature needs more energy. The instant heaters avoid the large volume / surface, so there is usually not big worries there. More like get to temperature once or twice  a year.
Anyway it is mainly an issue with the shower, not normal taps, as it is a problem to the lungs, not to the stomach. Keeping the shower head clean, to avoid fine mist may be as important than heating the water high enough.
 

Offline coppice

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #44 on: November 09, 2022, 06:12:04 pm »
Instant gas water heaters, whether the stand alone type or a feature of a combi boiler, only heat the water to 50 degrees or so. I've never heard of one of those leading to a disease problem.
I don't know about practice, but the official guidelines in the UK are 50 degrees or more at the tap, which pretty much means 55 or more at the heater. Here in the Netherlands it's the same, but the guidelines also say >60 at the tap is preferred. Of course at that point you're trading scalding risk against Legionella.
Your water cools 5 degrees in the few metres of pipe between boiler and tap? I think you need to learn about insulation.
 
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Offline DrGeoff

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #45 on: November 09, 2022, 10:24:09 pm »
Here's a plot of actual temperature readings from the panel inlet to the storage tank (Rheem Loline 3 collectors) located near Dave in Sydney. You can see the variation in temperatures over seasons.


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Offline Marco

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #46 on: November 10, 2022, 10:55:11 pm »
Your water cools 5 degrees in the few metres of pipe between boiler and tap? I think you need to learn about insulation.

Most homes here don't have circulation piping for sanitary water, unidirectional piping shouldn't be insulated because too much water volume will stay in the danger zone too long.
 

Offline EEVblogTopic starter

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #47 on: November 11, 2022, 08:05:40 am »
 

Offline Kleinstein

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #48 on: November 11, 2022, 08:46:02 am »
The tank will not have a uniform temperature.  Chances are, that in the morning the lower end of the tank is quite a bit colder (e.g. more like 25 C). So the amount of hot water / heat will be larger than the calculated 3 kWh. Chances are the actual heat needed is a bit under (e.g. 15% for thermal loss from the off gas) the 12.8 kWh needed for the old gas system. The thermal insulation on the new system likely is also a bit better than for the old one. There was likely some extra thermal loss through the internal Air path were the flame burns.

It is more like coincidence that the electric consumption was also 3 kWh. With a reasonable, though still not great COP of about 3 this would be some 9 kWh.

The efficiency will change with the seasons, not so much with the cold water temperature (that usually does not change much from the ground acting as a buffer, especially if the water pipe a burried rather deep as they have to in colder climate to avoid freezing). The point is a change in the air temperature that effects the COP.  In summer the heat pump may work from 30 to 60 C and in win winter it may be 5 to 60 C, so possibly nearly twice as hard.

So it absolutely makes sense to run the heat pump mainly during the day. It may even make sense to shift the time when to run it to later in the day as than the air temperature is usually higher.
 

Offline EEVblogTopic starter

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #49 on: November 12, 2022, 04:37:01 am »
The tank will not have a uniform temperature.  Chances are, that in the morning the lower end of the tank is quite a bit colder (e.g. more like 25 C). So the amount of hot water / heat will be larger than the calculated 3 kWh. Chances are the actual heat needed is a bit under (e.g. 15% for thermal loss from the off gas) the 12.8 kWh needed for the old gas system. The thermal insulation on the new system likely is also a bit better than for the old one. There was likely some extra thermal loss through the internal Air path were the flame burns.

It is more like coincidence that the electric consumption was also 3 kWh. With a reasonable, though still not great COP of about 3 this would be some 9 kWh.

Yep, this was my conclusion after watching it again and havign a think. I was fooled by the 3kWh coincidental. It clearly doesn't include the COP.
I've pulled the video and asked Reclaim for their input. They claim 3kWh for 315L, and we'd be lucky to be using 150L.
We might have been a bit better off with the smaller 315L tank in this case.
The element is effectively at the top of the tank (but an inlet from the heat pump in this case), so the benefit of that is that it will heat the water quickly in manual instant boost mode, but it sucks for storage. And the thermocouple seems to be near the bottom of the tank, so we are expendign extra energy heating up the entire tank even though we don't use it.
Yes, I also think the actual heat pump energy needed would be at least 9kWh, which kinda sucks for say 150L of water.

Two days later though I now have three data points: 3.0kWh, 2.8kWh, and 2.5kWh.
Still way higher than I expected based on Reclaims website value.
If this say doubles in winter then that's going to suck. Although I'm guessing would be more like 50% more than double on a bad winter day
But even then, the point is to use our "free" excess solar to heat out water and get rid of our gas connection entirely which would still be the case.

Quote
So it absolutely makes sense to run the heat pump mainly during the day. It may even make sense to shift the time when to run it to later in the day as than the air temperature is usually higher.

Yes, I planned on doing that, switching on at say 1pm instead of 10am.
« Last Edit: November 12, 2022, 04:43:01 am by EEVblog »
 
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