Energy Matters Forums

[garethh]

I hope that I'm not bringing up a topic that has already been discussed (I could not find an answer but maybe I missed it).

We are looking to get a new hot water system as our old electric one is about to give up the ghost. I have heard much conflicting information about Heat Pump and Solar Hot Water systems.

Given that all brands will be slightly different, is their any general rule of thumb about the ongoing cost of a Heat Pump vs. Solar Hot Water system? Say I was looking at two systems with the same capacity, waht would be cheaper to run?

[Roge]

The very question I joined, there is very little stuff around that gives a like for like comparison between aheat pump and straight solar system

[SR76]

There are probably a whole bunch of factors to consider, the main ones that spring to mind for me are:

a) heat pump uses SOME power ALL the time, solar can use as little as NO power in summer but quite a bit in winter (all depending on where you are and how much you use)
b) if you have the option of solar + gas boost, that's by far the best bet (all the advantages of instant gas, but you only burn it when required + lower lifecycle emissions and usually cheaper energy). Heat pump pretty much limits you to electric.
c) if you have to go electric boost on solar, you can use a timer to run the booster only at night, combine this with a time-based tariff (e.g. SmartPower in WA) for very cheap electricity. I think (and I may be wrong) heat pumps need to be operating more constantly and therefore might not work so well with a time-based tariff

There are heaps of pros and cons for each so any comparison is not so simple.

Good luck,

-SR

[Sandivee]

Evacuated Tubes are by far the most efficient followed by flat plate then heat pump.

Heat pumps are only really good in the northern states of AU where there is much hotter ambient air temperature, they don't really last long and because there is a lot of moving parts if something fails you could be looking at a whole new set up - unless of course you have a split system.

Flat plates are good, but if you have a thermo-siphon set up you have extreme weights on the roof and occasionally you will need to get additional roofing structures in place to support the tank on the roof with the flat plates filled with water. Flat plates do have the same issues as solar panels where they only work optimally in direct sunlight - when there is cloud or indirect sunlight ie. outside of 10.30am - 2.30pm you may not have too much production. If you have a split system set up it is very much the same deal with weight and hours of operation however you require a pump to move the fluid around. These flat plates are generally prone to frost also so if you are in an area that can slightly get a frost - make sure you have frost protection. Flat plates can also cost in excess of $1000 to replace just one plate + install.

Evacuated tubes, in my opinion are by far the greatest. Controlled water flow by a pump means you don't get any wastage from having too much hot water like in flat plate systems and not a lot of weight on the roof. If a tube breaks it is only approx $50 to replace and you could do it yourself. Because the tubes are circular they act as a self tracking system also providing heat from first thing in the morning till just as the sun goes down. Although it may not be as much heat in the early morning and late evenings it can still contribute 40% more production on an average day.

In all of these systems you will need to throw away your off peak power, but if your solar hot water system has been designed correctly then you should only need to use the boosters 10% of the time compared to 100% of the time with a standard electric boost.

In terms of pricing cheapest heat pumps > flat plates > evac tubes expensive although the efficiency goes in the reverse direction. For ongoing costs and actual reductions in power / gas consumption evac tubes will provide the greatest reduction over time > flat plate > heat pump (unless in a hot climate)

[Millsy]

Hi Guys,

I thought i would just add, its worth noting that if you go with gas boosted you have a guaranteed cost of $25 per month for the mains gas rental, Which unfortunately i didnt realise when i bought my gas ducted heating system also the electricity use for that system

My self personally when i update my electric hot water i will be going with electric boosted solar as i want to get rid of the gas cost as i can not produce gas myself unlike electricity via Solar panels.

Since your surrounded by trees, and may have limited solar access, the Heat pump systems would be quite good as although it does use a little energy every time the temperature of the water drops below a setting (NOT using electricity all the time) it also does heat 24 hours a day throughout the year. Also perhaps you could negate the electricity use with solar panels later on.

Hope that helps
Gareth.

[SR76]

Re: gas connection - good point, if you don't already have the gas connected for cooking / heating etc. And indeed if you want a PV system to offset everything then electric boost might be for you anyway.

Re: evac tubes - yep, hadn't got into the discussion on tubes vs plates but do be aware that again there are many pros and cons of both systems and there has been a fair bit of over-selling of the virtues of tubes. A very gross generalisation is that in cloudier climates tubes will be better and in sunnier climates the performance difference is negligible and won't pay the extra cost of the tubes. Replacment of a single tube is a selling point but the tank weight issue isn't much of a selling point (close-coupled tube systems might still have a big heavy tank on the roof, and the plates are not full of water - only a handful of pipes within them are).

"ALL the time" was an exaggeration for heat pumps, but indeed most of the time. And they work best when the ambient temp is highest (i.e. mid-day, when peak electricity tariffs might apply), so setting a timer to operate them at night won't help you at all (except encourage you to take very short showers).

[Roge]

Question re evacuated tubes, how well do they cope with hail especially the large stuff that we occasionally get?

I have no mains gas connected nor can we get it so it would have to be of the bottled variety (advantage would be its pre paid for I suppose)

I have a large North facing roof with a 25deg pitch, we are in the Hills north of Adelaide so get a frost in winter but plenty of sun most of the year, every plumber says heat pump but that may because they do an easy install at ground level.

As I have said earlier like for like comparisons are scarce so thanks for the responses so far food for thought.

[Sandivee]

there has been a fair bit of over-selling of the virtues of tubes

They are talked up evacuated tubes a little bit - however there is bugger all that can go wrong. I have never seen a tube lose its vacuum. Heard of it though - usually due to poor manufacturing their performance is good and as you said both the flat plates and tubes with work great in summer - once again if sized correctly.

Gas! Go for it. Be aware of using LPG bottles for your SHW and general usage such as cooking heating etc.
I have heard of issues with this set and I am confident that you would want a separate LPG bottle for your SHW.

and the plates are not full of water

Not FULL of water but still has a decent amount of water flowing through them. A lot more than an evacuated tube manifold. Not really an issue that needs to be brought up imo.

[Gordon-Loomberah]

Question re evacuated tubes, how well do they cope with hail especially the large stuff that we occasionally get?

Mine are the thick walled variety, supposedly good for 35mm hail. Also keep in mind than the greater the angle from horizontal you have them tilted, the less severe the hail impact will be. So far they have survived 20mm hail with no problems, but with 108 tubes I have plenty of exposure to big hail if it occurs- fortunately giant hail falls much more sparsely then the smaller stones.
I have thought about putting bird netting over them, pulled tight, in order to reduce any impacts. It would have minimal shading effect, although thats not a problem in summer anyway.

Gordon

[allenac]

Heat Pump vs. Solar Hot Water
I wish to mention that all Heat pump HWS have noise problem. The noise cames from its fan and the compressor. Some models (I read from the product review) also had ice deposits near the compressor.
The location of the heat pump is very important. It should not be placed near your bedrooms, or worse, near your neighbours. The biggest selling point for heat pump HWS is that it doesn't need to add anything on your roof.
Allenc